Study the Physical Properties of Aluminum and Boron

Republic of Iraq Ministry of Higher Education and Scientific Research Al – Mustansiriyah University College of Science

Study the Physical Properties of Aluminum and Boron co-doped Zinc Oxide Thin Films as Gas Sensor for NH3 and NO2 A Thesis Submitted to the Committee of College of Science, Al–Mustansiriyah University in a Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Physics By

RASHID HASHIM JABBAR M.Sc. 2009 B.Sc. 1989 Supervised by

Dr. Anwar H. Ali Al-Fouadi Assistant Professor

2015 A.D.

1436 A.H.

‫ب ِزْدِن‬ ‫َوقُل َّر ِّ‬ ‫ِع ْل ًما‬ ‫صدق اهلل العظيم‬ ‫ورةُ طه‬ ‫ُس َ‬

‫(جزء من اآلية ‪)144‬‬

Acknowledgements

On that memorable night in my life when I was going to finish the writing of my thesis, first of all my best thanks to Allah Almighty who help me with capability to complete this research work. I would like to express my deep sense of gratitude and appreciation to my supervisor Assistant Professor Dr. Anwar H. Ali Al-Fouadi. I am grateful to the College of Science at the University of Al- Mustansiriyah and special thanks go to the physics department for their helping me completing my studies. Many thanks and gratefulness to soul of my father and my mother who supported me and alleviated difficulties I have faced during their lives, mercy of Allah may be on them. Many thanks to everyone who helped me in carrying out this study in the Ministry of Science and Technology. I apologize for who I did not mention. My thanks to all who took part in this search achievement.

Rashid Hashim

Supervisor Certification I certify that this thesis entitled " Study the Physical Properties of Aluminum and Boron co-doped Zinc Oxide Thin Films as Gas Sensor for NH3 and NO2 " was prepared by "Rashid Hashim Jabbar" under my supervision at College of Science, University of Al-Mustansiriyah, in a partial fulfillment of the requirements for the degree of Doctor of philosophy in Physics.

Signature Name: Dr. Anwar H. Ali Al-Fouadi Title: Assistant Professor Address: Physics Department, Collage of Science, Al-Mustansiriyah University. Date:

/

/ 2015

In view of the available recommendation, I forward this thesis for debate by the examination committee.

Signature Name: Dr. Ali Abid D. AL-Zuky Title: Professor Address: Head of Physics Department, Collage of Science, Al-Mustansiriyah University. Date:

/

/ 2015

Committee Certification We certify that we have read this thesis entitled " Study the Physical Properties of Aluminum and Boron co-doped Zinc Oxide Thin Films as Gas Sensor for NH3 and NO2 " as an examine committee, examined the student (Rashid Hashim Jabbar) in its contents and that, in our opinion meets the standard of thesis for the degree of Doctor of Philosophy of Science in physics with an excellent degree. Signature:

Signature:

Name: Dr. Najiba Abdullah Hasan

Name: Dr. Abdulhussain K. Elttayef

Title: Professor

Title:

Date:

Date:

/

/ 2015

Researcher Chief /

(Chairman)

/ 2015 (Member)

Signature:

Signature:

Name: Dr. Aliyah A. A. Shehab

Name: Dr. Mahasin F. Hadi

Title: Professor

Title: Assistant Professor

Date:

Date:

/

/ 2015

/

(Member)

/2015 (Member)

Signature:

Signature:

Name: Dr. Dr. Batool D. Balwa

Name: Dr. Anwar H. Ali Al-Fouadi



Date:

Date:

/

/2015

(Member)

/

/2015

(Member/Supervisor) Approved by the Committee of the College of Science.

Signature: Name: Dr. Raad S. Sabry Title: Assistant Professor

Address: Dean of the Science College, Al-Mustansiriyah University Date:

/

/2015

Abstract In this research, pure ZnO and (aluminum and boron) co-doped ZnO(AZB)thin films were deposited with various doping concentration(2, 4, 6 and 8 %),[ZnO:(B 1%+Al 1%), ZnO:(B 2%+ Al 2%), ZnO:(B 3%,Al 3%), ZnO:(B 4%,Al4%)]. The thin films have been prepared by Spray Pyrolysis's method at (450 ±10 oC) substrate temperature with (150±5 nm) thickness on glass and porous silicon n-type p-type types with resistivity (ρ= 0.05–0.1) Ω.cm and (111) crystalline orientation. Heterojunctions of the types, such as :(( ZnO, AZB)/p-PS/Si, (ZnO, AZB)/n-PS/Si were also prepared. The porous silicon layer was prepared by the electrochemical for ptype and photo electrochemical etching for n-type under the effect of halogen light on silicon substrates with current density (30 mA) for (30 minutes). The surface topography of the films and the porous silicon are studied by using the Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and the Atomic Force Microscopy (AFM). The study shows that the surface structure of the deposited films is nano structural and the value of the grain size decreases with the increase of co-doping where the minimum value of grain size was (7.6 nm). It was found that the surface roughness increased with doping from (1.34 to 6.15) nm and (6.4 to 12.12) nm for a films deposited on glass and porous silicon substrate, respectively, the x-ray diffraction showed that the prepared doped and undoped ZnO thin films were of polycrystalline with hexagonal (wurtzite) structural. The optical properties of the prepared thin films have been studied by measured the transmittance and absorbance spectrum at room temperature for the wavelength range (300 – 1100) nm, the transmittance I

was (87 %) for the undoped ZnO, and decreased with the co-doped increased. The value of the optical energy gap increases with the co-doped increasing, the optical energy gap of the undoped film was found to be (3.22 eV) and increased to (3.31 eV) after doping with (8%) rate. The electrical measurements proves that doped and undoped thin films were ntype, and the carrier charge concentrations increases with the increasing of co-doped rate. The prepared thin films deposited on glass and porous silicon n-type and p-type types have been used to the sensitivity measurements for (NH3, NO2) gasses at room temperature for the (50, 100, 150 and 200) ppm concentrations. The results shows that the electrical resistance for the thin films decreases with the increasing of co-doped concentrations for (NH3) gas due to the reducing interaction, but it increases with the increasing of co-doped concentration for (NO2) gas due to the oxidation interaction. The results show that the sensitivity of the thin films deposited on porous silicon substrates is higher than the sensitivity of the thin films deposited on glass substrates. The sensitivity of n-type porous silicon is higher than p-type porous silicon and glass substrate. The study shows that the increases of gases concentrations leads to increasing of sensitivity.

II

CONTENTS CHAPTER ONE

Introduction and Review

Page No.

1-1

Introduction

1

1-2

Properties of ZnO

4

1-3

Literature Survey

7

1-4

The Aim Of The Study

13

CHAPTER TWO

Theoretical Background

2-1

Structural Properties

14

2-1-1

crystallite size

14

2-1-2

Dislocation density

15

2-1-3

Lattice Parameters

15

2-1-4

Micro Strain

16

2-1-5

Integral Breadth

16

2-1-6

Shape Factor

16

2-2

Williamson–Hall Method

17

2-3

The effect of Doping

18

2-4

Porous Silicon

18

2-4-1

Etching Parameters

18

2-5

Electrical Properties

20

2-5-1

D.C. Conductivity

20

2-6

Hall Effect

21

2-7

Optical Properties of Semiconductors

22

2-7-1

Optical Constants

24

2-7-2

Photoluminescence ( PL )

26

2-8

Morphological Properties

27

2-8-1

Atomic Force Microscopy (AFM)

27

2-8-2

Scanning Electron Microscope (SEM)

28

III

2-8-3

Transmission Electron Microscopy (TEM)

30

2-8-4

Fourier Transform Infrared (FTIR) Spectroscopy

31

2-8-5

Raman Spectroscopy

31

2-8-6

Applications of Porous Silicon

32

2-9

Capacitance-Voltage (C-V) Measurements

33

2-10

Heterojunctions

34

2-11

Gas Sensors

36

2-11-1

Metal Oxide Semiconductor Gas Sensors

36

2-11-2

Applications of Semiconductor Gas Sensors

36

2-11-3

Sensing Mechanism

37

2-11-4

Gas sensitivity

38

2-11-5

Response and Recovery Times

39

2-11-6

The effect of the Crystallite Size on the Sensitivity of 40 Metal-Oxide Gas Sensors

CHAPTER THREE

Experimental Work

3-1

Introduction

44

3-2

Experimental Details

45

3-2-1

Substrate Preparation

45

3-2-2

Porous Silicon Preparation

46

3-3

Chemical Spray Pyrolysis (CSP) Technique

47

3-4

Preparation of Solutions

49

3-5

Thin Film Deposition

51

3-6

Characterization of Films

52

3-6-1

Thin Film Thickness Determination

52

3-7

Structural and Morphological Investigations

53

3-7-1

X-Ray Diffraction (XRD) Measurements

53

3-7-2

Atomic Force Microscopy

54

IV

3-7-3

Scanning Electron Microscopy (SEM)

54

3-7-4

Transmission Electron Microscopy (TEM)

54

3-7-5

Optical Microscopic Measurement

54

3-8

Optical Properties Measurement

55

3-8-1

Fourier Transform Infrared Spectrophotometer (FTIF)

55

3-8-2

Raman Spectroscopy Measurement

56

3-9

Electrical Measurement

56

3-9-1

Masking Techniques

56

3-10

Capacitance –Voltage (C-V) Measurements

57

3-11

Photoluminescence Measurements

58

3-12

Gas Sensor Testing System

58

CHAPTER FOUR

Results and Discussions

4-1

Introduction

60

4-2

Structural Properties

60

4-2-1

Analysis of XRD for ZnO and AZB Thin Films

60

4-2-2

Micro strain (Crystal Distortion) Measurements

64

4-2-3

Average Crystallite Size Measurements

64

4-2-4

Integral Breadth

66

4-2-5

Shape Factor

67

4-2-6

William Hall's (W-H) Grain Size Measurements

68

4-3

Surface Morphology

70

4-3-1

Optical Microscopy

70

4-3-2

Atomic Force Microscopy (AFM) Investigation

71

4-3-3

Scanning Electron Microscope (SEM)

74

4-3-4

Transmission Electron Microscope (TEM)

79

4-4

FTIR Spectroscopy Measurements

80

4-5

Optical Properties

83 V

4-5 -1

Transmittance

83

4-5-2

Absorbance

84

4-5-3

Optical Energy Gap

86

4-5-4

Absorption Coefficient

90

4-5-5

Optical Conductivity

91

4-5-6

The Optical Constants

92

4-6

Photoluminescence

93

4-7


97

4-7-1

Hall Effect of ZnO and AZB Thin Film

97

4-7-2

Conductivity of ZnO and AZB Thin Films

99

4-8

Porous Silicon

102

4-8-1

Nanostructure Investigation

102

4-8-2

Atomic Force Microscope (AFM) for Porous Silicon

104

4-8-3

SEM Investigation for Porous Silicon

108

4-9


113

4-9-1

Hall Effect of AZB deposited on Porous Silicon

113

4-9-2

Capacitance-Voltage Measurements (C-V)

114

4-10

Raman Spectroscopy Measurement

118

4-11

Sensing Properties: Chemical Sensing Measurements

119

4-11-1 Sensing Properties of NH3 Gas

119

4-11-2 Sensing Properties of NO2 Gas

128

CHAPTER FIVE

Conclusions and Future Studies

5-1:

Conclusions

138

5-2:

Future Studies

139

References

140

VI

List of Symbols Symbol

Description

Unit

𝐴𝑏

Absorbance

-

a 𝛽𝐷

Lattice Constant

nm

Size Broadenings (intrinsic Full Width at

degree

Half Maximum (FWHM)) 𝛽𝑖

Instrumental Broadening

degree

𝛽𝑚

Measured Full Width at Half Maximum

degree

𝐵

Magnetic Field

Tesla

c

Lattice Constant

nm

C

HF concentration

𝐶𝐴𝑆𝑇𝑀

Lattice Constant of (ASTM)

-

𝐶𝑋𝑅𝐷

Lattice Constant Measured by (XRD)

-

d

Distance Between the Lattice Planes

nm

𝐷

crystallite Size

nm

e

Charge of Electron

Ec

Energy Bottom Of Conduction Band

eV

Ef

Fermi Energy Level

eV

EgOpt.

Optical Energy Gap

eV

Ev

Energy Top of Valence Band

eV

Ea

Activated Energy

eV

h

Planck Constant

J.s.

hν

Photon Energy

eV

𝐸

Electrical Field

N/Coul.

𝐸𝐻

Hall Field

N/Coul.

I

Current

Io

Incident Intensity

coul.

Amp. mW/cm2

VII

mW/cm2

It

Transmitting Light Intensity

𝐿

Distance Between the Electrodes

cm

𝐽𝑃𝑆𝑖

Current Density of Porous Silicon

A./cm2

𝐽

Current Density

A./cm2

kB

Boltzmann Constant

𝑀

Concentration of Molarities

m*

Effective Mass

𝑛𝑟

Refractive Index

𝑁∗

Complex Refractive Index

𝑁𝐷

Density of Carrier's Concentration

cm-3

𝑝

Holes Concentration

cm-3

𝑅𝑟

Reflectance

%

R

Resistance

Ω

𝑅𝐻

Hall Coefficient

Ra

Resistance of the Sensor in Air

Ω

𝑅𝑔

Resistance of the Sensor in Presence of Gas

Ω

Sensitivity

-

𝑇𝑃𝑠𝑖

J/K mole/liter kg -

cm3/coul.

Thickness of Porous Silicon Layer

cm o

T

Temperature

C, K

Ts

Substrate Temperature

o

Tr

Transmittance

%

t

Time

s

tt

Thickness Of Thin Film

S

Sensitivity

C

nm -

𝑣𝑃𝑆𝑖

Etching Rate

µm/min

𝑣𝐷

Drift Velocity

m/s

V

Voltage

V

𝑉𝑏𝑖

Built In Potential

V VIII

W

Depletion Layer Width

λ

Wavelength

Å

ρ

Resistivity

Ω.cm

δ

Dislocation Density

nm-2

𝛼

Absorption Coefficient

cm-1

K0

Extinction Coefficient

-

Weighted Average Strain

-

𝜀𝑟

Real Part of Dielectric Constant

-

𝜀𝑖

Imaginary Part of Dielectric Constant

-

𝜀∗

Complex Dielectric Constant

σd.c

Conductivity

(cm.Ω)-1

σ˳

Maximum Electrical Conductivity

(cm.Ω)-1

𝜀𝑠𝑡𝑟

𝜎𝑜𝑝𝑡𝑖𝑐𝑎𝑙

µm

s-1

Optical Conductivity

μh

Hall Mobility

cm2/V.s.

μe

Electron Mobility

cm2/V.s.

𝑣

Frequency

𝜀

Micro Strain

𝛥

Integral Breadth

Φ

Shape Factor

-

𝜽

Bragg's angle

degree

𝒏𝒕

Surface Electron Density

𝒏𝒃

Free Electron Density in the Grain Body

Hz degree

IX

List of Abbreviations ASTM a-Si AFM CSP FTIR FWHM IUPAC HF hν IC

American Society of Testing Materials Amorphous Silicon Atomic Force Microscopy Chemical Spray Pyrolysis Fourier Transform-Infrared Spectroscopy Full Width at Half Maximum International Union of Pure and Applied Chemistry Hydrofluoric Acid Photon Energy Integral circuit

Lx Ly Lz MOCVD PS PL PLD ppm QCE TCO TEM SEM Si XRD UV IR VIS EC PEC

Confined Region in X-axis Confined Region in Y-axis Confined Region in Z-axis Metal-Organic Chemical Vapor Deposition Porous Silicon Photoluminescence Spectroscopy pulsed laser deposition Parts Per Million Quantum Confinement Effect Transparent Conducting Oxides Transmission Electron Microscopy Scanning Electron Microscope Silicon X-Ray Diffraction Ultraviolet Infrared Visible Electrochemical Photo Electrochemical

X

List of Figures Figure No.

Title

Page No.

1-1

Unit Cells of ZnO Structure: (a) Hexagonal Wurtzite, (b): Zincblend and (c): Rock Salt

5

2-1

(a): Photoluminescence spectrum of ZnO thin film, (b): Photoluminescence spectrum from as prepared porous silicon sample PS etching time 60 min and current density 20mA/cm2

26

2-2

(a) 2-D AFM Image of ZnO on Glass Substrates, 3-D AFM Image of ZnO Thin Fim Deposition on Porous Silicon Layer

28

2-3

SEM Images of ZnO onto Glass Substrates

29

2-4

(a): SEM Micrographs Al and B co-doped ZnO Thin Films, (b): SEM of Porous Silicon Layer Produced by Illumination Intensity (40mw/Cm2)

29

2-5

(a) SEM Micrographs Al and B co-doped ZnO Thin Films, (b) CrossSectional SEM Images of ZnO:Al Thin Films Deposited on PS

29

2-6

Image of Unloaded ZnO; a) (HR-TEM) Bright-Field, (High-Resolution Transmission Electron Microscopy Showed Highly Crystalline ZnO), (b) TEM Image of ZnO

30

2-7

Raman Spectra of (a) PS (b) ZnO /PS

32

2-8

Classification of Heterojunctions :( a) Straddling Heterojunction, (b) Staggered Heterojunction, (c) Broken-Gap Heterojunction

35

2-9

Three Grains of a Semiconductor Oxide Showing How the Intergranular Contact Resistance Comes About, the Height of the Energy Barrier

38

2-10

A Sketch Showing How Response and Recovery Times are Calculated From a Plot of Sensor Conductance Versus Time

40

2-11

Schematic Model of the Effect of Crystallite Size on the Sensitivity Of MetalOxide Gas Sensors: (a) D >> 2L; (b) D ≥ 2L; (c) D < 2L

42

2-12

Response and Recovery of ZnO Sample

43

2-13

Dynamic Response of the Conductometric Sensor to Different NO2 Gas Concentrations in Air

43

3-1

Schematic of Experimental Work

44

3-2

(a)Electrochemical Etching (ECE) Cell Set-Up and (b) Photo Electrochemical Etching (PECE) Cell Set-Up

47

XI

3-3

General Schematic of a Spray Pyrolysis Deposition Process

48

3- 4

Spray Pyrolysis Droplets Modifying as they are Transported from the Atomizing Nozzle to the Substrate. Whether the Temperature or the Initial Droplet Size are Varied, there are Four Potential Paths Which the Droplet Can Take as it Moves Towards the Substrate

49

3-5

Mask Used in: (a) D.C. Conductivity and, (b) Hall Effect, (c) Gas Sensing

57

3-6

Gas Sensor Testing System

59

4-1

XRD patterns for thin films of undoped ZnO and AZB with different doping concentration (2, 4, 6, and 8 %) deposited on: (a) glass, (b) silicon, (c) p-type porous silicon substrate

62

4-2

Average Crystallite Size for (100, 002, 101) Planes of ZnO and AZB Thin Films Deposited on Glass Substrates as a Function of Doping Concentration

66

4-3

W-H plots of undoped ZnO and AZB with different doping concentration (2, 4, 6, and 8 %) thin films deposited on glass substrate

68

4-4

W-H Micro Strain of Undoped ZnO and AZB at Different Doping Concentration (2, 4, 6 and 8 %) Thin Films

69

4-5

Optical Micrographs of ZnO and AZB Thin Films with Doping Concentration: 0.0 to 8 at % Deposited on Glass Substrate at 450oC

70

4-6

Granularity Distribution, 2-D and 3-D AFM Image of the ZnO and AZB Thin Films with Doping Concentration (2, 4, 6, and 8%) Deposited on Glass Substrate at 450oC

74

4-7

SEM images of the ZnO and AZB with doping concentration (2, 4, 6 and 8) % deposited on glass substrate

75

4-8

Cross Sections of SEM Images for: A) ZnO /Glass, B) AZB (8%)/Glass

76

4-9

Average Grain Size Calculated by W-H Method, SEM and AFM of Undoped ZnO and AZB with Different Doping Concentration (2, 4, 6 and 8 %) Thin Films Deposited on Glass Substrate

78

4-10

TEM Image of Undoped ZnO for a Different Scales: (a) 80 nm, (b) 60 nm, (c) 40 nm, (d) 10 nm

79

4-11

FTIR Spectra of ZnO and AZB Thin Films Deposited on Silicon(111) Substrate of: a) ZnO, AZB with Doping Concentration: b) 2%, c) 4%, d)6%, e) 8%

82

4-12

(a): Transmittance Spectra, (b) Absorbance Spectra of ZnO and AZB Thin Films with Doping Concentration (2, 4, 6 and 8 %)

85

XII

4-13

Variation of (αhv) 2 vs. Photon Energy (hv) for: (a) Undoped ZnO, (b) AZB 2 %,( c) AZB 4 %, (d) AZB 6% and (e) AZB 8% Thin Film

89

4-14

Variation of energy Band Gap (𝑬𝒈 ) vs. ZnO and AZB Thin Film with Doping Concentration (2, 4, 6 and 8 %)

90

4-15

Absorbance Coefficient as a Function of Wavelength of ZnO and AZB Thin Films with Doping Concentration (2, 4, 6 and 8 %)

91

4-16

Optical Conductivity of the ZnO and AZB Thin Films with Doping Concentration (2, 4, 6 and 8 %)

91

4-17

PL Spectra of Undoped ZnO and AZB Thin Films with Doping Concentration : (a) ZnO, (b) ( c) 2%4 %,, (d) 6%, (e) 8%

95

4-18

PL Spectra of PS Layers Formed on n-type Regions with (111) Orientations at R.T.

