Temperature dependence of current-voltage ...

Microelectronics Reliability 78 (2017) 374–378

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Temperature dependence of current–voltage characteristics of MoS2/Si devices prepared by the chemical vapor deposition method Ting-Hong Su a, Chia-Hung Chiang b, Yow-Jon Lin a,⁎ a b

Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan Department of Physics, National Changhua University of Education, Changhua 500, Taiwan

a r t i c l e

i n f o

Article history: Received 22 August 2017 Received in revised form 3 October 2017 Accepted 3 October 2017 Available online xxxx Keywords: Molybdenum disulfide Temperature dependence Heterojunction Electrical properties Diode Si

a b s t r a c t Layers of MoS2 are directly deposited on the n-type Si (n-Si) substrate by chemical vapor deposition for fabricating a MoS2/n-Si heterojunction device. The rectification current–voltage (I–V) characteristics of MoS2/n-Si devices were measured in the temperature range from 80 to 300 K in steps of 20 K. The temperature-dependent forward-bias I–V characteristics can be explained on the basis of the thermionic emission theory by considering the presence of the interfacial inhomogeneous barriers at the MoS2/n-Si interfaces. The dominance of the induced carrier capture/recombination by states at the MoS2/n-Si interface that lead to the formation of the inhomogeneous barriers serves to influence the photo-response at room temperature. The fabricated MoS2/n-Si devices exhibit reversible switching between high and low current densities, when the simulated sunlight is turned on and off. The sensitivity of the I–V characteristics to temperature provides an opportunity to realize stable and reliable rectification behaviors in the MoS2/n-Si devices. It is found that the electron mobility in the n-Si layer reduces as temperature increases, which leads to the noticeably increased value of the series resistance of MoS2/n-Si devices. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Promoted by the discovery of graphene and its fascinating properties in the past few years, graphene-like layered transition metal dichalcogenides (LTMDs) have been the subject of intense investigation due to their unique properties, which can be employed in many applications, such as electronic and optoelectronic devices. Of these LTMDs, MoS2 has been extensively studied because it has good electrical and optical properties [1]. In addition, heterojunction device is playing an important role in electronics and optoelectronics. The integration of MoS2 on Si could lower the cost of electronic and optoelectronic devices and multifunctional devices would be possible [2,3]. A heterojunction that is composed of MoS2 and Si has many potential applications in electronic and optoelectronic devices and allows study of the interface effect in nanoscale and evolution of electrical transporting mechanisms. Interface states play key roles on the electrical output of the MoS2/Si device. Due to lattice mismatch or defect segregation, the interface between two heteropartners can exhibit a higher defect density than the bulk of each partner [4]. MoS2 is prepared by a variety of techniques such as an adhesive-tape-based micromechanical cleavage technique, lithium-based intercalation, chemical vapor deposition (CVD), metalorganic chemical vapor deposition, and thermal evaporation deposition [5–16]. The preparation process and the equipment for these methods ⁎ Corresponding author. E-mail address: [email protected] (Y.-J. Lin).

https://doi.org/10.1016/j.microrel.2017.10.002 0026-2714/© 2017 Elsevier Ltd. All rights reserved.

are very complex. In order to fabricate large-scale MoS2–based transistors, MoS2 that was prepared using CVD was developed [5,9,13–16]. The fabrication of large-area heterostructures is of fundamental and technological interest. The CVD growth of MoS2 is observed on bare Si surfaces in this study. The entire process for MoS2/Si devices that is demonstrated requires no transfer processing, which minimizes cost and leverages the existing Si manufacturing infrastructure to maximize performance. The investigation of the temperature-dependent currentvoltage (I–V) characteristics of MoS2/n-type Si (n-Si) devices to find the interfacial inhomogeneous barrier is developed in this study. Correlation effects were evaluated using the well-known expressions for thermionic emission (TE) [2,17–20]. To investigate the effect of the interfacial inhomogeneous barrier on the device performance, the photo-response measurement is performed on the MoS2/n-Si device. In addition, the sensitivity of the I–V characteristics to temperature provides an opportunity to realize stable and reliable rectification behaviors in the MoS2/n-Si devices. 2. Experimental procedure Four-inch 525 μm-thick n-Si (100) wafers with an electrical resistivity of about 3 Ω cm (Guv Team International Co., Ltd.) were used in the experiment. The n-Si samples were cleaned in chemical cleaning solutions of acetone and methanol. The n-Si sample was then chemically etched using a diluted HF solution for 1 min, rinsed with de-ionized water and blow-dried with N2 (referred to as as-cleaned n-Si samples).

