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Temperature insensitive conductance detection with surface-functionalised silicon nanowire sensors
Microelectronic Engineering 88 (2011) 1753–1756
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Temperature insensitive conductance detection with surface-functionalised silicon nanowire sensors Mohammad Adel Ghiass a,⇑, Silvia Armini b, Marta Carli b, Arantxa Maestre Caro b, Vladimir Cherman b, Jun Ogi c, Shunri Oda c, Zakaria Moktadir a, Yoshishige Tsuchiya a, Hiroshi Mizuta a a b c
School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, United Kingdom IMEC, Kapeldreef 75, B-3001 Heverlee, Leuven, Belgium Quantum Nanoelectronics Research Centre, Tokyo Institute of Technology, Tokyo 152-8552, Japan
a r t i c l e
i n f o
Article history: Available online 13 February 2011 Keywords: Silicon nanowire Functionalisation Chemical/biological sensing Temperature stability
a b s t r a c t The effects of suspension and functionalisation on electrical conduction of silicon nanowires (SiNWs) are experimentally investigated towards highly sensitive chemical and biological detection applications. The conductance is found to be affected by the suspension process, while it shows the similar trend against the temperature before and after the suspension. The reduction of conductance after the functionalisation with NH2 self-assembled monolayer/glutaraldehyde/biotin can be explained based on the assumption that there is a charge transfer from the SiNW to the molecular layers. The temperature insensitive conductance found in the functionalised SiNWs experimentally for the first time is expected to be extremely useful for practical sensing applications. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Chemical and biological sensors are crucial for a vast variety of applications such as medical diagnostics, food processing control, and environmental monitoring [1]. An appropriate transduction method can provide a detectable signal showing the existence of target species in a sample. Electrochemical transduction has been shown to be the proper choice to achieve label-free, fast, and highly sensitive sensors [2]. As targets to be detected, many chemical/biological molecules contain an intrinsic electrostatic charge under physiological conditions [3]. This charge can be used as the base for the detection mechanism. In accordance with the size of these particles, the magnitude of charge is very small. Therefore, the sensing element of a conduction-based sensor should be designed so that its electrical characteristics can be influenced reasonably in exposure to the target species. Silicon nanowires (SiNWs) are good candidates to be considered as the sensing element. Thanks to their large surface-to-volume ratio, even a small electric field induced by a charged molecule adsorbed on the surface can have a significant influence upon the electrical conductance [4,5]. The SiNWs can be made suspended to increase the surface area exposed to the target molecules, which in turn leads to a sensitivity enhancement. Compatibility with the conventional silicon technology is another advantage of utilising the SiNWs, as the peripheral circuitries can be easily integrated ⇑ Corresponding author. Tel.: +44 (0) 23 8059 3737. E-mail address: [email protected] (M.A. Ghiass). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.02.063
on the same chip. The conductance can be adjusted by changing the condition of the doping process so as to improve the sensitivity. Furthermore, the well-known surface chemistry of silicon and silicon dioxide facilitates the probable required functionalisation processes. Even in the case that the sensing environment contains a number of different charged particles, only the desired target molecules can be selectively adsorbed on the surface, since it is functionalised with specific chemical or biological molecules. One of the measures for assessing the sensors is their stability against variations of anything other than the desirable measurable parameter. For instance, temperature variation should not affect the operation of sensors except thermometers. In this paper, we have investigated the effects of functionalisation on the electrical transport properties of SiNWs, in particular, on the temperature dependence of conductance.
2. Experimental A schematic of the fabrication process is shown in Fig. 1, originally developed in Ref. [6]. The SiNW devices were fabricated on the silicon-on-insulator (SOI) platform, where the buried oxide layer is 200 nm thick. Electron beam lithography was used to pattern the NW structure on the SOI layer, which was heavily doped by phosphorous ion implantation with the doping concentration of 2 1019 cm3. A set of SiNWs was made suspended by applying isotropic and anisotropic etching to investigate the effects of suspension on the electrical characteristics. The samples were ther-
1754
M.A. Ghiass et al. / Microelectronic Engineering 88 (2011) 1753–1756
Conductance µ S
5 4 3 NW Suspended NW
2 1 0 0
20
40
60
80
100
Temperature °C Fig. 1. Fabrication process of SiNW devices.