96

4-19

Variation of Carrier Doping Concentration and Hall Mobility of ZnO and AZB Thin Films with Doping Concentration (2, 4, 6 and 8 %)

98

4-20

Ln (σ) of Electrical Conductivity Vs. (1000/T) for ZnO and AZB Thin Film with Doping Concentration (2, 4, 6 and 8 %)

100

4-21

Optical Micrographs of n-PS and p-PS Before and After Dissolution of the Porous Silicon Layer

102

4-22

Optical Photograph Top View of n-PS and p- PS Before and After Deposition of ZnO Thin Films

103

4-23

Granularity Distribution, 2-D and 3-D AFM Image of the ZnO and AZB Thin Films with Doping Concentration (2, 4, 6 and 8 %) Deposited on n-PS at 450°C

107

4-24

SEM Images of (X 1.5k) for p-PS and n-PS Before and After ZnO Deposition

108

4-25

SEM of: a) n- PS, b)p-PS, c) ZnO /n-PS, d) ZnO /p-PS, e) AZB 2%/n-PS, f) AZB 2%/p-PS, g) AZB 4%/n-PS, h) AZB 4%/p-PS, i) AZB 2%/ n-PS , j) AZB 8%/p-PS, k) AZB 8%/ n -PS, l) AZB 8%/p-PS Deposited at 450°C

110

4-26

Cross Sections of SEM Images for: a) AZB 4% /Glass, b) AZ 4% /PS

111

4-27

Variation of Carrier Doping Concentration and Hall Mobility of ZnO and AZB Thin Films with Doping Concentration (2, 4, 6 and 8 %) Deposited on pPS

113

4-28

Variation of Capacitance as a Function of Reverse Bias Voltage for ZnO and AZB Thin Films with Doping Concentration (2, 4, 6 and 8 %) Deposited on ntype PS Heterojunction

115

XIII

4-29

The Variation of 1/C2 As A Function of Reverse Bias Voltage for:(a) ZnO /PS, AZB2%/PS,(b) AZB 4%/PS, ,(c) AZB 6%/PS, ,(d) AZB 8%/PS

116

4-30

Raman spectra for ZnO and AZB thin films with doping concentration (2, 4, 6 and 8 %) deposited on p-PS

118

4-31

Resistance for ZnO, AZB with Doping Concentration (2, 4, 6, 8 %) Deposited on: A) Glass, B) p-PS and C) n-PS as a Function of Operating Time for NH3 Gas with Concentration (50, 100, 150 and 200) ppm

121

4-32

Sensitivity for ZnO, AZB with Doping Concentration: (2, 4, 6, 8 %) Deposited on: A) Glass, B) p-PS and C) n-PS as a Function of Operating Time for NH3 Gas with Concentration (50, 100, 150 and 200) ppm at RT

123

4-33

Change of Sensitivity with Temperature for Undoped ZnO and AZB with Doping Concentration (2, 4, 6, 8 %) Deposited on Glasssubstrate for NH3 Gas with Concentration (50, 100, 150 and 200) ppm

125

4-34

Change of Sensitivity with Temperature for Undoped ZnO and AZB with Doping Concentration (2, 4, 6, 8 %) Deposited On p-PS Substrate for NH3 Gas with Concentration (50, 100, 150 and 200) ppm

126

4-35

Change of Sensitivity with Temperature for Undoped ZnO and AZB with Doping Concentration (2, 4, 6, 8 %) Deposited on n-PS Substrate for NH3 Gas with Concentration (50, 100, 150 and 200) ppm

127

4-36

Resistance for ZnO, AZB with Doping Concentration (2, 4, 6, 8 %). Deposited On: A) Glass, B) p-PS and C) n-PS As A Function of Operating Time for NO2 Gas with Concentration (50, 100, 150 and 200) ppm

130

4-37

Sensitivity for ZnO and AZB with Doping Concentration (2, 4, 6, 8 %). Deposited on: a) Glass, b) p-PS and c) n-PS As A Function of Operating Time for NO2 Gas with Concentration (50, 100, 150 and 200) ppm At RT

132

4-38

Change of Sensitivity with Temperature for Undoped ZnO and AZB with Doping Concentration (2, 4, 6, 8 %) Deposited on Glass Substrate for NO2 Gas with Concentration (50, 100, 150 and 200) ppm

134

4-39

Change of Sensitivity with Temperature for Undoped ZnO and AZB with Doping Concentration (2, 4, 6, 8 %) Deposited on p-PS Substrate for NO2 Gas with Concentration (50, 100, 150 and 200) ppm

135

4-40

Change of Sensitivity with Temperature for Undoped ZnO and AZB with Doping Concentration (2, 4, 6, 8 %) Deposited on n-PS Substrate for NO2 Gas with Concentration (50, 100, 150 and 200) ppm

136

XIV

List of Tables Table No.

Title

Page No.

1-1

Some Properties of Hexagonal Wurtzite ZnO at Room Temperature

6

1-2

Some Physical Properties of Al and B

7

3-1

Optimum Thermal Spray Pyrolysis Deposition Condition for the

51

Preparation of ZnO, AZB Thin Films 3-2

Thicknesses of Thin Films Deposited on Glass and Porous Silicon

52

Obtained by TFProbeTM Spectroscopic Instrument 3-3

The Instrumental Broadening (βi) of Used XRD Instrument Given by

53

Using Standard Defect Free Si Sample 4-1

Structural Parameters of ZnO and AZB Thin Films Deposited on

63

Glass, Silicon and Porous Silicon Substrates at 450 oC 4-2

Average Crystallite Size and Micro Strain of ZnO and AZB thin Films

65

Deposited on Glass, Silicon and Porous Silicon Substrate 4-3

Maximum Intensity, Area Under the Curve, Integral Breadth and

67

Shape Factor of (100), (002) and ((101) Planes for ZnO and AZB Thin Films 4-4

W-H Grain Size DW-H, DSEM, DAFM, εW-H, Sq and Sz for Undoped ZnO and AZB

77

with Doping Concentration (2, 4, 6, 8 %) Thin Films

Deposited on Glass Substrate 4-5

Optical Constants at (λ=550 nm) for ZnO and AZB Thin Film with

92

Doping Concentration (2, 4, 6, 8 %) at Room Temperature 4-6

Variable of PL Wavelength with Doping Concentrations and

96

Wavelength of PS 4-7

Hall Coefficient (RH), Carrier Concentration (Nd) and Mobility (μH)

99

of ZnO and AZB Thin Films with Doping Concentration (2, 4, 6, 8 %), at Room Temperature 4-8

Activation Energy of ZnO and AZB Thin Films with Doping

101

Concentration: (2, 4, 6, 8 %), at Room Temperature 4-9

Grain Size Calculated by AFM, SEM, Sa, Sq and Sz of ZnO and AZB Thin Films Deposited on PS

XV

112

4-10

Thickness TPS, Porosity and Etching Rate 𝒗 of the Porous Silicon

112

Layer 4-11

Hall Coefficient, Carrier Concentration and Mobility of ZnO and AZB

114

Thin Films Deposited on p-PS with Doping Concentration 4-12

Values of 𝑾, 𝒏𝒅

and 𝑽𝒃𝒊 , for ZnO and AZB deposited on PS/Si

117

The Optimum Values of AZB at 8% Thin Films Deposited on Glass, p

142

junction 4-13

-PS And n –PS Substrate

XVI

Chapter One


1-1 Introduction: Among the family of transparent conductive oxides (TCO), Zinc oxide (ZnO) thin films are attractive in the semiconductor field due to their good optical characteristics, high stability and excellent electrical properties, among others. They have been frequently used in several electronic applications such as transparent conducting materials, piezoelectric transducers, solar cells, surface acoustic wave filters, heat mirrors, and liquid crystal displays. In addition to their potential in optoelectronic devices, nowadays they are being used as gas chemical sensors due to their high surface sensitivity [1]. Several techniques have been proposed and developed for the preparation of ZnO thin films such as magnetron sputtering, spray pyrolysis, metal-organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), arc plasma evaporation and ion plating. Among these, spray pyrolysis is one of the most convenient as well as commonly practiced methods for deposition of ZnO [2]. In the last decade, the number of publications on ZnO has been increased annually and in 2007 ZnO had become the second most popular semiconductor after Si, and it's popularity is still increasing with time [3]. A number of investigations have focused on transitional metals doped II-VI compound semiconductor ZnO. The industry is now tooling up for wide band gap-semiconductors such as Gallium-Nitride (GaN), Silicon Carbide (SiC) and Zinc-Oxide (ZnO). Besides wide band gap they also possess properties in optical and electrical characteristics. Only their direct band gap allows easier integration with other optical devices. But the global research interest in wide band gap semiconductors has been attracted towards zinc 1

Chapter One


oxide (ZnO) due to its properties as a semiconductor material

[4]

. The high

electron mobility, high thermal conductivity, good transparency, wide direct band gap (3.37 eV), large exciton binding energy (60 meV) and easiness of growing it in the nanostructure form make ZnO suitable for optoelectronics, transparent electronics, sensing, light emitting diodes, lasers diodes, photo detectors [5]. Porous Silicon is not a new material, but it is only recently under investigation due to its surprising properties. The formation of porous Si was first reported in 1956 by Uhlir at Bell Labs in USA during studies on electro polishing of Si in HF-based solutions. It was reported that a matter black, brown or red deposit is observed sometimes during the electro polishing of Si [6]

. One year after the observation of Uhlir, Fuller and Ditzenberger reported

about similar films chemically deposits on Si during immersing Si in HFHNO3 solution. In 1958, Turner studied for the first time electrochemically prepared porous Si, the termed as anodized porous Si layer. Chemically prepared films were studied by Archer in 1960. These films were not recognized as porous Si until Watanabe et al, first reported their porous nature and the fast oxidation of thick porous Si films [7]. In the 1970, the porous Si was utilized for dielectric isolation of active Si devices. The interest in porous Si has dramatically increased after the proposal of Canham in 1990, which efficient visible light emission from high porosity structures arises from quantum confinement effects. The main advantage of porous silicon light emitting diode (LED) is that it promises the integration of optoelectronics into the Si microchips. Lehman and Goesele reported in 1991 that porous Si exhibit a band gap increase compared with 2

Chapter One


bulk Si. The band gap increase explains the observed visible luminescence from porous Si [7, 8]. A gas sensor monitors the atmospheric air with the intention to prevent contamination of the surroundings, to protect people from hazardous gases in industrial milieu, in aircraft, and in living environments, also to detect the loss of the planetary atmospheric gases to outer space in exosphere. Gas detectors that identify the concentration of gas or gas mixtures are divided into optical, electrical, and chemical types. Optical gas detectors are based on detection of gas concentration by the power change of the light which was transmitted through the gaseous atmosphere [9]. Chiefly, detected light power is compared with a reference light, and the difference between the powers of transmitted and a reference light provides the information about the quantity of the gas in the atmosphere. Since, difference between the reference and transmitted light is proportional to the concentration of gas. Chemical gas detectors are based on the change of the current in the electrochemical cell attributable to the voltage variation between working and reference electrodes at different amounts of the gas. The electrolyte of the chemical gas sensor is usually aqueous acid or salt. Electrical gas detectors are divided into physical and chemical. Chemical type gas detector is based on the adsorption of the gas [10, 11]. The development of gas sensors to monitor the toxic and combustible gases is imperative due to the concerns for environmental pollution and the safety requirements for the industry. The sensors and sensors arrays are also used in medical applications, automotive and process control. In general, sensors provide an interface between the electronic equipment and the physical world typically by converting non electrical of physical or chemical 3

Chapter One


quantities into electrical signals. Recently, gas sensors based on the semiconducting metal-oxides (they also are known as resistive or chemo resistive sensors) such as tin dioxide (SnO2), zinc oxide (ZnO) and tungsten trioxide (WO3) were found to be very useful for detecting the toxic, harmful and hydrocarbons gases [12-14]. The principle mechanism for gas detection in metal oxides in ambient air is the ionosorption of oxygen at its surface, which produces a depletion layer (for n-type semiconductors), and hence reduces conductivity. Here ion sorption refers to the process where a species is adsorbed and undergoes a delocalized charge transfer with the metal oxide. This can then be used to measure reducing and oxidizing gases, as they will change the amount of ionosorbed oxygen, and then the conductivity of the metal oxide [15, 16]. 1-2: Properties of ZnO ZnO crystallize preferentially in the stable hexagonal wurtzite structure at room temperature (R.T.) and normal atmospheric pressure as shown in figure (1-1). Its wurtzite structure is very simple to explain, where each oxygen ion is surrounded tetrahedrally by four zinc ions, and vice versa, stacked alternatively along the c-axis. It is responsible for the spontaneous polarization observed in ZnO. The interaction among the polar charges at the surface depends on their distribution, therefore the structure is arranged in such a way to minimize the electrostatic energy, which is the main driving force for growing polar surface dominated nanostructures. This effect results in a growth of various ZnO nanostructures such as nanowires, nanosprings, nanocages, nanobelts, nanocombs, nanorings, and nanohelices. In addition to the wurtzite structure, 4

Chapter One


ZnO can also crystallize in the cubic zinc-blend and the rock-salt (NaCl) structures which are illustrated in figure (1-1) [17].

(a)

(b)

(c)

Fig. (1-1): Unit cells of ZnO structure: (a) hexagonal wurtzite, (b): zinc blend and (c): rock salt [17].

The basic physical parameters of ZnO at room temperature are shown in table (1-1).

5

Chapter One


Table (1-1): Some properties of hexagonal wurtzite ZnO at room temperature [18- 20]. Lattice parameters at 27oC Density Stable phase at 27 oC Melting point Static dielectric constant Refractive index Energy gap

a=0.32495 nm ; c=0.52069 nm 5.606 g/cm3 wurtzite 1975oC 8.656 2.008, 2.029 ~3.4 eV, direct max n-type ~1020 cm-3; max p-type ~1017 cm-3 60 meV 0.24

Intrinsic carrier Concentration Exciton binding energy Electron effective mass Electron Hall mobility at 27oC for low n-type conductivity Hole effective mass Hole Hall mobility at 27oC for low p-type conductivity Thermal conductivity Linear expansion coefficient(/oC) Bulk young's modulus Bulk hardness

200 cm2V-1s-1 0.59 5-50 cm2 V-1s-1 0.6 W/(m·K) ao:6.5 x 10-6 , co:3.0 x 10-6 111.2 ± 4.7 5.0 ± 0.1

A significant improvement of the thin films conductivity has been achieved by doping with group III elements: B, Al, Ga and In. Among them, Al has been proved as an excellent dopant, leading to a resistivity values of 1.2 ×10-4 Ω.cm of films with 1.0 at. % dopant concentration [21]. Table (1-2) shows some physical properties of aluminum and boron.

6

Chapter One


Table (1-2): Some physical properties of Al and B [22, 23]. Property Atomic number Atomic mass(g/mole) Atomic radius/pm Ionic radius Density /g cm–3at 25 oC Melting point / oC Boiling point / oC

Boron(B) 5 10.81 88 41 2.35 2030 3650

Aluminum(Al) 13 26.98 143 53.5 2.70 660 2467

1-3: Literature Survey: The literature survey in the field of interest can be summarized as follows: Lokhande et al.(2001)

[24]

have been studied structural, optical and

electrical properties of boron doped zinc oxide films prepared by spray pyrolysis technique, they found that the films with 0.8 wt.% boron doping concentration attains the lowest resistivity (10-4 Ω.m). The decrease in resistivity may be explained as follows: since the ionic radius of boron is smaller than that of zinc, the boron atoms doped into a ZnO lattice effectively acts as donors by supplying a single free electron when the B 3+ ions occupy Zn2+ ion sites. The optimum doping content of boron is 0.8 wt. %. The average transmittance was found to be 90% for all boron doped films. X-ray diffraction studies show that films are crystalline and well oriented along the (002) plane.

7

Chapter One


Romero et al. (2004) [18] studied electrical, structural and compositional properties of n-ZnO/c-Si heterojunction using chemical spray pyrolysis technique on monocrystalline silicon (100) substrates. The results showed that from C–V characteristics in the reverse bias region that the capacitance of the heterojunction is decreases with an increasing in the reverse bias with an approximately linear C−2–V bias. Also, the optical transmittance spectra of ZnO thin films exhibit an average transmittance over 95% in the visible range of the optical spectrum. Kang (2010)

[25]

has studied the optical and electrical properties of

aluminum and boron co-doped zinc oxide (AZB) thin films were prepared by DC magnetron sputtering as functions of the substrate temperature. The structural properties of the AZB thin films were improved by increasing the substrate temperature, and the lowest electrical resistivity was 5.16×10−4 Ω.cm in AZB thin films sputtered at 500ºC. AZB thin films sputtered at substrate temperatures above 300ºC showed a high average transmittance in the visible light wavelength range. The optical and the electric properties of the AZB thin films were found to significantly depend on the substrate temperature. The Al and the B co-doped ZnO thin films could be good candidates for optical device applications.

Lee et al. (2012)

[26]

studied the XRD spectra, electronic and optical

properties for the ZnO-based transparent conductive oxides (TCO) were investigated in the Al-Ga co-doped ZnO thin films. The 450ºC annealed sample showed the lowest resistivity and a Fermi-level shift of ~0.6 eV. The widening of optical-band gap for Al-Ga co-doped ZnO films was ~0.3 eV, and which was related to the conduction band filling up to ~0.45 eV in a 8

Chapter One


renormalized band gap. The lowest resistivity of 1.1×10-3 Ω.cm was observed at 450ºC annealed Al-Ga co-doped ZnO film. The Egopt. values increased from 3.25 eV (as-grown ZnO film) to 3.55 eV (450ºC -annealed Al-Ga co-doped ZnO film). The widening of optical-band gap is due to the filling of the states near the bottom of the conduction band. Yadav et al. (2012)

[27]

have been studied synthesis and properties of

Boron doped ZnO thin films by spray CVD technique at 200ºC substrate temperature. Has been studied the effect of variation of Boron doping concentration on the structural, electrical and optical properties. The structural analysis shows that the films are polycrystalline with preferential orientation along (002) direction. For 0.8 at% optimized Boron doping concentration the films exhibit maximum conductivity showing optical transmittance ≈ 90%. These films exhibit best values comparable with other depositions. The significant effect indicating enhanced electrical conductivity of the ZnO film is observed for the optimized B dopant concentration (0.8 at %). The films obtained at 200°C show the highest carrier concentration ≈ 10 20cm-3 with lowest resistivity of 0.39×10-3 Ω.cm.

Kumar et al. (2013)

[28]

have studied the structural, electrical, optical

properties of aluminum–boron co-doped ZnO (AZB) nanostructures on glass substrate using a low cost sol–gel process. The study was found that the concentration of Al plays a key role in varying the morphology and other properties of AZB films. A minimum resistivity of 6.8×10-4 Ω.cm and transmittance of ~88% was obtained at 0.6 at. % doping concentration of B and 0.4 at. % doping concentration of Al with a sheet resistance (Rs) of 24 Ω/sq. for Nanorods (NRs) in AZB films. This work shows that the co-doping 9

Chapter One


of ZnO with Al and B significantly improves the electrical properties of ZnO with no degradation in the optical properties. These AZB nanostructures can be potentially extended for the application of nano electronic devices. The increased carrier concentration is attributed to the free electrons donated by B3+ and co-doping of Al3+ which substituted Zn2+ ions in ZnO.

Bangbai et al. (2013) [29] have investigated effect of Al and N co-doped ZnO (ANZO) on structural and optical properties by sol-gel spin coating. Found (ANZO) thin films with hexagonal wurtzite type polycrystalline structure and good optical properties have been prepared on glass substrates. In addition, the Al and N doping significantly caused to increase of the direct bandgap of the films, which were beyond 3.22 eV of pure ZnO. XRD patterns of (AZNO) films exhibited unobvious XRD patterns of ZnO, suggesting that the co-doping of Al and N strongly affects the decrystallization of ZnO films leading to higher degree of amorphousity of ZnO with increasing doping content.

Castañeda (2013)

[30]

has investigated fabrication of transparent

conductive zinc oxide co-doped with fluorine and zirconium thin solid films by ultrasonic chemical pyrolysis, effects of precursor solution aging and substrate temperature, low resistive ZnO:F:Zr thin films with high transmittance by ultrasonic spray pyrolysis technique. While the lowest resistivity of the films was ∼1.2×10−2 Ω.cm, optical transmittance of all the samples remained high (80–85%) in the visible spectral range. All the samples are polycrystalline hexagonal wurtzite type and grow preferentially with (002) plane parallel to the substrate. 10

Chapter One

Park (2013)


[31]

have studied electrical and optical properties of In

and Al co-doped ZnO (IAZO) thin film, added (0.5, 1, 1.5, 2 %) amounts of indium (In) to Al doped ZnO thin films. Found the XRD analysis revealed that IAZO films have a hexagonal wurtzite crystal structure and a minimum sheet resistance of 37.5 ×103 Ω/sq. for 1.0 wt. % In doped AZO thin films. The transmittance of all IAZO thin films was about 85% in the visible region and band gap energy was broadened as the In doping concentration increased. Mereu et al. (2013) [32] have been studied synthesis and characterization of undoped, Al and/or Holmium (Ho) doped ZnO thin Films, The results show an increase of the ZnO thin films band gap energy value was registered by doping, both with Al3+ and Ho3+ thin films were in the range of 3.01–3.56 eV. The X-ray diffraction patterns confirmed the synthesis efficiency, showing that the films are polycrystalline with the wurtzite structure. Cuevas (2013) [33] have studied the characterization of ZnO films on glass substrate via spray pyrolysis deposition technique. The films show good transparency in the visible light spectrum. The thickness of the film affects the transparency due to the increases of light scattering from grain aggregates. Results from the XRD data show that the ZnO films are of wurtzite structure and have c-axis preferential growth. The ZnO films also exhibit high crystallinity and high phase purity. Masumdar and M. Barote (2013)

[1]

have investigated the effect of

solution molarity on the structural and opto-electric properties of ZnO thin films deposited by spray pyrolysis. The structural study of the films by X-ray 11

Chapter One


diffraction showed that films have a polycrystalline structure with an orientation according to the c-axis corresponding to the (002) plane crystallographic orientation. With the increase in molarity, the intensity of this peak increases. The optical measurements have shown an increase in the transmission T (%) with an increasing in the molarity up to 0.5 M and further it decreases. The band gap values were decreased from 3.25 eV to 3.02 eV. The film conductivity increases with increasing in molarity up to 0.5 M (3.22 × 10-1 Ω-1.cm-1) and further it decreases. Zhou (2013)

[34]

has investigated co-doped Zinc Oxide by a Novel Co-

Spray Deposition Technique. With this technique, ZnO films co-doped with one cationic dopant, Al, Cr, or Fe, and an anionic dopant, F, have been successfully synthesized, in which F is incompatible with all these three cationic dopants. The study shows the compared to singly-doped ZnO thin films, co-doped ZnO samples showed better electrical properties. Besides, a minimum sheet resistance, 55.4 Ω/sq., was obtained for Al and F co-doped ZnO films after vacuum annealing at 400°C. The transmittance for the Al and F co-doped ZnO samples was above 90% in the visible range. The study used co-spray deposition technique provides a simple and cost-effective way to synthesize co-doped TCOs with lower sheet resistance and high transmittance. Gupta et al.(2014)

[35]

have been studied impact of rapid thermal

annealing on structural, optical and electrical properties of DC sputtered doped and co-doped ZnO thin film, Al-B co-doped ZnO thin films were prepared on glass substrate. The X-ray diffraction (XRD) patterns of pristine films exhibits a preferable growth orientation in (002) phases, resistivity in

12

Chapter One


the order of 10−4 Ω.cm and more than 80% optical transmission in the visible range, the average grain size varies from 30 to 35 nm.