T.-H. Su et al. / Microelectronics Reliability 78 (2017) 374–378

The as-cleaned n-Si samples were then dipped in a (NH4)2Sx solution (with 6% S, Nippon Shiyaku Co., Ltd.) for 10 min at room temperature and dried in nitrogen. The MoS2 thin film was grown on the as-cleaned n-Si samples with (NH4)2Sx treatment using the CVD method. The detailed CVD-grown process is shown in Refs [21–23]. The MoS2 area was 1.0 × 0.5 cm2. The structural properties of the MoS2 films were determined using Raman spectroscopy (Ramboss 500i, DongWoo Optron). A 532-nm laser was used for excitation. The morphologies of the MoS2 films were studied using field emission scanning electron microscopy (FESEM). X-ray photoelectron spectroscopy (XPS) was used to identify the chemical bonding state of the samples. XPS measurements were performed using a monochromatic Al Kα X-ray source. These were calibrated by using the C 1 s peak as a reference. The electrical properties of the MoS2 and n-Si films were measured by the Van der Pauw method with a four-point contact configuration. In order to determine the electrical properties of MoS2/n-Si samples, gold (Au) ohmic contacts with a square pattern were deposited onto the MoS2 surface using a sputter coater and indium (In) ohmic contacts with a square pattern were deposited onto the n-Si surface using a sputter coater. The current–time (I–t) and I–V curves were measured, using a Keithley Model-4200 semiconductor characterization system. The I–V characteristics of the devices were measured in the temperature range from 80 to 300 K in steps of 20 K using a temperature controlled cryostat. The photo-response for the devices was measured at an illumination intensity of 100 mW/cm2, using a 150 W solar simulator with an AM 1.5G filter. The photo-response was measured by recording the current versus time and the simulated sunlight was turned on and off using a shutter. 3. Results and discussion Fig. 1(a) shows the Raman spectra for MoS2 films that were deposited on the n-Si substrate. The multilayer flakes show characteristic A1g and E12g Raman modes located at around 407 cm−1 and 383 cm−1, respectively. In the E12g mode, both S and Mo atoms vibrate along inplane direction, whereas the S atoms vibrate in the perpendicular-toplane direction in the A1g mode. Our Raman spectrum result is

consistent with the result shown in Ref. [24]. The frequency difference between the A1g and E12g peaks was about 24 cm− 1, suggesting that few layers of MoS2 were formed [21–24]. The full width at half maximum for the Raman peak the E12g (A1g) peak is about 10 (11) cm−1. The calculated values are similar to the reported values [25]. The carrier concentration, carrier mobility and conduction types for MoS2 samples were obtained from Hall-effect measurements. The Van der Pauw-Hall measurements (SWIN Hall8686 Hall Effect Measurement System) were performed at room temperature. The MoS2 thin film that is deposited on the Si substrate exhibits p-type behavior. The hole concentration and mobility for MoS2 films are respectively determined to be 2.0 × 1020 cm−3 and 354 cm2 V−1 s−1. Fig. 1(b) shows the Mo 3d and S 2 s XPS spectra for MoS2 films that were deposited on Si substrates. The XPS core-level peaks are deconvolved into their various components using an interactive least-squares computer program. Mixed Gaussian–Lorentzian peaks were used in this analysis. The peak for Mo 3d5/2 was measured at about 229.1 eV and the peak for S 2 s was measured at about 226.0 eV. These binding energies are in good agreement with the reported values for p-type MoS2 samples [25]. Both p-type and n-type conductivities have been reported in the MoS2 layers deposited on different substrates [2,6,12,15,21,22,25]. MoS2 shows both n- and p- type conductivities dependent on the experimental process [26]. The n-type and p-type conductivities have been reported in ultrathin MoS2 layers deposited on SiO2 [5,26,27–31]. It is worth noting that no intentional doping was introduced in these experiments. A relationship between the presence of large variations in the sulfur concentrations of MoSx and the conduction type has been reported [32,33]. McDonnell et al. [34] showed that MoS2 can exhibit both ptype and n-type conductivity at different positions on the same sample, which they attributed to variations in the local stoichiometry of MoS2 due to surface defects. Fig. 1(c) shows the FESEM image of MoS2 films. The scale bar is 1 μm. It is seen the shape of flake, indicating that MoS2 films were deposited on the n-Si substrate. The schematic of the MoS2/n-Si device with Au/In contacts is shown in Fig. 2(a). Fig. 2(b) shows the rectification | I |–V characteristics for MoS2/n-Si devices at 300 K in the dark. This result demonstrates direct and simple growth of p-type MoS2 on n-Si, which can be of high importance in future electronic and optoelectronic applications. Fig. 2(b) shows that the ratio of the forward to reverse current at a bias voltage of ±2 V is 128. For p-type MoS2/n-Si heterojunctions, the rectification conduction mechanism usually involves TE. According to TE theory, the rectification I–V characteristic is given by [2,17–20,35]. qV−IRs −1 I ¼ Is exp ηkT