Fig. 3. Temperature dependence of conductance in non-suspended and suspended SiNWs.
mally oxidised afterwards to provide appropriate surface passivation. Non-suspended and suspended SiNWs with the designed dimensions of 50 nm in thickness, 100 nm in width, and 200 nm in length were used in this study. To investigate the functionalisation effects, non-suspended SiNWs with the same thickness and width but a different length of 400 nm were prepared. A selective functionalisation method we used in this study is as follows [7]. First, the sample was completely covered with MET-2D resist. Later, the resist at the nanowire region was ablated by Joule heating caused by a current flowing through the SiNW, preparing the surface to be coated with a NH2 self-assembled monolayer (SAM), 3-aminopropyltriethoxysilane (APTES). The coating was followed by removal of the remaining resist, and introduction of the functional molecules, glutaraldehyde (GA), and biotin. The process is schematically shown in Fig. 2(a) and the scanning electron micrographs of the unfunctionalised and functionalised SiNWs are shown in Fig. 2(b) and (c), respectively. Electrical measurements were performed in the atmosphere by using a prober capable of changing the substrate temperature (Cascade M150) and a semiconductor parameter analyser (Agilent B1500).
plotted as a function of temperature in the range of 10–115 °C in Fig. 3. The incremental trend of conductance with increasing temperature can be explained by the carrier concentration increase [8]. The conductance decreased after the suspension, which is similar to the results at a lower temperature range, where the conductivity of suspended samples were reported to be orders of magnitude less than the non-suspended structure [9]. The suspended SiNW was oxidised thermally to provide appropriate surface passivation. Therefore, shrinkage in the final cross-sectional area is expected, which may play the key role in the conductance reduction. In addition, the surface traps introduced to the bottom of the SiNW due to the undercut process may contribute to the decrease in conductance. The same incremental tendency observed in the temperature dependence of conductance of the suspended SiNW can be considered a sign of similar conduction mechanism in both SiNWs. Next, we examined the effects of functionalisation on the conductance of SiNWs. The room temperature I–V characteristics of SiNW functionalised with NH2 SAM/GA/biotin are shown in Fig. 4 together with those of the unfunctionalised one. In this case, the conductance decreased after the functionalisation. This conductance reduction can be discussed in terms of a possible charge transfer mechanism between the SiNW and the molecular layers. In general, the surface characteristics change due to the SAM molecules covalently linked to substrates, where the altered work function of the inorganic layer can reduce the barriers for charge transfer into or out of the substrate [10]. The energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the inorganic layer with respect to
3. Results and discussion First, we investigated effects of suspension on the temperature dependence of the SiNW conductance. We then compared the I–V characteristics of the non-suspended and suspended SiNWs sharing the same designed dimensions and doping concentration. The conductance estimated from the slope of the linear I–V curve is
Fig. 2. (a) Schematic diagram of the selective NH2 SAM/GA/biotin functionalisation process by Joule heating method. (b) and (c) Scanning electron micrographs of unfunctionalised and functionalised SiNWs.
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M.A. Ghiass et al. / Microelectronic Engineering 88 (2011) 1753–1756
Drain current nA
40
29.65 µ S
o Before After
20 0 20 40 60 o o 2
oo
oo
o
oo
oo
oo
o
o oo
o ooo
oo
o
oo o
oo
o
oo
o
oo
o
oo
oo
40
o
Conductance µ S
60
16.53 µ S
T 1 0 1 Source drain voltage mV
30
AuNP func. Biotion func.
20
10
300 K 0
2
0
20
40
60
80
100
Temperature °C Fig. 6. Conductance of functionalised SiNWs as a function of temperature.
G G0
Fig. 4. Room temperature I–V characteristics of SiNWs before and after functionalisation with NH2 SAM/GA/biotin.
0.8
NW Suspended NW
0.6
AuNP func. NW GA Biotin func. NW
0.4 unfunctionalised 0.2 functionalised 0.0 0
20
40
60
80
100
Temperature °C Fig. 7. Normalised temperature dependence of conductance variation for different SiNWs.