1-4 The Aim of the Study: The aim of this study is to prepare thin films using spray pyrolysis technique of undoped ZnO and aluminum - boron co-doped ZnO (AZB) deposit on glass and porous silicon synthesized by using photo-electrochemical etching with (30 mA/cm2) for (30 minutes) and studying the properties (structural, optical, electrical, and morphological). The second aim is to use the thin films which prepared on glass and n-type and p-type porous silicon at different substrate temperature for (NH3 and NO2) gas sensing applications for different concentrations.

13

Chapter Two


2-1 Structural Properties: 2-1-1 crystallite size: The Crystallite size 𝑫 can be calculate by (Debye- Scherrer’s formula) Scherer equation [36]: 𝑫=

𝒌𝝀

…. (2-1)

𝜷𝑫 𝒄𝒐𝒔(𝜽)

Where:

𝒌 = 𝟐√

𝒍𝒏(𝟐) 𝝅

= 0.94 called(Scherer's constant), λ is the

wavelength of incident X-ray radiation, 𝜷𝑫 is the intrinsic Full Width at Half Maximum(FWHM) of the peak, and θ is the Bragg's diffraction angle of the respective XRD peak [37-41]. In the case considered curve X-ray diffraction is similar to function Lorentz and take the form of

𝟏 √𝟏+𝒌𝟐 𝒙𝟐

, the correction is given by the following

relationship, which was called (Scherer correction) [42]: 𝜷𝑫 = 𝜷𝒎 − 𝜷𝒊

. … (𝟐 − 𝟐)

Where 𝜷𝒎 the measured Full Width at Half Maximum of the peak is, 𝜷𝒊 is the instrumental broadening. Compensation equation (2-2) in the relationship (2-1) we get: 𝑫=

𝒌𝝀 (𝜷𝒎 −𝜷𝒊 ) 𝒄𝒐𝒔(𝜽)

. … (𝟐 − 𝟑)

In the case considered X-ray diffraction curve similar to the function Gauss which takes the form ( 𝒆−𝒌

𝟐 𝒙𝟐

) the accuracy to be higher because of the

great similarity between this function and the diffraction curves; it was suggested in the form [43]: 𝜷𝑫 𝟐 = 𝜷𝒎 𝟐 − 𝜷𝒊 𝟐

… … . . (𝟐 − 𝟒)

This correction called (Warren Correction). Compensation equation (2-4) in the relationship (2-1) we get: 14

Chapter Two


𝒌𝝀

𝑫=

… . . . (𝟐 − 𝟓) 𝟐

𝒄𝒐𝒔(𝜽)√𝜷𝒎 − 𝜷𝒊

𝟐

Warren was suggested a relationship takes into account the geometric meaning which is [43, 44]: 𝜷𝑫 = √(𝜷𝒎 − 𝜷𝒊 )√(𝜷𝒎 𝟐 − 𝜷𝒊 𝟐 )

… . (𝟐 − 𝟔)

Compensation equation (2-6) in the relationship (2-6) we get: 𝒌𝝀

𝑫=

. . . . (𝟐 − 𝟕)

𝒄𝒐𝒔(𝜽)√(𝜷𝒎 − 𝜷𝒊 )√(𝜷𝒎 𝟐 − 𝜷𝒊 𝟐 )

2-1-2 Dislocation density: The dislocation density (𝜹) which represents the defect in the film was determined from the formula [45, 46]: 𝜹=

𝟏 𝑫𝟐

… . (𝟐 − 𝟖)

2-1-3 Lattice Parameters: Bragg concluded that the path difference between the two X-rays diffracted from two consecutive lattice planes is 𝟐𝒅 𝒔𝒊𝒏𝜽, and it leads to Bragg’s law, which states that the condition for diffraction of X-rays for a crystalline material is [47]: 𝒏𝝀 = 𝟐𝒅 𝒔𝒊𝒏𝜽

…. (2-9)

Where 𝜽 is the Bragg's angle and 𝛌 is the wavelength of the X-rays, 𝒏 is an integer and it is the order of reflection, and 𝒅 is the distance between the

15

Chapter Two


lattice planes. From the 𝒅 values lattice parameters 𝒂 and 𝒄 were calculated from the XRD pattern using the equation [48]: 𝟏 𝒅𝟐

𝟒 𝒉𝟐 +𝒉𝒌+𝒌𝟐

= [

𝒂𝟐

𝟑

]+

𝒍𝟐 𝒄𝟐

. … (𝟐 − 𝟏𝟎)

2-1-4 Micro Strain: Biaxial strain model is used for the calculation of film stress. Strain (𝜺) for the films in direction of c- axis were calculated using the relation [49- 53]: 𝜺=[

|𝑪𝑿𝑹𝑫 −𝑪𝑨𝑺𝑻𝑴 | 𝑪𝑨𝑺𝑻𝑴

] × 𝟏𝟎𝟎%

. … (𝟐 − 𝟏𝟏)

Where: (𝑪𝑨𝑺𝑻𝑴 = 𝟎. 𝟓𝟐𝟎𝟕 𝒏𝒎 ) is the strain-free lattice parameter for the ZnO sample.

2-1-5 Integral Breadth (𝜟): Integral Breadth caused by non-ideal optics of the instrument, wavelength, dispersion and structural imperfections of the specimen which is given by [54]: 𝑨𝒓𝒆𝒂 𝑰𝒎𝒂𝒙

𝜟=

… . (𝟐 − 𝟏𝟐)

Where Area: area under peak, 𝑰𝒎𝒂𝒙 : Maximum intensity. 2-1-6 Shape Factor (Φ): The shape factor determined the line profile resulting from the XRD patterns, which was calculated using the following relation [55]: 𝜱=

𝜷𝑫 … . (𝟐 − 𝟏𝟑) 𝜟

16

Chapter Two


2-2 Williamson–Hall Method: XRD peaks are broadened by small crystallite size and lattice distortion caused by lattice dislocations. However, effect of strain and imperfection on the line broadening differs from the effect of crystalline size. The relation (Stokes and Wilson 1944):

𝜺𝒔𝒕𝒓 =

𝜷𝒔𝒕𝒓 𝟒 𝒕𝒂𝒏𝜽

.… (2-14)

Where: 𝜺𝒔𝒕𝒓 is the weighted average strain and 𝜷𝒔𝒕𝒓 the integral breadth of reflection in radians located at 2θ. Size broadenings 𝜷𝑫 and strain broadenings 𝜷𝒔𝒕𝒓 show a different θ that provides a way to separate each effect. It is possible to convolute each contribution by combining the two equations (Williamson and Hall 1953) [56, 57]: 𝜷 = 𝜷𝑫 + 𝜷𝒔𝒕𝒓

… . (𝟐 − 𝟏𝟓)

Substitution (2-1) and (2-14) in (2-15): 𝜷=

𝟎. 𝟗𝟒 𝝀 + 𝟒 𝜺𝒔𝒕𝒓 𝒕𝒂𝒏 𝜽 𝑫 𝒄𝒐𝒔 𝜽

… . (𝟐 − 𝟏𝟔)

Multiplying the above equation by 𝒄𝒐𝒔 𝜽 , gives [58-61]:

𝜷 𝑪𝑶𝑺 𝜽 =

𝟎. 𝟗𝟒 𝝀 + 𝟒 𝜺𝒔𝒕𝒓 𝒔𝒊𝒏 𝜽 … . (𝟐 − 𝟏𝟕) 𝑫

The plot of 𝜷 𝑪𝑶𝑺 𝜽 versus 𝟒𝒔𝒊𝒏 𝜽 taking (100), (002), (101) lattice planes corresponding to the wurtzite phase. From the linear fit to the data, the crystallite size 𝑫 was extracted from the y-intercept and the strain 𝜺𝒔𝒕𝒓 from the slope of the straight line. The strain is due to the incorporation of a dopant in the periodic lattice [59, 62].

17

Chapter Two


2-3 The Effect of Doping: The influence of doping on X-ray diffraction patterns is more effective than the others, because it may cause shift in the output peaks positions with higher or lower values depending on the ion size of dopant materials compared to the Zn+ (0.074 nm) ions in the lattice. For small Al + ions (0.054 nm), the 2θ values of the (002) plane are increased because Interplanner spacing (d) is decreased [62, 63]

. While for big Ag ions (0.122 nm) the intensity of (002) is gradually

decreased and the 2θ values of the (002) plane are decreased also (2-14) [64].

2-4 Porous Silicon: 2-4-1 Etching Parameters: Porosity is defined as the fraction of void within the porous silicon layer. An estimation of the porosity is obtained gravimetrically: first, the weight of the virgin wafer is measured before anodization (m1), then after anodization (m2), and finally after dissolution of the porous silicon layer (m3) in a 1M solution of sodium hydroxide (NaOH). After determining these values, the porosity is given by the following equation [34, 65]:

𝑷𝒐𝒓𝒐𝒔𝒊𝒕𝒚% =

𝒎 𝟏 − 𝒎𝟐 𝒎 𝟏 − 𝒎𝟑

… . (𝟐 − 𝟏𝟖)

This relationship works best in the medium porosity range. However, when the porous silicon layer is thin (200 nm), the mass difference is the same order of magnitude as the error in measurements, and the porosity value obtained is unreliable. Porosity is greatly affected by the anodization current density, etching time and hydrofluoric acid concentration. 18

Chapter Two


The thickness of the layers 𝑻𝑷𝒔𝒊 is another important parameter that is closely related to the etching parameters of anodization time and hydrofluoric acid concentration. Similar to porosity, one can also get the porous silicon layer thickness 𝑻𝑷𝒔𝒊 using the equation [66- 69]:

𝑻𝑷𝒔𝒊 =

𝒎𝟏 − 𝒎𝟏 … . (𝟐 − 𝟏𝟗) 𝝆𝑺

Where: 𝝆 is the Silicon density (2.33 g/cm3) and 𝑺 is the etched surface. Another way to measure the film thickness is by direct determination using the scanning electron microscopy (SEM) or atomic force microscopy (AFM) [35]

. Etching rate is defined as the ratio between PS layer thickness and the

etching time (t) and given as [7, 70]: 𝒗𝑷𝑺𝒊 =

𝑻𝑷𝑺𝒊 𝒕

…. (𝟐 − 𝟐𝟎)

Where: 𝑣 is etching rate (µm/min).

19

Chapter Two


2-5 Electrical Properties: 2-5-1 D.C. Conductivity: In case of semiconductors the resistance rapidly decreases with increasing the temperature. The conductivity is due to thermionic emission as illustrated in Arrhenius relation [66]: 𝝈𝒅.𝒄. = 𝛔˳ 𝐞𝐱𝐩 [−

𝑬𝒂 ] 𝒌𝑩 𝑻

… . (𝟐 − 𝟐𝟏)

This equation is basically derived to give the change of the electrical conductivity with temperature for most cases of intrinsic semiconductors. The quantity 𝑬𝒂 of a given semiconductor is called activation energy, 𝑻 is the absolute temperature, 𝒌𝑩 is the Boltzmann constant, 𝛔˳ is the maximum electrical conductivity, when T → very large. 𝛔˳ is not constant but it varies 𝟑

with 𝑻 𝟐 according to the following equation [67]. 𝟑

𝛔˳ = 𝐜𝐨𝐧𝐬𝐭. (𝝁𝒆 + 𝝁𝒉 )𝑻 𝟐

… . (𝟐 − 𝟐𝟐)

Where 𝝁𝒆 and 𝝁𝒉 are electron and hole mobility respectively. Ohm's law in terms of the shape independent resistivity (𝛒) or conductivity (𝝈𝒅.𝒄. ) state as following [69]. ⃗𝑬 ⃗ = 𝛒𝑱

𝒐𝒓

⃗ 𝑱 = 𝝈𝒅.𝒄. ⃗𝑬

… . (𝟐 − 𝟐𝟑)

⃗𝑬 ⃗ : is the electrical field and 𝑱 is the current density.

The resistivity (𝛒) of the films is calculated by using the following equation [63]: 𝛒=

𝑹𝑨 𝑳

… . (𝟐 − 𝟐𝟒)

20

Chapter Two


Where 𝑹 is the sample resistance, 𝑨 is the cross section area of the thin film and 𝑳 is the distance between the electrodes. The conductivity of the films was determined from the relation: 𝝈𝒅.𝒄. =

𝟏 𝝆

… . (𝟐 − 𝟐𝟓)

The activation energies could be calculated from the plot of 𝒍𝒏(𝝈) versus 103/T. 2-6 Hall Effect: The number of carriers and the sign of charge carriers contributing to the current flowing in a solid can be measured by Hall Effect. In this measurement, a magnetic field is imposed on the material perpendicular to the electric field in the direction. The force acting on the charge was given by equation (2-26) [71], so the forces acting on the charges are therefore: 𝑭𝒚 = −𝒆𝒗𝟎 𝑩𝒛 ; 𝑭𝒚 = −𝒆𝑬𝒚

… . (𝟐 − 𝟐𝟔 )

Where we have assumed 𝑩 = (𝟎, 𝟎, 𝑩𝒛 ) and

𝑬 = (𝟎, 𝑬𝒚 , 𝟎). Under steady

state condition these two forces on the charges must be balanced [72].So that: 𝑬𝒚 = 𝒗𝑫 𝑩𝒛 = 𝑬𝑯 = 𝑯𝒂𝒍𝒍 𝒇𝒊𝒆𝒍𝒅

Since we have 𝒗𝑫 = − ⃗𝑬 ⃗𝑯=−

𝑱𝒙 𝒆𝒏

𝟏 ⃗𝒛 𝑱 𝑩 = 𝑹𝑯 𝑱𝒙 ⃗𝑩 𝒆𝒏 𝒙 𝒛

… . (𝟐 − 𝟐𝟕)

, we obtain [73-76]: … . (𝟐 − 𝟐𝟖)

Where: 𝑹𝑯 = −

𝟏 𝒆𝒏

… . (𝟐 − 𝟑𝟓)

The simultaneous measurement of conductivity and Hall coefficient provides us the values of n the carrier concentration. The polarity of the Hall 21

Chapter Two


voltage indicates the sign of charge carriers, and it is used to distinguish whether a semiconductor is 𝒏 𝑜𝑟 𝒑 − 𝑡𝑦𝑝𝑒 [77]. So that: 𝝁𝑯 = 𝝈 |𝑹𝑯 |

… . (𝟐 − 𝟐𝟗)

In all situations, the Hall mobility of electrons (𝝁𝑯 ) is decreased by any disruption that occurs to the ordered crystalline structure. Defects in the materials are manipulated so that the properties of the material with respect to electrical conductivity and other desired properties could be optimized.

2-7 Optical Properties of Semiconductors: Based on the intrinsic location of the top of the valence band (V.B.) and the bottom of the conduction band (C.B.) in the band structure, the electron–hole pair generation occurs directly or indirectly [78]. ZnO is a direct band gap semiconductor, which means that the minimum of the conduction band is directly over the maximum of the valence band, then transition between the valence and conduction band requires only change in energy. This property made ZnO a very useful material for the manufacture optoelectronic device [67]. The absorption of radiation that leads to electronic transitions between the valence and conduction bands is split into direct and indirect transitions. These transitions are described by the empirical equation [69]. 𝜶𝒉𝒗 = 𝑩(𝒉𝒗 − 𝑬𝒈 )𝒓

… . (𝟐 − 𝟑𝟎)

22

Chapter Two


Where, α is the absorption coefficient, B is the edge width parameter, ℎ𝑣 is the incident photon energy, r is constant depending on the material and the type of the optical transition whether it is direct or indirect [72].

A: Direct Transitions: Direct transition which occurs between top of the valence band and bottom of the conduction band at the same wave vector (ΔK=0). When the change in wave vector is equal to zero is called the allowed direct transition (r=1/2). If the transition occurs also between states of the same wave vector, but the wave vector does not equal to zero then these transitions are called “direct forbidden transitions"(r=2/3) [73].

B: Indirect Transitions: In indirect transitions there is a large momentum difference between the points to which the transition takes place in valence and conduction bands, this means that the conduction- band minima are not at the same value of K as the valence band maxima (ΔK≠0) [74, 75]. For an allowed indirect transition, the transition occurs from the top of the valence band to the bottom of the conduction band(r=2), the forbidden indirect transitions occur from any point other than the bottom of conduction band(r=3).

23

Chapter Two


2-7-1 Optical Constants: The absorption coefficient of the material is a very strong function of the photon energy and band gap energy. Absorption coefficient represents the attenuation that occurs in incident photon intensity on the material for unit thickness, and the main reason for this attenuation is attributed to the absorption processes [76]. Two of the most important optical properties are the refractive index n and the extinction coefficient k, which are generally called optical constants [77]. Using the fundamental relations of photon transmittance Tr and absorbance 𝑨𝒃 . 𝑰𝒕 = 𝑰𝟎 𝒆−𝜶𝒕𝒕

… . (𝟐 − 𝟑𝟏)

Where It is the incident photon intensity at thickness (tt) inside the material and Io is incident photon intensity at surface of material and: 𝑰

𝑨𝒃 = 𝑳𝒐𝒈 ( 𝟎 ) 𝑰 𝒕

… . (𝟐 − 𝟑𝟐)

We have: 𝑨

𝜶 = 𝟐. 𝟑𝟎𝟑 ( 𝒕 𝒃) 𝒕

… . (𝟐 − 𝟑𝟑)

The refractive index 𝒏𝒓 value can be calculated from the formula [76]: 𝟒𝑹𝒓

𝒏𝒓 = √(𝑹

𝒓 −𝟏)

(𝑹 +𝟏)

− 𝒌𝟎 𝟐 − (𝑹𝒓 𝟐

𝒓 −𝟏)

… . (𝟐 − 𝟑𝟒)

Where 𝑹𝒓 is the reflectance, and can be expressed by the relation [76, 79]: 𝑹𝒓 =

(𝒏𝒓 −𝟏)𝟐 +𝒌𝟎 𝟐 (𝒏𝒓 +𝟏)𝟐 +𝒌𝟎 𝟐

… . (𝟐 − 𝟑𝟓)

The extinction coefficient is related to the absorption coefficient 𝜶 by the relation: 𝒌𝟎 =

𝜶𝝀 𝟒𝝅

… . (𝟐 − 𝟑𝟔)

24

Chapter Two


Where: 𝝀 is the incident photon wavelength

[80]

. The 𝜺𝒓 real

and

imaginary 𝜺𝒊 parts of dielectric constant respectively can be calculated as follows [68, 81]: 𝑵∗ = 𝒏𝒓 − 𝒊𝒌𝟎

Complex refractive index

Complex dielectric constant 𝜺∗ = 𝜺𝒓 − 𝒊 𝜺𝒊

… . (𝟐 − 𝟑𝟕) … . (𝟐 − 𝟑𝟖)

From the relation 𝑵∗ = √𝜺∗ , therefore: (𝒏𝒓 − 𝒊𝒌𝟎 )𝟐 = 𝜺𝒓 − 𝒊 𝜺𝒊

… . (𝟐 − 𝟑𝟗)

So that: 𝜺𝒓 = 𝒏𝒓 𝟐 − 𝒌𝟎 𝟐

… . (𝟐 − 𝟒𝟎)

𝜺𝒊 = 𝟐𝒏𝒓 𝒌𝟎

… . (𝟐 − 𝟒𝟏)

The optical conductivity (σoptical) depends directly on the wavelength and absorption coefficient [82]: 𝝈𝒐𝒑𝒕𝒊𝒄𝒂𝒍 =

𝜶𝒏𝒓 𝒄 𝟒𝝅

… . (𝟐 − 𝟒𝟐)

Where: c is speed of light.

25

Chapter Two


2-7-2 Photoluminescence ( PL ): Photoluminescence in simple terms is a reverse process of absorption. Photoluminescence occurs when an excited electron in an excited state returns to the initial state by emission of a photon whose energy gives the difference between the excited state and the initial state energies. The process can be direct or indirect depending on the gap energy [83]. As shown in figure (2-1), a photoluminescence (PL) spectrum of ZnO usually consists of two main peaks, near band-edge (380 nm) and green (530 nm) emissions. The origin of the green emission is still debatable, some authors attribute it to oxygen vacancies

[21]

while others attribute it to zinc

interstitials [84].

(a)

(b)

Fig. (2-1): (a): Photoluminescence spectrum of ZnO thin film

[85]

,

(b):

Photoluminescence spectrum from as prepared porous silicon sample PS etching time 60 min and current density 20mA/cm2 [62].

