11 Jun 2007 ... properties of zinc oxide (ZnO) and strontium titanate. (SrTiO. 3 or STO) thin films.
2. Determine how optical constants are affected by deposition ...
Optical Properties of Zinc Oxide and Strontium Titanate Thin Films by Reflectometry and Ellipsometry Dionne A. Miller Graduate School and City College City University of New York June 11, 2007 Mentor: Prof. Glen R. Kowach
Outline
Objectives and Motivation
Introduction to the Optical Constants
Reflection of Light
Reflectometry and Ellipsometry Techniques
Zinc Oxide and Strontium Titanate Thin Films Crystal Structure and Properties Film Preparation Results and Discussion
Summary
Objectives and Motivation
Objectives 1. Develop a reliable model to determine the optical properties of zinc oxide (ZnO) and strontium titanate (SrTiO3 or STO) thin films. 2. Determine how optical constants are affected by deposition conditions and substrates.
Motivation 1. Need for reliable data on optical constants of thin films for use in device applications. 2. Very few systematic studies of how deposition temperatures and substrate type affect optical properties of ZnO, STO thin films
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45
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35
% Reflectance
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25
20
15
Rainbow patterns of color on the wafer due to the thickness variation of the ZnO film. (substrates: Si, left; Pt on Si, right)
I
R
10
5
0 200
300
400
500
600
700
800
900
1000
Wavelength (nm)
Newton’s rings: concentric ring pattern of rainbow colors because different wavelengths of light interfere at different thicknesses
d
T
Optical Constants
sol.sci.uop.edu/.../incidentreflectrefract.jpg
When light is incident on a plane-parallel interface between two media it may be reflected or refracted The second medium is characterized by a complex index of refraction: Ň = n -ik
Optical Constants
n: (real) index of refraction; inverse measure of the phase velocity of light in the material
k: extinction coefficient; measure of how rapidly intensity decreases as the light passes through the material
ε: dielectric function; degree to which a material may be polarized by an applied electric field ε = ε1 + iε2 ; ε1 = n2 – k2; ε2 = 2nk
Both n and k are functions of wavelength; optical constant spectra show n and k as functions of wavelength or photon energy – dispersion spectra
Reflection of Light
Light incident on the sample has two components of polarization: in plane (p-waves) and perpendicular (swaves) to the plane of incidence Reflectance (R) :ratio of intensity of outgoing light to incoming light – measured in reflectometry Ratio of the amplitudes of outgoing and incoming light (r) is also of interest – measured in ellipsometry. For a single interface, this ratio is called the Fresnel Reflection Coefficient and is based on Snell’s Law.
Fresnel Reflection Coefficient for single interface: ~ ~ N b cos ! a " N a cos ! r rp = ~ ~ N b cos ! a + N a cos ! r
Reflectance,
Rp=│rp│2
Ratio of amplitude of outgoing resultant wave to amplitude of incoming wave is defined as the total reflection coefficient. For a single film (two interfaces) this is: I
R φa d
a φb
b φc
c
T
P P "i 2 ! r + r P ab bc e R = 1 + rabP rbcP e "i 2 !
d $ = 2# ( )( n b %ki "
b
) cos !b
d = film thickness; nb= index of refraction of layer b; kb = extinction coefficient of layer b
Reflectance,
Rp = │Rp│2
) N (
Ellipsometry
) K (
n o i t c a r f e R
Delta, Δ = δ1 – δ2 is the phase shift induced by reflection in the p and s-waves (0 – 360° or -180 to +180°)
Layer #1 N
2.8
K
0.8 0.7
2.6 0.6 2.4
0.5
f O
0.4 2.2
x e d n I
0.3 2
0.2 0.1
1.8
Phase shift also induces an amplitude reduction for both p- and s-waves: tan ψ = |Rp| / |Rs| (ψ: 0 to 90°)
t n e i c i f f e o C n o i t c n i t x E
0 1.6
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600
700
800
900
-0.1 1000
Wavelength (nm)
Cauchy Function n n n(! ) = n0 + 12 + 24 ! !