Fig. 5. Scanning electron micrographs of SiNW (a) before and (b) after functionalisation with AuNPs.
the Fermi level should be taken into account to determine the charge injection barriers. In the case of reduction in electron injection barrier, the electrons, especially those trapped at the interface, can be transferred to the molecular layer. Accordingly, the carrier density of the SiNW is reduced, resulting in the conductance decrease. Quantitative analysis is underway to estimate the amount of transferred charge. We also investigated the effects of functionalisation on the temperature dependence of conductance. In addition to the SiNWs functionalized with NH2 SAM/GA/biotin, we measured the samples in which surface NH2 groups were decorated with gold nanoparticles (AuNPs). The scanning electron micrographs of a SiNW before and after the functionalisation are shown in Fig. 5(a) and (b), respectively. The conductance of NH2 SAM/GA/biotin-functionalised and AuNPs-functionalised SiNWs are plotted as a function of temperature in Fig. 6. Virtually temperature independent conductance was observed for both cases. In order to compare all different groups of SiNWs, we defined the normalised conductance change,
DG=G0 ¼
G G0 G0
ð1Þ
where G0 is the conductance at T = 10 °C, and plotted the DG/G0 as a function of the temperature in Fig. 7. Regardless of the difference in functionalisation methods, DG/G0 is almost temperature independent after the functionalisation. One possible scenario is that the electron concentration increases with rising temperature as in unfunctionalised SiNWs, while the mobility reduction rate is amplified by the modified surface, and offsets the effects of increase in electron density. In addition, a charge transfer from SiNW to the outer layers, as suggested above can counteract the carrier concentration increase caused by rising temperature. Although the mechanism behind this phenomenon is not fully understood, this temperature insensitive conductance of the functionalised SiNWs is very useful, providing an ideal sensing platform tolerant to temperature variations of the environment. In other words, any temperature dependence observed in the actual sensing operation can be attributed to the interaction between the target molecules and the functionalised SiNW described by using the adsorption and desorption rates, which are generally temperature dependent [11]. Evaluation of the complete chemical/biosensor configurations utilising the functionalised SiNWs and their temperature dependence is left for our future studies.
4. Conclusion We investigated the effects of suspension and functionalisation on the electrical conduction of SiNWs towards sensing applications for chemical and biological molecules. The conductance was found to be altered by the suspension process, while it showed the similar trend against the temperature before and after the suspension. The reduction of the conductance after the NH2 SAM/GA/biotin functionalisation can be explained assuming the possible charge
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M.A. Ghiass et al. / Microelectronic Engineering 88 (2011) 1753–1756
transfer from the SiNW to the molecular layers. We observed the temperature insensitivity of the conductance in the differently functionalised SiNWs for the first time. Although the mechanism is not clear yet, this feature is expected to be extremely useful for practical highly sensitive sensing applications. Acknowledgement This work was financially supported by EU FP7 NEMSIC project. References [1] D. Grieshaber, R. MacKenzie, J. Vörös, E. Riemhult, Sensors 8 (2008) 1440– 1458.
[2] Z.Q. Gao, A. Agarwal, A.D. Trigg, N. Singh, C. Fang, C.H. Tung, Y. Fan, K.D. Buddharaju, J.M. Kong, Anal. Chem. 79 (2007) 3291–3297. [3] J. Gamby, J.P. Abid, M. Abid, J.P. Ansermet, H.H. Girault, Anal. Chem. 78 (2006) 5289–5295. [4] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289–1292. [5] G. Gruner, Anal. Bioanal. Chem. 384 (2006) 322–335. [6] J. Ogi, Y. Tsuchiya, S. Oda, H. Mizuta, Microelectron. Eng. 85 (2008) 1410–1412. [7] S. Armini et al., MRS 2010 Spring Meeting, K6.4, 5–9 April, USA. [8] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, John Wiley & Sons, Inc., New Jersey, 2007. [9] J. Ogi, M.A. Ghiass, Y. Tsuchiya, K. Uchida, S. Oda, H. Mizuta, Jpn. J. Appl. Phys. 49 (2010) 044001. [10] G. Heimel, L. Romaner, E. Zojer, J. L. Acc. Chem. Res. 41 (2008) 721–729. [11] S. Tlili, L.I. Nieto-Gligorovski, B. Temime-Rousell, S. Gligorovski, H. Wortham, J. Electrochem. Soc. 157 (2010) 43–48.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Temperature insensitive conductance detection with surface-functionalised silicon nanowire sensors Mohammad Adel Ghiass a,⇑, Silvia Armini b, Marta Carli b, Arantxa Maestre Caro b, Vladimir Cherman b, Jun Ogi c, Shunri Oda c, Zakaria Moktadir a, Yoshishige Tsuchiya a, Hiroshi Mizuta a a b c