26

Chapter Two


The average pore diameter (d) for the PS layer formed on the Si wafers can be calculated using the following equation [86, 87]: 𝑬(𝒆𝑽) = 𝑬𝒈 +

𝒉𝟐 𝟖𝒅𝟐

𝟏

𝟏

𝒆

𝒉

(𝒎 ∗ + 𝒎 ∗ )

…. (2-43)

Where 𝑬(𝒆𝑽) is the energy band gap of the PS layer calculated from the PL peak, 𝑬𝒈 = 1.16 𝑒𝑉 is the energy band gap of bulk c-Si, h is Planck's constant=4.13×10-15 eV·s., whereas 𝒎𝒆 ∗ and 𝒎𝒉 ∗ are the electron and hole effective masses, respectively (at 300 K, 𝒎𝒆 ∗ =0.19mo, 𝒎𝒉 ∗ =0.16mo, and mo=9.109x10-31 kg).

2-8 Morphological Properties 2-8-1 Atomic Force Microscopy (AFM): Atomic force microscopy (AFM) is very high resolution imaging technique and a non-destructive, with demonstrated resolution in the order of fractions of a nanometer. The AFM is one of the most widely used tools for imaging, measuring, and manipulating samples at the nanoscale. There is no special sample preparation required in this technique and the measurements can be carried out at ambient environment

[88]

.Typically, the deflection is

measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes. If the tip was scanned at a constant height, a risk would exist that the tip collides with the surface, causing damage. Figure (2-2) shows the AFM image samples of 2D and 3D AFM image of ZnO thin film deposition on porous silicon layer [89].

27

Chapter Two


(a)

(b)

Fig. (2-2):(a) 2-D AFM image of ZnO on glass substrates [90], (b) 3-D AFM image of ZnO thin film deposition on porous silicon layer [91].

2-8-2 Scanning Electron Microscope (SEM): SEM's are used for material characterization involving image and quantitative data representation. It offered an insight into the two dimensional and three dimensional imaging of the microstructure, chemical composition, crystallography and electronic properties. The SEM is relatively fast, and can image both at low (millimeter-scale) and high (nanometer-scale) magnifications. Additionally, sample cross-sections can be imaged easily by cleaving the sample and imaging it from the side [87]. Figure (2-3) shows SEM up view of ZnO deposited on glass substrate, figure (2- 4: a) shows Al and B co-doped ZnO deposited on glass, figure (2-4: b) shows samples of SEM images of PS before deposition and figure (2-5: a) SEM of Al and B co-doped ZnO thin films deposited on PS substrate and (2-5: b) Cross-sectional SEM images of ZnO: Al thin films deposited on PS [92].

28

Chapter Two


Fig. (2-3): SEM images of ZnO onto glass substrates [92].

(a)

(b)

Fig. (2-4): (a): SEM micrographs Al and B co-doped ZnO thin films/glass [35], (b): SEM of porous silicon layer produced by illumination intensity (40mW/cm2) [93].

(a)

(b)

Fig. (2-5) :(a) SEM micrographs Al and B co-doped ZnO thin films

[94]

Cross-sectional SEM images of ZnO: Al thin films deposited on PS [95]. 29

, (b)

Chapter Two


2-8-3 Transmission Electron Microscopy (TEM): In comparison to SEM, TEM is focus on the transmitted electron signal. In order to get any information using TEM, the specimens have to be electron transparent (< 100 nm). The accelerating voltage is considerably higher than in an SEM the benefits of high voltage include increased imaging resolution, due to the decreased electron wavelength, and also increased penetration and thus the ability to study thicker samples [96]. Figure (2-6, a, b) shows high crystalline TEM image of Unloaded ZnO for 20 nm scale and 50 nm.

(a)

(b)

Fig. (2-6): Image of Unloaded ZnO; a) (HR-TEM) bright-field. (Highresolution transmission electron microscopy showed highly crystalline ZnO)[97], b) TEM image of ZnO [57].

30

Chapter Two


2-8-4 Fourier Transform Infrared (FTIR) Spectroscopy: FTIR spectroscopy is a technique that provides information about the chemical bonding or molecular structure of materials, whether organic or inorganic. FTIR spectroscopy is very useful tools for investigating vibrational properties of synthesized materials. The band positions and absorption peak not only depend on the chemical composition and structure of the thin films but on the morphology of thin films also [48].

2-8-5 Raman Spectroscopy: Raman spectroscopy, and especially micro-Raman spectroscopy, can give indirect information on the microstructure of PS

[98]

. If the incident

photon imparts part of its energy to the lattice in the form of a phonon it emerges as a lower energy photon. This down converted frequency shift is known as Stokes-shifted scattering. Anti-Stokes shifted scattering results when the photon absorbs a phonon and emerges with higher energy. The antiStokes mode is much weaker than the Stokes mode so the Stokes-mode scattering is usually monitored. In Raman spectroscopy a laser beam, referred to as the pump, is incident on the sample. Figure (2-7, a) shows Raman spectral line of (111) where the broadening and the downshift of Raman peak toward lower energy indicates the presence of nanoscale features of the crystalline structures. As the size of nanocrystal decreases, the silicon optical phonon line shifts to lower frequency and becomes broader asymmetrically. The latter is more sensitive and distinct for high porosity layers of (PS). The absence of other peaks in Raman spectra confirms that the prepared sample retains the crystallinity of bulk silicon wafer. 31

Chapter Two


(a)

(b)

Fig. (2-7): Raman Spectra of (a) (PS) (b) ZnO/PS [91].

The optical phonon is observed in the central of Brillion zone with energy of 520 cm-1 and this is due to the conservation of quasi-momentum in crystals. Figure (2-7, b) shows Raman scattering spectra of ZnO/PS, the shapes of peaks at approximately 520cm-1 refer to first - order scattering phonons in cSi and this characterized the (PS) layers. 2-8-6 Applications of Porous Silicon: Porous Silicon has attracted noticeable attention of many scientists due to the possibility of developing (PS-based) devices including light emitting diode, waveguides, solar cells, photodetectors, photo modulators, photo resistor and sensors. The high specific surface area of the (PS) layer enables applications in chemical/biological sensing and can be exploited in the future heterogeneous chemical catalysis. A new technology based on (PS) layer uses the advantage of the macro porous structures formed on n-type (Si) wafers (higher specific 32

Chapter Two


capacitance). Maruska has reported the fabrication of visible p-n junction light emitting diode (LED) based on PS material which emits yellow-orange light under forward bias, with responsivity of up to (4 A/W). Anderson manufactured vapor sensor depending on (PS) layer. It was found the conductance of the (PS) layer increased to 600% in the presence of saturated ammonia [99].

2-9 Capacitance-Voltage (C-V) Measurements: When two semiconductors of different Fermi levels are brought into contact, the charge is transferred from one to the other until the Fermi levels are equalized. This gives rise to the formation of a depletion region on both sides of the junction.The potential barrier at the junction can be measured by small-signal capacitance–voltage (C–V) characteristics, the capacitance of the heterojunction are decreased with an increase in the reverse bias with an approximately linear (C−2–V) bias relationship and its extrapolated intercept on the voltage axis gives the built – in junction potential. This means that the depletion region in the vicinity of the heterojunction interface is expanded with an increase in the reverse bias. This C–V characteristic is also described by the conventional heterojunction theory: 𝟐 ∆𝑽 𝒏𝒅 = ( )( ) 𝟐 𝟏 𝒒𝜺𝒔 𝜺° 𝑨 ∆ 𝟐 𝑪

… . (𝟐 − 𝟒𝟒)

Where: 𝒏𝒅 is carrier concentrtion, 𝜺𝐬 the Relative permittivity of silicon, 𝜺° the Permittivity of free space and A is the effective area of the junction [100,101]: 𝟐𝜺𝒔 𝑽𝒃𝒊 𝑾 =√ 𝒒𝒏𝒅

… . (𝟐 − 𝟒𝟓)

Where: 𝑾 is depletion layer width. 33

Chapter Two


2-10 Heterojunctions: Heterojunction device consisting of a wide band gap semiconductor mated to a narrower band gap semiconductor have gained considerable prominence during the past few years. Most modern optoelectronic devices utilize heterojunctions between different semiconductor materials. They are used to restrict the flow of carriers or to collect carriers where they are needed [102]

. Junctions features the characteristics of electric and optoelectronic are

important for two different junctions, and that can be configured to divide [103]:

1- Those that are between two different semiconductors such as (GaAs) and (Ge). 2- Those that are between metals and semiconductors such as Schottky barrier formed by (Au) on (Si). 3- Those that are between metals and semiconductors that are contact Ohmic, such as (Al) and (CdS). Heterojunction in general is defined as the interface between two dissimilar materials in electron affinities, energy band gap and work function. Since the two materials used to form a heterojunction will have different energy band gaps. According to their band alignment, heterojunctions can be classified into three groups as shown in figure (2-8):  Type-1 or straddling heterojunction. 

Type-2 or staggered heterojunction.

 Type-3 or broken-gap heterojunction. In Type-1 (straddling) heterojunction, one material has both lower (Ec) and higher (Ev) and naturally it must have a smaller energy gap.

34

Chapter Two


In Type-2 (staggered) heterojunction, the locations of lower (Ev), and higher (Ec), are displaced, so electrons being collected at lower (Ev), and holes being collected at higher (Ec), are confined in different spaces. Type-3 (broken-gap) heterojunction is a special case of Type-2, but the (Ec), of one side is lower than the (Ev), of the other. The conduction band thus overlaps the valence band at the interface; hence the name is broken gap [73].

Fig. (2-8): Classification of heterojunctions :( a) Type-1 or straddling heterojunction. (b) Type-2 or staggered heterojunction. (c) Type-3 or broken-gap heterojunction [73].

The case of an isotype heterojunction is somewhat different. In an n-n heterojunction, since the work function of the wide-bandgap semiconductor is smaller, the energy bands will be bent oppositely to those for the n-p [73].

35

Chapter Two


2-11 Gas Sensors: 2-11-1 Metal Oxide Semiconductor Gas Sensors: Metal oxide semiconductor is one of the most important materials that could be applied for gas sensing measurements. It was first discovered decades ago that molecules interacting with semiconductor surfaces can influence the surface properties of semiconductors, such as conductivity and surface potential. The first chemo resistive semiconductor gas sensors were produced by Seiyama in 1962 since then, metal oxide semiconductors have been widely studied as gas sensors because of their low cost and relative simplicity. Metal oxides possess a broad range of electronic, chemical and physical properties that are often highly sensitive to the changes in their chemical environment. Due to these properties, metal oxides have become one the most popular commercial sensors. Numerous materials have been reported to be usable as metal oxide sensors including both single-component metal oxides such as ZnO [104]. 2-11-2 Applications of Semiconductor Gas Sensors: Numerous kinds of gases are emitted from various sources into our living space, working space, and outdoors. Many of them are hazardous to human beings and the environment. Most of the gases are present at very low concentrations so that extremely sensitive sensors are required for their monitoring. Metal oxide semiconductors are considered as one of the most promising gas sensor candidates. Since the first semiconductor gas sensor was created in 1960’s, much technological effort has been made to improve semiconductor gas sensors, in particular aiming at improve their sensitivity, selectivity, stability, and convenience for practical use. Nowadays, 36

Chapter Two


semiconductor gas sensors are widely used for domestic and industrial gas detectors, gas-leak alarms, process control and pollution control. In addition, semiconductor gas sensors have been successfully employed as sensors for the detection of different gases, such as CO, CO2, H2, alcohol, H2O, NH3, O2, NOx, and liquid petroleum gas [104].

2-11-3 Sensing Mechanism: Considering the influence factors on gas sensing properties of metal oxides, it is necessary to reveal the sensing mechanism of metal oxide gas sensor. The exact fundamental mechanisms that cause a gas response are still controversial, but essentially trapping of electrons at adsorbed molecules and band bending induced by these charged molecules are responsible for a change in conductivity. The negative charge trapped in these oxygen species causes an upward band bending and thus a reduced conductivity compared to the flat band situation. Figure(2-9) schematically shows the structural and band model of conductive mechanism upon exposure to reference gas with or without CO When gas sensors exposure to the reference gas with CO, CO is oxidized by O– and released electrons to the bulk materials. Together with the decrease of the number of surface O–, the thickness of space-charge layer decreases. Then the Schottky barrier between two grains is lowered and it would be easy for electrons to conduct in sensing layers through different grains. However, the mechanism is only suitable for n-type semiconducting metal oxides of which depletion regions are smaller than grain size.

37

Chapter Two


Fig. (2-9): Three grains of a semiconductor oxide showing how the intergranular contact resistance comes about. The height of the energy barrier [105].

2-11-4 Gas Sensitivity: This is the device characteristic of perception a variation in physical and or chemical properties of the sensing material under gas exposure. In order to improve it, it will be great interest to work with the most appropriate sensing material in every case, and reach its optimum detecting temperature. As suggested by several authors that working with nanostructured materials will give a higher surface area in front of gas. Taking into account that sensing reaction take place mainly on sensors layer surface, the control of semiconductor particle size will be one of the first requirements for enhancing the sensitivity of the sensor [106]. 38

Chapter Two


The sensitivity of sensors is defined as the relative variation of the resistance of the sensitive thin film in percent per ppm of applied gas concentration [107]. 𝑺=

⎸𝑹𝒈 −𝑹𝒂 ⎸ 𝒏𝑹𝒂

… . (𝟐 − 𝟒𝟔)

Where 𝑅𝑎 and 𝑅𝑔 are the electric resistance of the sensor in air and in presence of gas respectively and (𝑛) is the gas concentration. Although it can be calculated from conductance 𝝈 as in relation: 𝝈=

𝑮𝒈𝒂𝒔 𝑮𝒂𝒊𝒓

… . (𝟐 − 𝟒𝟕)

Where: 𝐺𝑎𝑖𝑟 is the conductance of the sensor in pure and dry air, and 𝐺𝑔𝑎𝑠 is the conductance of the sensor in the air containing a given concentration of reducing gas [108].

2-11-5 Response and Recovery Times: The response time (τres) of a gas sensor is defined as the time it takes the sensor to reach (90%) of maximum/minimum value of conductance upon introduction of the reducing/oxidizing gas [106]. Similarly, the recovery time (τrec) is defined as the time required to recover to within (10%) of the original baseline when the flow of reducing or oxidizing gas is removed. Figure (2-10) shows how this is measured from sensor data plotting the resistance as a function of time.

39

Chapter Two


Fig. (2-10): A sketch showing how response and recovery times are calculated from a plot of sensor resistance versus time [106].

2-11-6 The Effect of the Crystallite Size on the Sensitivity of Metal-Oxide Gas Sensors: The “small size effect” of metal oxides is one of the parameters that are very important for gas sensing, as shown in Figure (2-11), a sensor is considered to be composed of partially sintered crystallites that are connected to their neighbors by necks. Those interconnected grains form larger aggregates that are connected to their neighbors by grain boundaries [109, 110]. On the surface of the grains, adsorbed oxygen molecules extract electrons from the conduction band and trap the electrons at the surface in the form of ions, which produces a band bending and an electron depleted region called the space-charge layer. When the particle size of the sensing film is close to or less than double the thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably

[111]

. Three different cases can be

distinguished according to the relationship between the particle size (D) and 40

Chapter Two


the width of the space-charge layer (L) that is produced around the surface of the crystallites due to chemisorbed ions [112]. When D >> 2L, the conductivity of the whole structure depends on the inner mobile charge carriers and the electrical conductivity depends exponentially on the barrier height. It is not so sensitive to the charges acquired from surface reactions. When D ≥ 2L, the space-charge layer region around each neck forms a constricted conduction channel within each aggregate. Consequently, the conductivity not only depends on the particle boundaries barriers, but also on the cross section area of those channels and so it is sensitive to reaction charges. Therefore, the particles are sensitive to the ambient gas composition. When D < 2L, the space-charge layer region dominates the whole particle and the crystallites are almost fully depleted of mobile charge carriers. The energy bands are nearly flat throughout the whole structure of the interconnected grains and there are no significant barriers for intercrystallite charge transport and then the conductivity is essentially controlled by the intercrystallite conductivity. Few charges acquired from surface reactions will cause large changes of conductivity of the whole structure, so the crystalline metal oxide becomes highly sensitive to ambient gas molecules when its particle size is small enough [113].

41

Chapter Two


Fig. (2-11). Schematic model of the effect of the crystallite size on the sensitivity of metal-oxide gas sensors: (a) D >> 2L; (b) D ≥ 2L; (c) D < 2L [113]

.

Figure (2-12) shows Response and recovery of ZnO sample

[69]

, and

figure (2-13) shows the Dynamic response of ZnO thin film resistance for a different concentrations to different NO2 gas concentrations [114].

42

Chapter Two


Fig. (2-12): Response and recovery of ZnO sample [69].

Fig. (2-13): Dynamic response of the conduct metric sensor to different NO2 gas concentrations in air [114].

43

Chapter Three

Experimental Work

3-1 Introduction This chapter focuses on the experimental details used in fabrication and investigation of undoped ZnO and aluminum - boron co-doped ZnO (AZB) thin films which were deposited by chemical spray pyrolysis (CSP) technique on glass, silicon and n, p type porous silicon (PS) substrate. The main instruments that using to measure the structural, electrical, optical and sensing properties have been mentioned in this chapter, the experimental procedures are described and pointed out as shown in figure (3-1). undoped ZnO and AZB thin films prepared by chemical spray pyrolysis technique. Zinc nitrate+ Aluminum nitrite +boric acid + distill water deposited at 450 oC

Glass

silicon

Electrochemical etching (p-type)

PS

Photo electrochemical (etching (p-type)

PS prepared for (p and n) for 30 min, 30 A. /cm2

Film characterization

Electrical measurements Conductivity+ Hall effect +(C-V)

Structural investigation

XRD+AFM+SEM+TEM +FTIR

Optical measurements

Transmittance+ Raman+ PL

Sensing properties for NH3 and NO2 for (50, 100, 150 and 200) ppm

Figure (3-1): schematic of experimental work. 44

Chapter Three

Experimental Work

3-2 Experimental Details: 3-2-1 Substrate Preparation:

A: Glass Substrate: Glass slides of (2.5×2.5 cm2) area, were used as substrate. They were cleaned by distill water then by alcohol with ultrasonic bath in order to remove the impurities and residuals oil or dust from their surface.

B: Silicon Substrate: N-type and p-type Si with resistivity (ρ= 0.05 – 0.1 Ω.cm) and (111) crystalline orientation is employed as substrates with dimensions of (2×2cm2) area to prepare PS using electrochemical and photo electrochemical etching process. The thickness of the silicon wafer is about 500 μm.

C: Preparation of Porous Silicon Substrate: The substrate of silicon of (2×2cm2) area used to prepare porous silicon layer on the polished face of the silicon wafer. The silicon wafer cleaned by a mixture of HF and H2O (1:2) to remove oxide native. After chemical treatment, aluminum layer were deposited with approximately 0.1 μm, by using an evaporation method, on the backsides of the silicon wafer.

45

Chapter Three

Experimental Work

3-2-2 Porous Silicon Preparation: Process of photo electrochemical etching (PECE) is formed by the anodic electrochemical etching (ECE) of monocrystalline silicon in hydrofluoric acid (HF) with concentration of (50%wt) mixed with ethanol in ratio of (1:1). The porous silicon prepared by electrochemical and Photo electrochemical etching were carried out by using current density 30 mA/cm2 for 30 minutes. The anodization was carried out with the distance between the Si substrate to the Pt electrode fixed at 2.5 cm. PS layers were fabricated by PECE process for n-type c-Si wafer and by ECE for p-type. The area electrochemically etched for all sample is (1cm2). By using a platinum electrode as a cathode and silicon wafer as anode electrode the electrical circuit is completed in a parallel way to use it for prepare porous silicon (PS) layer on the frontal surface of the silicon. When the etching began by closing the electrical circuits, the bubbles of hydrogen (H2) are observed during the etching process, indicating the propagation of the etching process and hence the formation of porous silicon. Figure (3-2) shows a schematic cell of experimental set-up for electrochemical etching process to prepare the porous silicon. The setup consists of a Tungsten Halogen lamps (100W) integrated with diluted etching acid, the photons of light play an important effect in the reaction process between the silicon surface and the solution of (HF), one of the important factors is the photon energy which leads to have a different penetration depth within the silicon thereby affecting the photo-synthetization process.

46

Chapter Three

Experimental Work

(a)

(b)

Fig. (3-2): (a) Electrochemical etching (ECE) cell set-up and (b) photo electrochemical etching (PECE) cell set-up. 3-3 Chemical Spray Pyrolysis (CSP) Technique: During the several years ago, technologies of coating have a good attention for a devices applications, mainly due to their functional advantages over bulk materials, processing flexibility, and cost considerations. Thin film coatings may be deposited using physical methods or chemical methods. Spray pyrolysis is a technique which uses a liquid source for thin film coating. The main advantages of spray pyrolysis over other similar techniques are: - Cost effectiveness. - Substrates with complex geometries can be coated. - Relatively uniform and high quality coatings. - No high temperatures are required during processing. The chemical spray pyrolysis technique has been used in preparing the undoped ZnO and aluminum - boron co-doped ZnO (AZB) thin films samples on glass, silicon and PS substrates. Figure (3-3) shows the general schematic diagram of the thermal chemical spray pyrolysis deposition system. 47

Chapter Three

Experimental Work

Fig. (3- 3): General schematic of a spray pyrolysis deposition process [100]

.

Three processing steps can be viewed and analyzed: 1) Atomization of the solution. 2) Aerosol transport of the droplet. 3) Decomposition of the solution to initiate film growth. After the droplet leaves the atomizer, it travels through the ambient with an initial velocity determined by the atomizer. In the aerosol form, the droplets are transported with the aim of as many droplets as possible reaching the surface. As the droplets move through the ambient, they experience physical and chemical changes depicted in Figure (3-4).