Fig. 1. (a) Raman spectra and (b) Mo 3d and S 2 s XPS spectra for MoS2/n-Si samples and (c) a FESEM image of MoS2 films that were deposited on the n-Si substrate.

375

ð1Þ

where Is is the reverse-bias saturation current, Rs is the series resistance, q is the elementary charge, T is the absolute temperature, k is the Boltzmann constant, and η is the ideality factor. Based on TE theory, the forward-bias fitting curve is shown in Fig. 2(b). η is determined from the slope of the linear region of the forward bias ln (I)–V characteristics at low voltages. The derived value for η is 2.7. MoS2/n-Si devices exhibit non-ideal TE behaviors because of η N 2. This deviation can be attributed to the presence of the inhomogeneous barrier at the MoS2/n-Si interface that plays an important role in the conduction process. Due to lattice mismatch or defect segregation, the interface between two heteropartners can exhibit a higher defect density than the bulk of each partner [4]. In order to obtain a greater understanding of the rectification I–V characteristics, the photo-response measurements were performed on the MoS2/n-Si device. Fig. 2(c) shows the time-resolved response for the current to repeated light switching for cycling times between 0 and 130 s and for voltages from 0 to 1 μV. For Keithley Model-4200SCS semiconductor characterization system, it is difficult to observe the I–t characteristics for a constant voltage. However, the I–t characteristics could be easily obtained for applying V varied from 0 to 1 μV. The

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Fig. 3. |I|–V curves of a MoS2/n-Si device measured in the temperature range from 80 to 300 K in steps of 20 K.

Fig. 2. (a) Schematic view for the MoS2/n-Si device with Au/In contacts, (b) |I|–V curves of a MoS2/n-Si device at 300 K in the dark and the fitting curve based on TE theory and (c) time-resolved photocurrent measurements for MoS2/n-Si devices (switching on and off of the sunlight illumination is indicated).

