Chemical MIS Sensor with Nanoporous Carbon ...

ECS Transactions, 45 (14) 45-54 (2013) 10.1149/04514.0045ecst ©The Electrochemical Society

Chemical MIS Sensor with Nanoporous Carbon Adsorbed Layer Using Deep Level Transient Spectroscopy as Sensing Method V.I. Polyakova, A.I. Rukovishnikova, E.G. Shustina, V.V. Meriakria, B. Druzb, Y. Yevtukhovb a

Kotel’nikov Institute of Radio-engineering & Electronics of RAS, 11 Mohovaya str., Moscow 125009, Russia b Veeco Instruments Inc., Terminal Drive, Plainview, NY, 11803, USA

Sensitive and selective chemical sensor based on carrier transfer in metal/NPC/SiO2/p-Si structure with nanoscale metal and nanoporous carbon adsorbed layers were fabricated and studied. It was found that most powerfully and noticeably adsorbed molecules influence on whole injection from p-Si substrate on border traps in SiO2 layer and energy spectrum of trapping centers that lead to changes of sensor electrical characteristics. The most adsorption effect was found in Q-DLTS spectra. Moreover adsorption of the molecules of the different gases (vapor) induced different spectra changes. A physical model of operating the chemical sensor was proposed and discussed. The results are indicative of the possibility of fabricating sensitive and selective MIS chemical sensors for detection of adsorbed molecules using the isothermal Q-DLTS sensing method. Introduction The need for chemical sensor technology in applications such as monitoring of ecological parameters of human living environment, industrial processing, aerospace, and security has increased in recent years [1-3]. The majority of chemical sensors are solid state sensors that are based on the changes in conductivity of active adsorbed layers (sensing materials) or the capacity of the Schottky diodes or MOS (MIS) as primary transducers. With the emergence of nanotechnology, new reliable sensing materials have been developed. One of the most recent perspectives for use as sensing materials in sensors is nanoscale nanoporous materials or nanotubes and their composites. A variety of surface analysis tools have also been developed, however current sensors usually detect only one gas or vapor. For sensing multiple gases, an array of sensors, each sensor providing only partial information, is used. Often, interferences from other gases lowers both the sensitivity and reliability of the sensors and some chemical sensors are effectively operated only at increased temperatures. This is not sufficient for use in sensing new physical parameters and sensing methods. Therefore, fabrication and study of new types sensors based on other physical principles with better sensitivity indicate that adsorbed molecules are required for improvement of the sensor characteristics. In this paper, we present results of our study using metal/NPC/SiO2/p-Si structure with nanoscale metal and nanoporous carbon (NPC) adsorbed layers. To investigate adsorption and desorption processes we used isothermal deep level transient spectroscopy (DLTS) and capacitance-voltage (C-V) characterization.

45

Downloaded on 2015-10-22 to IP 194.34.56.135 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

ECS Transactions, 45 (14) 45-54 (2013)

Experimental details A schematic view of the fabricated sensor is shown in Fig. 1. NPC nanoscale films ~15 nm thick were deposited on SiO2/p-Si substrates by direct ion beam from hydrocarbon IC plasma [4] and beam plasma discharge at low magnetic field [5]. SiO2 insulating layer of the substrates was prepared by thermal oxidation and had thickness ~ 0.5 m. The layer thickness measurements were performed with use of the spectral ellipsometr. Metal (Ni or Co) electrodes were prepared by a magnetron sputtering. Gas (vapor) Adsorption

Desorption Metal thin layer (electrode )