48

Chapter Three

Experimental Work

Fig. (3- 4): Spray pyrolysis droplets modifying as they are transported from the atomizing nozzle to the substrate. Whether the temperature or the initial droplet size are varied, there are four potential paths which the droplet can take as it moves towards the substrate (A-D) [100].

3-4 Preparation of Solutions: A 0.075 Molarity (M) concentration used for the solution of zinc nitrate (Zn (NO3)2.6H2O) which is a solid material has a white color and its molecular weight (297.4 g/mole)) dissolved in distilled water. A magnetic stirrer was used for this purpose for about (10-15) minutes to facilitate the complete dissolution of the solute in the solvent. The weight is measured via an electrical balance sensitive four digits (10-4g). Equation (3-1) was used for calculating mass of ((Zn (NO3)2.6H2O).

49

Chapter Three 𝑀=

𝑊𝑡 𝑀𝑊𝑡 . 𝑉

Experimental Work

X 1000

… . (3 − 1)

Where: 𝑀 : Concentration of molarities (mole/liter). 𝑊𝑡 : Weight of ((Zn (NO3)2.6H2O) (g). 𝑀𝑊𝑡 : Molecular weight of ((Zn (NO3)2.6H2O) (g/mole). 𝑉: Volume of distilled water (ml).

The solution of zinc nitrate [(Zn (NO3)2.6H2O) - 99.9% purity] was used to prepare undoped ZnO thin films, the formation of ZnO thin film given by the following equations [77]: 2𝑍𝑛(𝑁𝑂3 )2 → 2𝑍𝑛𝑂 ↓ + 4 𝑁𝑂2 ↑ 𝑂2 ↑

… . (3 − 2)

For co-doped ZnO thin films by aluminum and boron the following steps were used: 1- Doping with boron: using a boric acid (H3BO3 – 99.8% purity, molecular weight: 61.83 g/mol.] dissolved in distilled water with 0.075M. 2- Doping

with

aluminum:

using aluminum nitrate [(Al

(NO3)3.9H2O) 99.9% purity, molecular weight: 375.13 g/mol.] dissolved in distilled water with 0.075M. By volume doping, to prepare aluminum-boron co-doped ZnO (AZB 2%), (1ml) of boric acid solution, (1ml) of aluminum nitrate solution and (98 ml) of zinc nitrate solution together will added in one piker, then mixed by the magnetic stirrer for about (10-15) minutes. The same way used for (AZB 4%), (AZB 6%) and (AZB 8%).

50

Chapter Three

Experimental Work

3-5 Thin Film Deposition: The prepared solutions were used as spray solution to preparing the undoped and dopant thin films by using spray pyrolysis deposition (SPD) technique. The spray pyrolysis substrate temperature is maintained within (450±10 oC) during the deposition. Film thickness is controlled by both the concentration of molarities and the number of sprays. (3) seconds spray time is maintained during the experiment then waiting for (42) seconds where this time was enough to return the substrate temperature to (450 ±10 o

C). The normalized distance between the spray nozzle and substrate was

fixed at 28cm. Table (3-1) summarizes the optimized thermal pyrolysis deposition conditions for the preparation of ZnO and AZB thin films that were employed in the current research.

Table (3-1): Optimum thermal spray pyrolysis deposition condition for the preparation of ZnO, AZB thin films. Spray Parameters

Values

Concentration of solution

0.075 mole/liter

Solvent

distilled water 450 ±10oC

Substrate temperature Spray rate

2 ml/min

Carrier gas pressure

3.5 bar

Nozzle – substrate distance

28 cm

51

Chapter Three

Experimental Work

3-6: Characterization of Films: 3-6-1 Thin Film Thickness Determination: The thickness of the films is an important parameter that affects the film properties, TFProbeTM spectroscopic reflectometer Film Thickness Measurement System used to measure the films thicknesses. The setting in laboratory of photocell center at ministry of science and technology. The thicknesses of thin films deposited on glass and porous silicon of n–type and p-type obtained by TFProbeTM spectroscopic instrument are listed in table (3-2):

Table (3-2): Thicknesses of thin films deposited on glass and porous silicon obtained by TFProbeTM spectroscopic instrument. Thin films thickness(nm) Sample

Substrate Glass

PS(p-type)

PS(n-type)

145.89

146.85

148.63

AZB 2% 148.75

147.32

148.76

AZB 4% 150.53

149.35

146.64

AZB 6% 151.53

150.39

149.49

AZB 8% 151.81

150.21

150.39

ZnO

52

Chapter Three

Experimental Work

3-7 Structural and Morphological Investigations: 3-7-1 X-Ray Diffraction (XRD) Measurements: The crystallite size (D) of the crystalline material which plays the important role in the material properties, can be estimated easily from the X-ray spectrum by means of full width at half maximum (FWHM) method that is often calculated by Scherrer's relation. The lattice constant can also calculated by X-ray diffraction measurement, where the obtained values can compared according to the ASTM (American Society of Testing Materials) cards. The setting in laboratory of XRD at ministry of science and technology. XRD instrument is of type (Shimadzu 6000) made in Japan, with the following specifications: Target Wavelength Current Voltage

: : : :

Cu Kα 1.5406 Å 30 mA 40 kV

The instrumental broadening (βi) correction was made using a standard defect free Si sample

[115]

. Where it is given by the used XRD

instrument in table (3-3): Table (3-3): The instrumental broadening (βi) of used XRD instrument given by using standard defect free Si sample. 2θ (deg.) 28.4154 47.2675 56.0857 69.1006 76.3417 87.9978 94.9175

53

FWHM (βi (deg.)) 0.1147 0.1098 0.1077 0.1140 0.1100 0.1215 0.1297

Chapter Three

Experimental Work

3-7-2 Atomic Force Microscopy: The surface morphology of ZnO and AZB thin films deposited on glass and porous silicon substrate has been analyzed by using atomic force microscopy (angstrom advance INC, SPM, AA-3000) made in USA. The setting in laboratory of nanotechnology at ministry of science and technology. 3-7-3 Scanning Electron Microscopy (SEM): Scanning electron microscopy (SEM) is basically a type of electron microscope. The SEM study has been carried out by Hitachi (S-4160). Scanning electron microscope the magnification power continuous form 6x to 100,000x. The setting in thin film laboratory at University of Tehran.

3-7-4 Transmission Electron Microscopy (TEM): Transmission electron microscopy (TEM) has been measured by the instrument type of (Nederland, manufactured by Philips, model of cm-30). The setting in thin film laboratory at University of Tehran.

3-7-5 Optical Microscopic Measurement: The prepared ZnO and AZB thin films investigated by optical microscope (1000X magnification power type Nikon ECLIPSE ME600). A digital Camera (DXM1200F) and a computer were added to store the surface image of the thin films prepared. The setting in laboratory of photocell center at ministry of science and technology.

54

Chapter Three

Experimental Work

3-8 Optical Properties Measurement: The Film Topography of the ZnO and AZB thin films were investigated with an optical microscope power type (NIKON, ECLIPSE, ME600). UV/VIS Spectrophotometer optima Tokyo, japan Model (SP3000 Plus) (S.N/ 340209), rate voltage (220VAC), rate current (1A) and a computer added to store the surface image of the prepared films. The wavelength range taken for the thin films deposited on glass substrate is (300-1100 nm), the setting in laboratory of photocell center at ministry of science and technology.

3-8-1 Fourier Transform Infrared Spectrophotometer (FTIR): Fourier transform of infrared spectra can be happens by the absorption of electromagnetic radiation dependence on probes the vibrations of molecular of ZnO and AZB thin films materials. The frequency of the incidence wavelength and intensity of absorption are the indication of the structures and structural of the molecule of the materials of thin films. The thin films (FTIR) has been measured by the instrument (IRPrestage-21/Fourier

Transform

Infrared

Spectrophotometer/

SHIMADZU), all ZnO and AZB thin films deposited on porous silicon has been measured for the Wavenumber (400-4000 cm-1). The setting in laboratory of nanotechnology at ministry of science and technology.

55

Chapter Three

Experimental Work

3-8-2 Raman Spectroscopy Measurement: Raman scattering measurement in fully backscattering geometry were conducted at R.T. on samples brought in air, using a microprobe setup (Sunterra Raman microscope Broker co. Germany). The setting in laboratory of nanotechnology at ministry of science and technology. 3-9 Electrical Measurement: Electrical measurement in our work includes dc conductivity eq.(2 − 25), Hall effect eq.(2 − 33), capacitance voltage (C-V) eq. (2 − 49) and (2 − 50), sensitivity eq. (2 − 51).

3-9-1 Masking Techniques: In the application of electrics and micro electrics, it is necessary to form films in specific patterns or shapes to measure the electrical properties. For this purpose a suitable mask is used to get the desired shape and to prevent the vapor condensation on the other areas of the substrate. The mask is usually formed by an Aluminum Ohmic contacts using vacuum at a pressure of 10-4 Torr. These masks are placed as closely as possible to the substrate on which the film is to be deposited to avoid the shadow effect. The mask shown in figure (3-5, a, b, c) was used in the measurements.

56

Chapter Three

Experimental Work

(a)

(b)

(c) Fig. (3-5): Mask Used in: (a) D.C. conductivity and, (b) Hall Effect, (c) gas sensing.

3-10: Capacitance –Voltage (C-V) Measurements: The heterojunction obtained by deposition of ZnO/ PS/Si/Al and the AZB/ PS/Si/Al thin films. Capacitance has been measured as a function of reverse voltage (C-V) using LC meter with a (1MHz) frequency. The (CV) measurements of the heterojunctions as a function of the bias voltage within the range (-5 to +5) Volts were carried out. By plot (1/C2) as a function of applied voltage and the value of the built-in (Vbi) potential were obtained from the intercept of the straight line with the applied voltage axis. The charge carrier density 𝒏𝒅 and the width of the depletion region (W) can be calculate from the equations (2-44) and (2-45). The measurements and the setting in laboratory of nanotechnology at ministry of science and technology. 57

Chapter Three

Experimental Work

3-11 Photoluminescence Measurements: Photoluminescence (PL) is an important physical phenomena used to characterize semiconductors which depicts a sample energy structure while possibly revealing other important material features. The transition energy of the samples was measured using the Perkin Elmer Spectrophotometer Luminescence LS 55 equipped with FL Win lab software. 3-12 Gas Sensor Testing System: Sensing properties of the ZnO and AZB thin films deposited on glass and porous silicon are used to sensing for (NH3 and NO2 gas) with concentration (50, 100, 150 and 200) ppm and prepared at different substrate temperature. Test chamber is illustrated schematically in figures (3-6) shows the sensor testing system. Unit consists of a vacuum –tight stainless steel cylindrical test chamber of diameter 20 cm and height 16 cm with the bottom base made removable and of O – ring sealed. The effective volume of the chamber is 5024cm3 and 50mbar pressure. The setting in laboratory of applied physics center at ministry of science and technology.

58

Chapter Three

Experimental Work

(a) Fig. (3-6): Gas sensor testing system.

59

Chapter Four

Results and Discussion

4-1 Introducion: This chapter deals with the results and analysis of the experimental measurements of the ZnO and aluminum-boron co-doped ZnO (AZB) thin films prepared by chemical spray pyrolysis(CSP) and deposited on glass, silicon and porous silicon substrate. The results include the structural, optical, electrical and sensing properties of ZnO and co-doped with aluminum and boron deposited on glass, n-type and p-type of porous silicon (PS).

4-2: Structural Properties: 4-2-1: Analysis of XRD for ZnO and AZB Thin Films: Figure (4-1, a, b and c) shows the X-ray diffraction analysis of undoped ZnO and AZB thin films co-doped with the different concentration of aluminum and boron deposited on glass, silicon and porous silicon respectively. The figures shows existence of three peaks, corresponding to (100), (002) and (101) directions respectively, this means the thin films are polycrystalline with a hexagonal wurtzite structure compared with standard (ASTM) of ZnO card, the (002) plane is the preferred orientation observed for all the thin films prepared.

60

002

101


100

Chapter Four

a

glass AZB 8%

Intensity(a. u.)

AZB 6%

AZB 4% AZB 2%

ZnO 34

36

101

32

002

30

38 40 2θ(deg.)

42

44

46

48

50

silicon

b

AZB 8%

Intensity(a.u.)

AZB 6% AZB 4%

AZB 2%

ZnO 30

32

34

36

38 40 2θ(deg.)

61

42

44

46

48

50

Chapter Four


Porous Silicon AZB 8% AZB 6% AZB 4% AZB 2%

100

101

002

Intensity(a.u.)

c

ZnO 30

32

34

36

38 40 2θ(deg.)

42

44

46

48

50

Fig. (4-1): XRD patterns for thin films of undoped ZnO and AZB thin films with different doping concentration (2, 4, 6, and 8 %) deposited on: (a) glass, (b) silicon, (c) p-type porous silicon substrate. From figure (4-1, a), the degree of crystallinity decreases with increasing of Al and B ratio due to the distortion in lattice parameter. In figure (4-1, b) the substrate of thin films transform to partial amorphosity with increases the Al and B concentration. From figure (4-1, c) the XRD pattern exhibit the nanostructure with increases the Al and B ratio because create new surface state as defect and distortion, also increases the surface area which lead to random reflection for XRD beam. It is well known that the (002) orientation of ZnO Wurtzite structure is generally observed, suggesting that the surface free energy of (002) plane is the lowest in ZnO films [116, 117]. The intensities of the peaks deposited on glass, silicon and PS substrate are changed with increasing of co-doping concentration in ZnO thin films. 62

Chapter Four


Imbalance the ion size of the Al3+ (r= 0.054 nm) and B3+ (r=0.041nm) interstitial in Zn2+ (r= 0.074 nm) which lead to many defects such as the stress and dislocation in the thin films [61]. Table (4-1) shows XRD data of diffraction angle(2θ), angular full width at half maximum β(FWHM), lattice constants (a and c), interplanner spacing(d) calculated by eq.(2-10) for ZnO and AZB thin films deposited on preheated glass substrate at temperature 450oC, silicon and porous silicon substrates, respectively. Table (4-1): Structural parameters of ZnO and AZB thin films deposited on glass, silicon and p-type porous silicon substrates at 450oC. 2θ β(FWHM) hkl Samples d(nm) a(nm) c(nm) (deg.) (deg.) ZnO AZB: 2% AZB: 4% AZB: 6% AZB: 8% ASTM ZnO AZB: 2% AZB: 4% AZB: 6% AZB: 8% ASTM ZnO AZB: 2% AZB: 4% AZB: 6% AZB: 8% ASTM

31.896 31.744 31.772 31.909 31.903 31.769 34.538 34.462 34.494 34.475 34.700 34.421 36.318 36.202 36.185 36.280 36.277 36.252

002

ZnO AZB: 2% AZB: 4%

34.8139 34.6667 34.9380

002 101

ZnO ZnO

34.6459 36.4679

100

002

101

Glass substrate 0.28035 0.29 0.32372 0.28166 0.60 0.32523 0.28141 0.87 0.32494 0.28023 1.11 0.32358 0.28029 1.12 0.32365 0.28143 0.32498 0.25948 0.24 0.32392 0.26004 0.69 0.32462 0.25994 0.86 0.32432 0.25994 0.77 0.32449 0.25831 1.17 0.32245 0.26033 0.32498 0.24716 0.27 0.32442 0.24792 0.61 0.32543 0.24803 0.84 0.32558 0.24741 0.98 0.32475 0.24742 1.13 0.32478 0.24759 0.32498 p-type Silicon substrate 0.25948 0.43 0.32181 0.26004 0.20 0.32357 0.25980 0.25 0.32287 p-type Porous silicon substrate 0.258701 0.2744 0.33957 0.246183 0.3481 0.32314

63

0.51864 0.52106 0.52060 0.51842 0.51852 0.52066 0.51896 0.52008 0.51960 0.51987 0.51661 0.52066 0.51977 0.52138 0.52161 0.52029 0.52033 0.52066 0.51558 0.51840 0.51728 0.54404 0.51771

Chapter Four


The shift in the (002) peak position for ZnO and AZB thin films to the higher values might be due to the substitution of Zn by the doped ions in the hexagonal lattice. These shifts agree with

[118–120]

. Such changes in

crystallinity might be the result of changes in the atomic environment due to extrinsic doping of ZnO samples. Where the substitution of Al3+ or B3+ for Zn2+ leads to decrease Interplanner spacing (d), so that the diffraction angle (2θ) must be increase according to Bragg's equation. However, the peak position not only depends on a substituted of Zn2+ in ZnO structure with Al3+ and B3+, but also is strongly related to other parameters, such as the preparation conditions [92]. 4-2-2: Micro Strain (Crystal Distortion) Measurements: Micro strain depends directly on the lattice constant (c), and its value related to the shift from the ASTM standard value and can be calculated using the equation (2-11) where (𝑪𝑨𝑺𝑻𝑴 = 0.520661nm) and the values of (𝑪𝑿𝑹𝑫 ) measured by XRD are shown in the table (4-1). The results listed in table (42) reveals that the micro strain (crystal distortion) of thin films grown on glass substrate is less than silicon and porous silicon substrates.

4-2-3: Average Crystallite Size Measurements: The results of the average crystallite size of (100), (002) and (101) of ZnO and AZB thin films deposited on glass, silicon and porous silicon were calculated by using equations (2-1, 2-3, 2-5and 2-7) are listed in table(4-2).

64

Chapter Four


Table (4-2): Average Crystallite size and micro strain of ZnO and AZB thin films deposited on glass, p-type silicon and p-type porous silicon substrate. average crystallite size(nm) Micro hkℓ

Samples

strain ε(x10-3)

100

002

101

002

Scherer

Scherer's

Warren

equation

correction

Correction

eq.(2-1)

eq.(2-3)

eq.(2-5)

ZnO

3.89

29.8

AZB 2%

0.76

14.2

AZB 4%

0.12

AZB 6%

Glass substrate 48

Warren

Dislocation

geometric

density(×1011)

meaning

(lines/cm2)

eq.(2-7)

32.2

39.3

1.1

17.4

14.4

15.8

4.9

9.81

11.2

9.89

10.5

10.4

4.30

7.76

8.61

7.80

8.20

16.6

AZB 8%

4.11

7.67

8.51

7.71

8.10

17.0

ZnO

3.27

35.8

65.5

40.2

51.3

0.8

AZB 2%

1.12

12.4

14.8

12.6

13.6

6.4

AZB 4%

2.03

10.1

11.5

10.1

10.8

9.8

AZB 6%

1.51

9.8

13.1

11.3

12.2

8.0

AZB 8%

7.78

7.40

8.17

7.43

7.79

18.3

ZnO

1.72

32.0

53.7

35.0

43.3

1.0

AZB 2%

1.39

14.1

17.1

14.3

15.6

5.0

AZB 4%

1.83

10.3

11.9

10.4

11.2

9.3

AZB 6%

0.71

8.85

9.95

8.90

9.41

12.8

AZB 8%

0.63

7.72

8.55

7.75

8.14

16.8

ZnO

10.91

p-type Silicon substrate 20.2 27.0 20.8

23.7

2.5

AZB: 2%

6.84

43.8

98.2

52.6

71.8

0.5

AZB: 4%

14.31

34.8

62.2

38.8

49.1

0.8

42.8

1.0

31.2

1.6

002

ZnO

6.26

p-type Porous Silicon substrate 31.7 52.9 34.6

101

ZnO

5.67

25.1

36.7

65

26.5

Chapter Four


The crystallite size of ZnO and AZB thin films deposited on glass substrate depending on Sherrer's equation (2-1) were shown in figure (4-2). The crystallite size values decreases with the increasing of aluminum and boron co-doped due to the increasing of dislocation density.

60

100 plane/glass

Crystallite size(nm)

50

002 plane/glass 40

101 plane/glass

30 20 10

0 0

2

4 Doping(%)

6

8

10

Fig. (4-2) Average crystallite size for (100, 002, 101) planes of ZnO and AZB thin films deposited on glass substrates as a function of doping concentration.

4-2-4 Integral Breadth (Δ): Integral breadth of the samples were obtained from the XRD pattern sheets and using the relation (2–12), the results in table (4-3) indicate that the integral breadth of undoped ZnO is less than the integral breadth of co-doped ZnO thin films for (100) and (101) due to decreasing of area under the curves of doped thin films as shown in figure(4-1, a), but for (002) plane of silicon

66

Chapter Four


substrate increasing aluminum and boron co-doped leads to decreasing the integral breadth. 4-2-5 Shape Factor (φ) The shape factor was calculated using the relation (2-13), the results show that the shape factor have a maximum value for AZB 2% for the planes due to the high value of the maximum intensity (IMAX) as shown in table (4-3). Table (4-3): Maximum Intensity, area under the curve, integral breadth and shape factor of (100), (002) and ((101) planes for ZnO and AZB thin films. Maximum hkℓ

Samples

Intensity(IMAX)

Area (a.u. ×deg.)

Integral breadth(Δ)

Shape factor

(deg.)

(a.u.)

(φ)

Glass substrate

100

002

101

ZnO

34

4.7

0.14

2.09

AZB 2%

90

46

0.51

1.17

AZB 4%

72

29

0.40

2.16

AZB 6%

56

24

0.43

2.59

AZB 8%

82

56

0.68

1.64

glass

Si

PS

glass

Si

PS

glass

Si

PS

glass

Si

PS

ZnO

238

60

2984

86

26

925

0.36

0.43

0.31

1.66

1

0.88

AZB 2%

68

88

60

31

0.88

0.35

0.68

0.58

AZB 4%

48

32

20

11

0.42

0.34

2.07

0.73

AZB 6%

32

16.5

0.52

2.17

AZB 8%

41

13

0.32

3.53

ZnO

80

25

Glass substrate 0.31

0.86

AZB 2%

118

64

0.54

1.13

AZB 4%

96

46

0.48

1.75

AZB 6%

56

29

0.52

1.88

AZB 8%

104

72

0.69

1.63

67

Chapter Four


4-2-6 William Hall's (W-H) Grain Size Measurements: The grain size of the ZnO and AZB thin film were calculated by using William Hall method (DW-H) using equation (2-17). Figure (4-3) shows the relation of (4sinθ) versus (FWHM cosθ) taking (100), (002), and (101) lattice

FWHM COS(θ)

planes corresponding to the wurtzite phase of ZnO. 0.025 0.024 0.023 0.022 0.021 0.02 0.019 0.018 0.017 0.016 0.015 0.014 0.013 0.012 0.011 0.01 0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0

ZnO pure AZB 6%

1

1.05

AZB 2% AZB 8%

1.1

1.15

AZB 4%

1.2

1.25

1.3

4SIN(θ) Fig. (4-3) W-H plots of undoped ZnO and AZB with different doping concentration (2, 4, 6, and 8 %) thin films deposited on glass substrate.