change in V should be small enough to suppress the induced current contribution. A negative photocurrent (IP) is observed, implying that the depletion region at the MoS2/n-Si interface contributes to the photo-response. The rise in IP was measured by switching on the simulated sunlight. The photocurrent rapidly decays to a stable dark current when the simulated sunlight is turned off. The fabricated diodes exhibit reversible switching between high and low current densities, when the simulated sunlight is turned on and off. Fig. 2(c) shows the IP decay. Such behavior is attributed to the occurrence of the induced carrier capture/recombination by interface states [36,37] that lead to the formation of the inhomogeneous barriers. The photo-response result supports the aforementioned conduction mechanism because there is a high density of states at the MoS2/n-Si interface. The deep understanding of conduction mechanism is helpful to control the performance for MoS2/n-Si devices. In order to understand this phenomenon, the temperature-dependent I–V characteristics were observed. Fig. 3 shows the temperature-dependent |I|–V characteristics of the MoS2/n-Si devices. The leakage current density increases with increasing temperature under a negative voltage. The reverse current is mainly due to thermal generation of electro-hole pairs in the depletion region [38]. Its increase with temperature is included in the TE theory. However, the current density increases with increasing temperature under a low positive voltage and the current density decreases with increasing temperature under high positive voltage. The behavior under high-forward bias may be suggestive of the dominance of Rs. In order to understand this phenomenon, the temperature dependence of Rs and η were further studied. Rs is determined from the data fitted to Eq. (1). The fitting curve is shown in Fig. 2(b). Fig. 4 shows the temperature-dependent values for Rs and η. It is found that the value of η decreases as temperature increases and the value of Rs increases as temperature increases. The variation in η with temperature is called the To effect [19]. η of the diodes showing this behavior varies with temperature as; η = ηo + (To / T) [19]. According to the Tung's model [39], both η of N1 and its linearity vs. 1000/T can be convincing evidence of an interfacial inhomogeneous barrier. The larger values of ηo and To correspond to more inhomogeneous barriers. The interfacial barrier

fluctuations consist of low and high barrier areas, that is, the current through the diode will flow preferentially through the lower barriers [40]. It is suggested that the existence of interface states leads to the formation of the interfacial barrier fluctuations. Fig. 4 shows η as a function of 1000/T and the linear fitting curve. This model is distinguished by a linear η–(1000/T) correlation [20,39]. Such behaviors of the diode ideality factor have been attributed to particular distribution of states at the MoS2/n-Si interface that serves to form the interfacial barrier fluctuations [40]. To verify the temperature dependence of Rs, the Hall-effect measurements were performed on the n-Si samples. The Van der PauwHall measurements (SWIN Hall8686 Hall Effect Measurement System) were performed in the temperature range from 120 to 300 K in steps of 30 K. Fig. 5 shows the temperature-dependent values of the electron concentration (ND), electron mobility and resistivity for n-Si samples. The n-Si sample shows that the electron concentration is insensitive to a change in temperature, whereas the n-Si sample exhibits that the electron mobility is significantly sensitive to a change in temperature. It is found that the value of resistivity for n-Si samples increases as temperature increases, because of the dominance of the electron mobility. Our Hall-effect measurement result is consistent with the results shown in Refs [41–43]. Li [41] found that the influence of electron-electron scattering is negligible for ND b 2 × 1016 cm−3, but is significant for ND ≥ 1 × 1017 cm−3. In addition, Rs is attributed to the ohmic loss in the whole device, including the resistance of MoS2 and n-Si films. However, the electrical resistivity for MoS2 (8.8 × 10−5 Ω cm at 300 K) is much lower than that for n-Si (3 Ω cm at 300 K), indicating that the resistance

Fig. 4. Temperature-dependent values of Rs and η.


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Fig. 5. Temperature-dependent electron concentration, electron mobility and resistivity for n-Si samples.

of MoS2 is a negligible contribution to Rs and the resistance of n-Si is a great contribution to Rs. It is suggested that the value of Rs noticeably increases as temperature increases, because of the dominance of the electron mobility in the n-Si layer. In order to derive the value of the barrier height (qϕ) at the MoS2/nSi interfaces at 300 K, we assume that the depletion layer is formed not in the MoS2 layer but in the n-Si layer. Almost the entire depletion layer extends into the low-doped region of the junction. The carrier density in the MoS2 layer (2.0 × 1020 cm−3) is much higher than that in the n-Si layer (2.7 × 1015 cm−3). Therefore, the depletion region takes place only n-Si side. According to TE theory, the Is–qϕ characteristic is given by [17,35] qφ Is ¼ Sa A T 2 exp − kT