Nanoporous car bon (NPC) thin film

V±

SiO2 layer

p-Si substrate

U~

Ohmic contact

Figure 1. Schematic view of the metal/NPC/SiO2/p-Si chemical sensor. All measurement including capacitance and charge based deep level transient spectroscopy (C- and Q-DLTS) with rate window (m IRE RAS using designed and manufactured in Institute diagnostic system ASEC-03E. The measured value of the DLTS signal by ASEC apparatus can be written as C = C(t1) - C(t2), or Q = Q(t1) - Q(t2), where t1 and t2 are the times counted from the beginning of the discharge upon applying a voltage pulse to the sample. The measurements of DLTS spectra or functional dependence C(m) and Q(m) were performed at m = (t2 – t1)/ln(t2/t1). The ratio t2/t1 in the present measurements was kept constant and equal to 2. The DLTS spectra maximums peaks were used to determine the value of transient charge and to study physical processes to investigate the structures [6-9]. DLTS spectra were formed by recharging electronic states (trapping centers) in the structure and the resulting spectra under several temperatures were used to define concentration, activation energy and capture cross-section. Results and discussion Used physical effects Our preliminary studies using the Q-DLTS method have shown that adsorption of gas (vapour) molecules on the surface of diamond and diamond-like carbon (DLC) materials leads to modification of DLTS spectra of investigated samples [10-14]. The observed changes were explained by physical process of the change energy spectrum of surface electronic states (trapping centers) of diamond and DLC films under adsorption of the molecules. Executed studies have shown that operation of the sensitive chemical sensor

46



can be founded on analysis of the changes induced by adsorbed molecules in the energy spectrum of surface trapping centers (TC). The first model of such sensor was designed and fabricated in Kotel’nikov Institute of Radio-engineering & Electronics of RAS (development team of the Dr. V.I. Polyakov) [10, 11]. Adsorbed properties of the sensor were investigated and more detailed are presented in [14]. However further studies (presented in given paper) have shown that essential contribution to measured DLTS spectra of specially prepared structures (such as shown in Fig. 1) give not only process of the change TC energy spectra but also, for example, hole transfer between p-Si and border traps into SiO2 layer and also minority carrier generation in the depleted region of p-Si. These physical processes were revealed in the C-V characteristics that were obtained at different test signal frequencies for sample 1 with border traps into SiO2 layer (solid curves in Fig. 2) and sample 2 with practical absence border traps into SiO2 layer (dotted curve). Area of cutting the change of the C-V curves under small positive bias voltages (Fig. 2, area A(I)) related with avalanche hole injection from p-Si substrate on border traps in SiO2 layer while area B(G) as is known to be related with minority carrier generation in the depleted region of p-Si. As can be seen from Fig. 2 area A(I) explained by hole injection effect is absent in C-V characteristic for sample 2.

105

f = 0.05 kHz (sample 1) 90

0.5

1.5 5

Capacitance, pF

75 60

15

45

250 (s.1)

B (G) 30

250 (s.2) A (I)

15 0

-10

-5

0

5

10

Bias, U

Figure 2. C-V characteristics of the metal/NPC/SiO2/p-Si structure at different testing signal frequencies f, kHz. A(I) – area on C-V curves related with avalanche hole injection from p-Si substrate at border traps in SiO2 layer; B(G) – area related with minority carrier generation in the depleted region of p-Si.

The hole injection effect is revealed in the C-DLTS spectra (Fig. 3). As can be seen from Fig. 3 (solid curves for sample 1) two peaks E1 and E2 are observed. We expect that peak E1 is related with hole transfer between p-Si and border traps into SiO2 layer. High frequency C-DLTS spectrum for sample 2 with practical absence border traps is exhibited

47



in Fig. 3 by dotted line and peak E1 is absent while peak E2 appeared. Joint analysis of the frequency dependencies of the C-V characteristics (Fig. 2) and C-DLTS spectra (Fig. 3) shows that peak E2 is most probably related with minority carrier generation in pSi substrate near SiO2/p-Si interface forming the inversion channel. 100

peak E2

C-DLTS signal C, pF

80

f = 5 kHz (sample 1)

60

10 50

40

peak E1 100 250 (s.1)

20

0 3,5

250 (s.2) 4,0

4,5

5,0

5,5

Log(m, s)

6,0

6,5

Figure 3. C-DLTS spectra of the metal/NPC/SiO2/p-Si structure at different testing signal frequencies f, kHz. Peak E1 – area related with carrier transfer between p-Si substrate and border traps into SiO2 layer peak E2 – area related with minority carrier generation in the depleted region of p-Si.