From the linear fit to the data, the grain size (DW-H) was extracted from the y-intercept and the micro strain εW-H from the slope of the straight line as shown in figure (4-4).

68

Chapter Four


10

micro strain

9

Micro strain(X10-3)

8

7 6

5 4

3 2 1 0 0

2

4 Doping(%)

6

8

10

Fig. (4-4): W-H micro strain of undoped ZnO and AZB at different doping concentration (2, 4, 6, 8 %) thin films.

The structure disorder concentration becomes higher with doping concentration in the thin films. Increasing of doping concentration leads to the stress formation as the result of ionic radius difference between Zn2+ and Al3+, and the strain is due to the incorporation of a dopant in the periodic lattice. The W-H plots show a strain for ZnO and AZB thin films which is an indication of lattice Shrinkage [121].

69

Chapter Four


4-3 Surface Morphology: 4-3-1 Optical Microscopy: Figure (4-5) shows the optical micrographs of the ZnO and AZB thin films deposited on glass substrate at 450 oC. These micrographs revealed that the thin film morphology can be easily recognized through the thin film homogeneity and color. It is clear that the thin film homogeniety and colour are changed gradually with respect to the doping concentration of aluminum and boron.

ZnO

AZB 2%

AZB 6%

AZB 4%

AZB 8%

Fig. (4-5): Optical micrographs of ZnO and AZB thin films with doping concentration: 0.0 to 8 % deposited on glass substrate at 450 oC.

70

Chapter Four


4-3-2 Atomic Force Microscopy (AFM) Investigation: Figure (4-6) reveals the granularity distribution, (2-D) and (3-D) AFM images of ZnO and AZB thin films deposited on glass substrate preheated to 450 oC. The surface characteristic is important for applications such as gas sensors and catalysts [122, 123]. The atomic force microscopy was used to determine roughness average and average size of the grains nucleated in the thin films. The average grain size obtained by AFM(DAFM) of co-doped and undoped of ZnO thin films is found to be around (38-72) nm as shown in granularity distribution in figure(4-6), the average grain size decrease with increases of Al and B doping concentration. The average roughness of ZnO and AZB thin films increase within the range (1.34-6.15 nm), the results of this test are listed in table (44).

71


Count, counts

Chapter Four

20 18 16 14 12 10 8 6 4 2 0 0

20

40

60

80 100 120 140 160 180

Average size range, nm

Count, counts

ZnO

20 18 16 14 12 10 8 6 4 2 0

0

20

40

60

80 100 120 140 160 180


AZB 2% 72


Count, counts

Chapter Four

20 18 16 14 12 10 8 6 4 2 0

0

20

40

60

80 100 120 140 160 180


Count, counts

AZB 4%

20 18 16 14 12 10 8 6 4 2 0

0

20

40

60

80 100 120 140 160 180


AZB 6% 73


Count, counts

Chapter Four

20 18 16 14 12 10 8 6 4 2 0

0

20

40

60

80 100 120 140 160 180


AZB 8% Fig. (4-6): Granularity distribution, 2-D and 3-D AFM image of the ZnO and AZB thin films with doping concentration (2, 4, 6, and 8%) deposited on glass substrate at 450 oC. 4-3-3 Scanning Electron Microscope (SEM): Figure (4-7) shows the top view SEM images of ZnO and AZB thin films with co-doped (2, 4, 6 and 8%) nanostructure deposited on glass substrate at 450 oC by spray pyrolysis technique. As clear in SEM images the grain size was decreases with the increasing of Al and B doping concentration in ZnO thin films and also some porosity is observable on the surface, where the values of the grain size (DSEM) decreases from 43 nm in ZnO thin films to 15 nm in AZB 8% as shown in table (4-4), this decrease of grain size due to substituted of Al3+ and B3+ in the locations of Zn2+ in ZnO structure. The grain sizes determined by scanning electron microscopy are different of those

74

Chapter Four


measured by XRD. This different related to the different grain size criteria underlying the different methods [92].

ZnO

AZB 2%

AZB 4%

AZB 6%

AZB 8% Fig. (4-7) SEM images of the ZnO and AZB with doping concentration (2, 4, 6 and 8%) deposited on glass substrate.

75

Chapter Four


This increasing of roughness with increase of co-doped ratio leads to effect on gas sensitivity of the films as will show in the measurements of gas sensitivity in the last part of this chapter. Figure (4-8) shows cross sections of SEM images of undoped ZnO and Al- B co-doped AZB 8% deposited on glass substrate.

(a)

(b) Fig. (4-8) Cross sections of SEM images in (60 and 100 xk) for: a) ZnO /glass, b) AZB (8%)/glass. As clear from figure (4-8, a) the crystallinity of undoped ZnO more than AZB 8% in the figure (4-8, b) due to Al-B co-doping concentrations. This 76

Chapter Four


phenomenon is consistent with the results of the XRD pattern as mentioned by XRD in figure (4-1, a), and increases the roughness value with increasing the co-doped concentrations as illustrated in figure (4-6) and in table (4-4). The SEM images show a homogeneous of the thin films pattern agree with [92,124]. Table (4-4) shows the values of micro strain, grain size calculated by WH method (DW-H), and average grain size measured by SEM (DSEM) and by AFM (DAFM), roughness average, Root Mean Square and Ten Point Height of ZnO and AZB thin films. This results are in agreement with [125- 127]. Table (4-4): W-H grain size DW-H, DSEM, DAFM, εW-H, Sq and Sz for undoped ZnO and AZB with doping concentration (2, 4, 6, 8 %) thin films deposited on glass substrate.

Samples

DW-H (nm)

DSEM (nm)

DAFM (nm)

εW-H (×10-3)

Sa(roughness

Sq (R. M.S.)

Average)(nm)

(nm)

Sz (Ten Point Height) (nm)

ZnO

60.3

43

72

9

1.34

1.85

13.3

AZB 2%

14.5

30.5

53

3.7

1.43

1.75

6.6

AZB 4%

9.5

22

58

5.5

2.24

2.77

7.74

AZB 6%

8.0

18

44

9

2.73

3.46

3.92

AZB 8%

7.6

15

38

3.3

6.15

6.52

3.21

Figure (4-9) shows the grain size obtained by W-H method, SEM, AFM. Where the average grain size measured by AFM (DAFM) is higher than (DSEM) and (DW-H), so that the ZnO and AZB thin films are, and the values of average grain size calculated by W-H (DW-H) are approximately closer to the (DSEM). 77

Chapter Four


The grain sizes determined by AFM are larger than those measured by SEM and XRD. This is related to the different grain size measured by the different methods. In the XRD pattern obtained for the samples the crystallites in the sample determined using Scherer's formula after applying particle strain corrections using Williamson's Hall analysis. In the TEM and SEM analyses, the grain size is measured by the distances between the visible grain boundaries, it is determined by the extent of the crystalline regions [95].

80

glass

Grain size(nm)

70 60

50 40

W-H

30

SEM

AFM

20 10

0 0

2

4

6

8

10

doping(%)

Fig. (4-9)Average grain size calculated by W-H method, SEM and AFM of undoped ZnO and AZB with different doping concentration (2, 4, 6 and 8 %) thin films deposited on glass substrate.

78

Chapter Four


4-3-4 Transmission Electron Microscope (TEM): TEM images results like XRD (Scherer’s formula) results confirm the nanometric sizes of the samples, but TEM shows the mean size of the particles more than XRD. This difference in the size refers to this fact that “TEM shows the size of the particles and XRD shows the size of the crystallites” [57]. Figure (4-10) shows the high resolution TEM lattice images of the ZnO films with the present of spherical nanoparticles. The average grain sizes of this spherical particles in the range of (6.5–23) nm as measured by imagej software program. Also some porosity are appearance at the surface of the TEM images. This results and TEM images of ZnO thin film pattern agree with [95, 128].

(a)

(b)

(c) (d) Fig. (4-10)TEM image of undoped ZnO for a different scales: (a) 80 nm, (b) 60 nm, (c) 40 nm, (d) 10 nm. 79

Chapter Four


4-4 FTIR Spectroscopy Measurements: Figure (4-11) shows the FTIR transmittance spectra for ZnO and AZB thin films deposited on p-type silicon (111) substrates, the absorption peak is observed around (400–800) cm-1. An absorption bands observed at (405, 422, 447, 474, 501, 628) cm-1 corresponds to the Zn-O stretching vibration of hexagonal ZnO, this absorption decreases with increasing of Al and B concentration may be due to substitution of Al3+ or B3+ for Zn2+. These results

400

450

628

501

474

447

a

422

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

405

Transmittance(%)

are agree with result [96, 129, 130].

500 550 600 650 700 Wavenumber(cm-1)

80

750

800

Chapter Four


0.5

b

0.3 0.2 0.1

416 428 462

0

611

Transmittance(%)

0.4

-0.1

400

450

500

550

600

650

700

750

800

Wavenumber(cm-1) 18.5

C

17.5

17

15.5 15

665

622

470

16

601

16.5 416

Transmittance(%)

18

400 450 500 550 600 650 700 750 800 Wavenumber(cm-1)

81

Chapter Four


3

d 665

405

1.5

541

2 418

Transmittance(%)

2.5

1 0.5

0 400

450

500

550

600

650

700

750

800

750

800

Wavenumber(cm-1) 35

e 667

25 20 609

Transmittance(%)

30

15

10 5 0

400

450

500

550

600

650

700

Wavenumber(cm-1) Fig.(4-11): FTIR spectra of ZnO and AZB thin films deposited on p-type silicon(111) substrate of: a) ZnO, AZB with doping concentration: b) 2%, c) 4%, d)6%, e) 8%.

82

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4-5 Optical Properties: The optical properties of the ZnO and AZB thin films deposited on a glass substrate synthesized by chemical spray pyrolysis (CSP) technique at a substrate temperature 450 oC were measured at room temperature. The optical energy gap ( 𝑬𝒈 𝑶𝒑𝒕 ) and the optical constants which include the refractive index (𝒏), extinction coefficient (𝑲𝒐 ), real dielectric constant ( 𝜺𝒓 ) and imaginary dielectric constant ( 𝜺𝒊 ) have been determined by the transmission and absorbance spectra at room temperature in the range from (300 to 1100) nm.

4-5 -1 Transmittance (Tr): Figure (4-12, a) shows the optical transmittance spectra of ZnO and AZB thin films by using UV-VIS spectrophometer as a function of wavelength. It is found that the transmittance of undoped ZnO are (87 %) in the visible region then decreases as the aluminum and boron co-doped concentration is increasing as clear in 8%. The transmittance is reduces to (85.4 %) as 2% Al-B co-doped is added. By further increasing the Al-B contents to 8% the transmittance of the films decreased to 79.8%. This result may be described to the light being scattered by large amounts of grain boundaries as well as the Al-B clusters also reflecting the incident light, therefore the transmittance of co-doped ZnO films decreases with the increase in Al-B content. This is indicate that AZB thin film can be used as a window material in solar cells. This behavior agrees with Singh [131, 132].

83

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4-5-2 Absorbance (Ab): The Absorbance spectra of the ZnO and AZB thin films are shown in Figure (4-12, b). It has been found that the absorbance decreases abruptly up to 450 nm i.e. in the visible region. In the UV region the absorbance decreases slowly. The absorbance has been found to increases with increasing of aluminum and boron co-doped and it is maximum for 8% of Al-B due to increases of roughness and grain boundaries.

84

100 90 80 70 60 50 40 30 20 10 0


a ZnO pure

AZB 2%

AZB 4% 90

AZB 6%

AZB 8%

Transmittance(%)

Transmittance(%)

Chapter Four

85

ZnO pure AZB 4% AZB 8%

80

75 300

500

700

900

1100

Wavelength(nm)

300

500

700 Wavelength(nm)

900

1100

b

0.14 0.12

Absorbance

AZB 2% AZB 6%

0.1

0.08 0.06 ZnO pure

0.04

AZB 2%

AZB 4%

AZB 6%

AZB 8%

0.02 0 300

500

700

900

1100

Wavelength(nm)

Fig. (4-12) (a): Transmittance spectra, (b) Absorbance spectra of ZnO and AZB thin films with doping concentration (2, 4, 6 and 8 %).

85

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4-5-3 Optical Energy Gap (Egopt): Figure (4-13) shows the plot of (αhν) 2 versus hν (photon energy) of ZnO and AZB thin films as a function of doping level. The optical band gap can be determined from extrapolating the straight line of the plot to α=0. This shift of the optical band gap of TCO thin films by varying growth conditions can be explained by the Burstein-Moss model due to the different carrier co-doped of these thin films. Explained that the band gap would rise with increasing carrier co-doped. The Burstein-Moss shift ∆𝑬𝒈 is given by [133]: 𝟐 𝒉𝟐 𝟐 𝟑 … . (𝟒 − 𝟏) (𝟑𝝅 ∆𝑬𝒈 = ( 𝒏) ) 𝟐𝒎∗

Where ∆𝑬𝒈 is the shift of the doped semiconductor with respect to the undoped semiconductor, 𝒎∗ the electron reduced effective mass, 𝒏 carrier co-doped and 𝒉 is the Planck constant. The optical band gap increased from 3.22 eV for pure ZnO films to 3.31 eV when the Al-B content increased to 8%. This shift of the energy gap was mainly due to both the grain size effect and the existence of partial amorphous phase in thin films co-doped with Al-B. And also may be related to decreasing the density of localized states in the bond structure, can sequently increased the optical band gap.

86

Chapter Four


1.2E+11

(αh )2 (eV/cm)2

a

ZnO pure

1E+11 8E+10 6E+10 4E+10

2E+10 0 3.2

3.25

3.3

h

3.35

3.4

(eV)

1.2E+11

(αh )2 (eV/cm)2

b

AZB 2%

1E+11 8E+10 6E+10 4E+10

2E+10 0

3.2

3.25

3.3

h

87

(eV)

3.35

3.4

Chapter Four


1.2E+11

(αh )2 (eV/cm)2

c

AZB 4%

1E+11 8E+10 6E+10 4E+10

2E+10 0 3.2

3.25

3.3

h

3.35

3.4

(eV)

1.2E+11

(αh )2 (eV/cm)2

d

AZB 6%

1E+11 8E+10 6E+10 4E+10

2E+10 0

3.2

3.25

3.3

h

88

(eV)

3.35

3.4

Chapter Four


1.2E+11

(αh )2 (eV/cm)2

e

AZB 8%

1E+11 8E+10 6E+10 4E+10

2E+10 0

3.2

3.25

3.3

h

3.35

3.4

(eV)

Fig. (4-13): Variation of (αhv) 2 vs. photon energy (hv) for: (a) undoped ZnO, (b) AZB 2 %,( c) AZB 4 %, (d) AZB 6% and (e) AZB 8% thin film.

Fundamental absorption Edge of undoped ZnO obtained by plot absorption coefficient versus wavelength is (λc=387 nm).The optical band gap of the ZnO and AZB thin films apparently various with the Al and B as in figure (4-14).

89

Eg(eV)

Chapter Four


3.32 3.31 3.3 3.29 3.28 3.27 3.26 3.25 3.24 3.23 3.22 3.21 0

2

4 6 Doping(%)

8

10

Fig. (4-14) Variation of optical energy band gap (𝑬𝒈 ) vs. ZnO and AZB thin film with doping concentration (2, 4, 6 and 8 %).

4-5-4 Absorption Coefficient (α): The absorption coefficient α was determined from the region of high absorption at the fundamental absorption edge of the thin films. The transmittance coefficient α of the ZnO and AZB thin films is calculated using Lambert's as in equation (2-31) is shown in figure (4-15). The Absorption coefficient of ZnO and AZB thin films decreases sharply in the UV/VIS boundary, and then decreases gradually in the visible region because it is inversely proportional to the absorption.

90

Chapter Four


Fig. (4-15): Absorption coefficient as a function of wavelength of ZnO and AZB thin films with doping concentration (2, 4, 6 and 8 %). 4-5-5 Optical Conductivity (σoptical): Figure (4-16) shows the variation of optical conductivity as a function of wavelength for different of aluminum and boron co-doped. The optical conductivity is calculated by using equation (2-42).

Optical conductivity(s-1)

100 90

80 70

ZnO

AZB 2%

AZB 6%

AZB 8%

AZB 4%

60 50 40

30 20 10 0

300

500

700

900

1100

Wavelength(nm) Fig. (4-16): Optical conductivity of the ZnO and AZB thin films with doping concentration (2, 4, 6 and 8 %). 91

Chapter Four


From Figure (4-16), can see that the optical conductivity increases with increasing of aluminum and boron co-doped. This suggests that the increases in optical conductivity is due to electron excited by photon energy.

4-5-6 The Optical Constants: Table (4-5) shows optical constants of undoped ZnO and co-doped AZB thin films calculated for the wavelength (λ=550 nm), Refractive Index (𝒏𝒓 ), Reflectance (𝑹𝒓 ), Extinction coefficient (𝑲𝒐 ), Real and imaginary part of dielectric constant (εr, εi), are calculated using equations (2-34, 35, 36, 40 and 41), respectively.

Table (4-5): Optical constants at (λ=550 nm) for ZnO and AZB thin film with doping concentration (2, 4, 6, 8 %) at room temperature. Samples

ZnO

AZB 2%

AZB 4%

AZB 6%

AZB 8%

𝑬𝒈 (eV)

3.222

3.250

3.255

3.305

3.31

α ×104 (cm-1)

3.63

3.75

3.77

5.99

6.24

𝑲𝒐

0.110

0.189

0.114

0.114

0.185

𝜺𝒓

6.20

6.78

6.30

6.29

6.84

𝜺𝒐

0.54

0.99

0.57

0.57

0.95

𝑹𝒓

0.34

0.37

0.39

0.48

0.55

𝒏𝒓

2.015

2.082

2.15

2.36

2.82

92

Chapter Four


4-6 Photoluminescence (PL): Photoluminescence (PL) studies provide information of different energy state available between valence and conduction bands responsible for irradiative recombination. PL measurements have been performed in order to evaluate the optical properties of the undoped ZnO and Al- B co-doped ZnO (AZB) using xenon lamp with (λ=325 nm) at RT. Figures(4-17 ,a, b, c, d and e) shows the PL spectra obtained at room temperature of undoped ZnO and Al- B co-doped ZnO (AZB) with (2, 4, 6 and 8 %), respectively. The spectrum displays two luminescence peaks. The first peak position at 372nm UV emission was caused by the (near-band edge (NBE)) in the wide band gap of the ZnO which resulted from the direct recombination of photo generated charge carriers. The original value of the PL peak of the bulk ZnO was 367.655 nm [134]. Therefore the red shift in the PL peak of ZnO film toward a longer wavelength of 372 nm attributed to Zn vacancies in ZnO energy band gap. Moreover the defect in band structure caused the shift in the PL peak of ZnO and Al-B co-doped ZnO films as listed in table (4-6). The intrinsic band to band transition and it is originated from the recombination of the free exciton. The second peak position at 550nm green emission resulted from the recombination of electrons with holes while the green emission broadening refers to the defects in the film. Figure (4-17, e) ZnO 8% due to high doping concentrations of adding aluminum and boron atoms and the recombination of free exciton.

93

Chapter Four


700

a

ZnO

600

PL, Intensity(a.u.)

500

400 300 200 100 0

200

300

400

500

600

700

Wavelength(nm)

1200

PL, Intensity(a.u.)

b

AZB 2%

1000 800 600 400

200 0 200

300

400

500

600

700

Wavelength(nm)

PL, Intensity(a.u.)

700

c

AZB 4%

600

500 400 300

200 100 0

200

300

400 500 Wavelength(nm)

94

600

700

Chapter Four


PL, Intensity(a.u.)

1200

d

AZB 6% 1000 800 600

400 200 0 200

300

400

500

600

700

Wavelength(nm)

PL, Intensity(a.u.)

1200

e

1000

AZB 8%

800 600 400

200 0 200

300

400

500

600

700

Wavelength(nm)

Fig.(4-17 , a, b, c, d, e) PL spectra of undoped ZnO and AZB thin films with doping concentration : (a) ZnO, (b) ( c) 2%4 %,, (d) 6%, (e) 8% . Figure (4-18) shows the intensity PL peak at 518 nm is attributed to the PS layer of p-type regions with (111) orientations using xenon lamp with (λ=325 nm) at R.T.. The blue shift in the PL peak of the PS layer is attributed to the effect of electrons in nanosize of particles in the PS layer.

95

Chapter Four


350 300

PS

PL, Intensity(a.u.)