ð2Þ

turned on and off. The observed IP decay is attributed to the induced carrier capture/recombination by states at the MoS2/n-Si interface that lead to the formation of the interfacial inhomogeneous barriers. The I–V characteristics of MoS2/n-type Si devices were measured in the temperature range from 80 to 300 K in steps of 20 K. The temperature dependence of forward-bias I–V characteristics can be explained on the basis of the TE theory by considering the presence of the inhomogeneous barriers at the MoS2/n-Si interfaces. This observation clearly shows the importance of the spatially inhomogeneous barriers on the device performance. In addition, the electron mobility in the n-Si layer has a noticeable effect on the series resistance for MoS2/n-Si devices. It is shown that the value of the electron mobility in the n-Si layer decreases as temperature increases, which leads to the significantly increased value of the series resistance of MoS2/n-Si devices. This study represents

where A⁎ is the effective Richardson constant for n-Si samples (A⁎ = 114 A cm− 2 K− 2 [17]), and Sa is the Schottky contact area (Sa = 0.5 cm2). The derived value for qϕ is 0.63 eV. Fig. 6 shows the band alignment at the interface between n-Si and MoS2 at equilibrium at 300 K. 4. Conclusion This study details the CVD growth of MoS2 with few layers on the nSi substrate and the fabrication of a MoS2/n-Si device. For MoS2/n-Si devices, the behavior of the carrier transports and the responsivity to solar irradiation are studied. A MoS2/n-Si device exhibits stable rectification behavior. The fabricated MoS2/n-Si devices exhibit reversible switching between high and low current densities, when the simulated sunlight is

Fig. 6. The band alignment at the interface between n-Si and MoS2 at equilibrium at 300 K.

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a step forward toward the integration of MoS2 into existing Si technology for new generation electronic/optoelectronic devices. Acknowledgment The authors acknowledge the support of the Ministry of Science and Technology, Taiwan (Contract Nos. 103-2112-M-018-003-MY3 and 106-2112-M-018-001-MY3) in the form of grants. References [1] Z.M. Wang, MoS2: Materials, Physics, and Devices, Springer, Berlin, 2013. [2] L. Hao, Y. Liu, W. Gao, Z. Han, Q. Xue, H. Zeng, Z. Wu, J. Zhu, W. Zhang, Electrical and photovoltaic characteristics of MoS2/Si p-n junctions, J. Appl. Phys. 117 (2015) 114502. [3] A.D. Bartolomeo, Graphene Schottky diodes: an experimental review of the rectifying graphene/semiconductor heterojunction, Phys. Reports 606 (2016) 1–58. [4] R. Scheer, Activation energy of heterojunction diode currents in the limit of interface recombination, J. Appl. Phys. 105 (2009) 104505. [5] Y.H. Lee, X.Q. Zhang, W. Zhang, M.T. Chang, C.T. Lin, K.D. Chang, Y.C. Yu, J.T.W. Wang, C.S. Chang, L.J. Li, T.W. Lin, Synthesis of large-area MoS2 atomic layers with chemical vapor deposition, Adv. Mater. 