Two possible mechanisms that can induce minority carrier generation are: tunnel current of the carriers from valence band (Fig. 4, G(1)) and diffusion from the p-Si bulk (Fig. 4, G(2)). Increase in the frequency of testing signal reduces contribution of the minority carrier generation in measured C-V characteristics as well as in C-DLTS spectra that leads to observed decrease of the signal within range of peak E2 (Fig. 3). The energy band diagram of investigated chemical sensors is shown in Fig. 4. Influence of adsorbed molecules on C-V characteristics and DLTS spectra In the first place, it is necessary to note essential influence geometric and structured parameters of the chemical sensor layers on all measured characteristics. Therefore, for the reason increasing of sensitivity of the proposed sensor, different adsorbed diamondlike carbon films were investigated. A high-resolution atomic force and scanning tunnelling microscopic study of the carbon films and also deposited metal electrodes were performed. Analysis of obtained results has shown that best adsorbed characteristics and sensitivity have sensors with nanoporous carbon films and at NPC film thicknesses approximately 15 nm. The enhanced sensitivity sensors using NPC film as sensing materials attributed to the larger adsorbed surface area of the NPC films [1].

48



_ _

_ _ _ _

G (2)

S

_ _ _ _ + + +

EC EF EV

G (1)

I

Metal electrode

p-Si substrate X

W

NPC thin film

SiO 2

Figure 4. Energy band diagram of the metal/NPC/SiO2/p-Si structure. I – carrier injection from p-Si substrate into SiO2; G(1) – carrier generation from valence band, G(2) - carrier diffusion from the p-Si.

Morphology studied of the ultrathin metal (Ni and Co) electrodes on surface of NPC films has shown (also as in [15, 16]) that metal was deposited in the form of grains with typical lateral diameter of 100-200 nm and height 10-50 nm. The physical model of the prepared sensor is illustrated by figure 5. Gas (vapor) molecules Metal electrode NPC thin film i

p-Si substrate

SiO2 layer

Border traps

Figure 5. Physical model of the metal/NPC/SiO2/p-Si chemical sensor. We assume that adsorbed molecules of the gas (vapor) are diffused through metal electrode and are trapped by trapping centers in nanoscale NPC film that leads to induce new electrically active centers and change TC energy spectrum of sensor structures. Moreover essential part of the molecules run to SiO2 surface and are trapped by interface

49



TC. As result, this process creates a change of the charge distribution and change of potential barriers (band banding s of energy band diagram which is shown in Fig. 4) in the chemical sensor structure. Obviously the change of the electrical characteristics must be also. The influence of adsorbed alcohol molecules on high frequency C-V characteristics is shown in Fig. 6. C-V measurements were performed under different concentration (N) of the adsorbed molecules. Adsorbed molecule concentration increased from zero (curve 1) up to N = N4 (curve 4). The observed changes we explain by shifts contribution of the hole injection process in C-V characteristics.

90

f = 250 kHz

Capacitance, pF

75 60

1

45

2 3 4

30 15 0

-10

-5

0

5

10

Bias, U

Figure 6. C-V characteristics of the sensor. 1 – in air, 2-4 – at different concentration (N) adsorbed alcohol molecules. N(1) = 0, N(2) < N(3) < N(4).

More observably adsorbed alcohol molecules influence on high frequency C-DLTS spectra (see Fig. 7). Concentration of the adsorbed alcohol molecules increased at measurements from zero (curve 1) up to N = N3 (curve 3). As can be seen from figure peak E1 is displaced from the range of low m (log m ~ 4.7, or m ~ 0.47 ms) to the range of high (m ~ 5.7 ms). The observed changes DLTS spectra we explain by increase the band banding s (see Fig. 4) and the time of beginning discharge border traps. In favour of given suggestions indicate similar change the DLTS spectra observed under constant applied voltage which changes band banding s. However the most adsorption shift effect was found in Q-DLTS spectra (Fig. 8). The molecule adsorption is induced displacement E1 peak from rate window m ~ 0.34 ms to m ~ 5.5 ms. Moreover the change of Q-DLTS spectra (if compare to similar change of C-DLTS spectra) is observed under vastly greater measured signal and electronic interface for sensitive sensor can be more simply in fabrication if use Q-DLTS measurements as sensing method.