250 200 150

100 50

0 400

450

500

550

600

Wavelength(nm)

Fig. (4-18) PL spectra of PS layers formed on p-type regions with (111) orientations at R.T. The average pore diameter (d) for the PS layer formed on the p-type Si (111) wafers was 4.84 nm calculated using equation (2-43) at peak (λ=538 nm), Where 𝑬(𝒆𝑽) is the energy band gap of the PS layer calculated from the PL peak was (2.308 eV). Table (4-6) shows the variable of PL peaks wavelength with doping concentrations and wavelength of PS. Table (4-6) Variable of PL wavelength with doping concentrations and wavelength of PS. Samples ZnO AZB 2% AZB 4% AZB 6% AZB 8% PS

E1(eV) 3.34 at 372 3.34 at 372 3.39 at 366 3.34 at 372 3.43 at 362 2.308 at 538

96

E2(eV) 2.28 at 550 2.25 at 552 2.25 at 550 2.52 at 551 2.24 at 554

Chapter Four


4-7 Electrical Properties: For the application of transparent conductors in solid state display devices, the electrical properties of the conducting films such as resistivity and resistance change during operation are important factors. Therefore emphasis has been given to preparing low resistive and highly stable films. 4-7-1 Hall Effect of ZnO and AZB Thin Film: Hall effect on the ZnO and AZB thin film deposited on glass substrate shows all thin films are n-type semiconductor. The variation of carrier concentration (n) and Hall mobility (μH) of ZnO and AZB thin films as the function of AlB: co-doped concentration were measured with the Hall system, the resists are shown in the figure (4-19). It is seen that the carrier concentration is increases from 0.1205×1012 to 1.70×1012 cm-3 as the Al-B content are increasing to 8%. The increasing is attributed to the free electrons denoted by Al3+ and B3+ which substituted Zn2+ ions in ZnO. Carrier co-doped is seen to increases with increasing in doping concentration. This is due to the substitutional incorporation of Al 3+ and B3+ ions at Zn2+ cation sites, or the incorporation of Al and B ions in interstitial positions [31].

97


1.8

35

1.6

30

1.4 25

1.2

1

20

0.8

15

0.6

10

0.4 0.2

5

0

0

0

2

4

6

8

Mobility (cm2 .V-1.s-1)

carrier concentration X1012 (cm-3)

Chapter Four

10

Doping(%)

Fig. (4-19): Variation of carrier concentration and Hall mobility of ZnO and AZB thin films with varying concentration (2, 4, 6 and 8 %).

The carrier concentration (n) is related to the Hall coefficient (RH) as expressed by the equation (2-35). As more dopant is introduced, more free electrons will be generated and the higher will be carrier concentration in thin films. Hall mobility of electrons (μH) calculated simply from the equation (236). These results shows in Table (4-7). The carrier mobility decreases from 31.3 for pure ZnO to 0.24 (cm2/V.s) as Al-B co-doped concentration are increased up to 8%. The possible reason is that fine grains with large amount of grain boundaries in AZB films that have a higher Al-B contents, which would hinder the movement of the carrier. Also the lattice distortion in AZB films with higher concentration of Al-B contents could prevent carrier progress.

98

Chapter Four


Table (4-7): Hall coefficient (RH), carrier concentration (nd) and mobility (μH) of ZnO and AZB thin films with doping concentration: 2 to 8 %, at room temperature. Samples ZnO AZB 2% AZB 4% AZB 6% AZB 8%

RH (cm3/C) ×106 51.79 9.294 5.565 4.759 1.039

nd (1/cm3) ×1012 0.120 0.678 1.122 1.312 1.7

µH (cm2/V.s) 31.30 13.88 5.00 3.66 0.24

Type n n n n n

Increasing of aluminum-boron caused to increase of free conducting electrons or carrier concentrations (n) because of Al3+ and B3+ substitutions in the locations of Zn2+. Hall coefficient (RH) decreases due to increasing of carrier concentrations according to the equation (2-35) so that the Hall mobility decreases as shown in value of table (4-7).

4-7-2 Conductivity of ZnO and AZB Thin Films: The electrical properties of (Al and B) co-doped ZnO (AZB) thin films have been reported The logarithmic conductivity ln(σ) as a function of temperature for ZnO and AZB thin films are illustrated in figure (4-20), where the electrical conductivity(σ) of the thin films had been measured for temperature range (300–443)ºK. As shown in the figure (4-20) the values of conductivity increases with temperature, such as the general characteristics of the semiconductor with increases temperature lead to an increasing in the number of electron-hole pairs resulting on increases conductivity [127].

99

Chapter Four


It is noticed that the thin films have two values of activation energy (the first region is high temperature and the second region is low temperature), the values of the first and second regions of activation energy are shown in table (4-8). In the case of a ZnO: Al, the Al3+ ion is required to occupy a Zn2+ lattice site in order to provide a free electron(charge carrier) and enhance the conductivity of the ZnO [135, 136].

Fig. (4-20): ln (σ) of electrical conductivity vs. (1000/T) for ZnO and AZB thin film with doping concentration (2, 4, 6 and 8 %).

100

Chapter Four


Table (4-8): Activation energy of ZnO and AZB thin films with varying doping concentration: from 2 to 8 %, at room temperature. Samples ZnO AZB 2% AZB 4% AZB 6% AZB 8%

Ea1(eV) 0.021 0.061 0. 072 0. 093 0.104

Ea2(eV) 0.108 0. 215 0.236 0.293 0.308

N-type doping in ZnO is generally done using a column 13 element, specifically boron, aluminum. These elements have one additional valence electron compared to the column 12 element zinc. During deposition, B3+, Al3+ ions are substituted for the Zn2+, leaving one additional weakly bound electron [137]

.

101

Chapter Four


4-8 Porous Silicon: 4-8-1 Nanostructure Investigation: The surface morphology of the porous silicon (PS) layers was investigated using Atomic Force Microscope (AFM) studies focus entirely on the nanoscale characterization of PS layers. Porous silicon of the samples ntype and p-type prepared for 30 minutes at 30 mA/cm2 etching current densities is investigated by using optical microscopy. Figure (4-21) display the optical micrographs of porous silicon on n-type and p-type Si with resistivity (ρ= 0.05 – 0.1 Ω.cm) and (111) crystalline orientation, before and after dissolution of the porous silicon layer (m3) in a 1M solution of sodium hydroxide (NaOH).

n-type(before)

n-type(after)

p -type(before)

p -type(after)

Fig. (4-21): Optical micrographs of n-PS and p-PS before and after dissolution of the porous silicon layer. 102

Chapter Four


Figure (4-22) display optical photographs of the n-type and p-type porous silicon before and after deposition of ZnO thin films, the different color of the samples n-type and p-type is due to reflection from surface of ZnO thin film.

n-type(before)

n-type(after)

p-type(before)

p-type(after)

Fig. (4-22): Optical photograph top view of n-PS and p- PS before and after deposition of ZnO thin films.

103

Chapter Four


4-8-2: Atomic Force Microscope (AFM) for Porous Silicon: Figure (4-23) shows the Granularity distribution, (2-D) and (3-D) AFM images of PS, ZnO and AZB thin films with the co-doped (2, 4, 6 and 8) %, deposited on PS prepared by spray pyrolysis technique at 450ºC.

PS

104

Chapter Four


ZnO

AZB 2%

105

Chapter Four


AZB 4%

AZB 6%

106

Chapter Four


AZB 8% Fig. (4-23): Granularity distribution, 2-D and 3-D AFM image of the ZnO and AZB thin films with doping concentration (2, 4, 6 and 8 %) deposited on p-PS at 450 °C.

All films was homogenous and the grain shapes, from the granulite accumulation distribution it's clear that high ratio of the average grain size lies between 60-120 nm while the roughness average of the surface increases from 6.4 nm for ZnO to 12.12 nm for AZB 8% as listed in table (4-9), indicating that the roughness increases with the increasing of Al-B co-doped.

107

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4-8-3 SEM Investigation for Porous Silicon: Figures (4-24) and (4-25) shown the Images of p-type and n-type of porous silicon (PS) prepared by anodized etching method at 30 minutes etching time and 30 mA/cm2 etching current density before and after deposit ZnO at 450ºC, the grain size of thin films calculated by SEM using Imagej software program as shown in table(4-9). This behavior of grains arrangements is good agreement with reported [138].

ZnO/n- PS ZnO/p- PS Fig. (4-24) SEM images of n-PS and p- PS after ZnO deposition.

(a)

(b)

108

Chapter Four


(c)

(d)

(e)

(f)

(g)

(h)

109

Chapter Four


(i)

(J)

(k) (l) Fig.(4-25) SEM of (× 60k): a) n- PS, b)p- PS, c) ZnO/n-PS, d)ZnO/p-PS, e) AZB 2%/n-PS, f) AZB 2%/p-PS, g) AZB 4%/n-PS, h) AZB 4%/p-PS, i) AZB 6%/ n -PS , j) AZB 6%/p-PS, k) AZB 8%/ n -PS, l) AZB 8%/p-PS.

As clear from SEM images of ZnO and AZB thin films deposited on ntype and p-type porous silicon that the grain size decreases and the roughness increases with the increasing of co-doped ratio due to crystal distortion caused by the adding aluminum and boron in the thin films, values of the grain size and roughness measured by ImageJ software program are shown in the table (4-9).

110

Chapter Four


Figure (4-26) shows SEM images of cross section PS prepared by (30 minutes) and (30 mA/cm2) of AZB with doping concentration (4%) of codoped (Al and B) deposited on glass and porous silicon. The figure shows a columnar structure with no voids and with a preferred c-axis orientation. As clear from the figure the crystalline of AZB 4% thin films deposited on glass is higher than AZB 4% deposited on PS and the roughness is increases for films deposited on PS.

(a)

(b)

Fig. (4-26) Cross sections of SEM images for: a) AZB 4% /glass, b) AZB 4% /n-PS.

111

Chapter Four


Table (4-9): Grain size calculated by AFM, SEM, Sa, Sq and Sz of ZnO and AZB thin films deposited on p-PS. DSEM-PS

DAFM-PS

sample

(nm)

(nm)

ZnO AZB 2% AZB 4% AZB 6% AZB 8%

36.4 28.7 22.3 19.2 12.3

53.5 42.7 37.4 26.3 18.6

Sa(roughness Average)(nm)

Sq (Root Mean Sz (Ten Square) Point (nm) Height) (nm)

6.40 6.44 9.76 10.04 12.12

7.16 7.79 10.24 11.67 12.6

15.4 9.45 14.4 7.54 7.59

According to measurement of silicon wafer before anodization (m1), then after anodization (m2), and finally after dissolution of the porous silicon layer (m3) the porosity of the porous silicon calculated using equation (2-18), that means medium level porosity dependence on classification of porosity. The grain size measured by SEM and AFM for thin films deposited on glass substrate is larger than grain size deposited on PS, and roughness of the films deposited on glass substrate is less than the roughness of the films deposited on PS as clear from table (4-4) and (4-9), The thickness of the porous silicon layer is calculated using equation (219), etching rate is calculated using equation (2-20) all the results are listed in table (4-10). Table (4-10): Thickness TPS, Porosity and etching rate 𝒗 of the porous silicon layer. Samples

TPS (μm)

Porosity (%)

𝑣(μm/min)

PS

46.48

61-66

1.55

112

Chapter Four


4-9 Electrical Properties: 4-9-1 Hall Effect of AZB deposited on Porous Silicon: Figure (4-27) shows the Variation of carrier concentrations and Hall mobility with co-doping concentration of ZnO and AZB thin films deposited on p-PS. The Hall mobility of the thin films shows a decrease with increasing of aluminum-boron concentration where the mobility calculated according to the equation (2-35). The actual value of mobility is determined by the interaction between the various scattering centers and free carriers. Carrier concentrations are increases with the increasing of aluminum-boron due to

70

3000

60

2500

50

2000

40 1500

30 1000

20

500

10 0

Mobility (cm2 .V-1.s-1)

carrier concentration X1014 (cm-3)

increases of electrons concentration caused by Al and B.

0

0

2

4

6

8

10

Doping(%)

Fig. (4-27): Variation of carrier doping concentration and Hall mobility of ZnO and AZB thin films with doping concentration (2, 4, 6 and 8 %) deposited on p-PS.

113

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Table (4-11) shows the result of Hall measurements of thin films deposited on p-type porous silicon, the results shows all the thin films are ntype and Hall coefficient is decreases with the increasing of doping concentrations because of the increasing of carrier concentration according to the equation (2-35).

Table (4-11): Hall coefficient, carrier concentration and mobility of ZnO and AZB thin films deposited on p-PS with doping concentration. Samples ZnO AZB 2% AZB 4% AZB 6% AZB 8%

RH (cm3/C) ×105 0.182 0.133 0.054 0.011 0.010

n (1/cm3) ×1014 3.429 4.684 11.56 55.4 62.79

µH (cm2/V.s.) 2598 1968 927.8 353.4 69.85

Type n n n n n

4-9-2 Capacitance-Voltage Measurements (C-V): C-V characteristic is one of the most important parameters of (ZnO PS/Si/Al) heterojunction, since it determines different parameters such as built-in potential, depletion width layer. The variation of capacitance as a function of applied voltage for ZnO and AZB/Si heterojunction are shown in figure (4-28), where the junction capacitance is decreases with increasing bias voltage due to increasing of depletion layer width as shown in table (4-12). This behavior is similar to many of C-V characteristics of porous silicon fabricated by the electrochemical and photo-electrochemical etching process [139].

114

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7.5E-11

ZnO

7E-11

6E-11

C(F/cm2)

C(F/cm2)

6.5E-11

5.5E-11 5E-11

4.5E-11 4E-11 -6 -5 -4 -3 -2 -1 0

1

2

3

4

5

6

5.2E-11 5.1E-11 5E-11 4.9E-11 4.8E-11 4.7E-11 4.6E-11 4.5E-11 4.4E-11 4.3E-11 -6 -5 -4 -3 -2 -1 0

1

2

Applied voltage(V)

Applied voltage(V)

4.4E-11

3.55E-11

4.3E-11

AZB 4% C(F/cm2)

4E-11

3.9E-11

5

6

AZB 6%

3.4E-11 3.35E-11 3.3E-11

3.8E-11

3.25E-11 1

2

3

4

5

6

3.2E-11 -6 -5 -4 -3 -2 -1 0

Applied voltage(V)

C(F/cm2)

4

3.45E-11

4.1E-11

3.7E-11 -6 -5 -4 -3 -2 -1 0

3

3.5E-11

4.2E-11 C(F/cm2)

AZB 2%

1

2

3

4

5

6

Applied voltage(V)

1.13E-11 1.12E-11 1.11E-11 1.1E-11 1.09E-11 1.08E-11 1.07E-11 1.06E-11 1.05E-11 1.04E-11 -6 -5 -4 -3 -2 -1 0

AZB 8%

1

2

3

4

5

6

Applied voltage(V)

Fig. (4-28): Variation of capacitance as a function of reverse bias voltage for ZnO and AZB thin films with doping concentration (2, 4, 6 and 8 %) deposited on p-type PS heterojunction. Figure (4-29) shows 1/C2 versus applied voltage. This plot shows a linear relationship of 1/C2 with bias voltage indicating that these junctions are an abrupt type. The linear relationship represents Schottky- like barrier between 115

Chapter Four


Al layer and porous silicon. This behavior is similar to the many of the C-V characteristics of the porous silicon fabricated by the electrochemical and photo-electrochemical etching process [140, 141]. 8.5E+21

4.8E+20

a

8.45E+21

b

4.7E+20

1/C2(Farad)

1/C2(Farad)

4.6E+20

8.4E+21 8.35E+21 8.3E+21

4.5E+20 4.4E+20 4.3E+20 4.2E+20

8.25E+21

4.1E+20

8.2E+21 0

0.5

1

1.5

2

4E+20

2.5

0

0.5

Applied voltage(V)

1.5

2

2.5

Applied voltage(V) 6.5E+20

2.35E+20

c

2.3E+20

d

6.45E+20

2.25E+20

1/C2(Farad)

1/C2(Farad)

1

2.2E+20

2.15E+20 2.1E+20

6.4E+20

6.35E+20 6.3E+20 6.25E+20

2.05E+20 2E+20

6.2E+20

0

0.5

1

1.5

2

2.5

0

Applied voltage(V)

0.5

1

1.5

2

2.5

Applied voltage(V)

8.31E+20

e

8.29E+20

1/C2(Farad)

8.27E+20 8.25E+20

8.23E+20 8.21E+20 8.19E+20 8.17E+20 8.15E+20 0

0.5

1

1.5

2

2.5

Applied voltage(V)

Fig. (4-29): The variation of 1/C2 as a function of reverse bias voltage for:(a) ZnO/PS, (b) AZB2%/PS,(c) AZB 4%/PS, ,(d) AZB 6%/PS, ,(e) AZB 8%/PS.

116

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From the (1/C2-V) plot built in potential 𝑽𝒃𝒊 can be calculate by extrapolating to (1/C2=0), the values of 𝑽𝒃𝒊 are shown in table (4-12) According to the to the capacitance-voltage measurements, the charge carrier density 𝒏𝒅 can be calculated by using equation (2-44), and the porous silicon layer 𝑾 by using the equation (2-45) where the values of 𝒏𝒅 and 𝑾 are shown in table (4-12).

Table (4-12): Values of 𝑾, 𝒏𝒅 and 𝑽𝒃𝒊 , for ZnO and AZB deposited on pPS/Si junction. Samples 𝑽𝒃𝒊 (Volt) ZnO 1.8 AZB2 1.9 AZB4 1.2 AZB6 1.2 AZB8 1

𝒏𝒅 (cm-3) 𝑾(μm) 1.14×1012 10.41 12 8.56×10 3.906 12 8.68×10 3.082 13 1.33×10 2.489 13 2.04×10 1.837

117

Chapter Four


4-10 Raman Spectroscopy Measurement: Raman spectroscopy has been widely used to study the optical phonon spectrum modification in ZnO nanostructures. Fig. (4-30) show Raman spectra for 30 min etched PS. The optical phonon is observed in the central of Brillouin zone with energy of wavenumber 520 cm-1 and this is due to the conservation of quasi-momentum in crystals. Raman scattering spectra of ZnO/PS, the shapes of peaks at approximately 520 cm-1 refer to first - order scattering phonons in c-Si and this characterized the PS layers. This feature of Si nanocrystals is attributed to the quantum confinement of optical phonons in the electronic wave function of Si nanocrystals, the low frequency comes from optical phonon confinement in Si nanocrystals. No shift in peak of Raman spectroscopy has been noticed after doping. This result is agree well with that presented in reference [84].

PS Raman intensity(a. u.)

ZnO/PS AZB 2%/PS AZB 4%/PS AZB 6%/PS AZB 8%/PS

0

500

1000

1500

2000

2500

3000

3500

Wavenumber(cm-1)

Fig. (4-30): Raman spectra for ZnO and AZB thin films with doping concentration (2, 4, 6 and 8 %) deposited on p-PS. 118

Chapter Four


4-11 Sensing Properties: Chemical Sensing Measurements: In this work, the sensing properties of ZnO/glass, ZnO/PS pure and codoped with (Al and B) were investigated as a function of time for (NH3, NO2,) gases with co-doped of (50, 100, 150 and 200) ppm at room temperature and varying substrate temperature. Several types of gases, such as liquefied petroleum gas, alcohol, NH3 and NO2, are toxic, harmful, or flammable [142].

4-11-1 Sensing Properties of NH3 gas: Ammonia (NH3) is harmful and toxic in nature. The exposure of ammonia causes chronic lung illnesses, irritating and even burning the respiratory track, etc. Environmental pollution is a burning global issue. Therefore, all industries working on and for ammonia should have an alarm system detecting and warning form dangerous ammonia concentration levels. Detection of low concentration of ammonia is not only important form the points discussed above, but also it is very important form the view of chemical pollution in the production of silicon devices in clean rooms [143]. Applications for NH3 gas sensing have been published concerning ZnO or have also introduced doped metal oxides. Hence the sensors operable at room temperature with low cost metal additives must be developed for large applicability [143]. Figure (4-31, a, b, c) shows the electrical resistance of ZnO/glass, ZnO/p-PS and ZnO/n-PS pure and co-doped with (Al and B) sensor as function of operating time at RT, The resistance is decreased with increasing 119

Chapter Four


of co-doped of aluminum and boron due to increasing of electrons caused by the interaction between (NH3) and O2− ions on ZnO as in interaction eq. (4-3). It is known that the sensing mechanism of ZnO towards NH3 gas depends on the interaction between the reducing gas and the negatively charged O2− ions on the ZnO thin films surface, thereby causing a variation in conductance, as described by equation [144]: 2NH3 + 3O2− → 3H2O + N2 + 3e−

…. (4-3)

So that, by the electrons released back into the ZnO conduction band and increasing the carrier co-doped in the ZnO active layer, the resistance of the sensor is decreased upon exposure to a reducing gas [145]. NH3 is the most common and is used in food processing, environmental remediation, agriculture, and medical diagnostics [142]. 6.5

NH3

6

(a)

glass

50 ppm

100 ppm

150 ppm

200 ppm

Resistance(MΩ)

5.5 5

4.5 4

gas on gas off

3.5

ZnO

gas on gas off

AZB 2%

gas on gas off gas on gas off

AZB 4%

AZB 6%

AZB 8%

3 0

500

1000

1500

Time(s.)

120

2000

2500

Chapter Four


4 NH3

3.5

50 ppm

Resistance(MΩ)

3

(b)

p-PSi 100 ppm

150 ppm

200 ppm

2.5

ZnO

2

AZB 2%

1.5

AZB 4%

1

AZB 6%

0.5

AZB 8%

gas on gas off gas on gas off gas on gas off gas on gas off

0 0

500

1000

1500

2000

2500

3000

Time(s.)

1.2 NH3

1.15

50 ppm

1.1

Resistance(MΩ)

n-PSi 100 ppm

150 ppm

(c)

200 ppm

1.05 1 0.95

0.9 gas on

0.85 0.8

gas off gas on gas off gas on gas off gas on gas off

ZnO

0.75 0

AZB 2% 500

AZB 4%

1000

AZB 6% 1500

AZB 8% 2000

2500

Time(s.)

Fig. (4-31, a, b, c) Resistance for ZnO, AZB with doping concentration (2, 4, 6, 8 %). Deposited on: a) glass, b) p-PS and c) n-PS as a function of operating time for NH3 gas with concentration (50, 100, 150 and 200) ppm.