School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, United Kingdom IMEC, Kapeldreef 75, B-3001 Heverlee, Leuven, Belgium Quantum Nanoelectronics Research Centre, Tokyo Institute of Technology, Tokyo 152-8552, Japan
a r t i c l e
i n f o
Article history: Available online 13 February 2011 Keywords: Silicon nanowire Functionalisation Chemical/biological sensing Temperature stability
a b s t r a c t The effects of suspension and functionalisation on electrical conduction of silicon nanowires (SiNWs) are experimentally investigated towards highly sensitive chemical and biological detection applications. The conductance is found to be affected by the suspension process, while it shows the similar trend against the temperature before and after the suspension. The reduction of conductance after the functionalisation with NH2 self-assembled monolayer/glutaraldehyde/biotin can be explained based on the assumption that there is a charge transfer from the SiNW to the molecular layers. The temperature insensitive conductance found in the functionalised SiNWs experimentally for the first time is expected to be extremely useful for practical sensing applications. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Chemical and biological sensors are crucial for a vast variety of applications such as medical diagnostics, food processing control, and environmental monitoring [1]. An appropriate transduction method can provide a detectable signal showing the existence of target species in a sample. Electrochemical transduction has been shown to be the proper choice to achieve label-free, fast, and highly sensitive sensors [2]. As targets to be detected, many chemical/biological molecules contain an intrinsic electrostatic charge under physiological conditions [3]. This charge can be used as the base for the detection mechanism. In accordance with the size of these particles, the magnitude of charge is very small. Therefore, the sensing element of a conduction-based sensor should be designed so that its electrical characteristics can be influenced reasonably in exposure to the target species. Silicon nanowires (SiNWs) are good candidates to be considered as the sensing element. Thanks to their large surface-to-volume ratio, even a small electric field induced by a charged molecule adsorbed on the surface can have a significant influence upon the electrical conductance [4,5]. The SiNWs can be made suspended to increase the surface area exposed to the target molecules, which in turn leads to a sensitivity enhancement. Compatibility with the conventional silicon technology is another advantage of utilising the SiNWs, as the peripheral circuitries can be easily integrated ⇑ Corresponding author. Tel.: +44 (0) 23 8059 3737. E-mail address: [email protected] (M.A. Ghiass). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.02.063
on the same chip. The conductance can be adjusted by changing the condition of the doping process so as to improve the sensitivity. Furthermore, the well-known surface chemistry of silicon and silicon dioxide facilitates the probable required functionalisation processes. Even in the case that the sensing environment contains a number of different charged particles, only the desired target molecules can be selectively adsorbed on the surface, since it is functionalised with specific chemical or biological molecules. One of the measures for assessing the sensors is their stability against variations of anything other than the desirable measurable parameter. For instance, temperature variation should not affect the operation of sensors except thermometers. In this paper, we have investigated the effects of functionalisation on the electrical transport properties of SiNWs, in particular, on the temperature dependence of conductance.
2. Experimental A schematic of the fabrication process is shown in Fig. 1, originally developed in Ref. [6]. The SiNW devices were fabricated on the silicon-on-insulator (SOI) platform, where the buried oxide layer is 200 nm thick. Electron beam lithography was used to pattern the NW structure on the SOI layer, which was heavily doped by phosphorous ion implantation with the doping concentration of 2 1019 cm3. A set of SiNWs was made suspended by applying isotropic and anisotropic etching to investigate the effects of suspension on the electrical characteristics. The samples were ther-
1754
M.A. Ghiass et al. / Microelectronic Engineering 88 (2011) 1753–1756
Conductance µ S
5 4 3 NW Suspended NW
2 1 0 0
20
40
60
80
100
Temperature °C Fig. 1. Fabrication process of SiNW devices.