50



12

peak E1 displacing

C-DLTS signal C, pF

E1

E1

10

3

2 8 6

E1

E2 1

4

E3 2 0 3,5

4,0

4,5

5,0

5,5

Log(m, s)

6,0

6,5

Figure 7. C-DLTS spectra of the sensor. 1 – in air, 2, 3 - at different concentration (N) adsorbed alcohol molecules. N(1) = 0, N(2) < N(3).

240

peak E1 displacing

Q-DLTS signal Q, pC

200

E1

E1

3

2 160

E1

120

1 80

E3

40

0

2

3

4

Log(m, s)

5

6

Figure 8. Q-DLTS spectra of the sensor. 1 – in air, 2, 3 - at different concentration (N) adsorbed alcohol molecules. N(1) = 0, N(2) < N(3).

51



The effect of mixture of the various adsorbed molecules on the Q-DLTS spectrum was investigated also. The Q-DLTS spectrum obtained in mixture of water and alcohol vapor is demonstrated in Fig. 9 together with spectra obtained separately in water and alcohol vapour. As can be seen from Fig. 9 adsorbed water and alcohol molecules induced different spectra changes and can be detected separately.

peak E1 displacing

Q-DLTS signal Q, pC

60

E1

E1

2

3

4

E3

E1

50

1 40 30

E3

20 10 0 1

2

3

4

Log(m,s)

5

6

Figure 9. Q-DLTS spectra of the sensor. 1 - on air, 2 – in mixture of water and alcohol vapor, 3 - in alcohol vapor, 4 - in water vapor.

The found selectivity effect we explain by difference of the influence of the various adsorbed molecules on physical processes in chemical sensor structure. We suppose that one of the reasons there is operation of nanoscale metal/NPC layers as gas-diffusion membrane with different transport diffusivity for various molecules. As a result we observe primary offset peak E1 in alcohol vapour (Fig. 9, curves 1, 3) which related with hole transfer between p-Si and border traps into SiO2 layer. In contrast, in the case of water molecule adsorption, new peak E3 is appeared in Q-DLTS spectra (Fig. 9, curves 1, 4) and measured Q-DLTS signal in this range increased in 23 orders. We assume that peak E3 is related with change energy spectrum of trapping centers in adsorbed nanoscale NPC film. The Q-DLTS spectrum obtained in mixture of water and alcohol vapour have simultaneously two peaks E1 and E3 (Fig. 9, curves 1, 2). The observed sharp form of peak E1 is indicate on relationship peak E1 with hole injection in contrast with theoretical and experimental (peak E3) form peak which related with hole (or electron) thermo emission from TC (Figs 8, 9). It is necessary also to note that contribution to DLTS spectra of described above physical processes possible to change using planar electrodes for measurements in particular to increase role of TC. Our preliminary estimations founded on got data have shown that proposed sensor using for detected of adsorbed molecules the Q-DLTS

52



measurements as sensing method are able to differentiate a few hundred molecules of any gases or vapor. Conclusions Sensitive and selective chemical sensor based on carrier transfer in metal/NPC/SiO2/p-Si structure with nanoscale metal and NPC adsorbed layers were fabricated and studied. The influence of adsorbed alcohol and water molecules on physical processes in sensor structures such as change of density and energy spectrum of trapping centers, hole transfer between p-Si and border traps into SiO2 layer and minority carrier generation in p-Si substrate near SiO2/p-Si interface were investigated. It was found that most powerfully and noticeably adsorbed molecules influence on energy spectrum of trapping centers and hole transfer between p-Si and border traps into SiO2 layer that lead to changes of electrical characteristics. The most adsorption effect was found in Q-DLTS spectra. Moreover adsorbed molecules of the different gases (vapor) induced different spectra changes and can be detected separately. A physical model of operating the chemical sensor was proposed and discussed. The results are indicative of the possibility of fabricating sensitive and selective MIS chemical sensors for detection of adsorbed molecules using the isothermal Q-DLTS sensing method.