121

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Figure(4-32, a, b, c) shows the sensitivity as function of operating time for NH3 gas at room temperature for ZnO/glass, ZnO/Si, ZnO/PS pure and co-doped with (Al and B) prepared at 450 oC. 16

glass

NH3

14 12

(a) ZnO

Sensitivity(%)

10 AZB 2%

8 6

AZB 4%

4

AZB 6%

2

AZB 8%

0 0

500

1000

1500

2000

2500

3000

Sensitivity(%)

Time(s.)

22 20 18 16 14 12 10 8 6 4 2 0

(b)

p-PSi

NH3

ZnO AZB 2% AZB 4%

AZB 6% AZB 8%

0

500

1000

1500 Time(s.)

122

2000

2500

3000

Chapter Four


25

(c)

n-PSi

NH3 20

Sensitivity(%)

15 10 5

ZnO

0 0

AZB 2% 500

AZB 4% 1000

AZB 6% 1500

AZB 8% 2000

2500

Time(s.)

Fig. (4-32, a, b, c) Sensitivity for ZnO, AZB with doping concentration: 2 to 8 %. Deposited on: a) glass, b) p-PS and c) n-PS as a function of operating time for NH3 gas with concentration (50, 100, 150 and 200) ppm at R.T.

As shown in the Figure (4-32, a, b, c) the sensitivity is increases with the increasing of aluminum and boron co-doped and concentration of NH3 gas, for glass and porous silicon substrate. The sensitivity of porous silicon substrate is larger than the glass substrate for all thin films, so that the increase of co-doped (Al and B) co-doped leads to improve the sensitivity. This result is good agreement with [145-147]. The response time and recovery time for 50 ppm were (230-280) and (120-240) respectively for 50 ppm of undoped ZnO and AZB nanostructure thin films deposited on glass and n-type and p-type porous silicon (PS) prepared for 30 minutes at 30m A/cm2 etching current densities, for glass substrate the response time is longer than PS substrate, for n-PS substrate the 123

Chapter Four


response time of the thin films is less than p-PS substrate, response time decrease with the increase of (Al and B) concentration. Figures (4-33), (40-34) and(4-35)

shows the sensitivity of films

deposited on glass, p-porous silicon and n-porous silicon respectively for the concentrations of (50, 100, 150 and 200) ppm, sensitivity increases with increasing temperature until reach to highest sensitivity at 150 oC then decreases above this temperature, also the sensitivity increases with increasing of aluminum-boron co-doped concentration.

124

Chapter Four


50 45

NH3

glass

a

50 ppm

35 30

Sensitivity(%)

Sensitivity(%)

40


25 20 15 10

5 0 0

50

100

150

200

250

50 45 40 35 30 25 20 15 10 5 0

NH3

50

50

NH3

glass

c

150 ppm

35 Sensitivity(%)

Sensitivity(%)

40

30 25


20 15 10 5 0

0

50

100

150

100

150

200

250

300

Temperature (oC)

Temperature (oC)

45

b

100 ppm


0

300

glass

200

250

300

50 45 40 35 30 25 20 15 10 5 0

NH3

glass

d

200 ppm

ZnO AZB 2% AZB 4% AZB 6%

AZB 8% 0

50

100

150

200

250

300

Temperature (oC)

Temperature (oC)

Fig. (4-33, a, b, c, d): Change of sensitivity with temperature for undoped ZnO and AZB with doping concentration (2, 4, 6, 8 %) deposited on glass substrate for NH3 gas with concentration (50, 100, 150 and 200) ppm.

125

Chapter Four


50 45

NH3

p-PS

a

50 ppm

35 30

Sensitivity(%)

Sensitivity(%)

40


25 20 15

10 5 0 0

50

100

150

200

250

50 45 40 35 30 25 20 15 10 5 0

NH3

p-PS


0

300

50

50

NH3

p-PS

c

150 ppm

35 Sensitivity(%)

Sensitivity(%)

40 30

25


20 15

10 5 0 0

50

100

150

100

150

200

250

300

Temperature (oC)

Temperature (oC)

45

b

100 ppm

200

250

300

50 45 40 35 30 25 20 15 10 5 0

NH3

p-PS

d

200 ppm


0

50

100

150

200

250

300

Temperature (oC)

Temperature (oC)

Fig. (4-34, a, b, c, d): Change of sensitivity with temperature for undoped ZnO and AZB with doping concentration (2, 4, 6, 8 %) deposited on p-PS substrate for NH3 gas with concentration (50, 100, 150 and 200) ppm.

126

Chapter Four


50 45

NH3

n-PS

a

50 ppm

35 30

Sensitivity(%)

Sensitivity(%)

40


25 20 15

10 5 0 0

50

100

150

200

250

50 45 40 35 30 25 20 15 10 5 0

NH3

50

50

NH3

n-PS

c

150 ppm

35 Sensitivity(%)

Sensitivity(%)

40 30 25


20 15 10 5 0

0

50

100

150

100

150

200

250

300

Temperature (oC)

Temperature (oC)

45

b

100 ppm


0

300

n-PS

200

250

300

50 45 40 35 30 25 20 15 10 5 0

NH3

n-PS

d

200 ppm

ZnO AZB 2% AZB 4%

AZB 6% AZB 8% 0

50

100

150

200

250

300

Temperature (oC)

Temperature (oC)

Fig. (4-35, a, b, c, d): Change of sensitivity with temperature for undoped ZnO and AZB with doping concentration (2, 4, 6, 8%) deposited on n-PS substrate for NH3 gas with concentration (50, 100, 150 and 200) ppm.

127

Chapter Four


4-11-2 Sensing Properties of NO2 Gas: Nitrogen dioxide, NO2, is one of the most dangerous gases emitted from burning of the exhaust of cars engines, home heaters, furnaces, plants. Therefore the development of portable fast-response sensors that are robust small sized, long lifetime, quick in response and with sufficient sensitivity for the detection of nitrogen dioxide in low concentrations, in the environment is so important and needed. The surface of the ZnO and AZB grains is responsible for the gas sensing properties. The chemical reactions with target gases in the metal oxide surface changes the co-doped of the conduction electrons. Chemisorption changes the defect states of the metal oxide’s surface. The detection of an oxidizing gas such as NO2 for ZnO and AZB can be associated with a reaction in which conduction electrons are consumed and the subsequent detection reaction leads to increase in the barrier height and the surface resistance. Exposure to NO2 revealed an increase in the resistivity for the ZnO and AZB. The response to NO2 at the tested range of temperature can be explained by the following reactions [148]: NO2 (gas) + e− → NO−2 (ads) NO2 (gas) + O−2 (ads) + 2e− → NO−2 (ads) + 2O− (ads)

…. (4-4)

NO2 molecules can interact mainly with the Zn atoms or with oxygen species previously adsorbed, the main oxygen species at ZnO surface is O−2. During the adsorption of oxygen species on the surface of sensing element, capturing of electrons from conduction band and the associated decrease in the charge carrier concentration (e-) leads to an increase in the 128

Chapter Four


resistance of the n-type sensing element until it attains equilibrium. Thus, the surface resistance increases and attains equilibrium during the chemisorption process. So when ZnO film exposure to NO2 gas oxygen molecules in the atmosphere were adsorbed on to the surface of the film due to which the free electrons were removed from the conduction band of ZnO resulting an increasing in the resistance of the film. Figure (4-36, a, b, c) shows the electrical resistance of ZnO/glass, ZnO/pPS and ZnO/n-PS pure and co-doped with (Al and B) sensor as function of operating time, The resistance is increasing with increasing of co-doped of aluminum and boron due to decreasing of electrons caused by the interaction between (NO2) and O2− ions on ZnO as in interaction eq. (4-4).

129

Chapter Four


Fig. (4-36, a, b, c) Resistance for ZnO, AZB with doping concentration (2, 4, 6, 8 %). Deposited on: a) glass, b) p-PS and c) n-PS as a function of operating time for NO2 gas with concentration (50, 100, 150 and 200) ppm.

130

Chapter Four


Figure(4-37, a, b, c) shows the sensitivity as function of operating time for NO2 gas with the concentration (50, 100, 150 and 200) ppm at room temperature for ZnO/glass, ZnO/Si, ZnO/PS pure and co-doped with (Al and B) prepared at 450 oC.

131

Chapter Four


Fig. (4-37, a, b, c) Sensitivity for ZnO and AZB with doping concentration (2, 4, 6, 8 %). Deposited on: a) glass, b) p-PS and c) n-PS as a function of operating time for NO2 gas with concentration (50, 100, 150 and 200) ppm at R.T. As shown in the figure (4-37) the sensitivity is increase with the increase of aluminum and boron co-doped for glass, silicon and porous silicon substrate. The sensitivity of n-PS substrate is larger than the p-PS and glass substrate for all thin films, so that the increase of (Al and B) co-doped leads to improve the sensitivity. This result is good agreement with [147, 149].

The response time and recovery time of undoped ZnO and AZB nanostructure thin films deposited on glass and n-type and p-type porous silicon(PS) prepared for 30 minutes at 30m A/cm2 etching current densities for 50 ppm were (110-170) and (165-360) respectively for 50 ppm, for glass substrate the response time is longer than PS substrate, for n-PS substrate the 132

Chapter Four


response time of the thin films is less than p-PS substrate, response time decrease with the increase of (Al and B) concentration.

Figure (4-38), (4-39) and (4-40) shows the sensitivity of films deposited on glass, p-porous silicon and n-porous silicon respectively for the concentrations of (50, 100, 150 and 200) ppm, sensitivity increase with increasing temperature until reach to highest sensitivity at 200 oC in general for glass and p-PS substrate then decrease, for n-PS substrate, sensitivity increase with the increase of temperature in general, also the sensitivity increase with increasing of aluminum-boron co-doped concentration.

133

Chapter Four


50 45

NO2

glass

a

50 ppm

35 30

Sensitivity(%)

Sensitivity(%)

40


25 20 15

10 5 0 0

50

100

150

200

250

50 45 40 35 30 25 20 15 10 5 0

NO2

50

50

NO2

glass

c

150 ppm

35 Sensitivity(%)

Sensitivity(%)

40

30 25


20 15 10 5 0

0

50

100

150

100

150

200

250

300

Temperature (oC)

Temperature (oC)

45

b

100 ppm

ZnO AZB 2% AZB 4% AZB 6% AZB 8% 0

300

glass

200

250

300

50 45 40 35 30 25 20 15 10 5 0

NO2

glass

d

200 ppm


50

100

150

200

250

300

Temperature (oC)

Temperature (oC)

Fig. (4-38, a, b, c, d): Change of sensitivity with temperature for undoped ZnO and AZB with doping concentration (2, 4, 6, 8 %) deposited on glass substrate for NO2 gas with concentration (50, 100, 150 and 200) ppm.

134

Chapter Four


50 45

NO2

p-PS

a

50 ppm

35

Sensitivity(%)

Sensitivity(%)

40 30 25


20 15

10 5 0 0

50

100

150

200

250

50 45 40 35 30 25 20 15 10 5 0

NO2

p-PS


300

50

50

NO2

p-PS

c

150 ppm

35 Sensitivity(%)

Sensitivity(%)

40

30 25


20 15 10 5 0

0

50

100

150

100

150

200

250

300

Temperature (oC)

Temperature (oC)

45

b

100 ppm

200

250

300

50 45 40 35 30 25 20 15 10 5 0

NO2

p-PS

d

200 ppm


50

100

150

200

250

300

Temperature (oC)

Temperature (oC)

Fig. (4-39, a, b, c, d): Change of sensitivity with temperature for undoped ZnO and AZB with doping concentration (2, 4, 6, 8 %) deposited on p-PS substrate for NO2 gas with concentration (50, 100, 150 and 200) ppm.

135

Chapter Four


50

NO2

45

n-PS

a

50 ppm

35

Sensitivity(%)

Sensitivity(%)

40 30 25 20 15

10

ZnO

5

AZB 2%

AZB 4%

AZB 6%

150

200

AZB 8%

0 0

50

100

250

50 45 40 35 30 25 20 15 10 5 0

NO2

n-PS

ZnO

AZB 2%

AZB 4%

50

100

150

0

300

50

NO2

n-PS

c

150 ppm

35 Sensitivity(%)

Sensitivity(%)

40

30 25 20 15 10

ZnO

5

AZB 2%

AZB 4%

100

150

AZB 6%

AZB 8%

0

0

50

b

AZB 6%

AZB 8%

200

250

300

Temperature (oC)

Temperature (oC)

45

100 ppm

200

250

300

50 45 40 35 30 25 20 15 10 5 0

NO2

n-PS

ZnO 0

50

d

200 ppm

AZB 2%

AZB 4%

100

150

AZB 6% 200

AZB 8% 250

300

Temperature (oC)

Temperature (oC)

Fig. (4-40, a, b, c, d): Change of sensitivity with temperature for undoped ZnO and AZB with doping concentration (2, 4, 6, 8 %) deposited on n-PS substrate for NO2 gas with concentration (50, 100, 150 and 200) ppm.

136

Chapter Four


Table (4-13): The Optimum values of AZB at 8% thin films deposited on glass, p -PS and n –PS substrate. Substrate glass

p-PS

n-PS

Sensitivity at R.T. Gas

NH3

NO2

NH3

NO2

NH3

NO2

50 ppm

11.34

10.57

13.04

16.43

14.54

25.02

100 ppm

11.54

13.26

13.24

17.28

14.44

30.30

150 ppm

11.55

14.26

13.63

20.6

15.13

30.36

200 ppm

13.31

15.66

15.01

21.39

16.51

31.51

Sa(roughness 6.15

12.12

DSEM (nm)

15

12.3

DAFM (nm)

38

18.6

𝑬𝒈 (eV)

3.31

α ×104 (cm-1)

6.24

RH (cm3/C) ×106

1.039 ×106

0.010×105

nd (1/cm3) ×1012

1.7 ×1012

62.79×1014

µH (cm2/V.s)

0.24

69.85

Average)(nm)

137

Chapter Five


5-1 Conclusions: The main conclusions will be shown and also be submitted some suggestions for future works. 1- The sensitivity of heterojunction device of ZnO/n-PS is higher than that ZnO/p-PS and ZnO/glass for gas sensor applications (NH3 and NO2). 2- Increases of operating temperature leads to increasing of sensitivity of the films, with optimum value 150oC for NH3 and increases sensitivity with the increase of operating temperature for NO2 gas. 3- Spray pyrolysis technique is used efficiently grow ZnO and (Al, B) codoped thin films to deposited on glass and porous silicon used for gas sensor. 4- The optical transitions in ZnO are direct and the value of optical energy gap increases with the increase of co-doped of Al and B concentrations. 5- The capacitance, depletion layer and Vbi decreases with increasing of the co-doped. Hall measurements show that the all the films are n-type. 6- Morphological properties of thin films deposited on glass and PSi are found to be dependent on the co-doped of Al and B concentrations. 7- The porous silicon (PS) substrate is macropores (pore size > 50 nm). 8- The fabricated sensors have good sensitivity to NH3 and NO2 gases at R.T.

138

Chapter Five


5-2: Future Studies: Further investigation can be suggested as a future studies: 1- Studying the effect of annealing of ZnO pure and (Al, B) co-doped thin films on the structural, electrical and optical properties. 2- Fixing one of the co-doped materials and varying the second one for a several doping to study the variable of grain size, roughness and sensitivity of the thin films. 3- Studying the effects of thickness on sensing properties to choice a proper thickness for gas sensing preparing the thin films by using other technique. 4- Studying the thin films for other applications like solar cell and photo detector. 5- Study the effect of new dopants on the sensing of ZnO/Si and ZnO/PSi. 6- Using the thin films for other gasses with different concentrations and varying the substrate temperature.

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151

‫الخالصة‪:‬‬ ‫تم في هذا البحث تحضير أغشية أوكسيد الخارصين النقي ‪ ZnO‬والمشوبة تشويبا مشتركا‬ ‫بالبورون‬

‫واأللمنيوم‬

‫(‬

‫‪)AZB‬‬

‫بنسب‬

‫تشويب‬

‫(‪،2‬‬

‫‪،4‬‬

‫‪،6‬‬

‫‪)%8‬‬

‫اي‬

‫‪،ZnO:(B 3%,Al 3%(،ZnO:(B 2%+ Al 2%) ،ZnO:(B 1%+Al 1%) ].‬‬ ‫)‪ .[ ZnO:(B 4%,Al4%‬إذ تم تحضير األغشية بطريقة التحلل الكيميائي الحراري بدرجة‬ ‫حرارة القاعدة مقدارها )‪ (450 ±10 oC‬وسمك )‪ (150±5 nm‬على قواعد زجاجية وسليكون‬ ‫مسامي من النوعين )‪ (n, p‬بمقاومية مقدارها)‪ (0.05–0.1 Ω.cm‬واتجاه بلوري )‪ .(111‬تم‬ ‫تصنيع مفارق هجينة من انواع‪ . ((ZnO, AZB)/p-PS/Si, (ZnO, AZB)/n-PS/Si ( :‬وتم‬ ‫تحضير طبقة السليكون المسامي بطريقة التنميش الكهروكيميائية والكهروكيميائية تحت تأثير ضوء‬ ‫الهالوجين على قواعد السليكون للنوعين )‪ (n, p‬بكثافة تيار مقدارها )‪ (30 mA‬وزمن مقداره‬ ‫)‪. (30 minutes‬‬ ‫تم دراسة طوبوغرافية سطح األغشية المرسبة على الزجاج والسليكون المسامي باستعمال‬ ‫المجهر االلكتروني الماسح )‪ ،(SEM‬مجهر نفاذ االلكترونات )‪ (TEM‬ومجهر القوة الذري‬ ‫)‪ (AFM‬فتبين ان التركيب السطحي لألغشية المرسبة هو تركيب نانوي يتناقص فيه الحجم الحبيبي‬ ‫مع زيادة نسب التشويب المشترك إذ بلغت قيمة أصغر حجم حبيبي )‪ ،)7.5 nm‬وان خشونة‬ ‫السطح تزداد مع زيادة التشويب المشترك وتتغير من )‪ (6.15 nm -1.34‬ومن ‪(12.12 nm -‬‬ ‫)‪6.4‬لألغشية المرسبة على الزجاج والسليكون المسامي على التوالي‪ ،‬وأظهر جهاز حيود األشعة‬ ‫السينية‬ ‫(‪ ) XRD‬ان أغشيية ‪ ZnO‬المحضرة النقية والمشوبة ذات تركيب متعدد التبلور ومن النوع‬ ‫السداسي المتراص (‪.)Hexagonal Wurtzite‬‬ ‫تم دراسة الخصائص البصرية لألغشية المحضرة بوساطة قياس طيف النفاذية‬ ‫واالمتصاصية عند درجة حرارة الغرفة لمدى طول موجي يتراوح من )‪ (1100 nm –300‬تبين‬ ‫ان النفاذية البصرية لألغشية النقية )‪ (87 %‬وانها تتناقص مع زيادة نسب التشويب‪ ،‬وظهر إن‬ ‫فجوة الطاقة البصرية تزداد بزيادة نسب التشويب حيث بلغت قيمة فجوة الطاقة البصرية للغشاء‬ ‫النقي )‪ (3.22 eV‬وتصل الى )‪ (3.31 eV‬عند التشويب بنسبة )‪.(8%‬‬

‫اظهرت القياسات الكهربائية ان االغشية النقية و المشوبة من النوع السالب )‪،(n-type‬‬ ‫وتركيز حامالت الشحنة يزداد مع زيادة نسب التشويب المشترك‪.‬‬ ‫االغشية‬

‫المحضرة‬

‫المرسبة‬

‫على‬

‫الزجاج‬

‫والسليكون‬

‫المسامي‬

‫للنوعين‬

‫)‪ (n, p‬تم استخدامها في قياس التحسسية للغازين )‪ ،(NH3, NO2‬بدرجة حرارة الغرفة وتغيير‬ ‫درجة حرارة القاعدة وبتراكيز(‪ )200 ،150 ،100 ،50‬جزء بالمليون (‪ .)ppm‬أظهرت النتائج‬ ‫ان المقاومة الكهربائية لألغشية تتناقص مع زيادة التشويب بالنسبة للغاز )‪ (NH3‬بسبب تفاعل‬ ‫االختزال‪ ،‬لكنها تزداد مع زيادة التشويب بالنسبة للغاز )‪ (NO2‬بسبب تفاعل االكسدة‪ .‬بينت النتائج‬ ‫ان تحسسية األغشي ة المرسبة على السليكون المسامي تكون اكبر من تحسسية األغشية المرسبة‬ ‫على الزجاج‪ ،‬وتكون تحسسية االغشية المرسبة على السليكون المسامي نوع )‪ (n‬اكبر منها في‬ ‫حالة السليكون المسامي لنوع )‪ .(p‬وبينت الدراسة ان زيادة تراكيز الغازات تؤدي الى زيادة‬ ‫التحسسية‪.‬‬

‫جمهورية العراق‬ ‫وزارة التعليم العالي والبحث العلمي‬ ‫الجامعة المستنصرية‬ ‫كلية العلوم‬

‫دراسة الخصائص الفيزيائية ألغشية أوكسيد‬ ‫الخارصين الرقيقة المشوبة تشويبا مشتركا‬ ‫بالبورون واأللمنيوم واستخدامها كمتحسس لغازي‬ ‫األمونيا وثاني أوكسيد النيتروجين‬ ‫رسالة مقدمة الى مجلس كلية العلوم‪ -‬الجامعة المستنصرية‬ ‫كاستكمال جزئي لمتطلبات نيل شهادة دكتوراه فلسفة في الفيزياء‬ ‫مقدمة من قبل‬

‫رشيد هاشم جبار‬ ‫ماجستير ‪2009‬‬ ‫بكالوريوس ‪1989‬‬ ‫إشراف‬

‫د‪.‬أنوار حسين علي الفؤادي‬ ‫أستاذ مساعد‬

‫‪ 1436‬ه‬

‫‪ 2015‬م‬