ρ = Rp/Rs = tan ψeiΔ or tan ψeiΔ = Rp/Rs (fundamental equation of ellipsometry)
Lorentz Oscillator Model
Ellipsometers measure Δ and ψ: from these quantities optical constants and thickness can be calculated using a 2 d x dx theoretical model. m 2 +# ' + "x = eE0 exp(i!t ) dt
dt
Zinc Oxide ZnO
Zinc Oxide Structure
(0001) - Zn2+
( 0001 )- O2-
Hexagonal wurtzite structure – noncentral symmetry Alternating planes of Td coordinated zinc and oxide ions stacked alternately along the c-axis Positively charged (0001)-Zn2+ and negatively charged (0001 )-O2- polar surfaces result in a normal dipole moment and spontaneous polarization along the c-axis Uniaxial with optical axis being the hexagonal c-axis – gives rise to two independent refractive indices, n0 and ne
Zinc Oxide Properties and Applications
II-IV, n-type semiconductor (band gap 3.3 eV)
High excitonic binding energy (60 eV) - optically pumped UV lasing at room temperature demonstrated by Yu et al and Bagnall et al1
Due to the non-central crystal symmetry, ZnO is piezoelectric: useful for electromechanical coupled sensors, varistors and transducers2
Transparent conductive oxide, exhibiting high transmittance in the visible region and low electrical resistivity: ideal material for solar cells and flat panel displays3
1.
Yu et al, 23rd Intl. Conf. On Physics of Semiconductors, M. Scheffler and R. Zimmermannn, Eds., World Scientific 1996. Bagnall, D.M., et al., Appl Phys Lett, 1997, 70(17): p. 2230-2232.
2.
Rebien, M., et al., Appl Phys Lett, 2002. 80(19): p. 3518-3520. Wang, Z.L., Nanostructures of Zinc Oxide, in Materials Today. 2004. p. 26-33.
3.
Liu et al, Thin Solid Films, 2006. 510(1-2): p. 32-38.
Zinc Oxide Thin Film Preparation RF
3 inch target
shutter Ar
95 cm substrate heater
to vacuum
O2
Schematic of ANELVA SPF-332H sputtering system
Films were deposited using radio frequency (RF) planar magnetron sputtering using a 3 inch diameter Zn target reactively sputtered in Ar-O2 (1:1) plasma discharge Substrates were placed parallel to the target surface in a sputter down geometry and heated with quartz lamps to temperatures varying from 200°C to 600°C
Data Analysis Models of Film Structure
EMA Layer void ZnO
ZnO thin film (L1) Platinum film Silicon substrate
Multi-parameter regression analysis: Build model of the thin film structure: estimate thickness, optical characteristics Determine actual change in reflection/polarization over a wide spectral range Measured change is compared to the calculated change and regression analysis based on selected model parameters is performed until a ‘best fit’ is achieved Best fit measured by minimizing the root mean square error (RMSE)
Sample Data: ZnO on Si 80
50
70
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40
60
35
30 Psi (degrees)
% Reflectance
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15
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5 0 200
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0
Wavelength (nm)
Figure 1: Measured (-) and simulated (--) 0 and 70 degree reflection data for ZnO film on Si substrate deposited at 500C.
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950
Wavelength (nm)
Figure 3: Measured (-) and simulated (--) psi ellipsometric data for the film shown in Fig.