Fig. 3. Temperature dependence of conductance in non-suspended and suspended SiNWs.
mally oxidised afterwards to provide appropriate surface passivation. Non-suspended and suspended SiNWs with the designed dimensions of 50 nm in thickness, 100 nm in width, and 200 nm in length were used in this study. To investigate the functionalisation effects, non-suspended SiNWs with the same thickness and width but a different length of 400 nm were prepared. A selective functionalisation method we used in this study is as follows [7]. First, the sample was completely covered with MET-2D resist. Later, the resist at the nanowire region was ablated by Joule heating caused by a current flowing through the SiNW, preparing the surface to be coated with a NH2 self-assembled monolayer (SAM), 3-aminopropyltriethoxysilane (APTES). The coating was followed by removal of the remaining resist, and introduction of the functional molecules, glutaraldehyde (GA), and biotin. The process is schematically shown in Fig. 2(a) and the scanning electron micrographs of the unfunctionalised and functionalised SiNWs are shown in Fig. 2(b) and (c), respectively. Electrical measurements were performed in the atmosphere by using a prober capable of changing the substrate temperature (Cascade M150) and a semiconductor parameter analyser (Agilent B1500).
plotted as a function of temperature in the range of 10–115 °C in Fig. 3. The incremental trend of conductance with increasing temperature can be explained by the carrier concentration increase [8]. The conductance decreased after the suspension, which is similar to the results at a lower temperature range, where the conductivity of suspended samples were reported to be orders of magnitude less than the non-suspended structure [9]. The suspended SiNW was oxidised thermally to provide appropriate surface passivation. Therefore, shrinkage in the final cross-sectional area is expected, which may play the key role in the conductance reduction. In addition, the surface traps introduced to the bottom of the SiNW due to the undercut process may contribute to the decrease in conductance. The same incremental tendency observed in the temperature dependence of conductance of the suspended SiNW can be considered a sign of similar conduction mechanism in both SiNWs. Next, we examined the effects of functionalisation on the conductance of SiNWs. The room temperature I–V characteristics of SiNW functionalised with NH2 SAM/GA/biotin are shown in Fig. 4 together with those of the unfunctionalised one. In this case, the conductance decreased after the functionalisation. This conductance reduction can be discussed in terms of a possible charge transfer mechanism between the SiNW and the molecular layers. In general, the surface characteristics change due to the SAM molecules covalently linked to substrates, where the altered work function of the inorganic layer can reduce the barriers for charge transfer into or out of the substrate [10]. The energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the inorganic layer with respect to
3. Results and discussion First, we investigated effects of suspension on the temperature dependence of the SiNW conductance. We then compared the I–V characteristics of the non-suspended and suspended SiNWs sharing the same designed dimensions and doping concentration. The conductance estimated from the slope of the linear I–V curve is
Fig. 2. (a) Schematic diagram of the selective NH2 SAM/GA/biotin functionalisation process by Joule heating method. (b) and (c) Scanning electron micrographs of unfunctionalised and functionalised SiNWs.
1755
M.A. Ghiass et al. / Microelectronic Engineering 88 (2011) 1753–1756
Drain current nA
40
29.65 µ S
o Before After
20 0 20 40 60 o o 2
oo
oo
o
oo
oo
oo
o
o oo
o ooo
oo
o
oo o
oo
o
oo
o
oo
o
oo
oo
40
o
Conductance µ S
60
16.53 µ S
T 1 0 1 Source drain voltage mV
30
AuNP func. Biotion func.
20
10
300 K 0
2
0
20
40
60
80
100
Temperature °C Fig. 6. Conductance of functionalised SiNWs as a function of temperature.
G G0
Fig. 4. Room temperature I–V characteristics of SiNWs before and after functionalisation with NH2 SAM/GA/biotin.
0.8
NW Suspended NW
0.6
AuNP func. NW GA Biotin func. NW
0.4 unfunctionalised 0.2 functionalised 0.0 0
20
40
60
80
100
Temperature °C Fig. 7. Normalised temperature dependence of conductance variation for different SiNWs.