24 (2012) 2320–2325. [6] N.R. Pradhan, D. Rhodes, Q. Zhang, S. Talapatra, M. Terrones, P.M. Ajayan, L. Balicas, Intrinsic carrier mobility of multi-layered MoS2 field-effect transistors on SiO2, Appl. Phys. Lett. 102 (2013) 123105. [7] P. Joensen, R.F. Frindt, S.R. Morrison, Single-layer MoS2, Mater. Res. Bull. 21 (1986) 457–461. [8] A. Schumacher, L. Scandella, N. Kruse, R. Prins, Single-layer MoS2 on mica: studies by means of scanning force microscopy, Surf. Sci. Lett. 289 (1993) L595–L598. [9] Y.H. Lee, L. Yu, H. Wang, W. Fang, X. Ling, Y. Shi, C.T. Lin, J.K. Huang, M.T. Chang, C.S. Chang, M. Dresselhaus, T. Palacios, L.J. Li, J. Kong, Synthesis and transfer of singlelayer transition metal disulfides on diverse surfaces, Nano Lett. 13 (2013) 1852–1857. [10] A. Castellanos-Gomez, M. Barkelid, A.M. Goossens, V.E. Calado, H.S.J. van der Zant, G.A. Steele, Laser-thinning of MoS2: on demand generation of a single-layer semiconductor, Nano Lett. 12 (2012) 3187–3192. [11] W.K. Hoffman, Thin films of molybdenum and tungsten disulphides by metal organic chemical vapour deposition, J. Mater. Sci. 23 (1988) 3981–3986. [12] X. Ma, M. Shi, Thermal evaporation deposition of few-layer MoS2 films, Nano-Micro Lett. 5 (2013) 135–139. [13] M. Amani, M.L. Chin, A.G. Birdwell, T.P. O'Regan, S. Najmaei, Z. Liu, P.M. Ajayan, J. Lou, M. Dubey, Electrical performance of monolayer MoS2 field-effect transistors prepared by chemical vapor deposition, Appl. Phys. Lett. 102 (2013) 193107. [14] J. Zhang, H. Yu, W. Chen, X. Tian, D. Liu, M. Cheng, G. Xie, W. Yang, R. Yang, X. Bai, Scalable growth of high-quality polycrystalline MoS2 monolayers on SiO2 with tunable grain sizes, ACS Nano 8 (2014) 6024–6030. [15] A. Sanne, R. Ghosh, A. Rai, H.C.P. Movva, A. Sharma, R. Rao, L. Mathew, S.K. Banerjee, Top-gated chemical vapor deposited MoS2 field-effect transistors on Si3N4 substrates, Appl. Phys. Lett. 106 (2015), 062101. [16] M.L. Tsai, S.H. Su, J.K. Chang, D.S. Tsai, C.H. Chen, C. Wu, L.J. Li, L.J. Chen, J.H. He, Monolayer MoS2 heterojunction solar cells, ACS Nano 8 (2014) 8317–8322. [17] Y.J. Lin, B.C. Huang, Y.C. Lien, C.T. Lee, C.L. Tsai, H.C. Chang, Capacitance–voltage and current–voltage characteristics of Au Schottky contact on n-type Si with a conducting polymer, J. Phys. D. Appl. Phys. 42 (2009) 165104. [18] Y.J. Lin, J.J. Zeng, H.C. Chang, Temperature-dependent electrical properties for graphene Schottky contact on n-type Si with and without sulfide treatment, Appl. Phys. A Mater. Sci. Process. 118 (2015) 353–359. [19] Ş. Karataş, Ş. Altındal, Analysis of I–V characteristics on Au/n-type GaAs Schottky structures in wide temperature range, Mater. Sci. Eng. B 122 (2005) 133–139. [20] Y.J. Lin, C.H. Ruan, Y.J. Chu, C.J. Liu, F.H. Lin, Correlation between interface modification and rectifying behavior of p-type Cu2ZnSnS4/n-type Si diodes, Appl. Phys. A Mater. Sci. Process. 121 (2015) 103–108.