Acknowledgements The authors would like to Prof. V.A. Sablicov for valuable discussions. The research was supported by RFBR (grants No 11-02-93965 SA_a, and 11-08 –00257-a). References 1. G. Korotcenkov, B.K. Cho, Porous Semiconductors: Advanced Material for Gas Sensor Applications, Critical Reviews in Solid State and Materials Sciences, 1, pp. 1-37 (2010). 2. M.R. Plata, A.M. Contento, A.Rios, State-of-the-Art of (Bio), Chemical Sensor Developments in Analytical Spanish Groups Sensors 2010, 10, pp. 2511-2576 (2010). 3. S.N. Shtykov, T.Yu. Rusanova, Nanomaterials and nanotechnologies in chemical and biochemical sensors: Capabilities and applications Russian journal of general chemistry, 78, pp. 2521-2531 (2008). 4. B. Druz, E. Ostan, S. Distefano, A. Hayes, V. Kanarov, V.I. Polyakov, A.I. Rukavishnikov, N. Rossukanyi, A. Khomich, Diamond and Related Materials, 7, pp. 965-972 (1998). 5. E.G. Shustin, N.V. Isaev, M.P.Temiryazeva, Yu.V. Fedorov, Vacuum 2009, 83, pp.1350-1354 (2009). 6. S. Nath, J.I.B. Wilson, Impedance measurements on CVD diamond, Diamond and Related Materials, 1, pp. 65-75 (1996). 7. V.I. Polyakov, N.M. Rossukanyi, A.I. Rukovishnikov, S.M. Pimenov, A.V. Karabutov, V.I. Konov, J. Appl. Phys. 84, pp. 2882-2889 (1998).

53



8. V.I. Polyakov, A.I. Rukovishnikov, N.M. Rossukanyi, B. Druz, Materal Rerearch Society, “Electrically Based Microstructural Characterization III”, Warrendale, PA, 699, pp. 219-224 (2002). 9. V.I. Polyakov, A.I. Rukovishnikov, N.M. Rossukanyi, V.G. Pereverzev, S.M. Pimenov, J.A. Carlisle, D.M. Gruen, E.N. Loubnin, Diamond and Related Materials, 12, pp. 1776-1782 (2003). 10. V.I. Polyakov, A.I. Rukovishnikov, N.M. Rossukanyi, V.P. Varnin, B. Druz, et al, in Proceedings book of the 5th Applied Diamond Conference/ 1st Frontier Carbon Technology Joint Conference, Tsukuba, Japan, pp.645-648 (1999). 11. V.I. Polyakov, A.I. Rukovishnikov, A.V. Khomich, B.L. Druz, D. Kania, A. Hayes, M.A.Prelas, R.V. Tompson, T.K. Ghosh, S.K. Loyalka, in Proceedings book: W.Y.L. R. N. Johnson, M. A. Pickering, B.W. Sheldon (Ed.) Properties and Proceedings of Vapor-Deposited Coatings, Material Research Society, Warrendale, PA, 555, pp. 345-348 (1999). 12. V. I. Polyakov, A. I. Rukovishnikov, N. M. Rossukanyi, V. G. Pereverzev, S. M. Pimenov, J. A. Carlisle, D. M. Gruen and E. N. Loubnin, Diamond and Related Materials, 10-11, pp. 1776-1782 (2003). 13. V. I. Polyakov, A. I. Rukovishnikov, V.G. Ralchenko, Proceedings of Joint Intern. Meeting, Ninth Intern. Symp. On Diamond Materials , Honolulu, Hawaii, p. U1-1801 (2004). 14. V.I. Polyakov, A. Yu. Mityagin, A.I. Rukovishnikov, B. Druz, I. Zaritsky, Y. Yevtukhov, Diamond and Related Materials, 15, pp. 1926-1929 (2006). 15. B.L. Druz, V.I. Polyakov, A.V. Karabutov, N.M. Rossukanyi, A.I. Rukovishnikov, E. Ostan, A. Hayes, V.D. Frolov, V.I. Konov, Field electron emission from diamond-like carbon films deposited using RF inductively coupled CH4 -plasma source”, Diamond and Related Materials, 7, pp. 695-698 (1998). 16. V.D.Frolov, S.M.Pimenov, V.I.Konov, V.I.Polyakov, A.I.Rukovishnikov, N.M.Rossukanyi, J.A.Carlisle, D.M.Gruen, Surface and Interface Analysis, 36, pp. 449-454 (2004).

54