Fig. 5. Scanning electron micrographs of SiNW (a) before and (b) after functionalisation with AuNPs.
the Fermi level should be taken into account to determine the charge injection barriers. In the case of reduction in electron injection barrier, the electrons, especially those trapped at the interface, can be transferred to the molecular layer. Accordingly, the carrier density of the SiNW is reduced, resulting in the conductance decrease. Quantitative analysis is underway to estimate the amount of transferred charge. We also investigated the effects of functionalisation on the temperature dependence of conductance. In addition to the SiNWs functionalized with NH2 SAM/GA/biotin, we measured the samples in which surface NH2 groups were decorated with gold nanoparticles (AuNPs). The scanning electron micrographs of a SiNW before and after the functionalisation are shown in Fig. 5(a) and (b), respectively. The conductance of NH2 SAM/GA/biotin-functionalised and AuNPs-functionalised SiNWs are plotted as a function of temperature in Fig. 6. Virtually temperature independent conductance was observed for both cases. In order to compare all different groups of SiNWs, we defined the normalised conductance change,
DG=G0 ¼
G G0 G0
ð1Þ
where G0 is the conductance at T = 10 °C, and plotted the DG/G0 as a function of the temperature in Fig. 7. Regardless of the difference in functionalisation methods, DG/G0 is almost temperature independent after the functionalisation. One possible scenario is that the electron concentration increases with rising temperature as in unfunctionalised SiNWs, while the mobility reduction rate is amplified by the modified surface, and offsets the effects of increase in electron density. In addition, a charge transfer from SiNW to the outer layers, as suggested above can counteract the carrier concentration increase caused by rising temperature. Although the mechanism behind this phenomenon is not fully understood, this temperature insensitive conductance of the functionalised SiNWs is very useful, providing an ideal sensing platform tolerant to temperature variations of the environment. In other words, any temperature dependence observed in the actual sensing operation can be attributed to the interaction between the target molecules and the functionalised SiNW described by using the adsorption and desorption rates, which are generally temperature dependent [11]. Evaluation of the complete chemical/biosensor configurations utilising the functionalised SiNWs and their temperature dependence is left for our future studies.
4. Conclusion We investigated the effects of suspension and functionalisation on the electrical conduction of SiNWs towards sensing applications for chemical and biological molecules. The conductance was found to be altered by the suspension process, while it showed the similar trend against the temperature before and after the suspension. The reduction of the conductance after the NH2 SAM/GA/biotin functionalisation can be explained assuming the possible charge
1756
M.A. Ghiass et al. / Microelectronic Engineering 88 (2011) 1753–1756
transfer from the SiNW to the molecular layers. We observed the temperature insensitivity of the conductance in the differently functionalised SiNWs for the first time. Although the mechanism is not clear yet, this feature is expected to be extremely useful for practical highly sensitive sensing applications. Acknowledgement This work was financially supported by EU FP7 NEMSIC project. References [1] D. Grieshaber, R. MacKenzie, J. Vörös, E. Riemhult, Sensors 8 (2008) 1440– 1458.
[2] Z.Q. Gao, A. Agarwal, A.D. Trigg, N. Singh, C. Fang, C.H. Tung, Y. Fan, K.D. Buddharaju, J.M. Kong, Anal. Chem. 79 (2007) 3291–3297. [3] J. Gamby, J.P. Abid, M. Abid, J.P. Ansermet, H.H. Girault, Anal. Chem. 78 (2006) 5289–5295. [4] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289–1292. [5] G. Gruner, Anal. Bioanal. Chem. 384 (2006) 322–335. [6] J. Ogi, Y. Tsuchiya, S. Oda, H. Mizuta, Microelectron. Eng. 85 (2008) 1410–1412. [7] S. Armini et al., MRS 2010 Spring Meeting, K6.4, 5–9 April, USA. [8] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, John Wiley & Sons, Inc., New Jersey, 2007. [9] J. Ogi, M.A. Ghiass, Y. Tsuchiya, K. Uchida, S. Oda, H. Mizuta, Jpn. J. Appl. Phys. 49 (2010) 044001. [10] G. Heimel, L. Romaner, E. Zojer, J. L. Acc. Chem. Res. 41 (2008) 721–729. [11] S. Tlili, L.I. Nieto-Gligorovski, B. Temime-Rousell, S. Gligorovski, H. Wortham, J. Electrochem. Soc. 157 (2010) 43–48.