[21] T.H. Su, Y.J. Lin, Interface modification of MoS2/SiO2 leading to conversion of conduction type of MoS2, Appl. Sur. Sci. 387 (2016) 661–665. [22] Y.J. Lin, T.H. Su, SiO2 substrate passivation effects on the temperature-dependent electrical properties of MoS2 prepared by the chemical vapor deposition method, J. Mater. Sci. Mater. Electron. 28 (2017) 10106–10111. [23] Y.J. Lin, T.H. Su, S.M. Chen, Behavior of carrier transports and their sensitivity to solar irradiation for devices that use MoS2 that is directly deposited on Si using the chemical vapor method, J. Mater. Sci. Mater. Electron. 28 (2017) 14430–14435, https:// doi.org/10.1007/s10854-017-7304-9. [24] C. Muratore, J.J. Hu, B. Wang, M.A. Haque, J.E. Bultman, M.L. Jespersen, P.J. Shamberger, M.E. McConney, R.D. Naguy, A.A. Voevodin, Continuous ultra-thin MoS2 films grown by low-temperature physical vapor deposition, Appl. Phys. Lett. 104 (2014) 261604. [25] M.R. Laskar, L. Ma, S. Kannappan, P.S. Park, S. Krishnamoorthy, D.N. Nath, W. Lu, Y. Wu, S. Rajan, Large area single crystal (0001) oriented MoS2, Appl. Phys. Lett. 102 (2013) 252108. [26] Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, F. Boey, H. Zhang, Single-layer semiconducting nanosheets: High-yield preparation and device fabrication, Angew. Chem. Int. Ed. 50 (2011) 11093–11097. [27] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150. [28] H. Li, G. Lu, Z. Yin, Q. He, H. Li, Q. Zhang, H. Zhan, Optical identification of single- and few-layer MoS2 sheets, Small 8 (2012) 682–686. [29] M. Berg, K. Keyshar, I. Bilgin, F. Liu, H. Yamaguchi, R. Vajtai, C. Chan, G. Gupta, S. Kar, P. Ajayan, T. Ohta, A.D. Mohite, Layer dependence of the electronic band alignment of few-layer MoS2 on SiO2 measured using photoemission electron microscopy, Phys. Rev. B 95 (2017) 235406. [30] K. Dolui, I. Rungger, S. Sanvito, Origin of the n-type and p-type conductivity of MoS2 monolayers on a SiO2 substrate, Phys. Rev. B 87 (2013) 165402. [31] T.H. Su, Y.J. Lin, Effects of nitrogen plasma treatment on the electrical property and band structure of few-layer MoS2, Appl. Phys. Lett. 108 (2016), 033103. [32] R. Addou, S. McDonnell, D. Barrera, Z. Guo, A. Azcatl, J. Wang, H. Zhu, C.L. Hinkle, M. Quevedo-Lopez, H.N. Alshareef, L. Colombo, J.W.P. Hsu, R.M. Wallace, Impurities and electronic property variations of natural MoS2 crystal surfaces, ACS Nano 9 (2015) 9124–9133. [33] R. Addou, L. Colombo, R.M. Wallace, Surface defects on natural MoS2, ACS Appl. Mater. Interfaces 7 (2015) 11921–11929. [34] S. McDonnell, R. Addou, C. Buie, R.M. Wallace, C.L. Hinkle, Defect-dominated doping and contact resistance in MoS2, ACS Nano 8 (2014) 2880–2888. [35] A.D. Bartolomeo, G. Luongo, F. Giubileo, N. Funicello, G. Niu, T. Schroeder, M. Lisker, G. Lupina, Hybrid graphene/silicon Schottky photodiode with intrinsic gating effect, 2D Mater. 4 (2017), 025075. [36] Y.J. Lin, W.M. Cho, H.C. Chang, Y.H. Chen, Interface modification and trap-type transformation in Al-doped CdO/Si-nanowire arrays/p-type Si devices by nanowire surface passivation, Curr. Appl. Phys. 15 (2015) 213–218. [37] Y.J. Lin, J. Luo, H.C. Chang, Electronic transport and Schottky barrier heights of p-type CuAlO2 Schottky diodes, Appl. Phys. Lett. 102 (2013) 193511. [38] A.D. Bartolomeo, F. Giubileo, G. Luongo, L. Iemmo, N. Martucciello, G. Niu, M. Fraschke, O. Skibitzki, T. Schroeder, G. Lupina, Tunable Schottky barrier and high responsivity in graphene/Si-nanotip optoelectronic device, 2D Mater. 4 (2017), 015024. [39] R.T. Tung, Electron transport at metal-semiconductor interfaces: general theory, Phys. Rev. B 45 (1992) 13509. [40] S. Zhu, R.L. Van Meirhaeghe, C. Detavernier, F. Cardon, G.P. Ru, X.P. Qu, B.Z. Li, Barrier height inhomogeneities of epitaxial CoSi2 Schottky contacts on n-Si (100) and (111), Solid State Electron. 44 (2000) 663–671. [41] S.S. Li, The dopant density and temperature dependence of electron mobility and resistivity in n-type silicon (U.S. DEPARTMENT OF COMMERCE), National Bureau of Standards, Washington, 1977. [42] S.S. Li, W.R. Thurber, The dopant density and temperature dependence of electron mobility and resistivity in n-type silicon, Solid State Electron. 20 (1977) 609–616. [43] P. Norton, T. Braggins, H. Levinstein, Impurity and lattice scattering parameters as determined from Hall and mobility analysis in n-type silicon, Phys. Rev. B 8 (1973) 5632–5653.