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Delta (degrees)
120
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0 300
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1000
Wavelength (nm)
Figure 2: Measured (-) and simulated (--) delta ellipsometric data for the film shown in Fig. 1
o Quality of fit: RMSE 1.437 o Strong interference oscillations below the band edge due to multiple internal reflections between the film and the substrate o Model is able to fit data above and below the band gap
) K (
Discussion
) N ( n o i t c a r f e R
Layer #1 N
2.8
K
0.8 0.7
2.6 0.6 2.4
0.5
f O
0.4 2.2
x e d n I
0.3 2
0.2 0.1
1.8
t n e i c i f f e o C
Optical constant spectra show strong peaks which may be attributed to interband transitions (375 nm) as well as a smaller peak (338 nm) usually attributed to free excitonic and exciton-photon complex transitions1
n exhibits typical semiconductor dispersion: n decreases with increasing λ and resonant frequency close to visible region (~370 nm)
ZnO film is transparent in region 400 – 920 nm
n o i t c n i t x E
0 1.6
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900
-0.1 1000
Wavelength (nm)
Refractive index and extinction coefficient obtained using SCI TM dispersion model for the film in Fig. 1 Extracted Data Thickness Layer 1
540.14 nm
Thickness Layer 2
10.83 nm
Refractive Index @ 632 nm
1.99575
Extinction coefficient @ 632 nm
0.0000
RMSE
1.437
1. Washington et al, Appl. Phys. Lett. 1988, 72, 25, 3261-63. Liu et al, Thin Solid Films, 2006, 510, 32-38.
Discussion – Si Substrate
Refractive Index, n
2.03 2.02 2.01 2 1.99 1.98 0
200
400
600
800
Temperature (deg C)
Refractive index (n) vs temperature (°C) for ZnO (Si substrate) for films of similar thickness (650 nm ± 30 nm)
EMA layer thickness/nm
20 15 10 5
0 0
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400
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Temperature/deg C
EMA layer thickness (nm) vs temperature (°C) for ZnO (Si substrate) for films of similar thickness (650 nm ± 30 nm)
n – indicator of packing density and film stoichiometry. Increasing temperature results in increased surface mobility of impinging species and a more regular lattice structure. Index also increases with O: Zn ratio – film is nearly stoichiometric. EMA layer has no correlation to temperature – real
Discussion – Pt Substrate 2.02
Refractive index, n
2.01
2 1.99 1.98
1.97 1.96 1.95 0
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Substrate temperature/deg C
Refractive index (n) vs temperature (°C) for ZnO (Pt substrate for films of similar thickness (640 nm ± 40 nm)
EMA layer thickness/nm
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Temperature (deg C)
EMA layer thickness (nm) vs temperature (°C) for ZnO (Pt substrate for films of similar thickness (640 nm ± 40 nm)
n increases with substrate temperature EMA layer has no correlation to temperature – real. FESEM images confirm significantly higher roughness in RT, 550°C films Low temperature films on both substrates exhibit low n and high EMA thickness, i.e., are rough and not very dense.
Discussion: Refractive Index 2.04
(1.99 on sapphire1;1.9985 single
2.03
crystal2 (ord. ray); 1.997 uniaxial thin film material3 (ord. ray)
Refractive index,n
2.02 2.01 2
Platinum Silicon
1.99 1.98
Measured values are close to ordinary bulk values (1.99 2.03 for ZnO/Si and 1.96 – 2.01 ZnO/Pt) indicating a high quality film
Comparison of substrates indicate high quality films can be grown on both materials
1.97 1.96 1.95 0
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Temperature/deg C
1. 2. 3.
Bulk refractive index @ 600 nm ~ 2.00
Postava et al, J. of Appl. Phys., 2000, Vol. 87, No. 11, 7820. Bond et al, J. Appl. Phys., 1965, 36, 1674 Jellison et al, Phys. Rev. B, 1998, 58, 7, 3586
Strontium Titanate SrTiO3
Structure and Properties
Sr
Ti
O
Perovskite crystal structure Important dielectric material with incipient ferroelectric transition (tetragonal cubic) at 110K and bandgap of 3.22 eV1 High dielectric constant, excellent optical properties and stable stoichiometry at RT offers prospective applications in microelectronics and optoelectronics in thin film morphology Isotropic optical behavior as a result of cubic crystal structure
1. M.-S. Zhang et al, Applied Physics A, 76, 2003, 1105-8. J.H. Ma et al, J. Appl. Phys. 99, 2006, 033515.
Thin Film Preparation
Films were deposited using radio frequency (RF) planar magnetron sputtering using a 3 inch diameter SrTiO3 target (IC Mechanics 99.9%) in Ar-O2 (4:1) plasma discharge in an Anelva SPF332H sputtering system
Substrates were placed parallel to the target surface (target to substrate distance of 4.5 cm) in a sputter down geometry and heated with quartz lamps to temperatures varying from room temperature to 800°C
Sample Data: SrTiO3 Film Simulated @ 70.775 o Simulated @ 0 o
110
n o i t c e l f e R
Measured @ 70.775 o Measured @ 0 o
90
70
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30
% 10
-10
-30
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400.0
500.0
600.0
700.0
800.0
900.0
1000.0
1100.0
Wavelength (nm)
Measured and simulated 0° and 70° reflection data for SrTiO3 film on silicon substrate (deposition temperature 700°C)
Extracted Data Thickness Layer 1 (STO)
564.88 nm
Maximum Refractive Index @ 632 nm
2.32958
Extinction coefficient @ 632 nm
0.0000
RMSE
1.638
Analysis using multi-angle reflectometry Graded model Quality of fit: RMSE 1.638 Strong interference oscillations due to multiple internal reflections between the film and substrate
Discussion Layer #1 N
2.5
K
2.4
m n 2 3 6
8
0 1 x 1
7
(
6
)m
9
2.3 2.2
5n 5 -
2.1
4
@ N
3
2
2
@
1
K
1.9 1.8
2 3 6
0
1.7 -100
0 100 Substrate
200
300
Thickness (nm)
400
500 Surface
-1 600
Graded refractive index and extinction coefficient spectra at 632 nm obtained for film
EMA layer higher index film lower index film substrate
STO film
Data analysis indicates a graded refractive index – n increases linearly in the normal direction, reaching a maximum at the film surface Graded index occurs only in films deposited at 400°C and above Applications: multi-band rugate filters, anti-reflective coatings, edge and dichroic filters
Discussion
Clear trend of increasing n with increasing deposition temperature: increased packing density
Significant increase in n above 300°C (1.989 to 2.274 at 400°C)
Bulk refractive index of SrTiO3 is 2.394 at 620 nm; maximum value of 2.385 at 800°C which is very close to the bulk values, indicating very high crystallinity and film quality.
2.7
Maximum Refractive Index, n
2.5
2.3
2.27359
2.38458
2.36684 2.32958
2.31942 2.28099
2.1 1.9894 1.9 1.84401
1.7
1.5 0
100
200
300
400
500
600
700
Substrate Tem perature (deg C)
Maximum refractive index versus substrate deposition temperature for SrTiO3 films on silicon substrate
800
900
Summary
High quality ZnO and SrTiO3 films can be produced via sputtering on Si and Pt substrates Film density, as measured by an increase in the refractive index, increases with increasing substrate temperature during deposition For ZnO films, model is able to fit data both above and below the band gap For STO films, a marked change in crystallinity of the deposited films occurs above 300°C; films also exhibit a graded index at higher deposition temperatures
Reflectometry Instrumentation
Intensity of a monochromatic beam of light is measured before and after it reflects from the sample
DETECTOR
BEAMSPLITTER
I Rrelative = I standard sample _________
Absolute reflectance of the sample can be calculated from absolute reflectance of the standard (bare Si wafer)
LIGHT SOURCE
SAMPLE
Ellipsometry Instrumentation
Measurement of the change in polarization state of a beam of light upon reflection from the sample Light source
polarizer
sample
analyzer
detector
Detector signal is measured as a function of time then Fourier analyzed to obtain psi and delta
