p-InP enhanced Schottky barrier contacts

Thin Solid Films 616 (2016) 145–150

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Electronic properties of Al/MoO3/p-InP enhanced Schottky barrier contacts Jun Chen ⁎, Jiabing Lv, Qingsong Wang School of Electronic and Information Engineering, Soochow University, Suzhou 215006, China

a r t i c l e

i n f o

Article history: Received 26 June 2016 Received in revised form 5 August 2016 Accepted 7 August 2016 Available online 08 August 2016 Keywords: Al/MoO3/p-InP Schottky barrier heights Gaussian distribution Interfacial insulator layer

a b s t r a c t The Al/MoO3/p-InP metal/insulator/semiconductor (MIS) and Al/p-InP metal/semiconductor (MS) Schottky Barrier Diodes (SBDs) have been fabricated to confirm Schottky barrier heights (SBHs) enhancement by inserting an ultrathin insulator layer between the MS contact. The current-voltage (I-V) and capacitance-voltage (C-V) measurements have been performed in the temperature range of 310–400 K. The modified Richardson plot gives the Richardson constant of 66.16 and 59.07 A cm−2 K−2 for Al/MoO3/p-InP MIS SBDs and Al/p-InP MS SBDs, respectively, which are both close to the theoretical value known for p-InP (60 A cm−2 K−2). The SBHs for these two diodes have decreased with decreasing temperature while the ideality factor values have increased with decreasing temperature, obeying the barrier height Gaussian distribution model based on thermionic emission current theory. In addition, the experimental barrier height shows an obvious enhancement and ideality factor does not show a considerable increase, verifying that MoO3 is a good candidate for the interfacial insulator layer. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Metal/semiconductor (MS) contacts have attracted a considerable research interest over the past decades because of their attractive applications in integrated microelectronics technology such as phototransistor, field effect transistors, photovoltaics and optoelectronic devices [1–3]. In general, MS contacts perform either Ohmic contacts or Schottky characteristics relying mainly on the interfacial characterization [4–5]. In order to obtain a qualified Schottky Barrier Diodes (SBDs) with good rectifying behavior, a thin interfacial insulator layer is commonly introduced to be inserted between the MS contact. Converting the MS contact to metal/insulator/semiconductor (MIS) contacts will lead to an enhanced Schottky barrier height (SBH) due to the tunneling barrier of the interfacial layer [6–8]. Especially for the compound semiconductor such as InP, low SBH will cause serious leakage current problem owing to Fermi level pinning [9–10]. It is found that, the properties of the MIS contacts are usually considered to be crucial elements in evaluating the efficiency, performance and reliability of the devices [11–12]. While, the most important electronic property for the MIS contacts is barrier height (BH), which will be deviated from ideal thermionic emission (TE) theory by the interfacial insulating layer through affecting the space charge region of the semiconductor [13–15]. Thus, barrier inhomogeneities occur on the MIS interfaces,

⁎ Corresponding author. E-mail address: [email protected] (J. Chen).

http://dx.doi.org/10.1016/j.tsf.2016.08.019 0040-6090/© 2016 Elsevier B.V. All rights reserved.

having a profound impact on the current conduction mechanisms across the contact. For deeply understanding the nature of barrier inhomogeneities at the Schottky contact, extensive researches about the interfacial layers sandwich between the metal and InP, such as Rhodamine [16], POxNyH [17], pyronine [18], and PVC [19] have been performed through the analysis of the temperature-dependent current-temperature (I-V) and capacitance-voltage (C-V) characterization based on SBDs. Molybdenum oxide (MoOx) with a wide band gap from 2.75 to 2.95 eV [20] and perfect thermal stability has been a promising candidate for the interfacial layer to obtain an enhanced BH Schottky contacts. Furthermore, MoOx is widely used as hole injection layer (HIL) to improve the carriers injection efficiency for OLEDs [21–23], and several researchers have demonstrated that the thin MoO3 interfacial films is indispensable in the inorganic/organic hybrid upconverters integrated by InP based compound semiconductor substrate and OLEDs [24–26]. However, electronic properties of MoO3/p-InP contact have never been investigated. The main purpose of this study is to clarify the SBHs enhancement by exploring comparatively the fabricated Al/p-InP SBDs in the range of 310–400 K with and without the insulator layer (MoO3 layer). For better understanding the electronic parameters of Al/MoO3/p-InP Schottky contacts, the temperature dependence behavior of the I-V and C-V characteristics were systematically investigated and compared with the Al/p-InP SBDs. Zero-bias BH φap and ideality factor n have been obtained by TE theory, while φap increases and n decreases with increasing temperature, which can be successfully interpreted by assuming the spatial SBHs inhomogeneities at the interfaces. The Gaussian

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distribution model was employed to describe the barrier inhomogeneities, which was verified by the mean BH φb0 and standard deviation σs0. 2. Device fabrication details For fabricating the Al/MoO3/p-InP SBDs, a p-InP (100) wafer with carrier concentration of 1.2 × 1018 cm−3 was employed as the substrate in this work. Firstly, the p-InP substrate was ultrasonically cleaned using acetone, isopropanol, and deionized (DI) water, sequentially. The wafer was then etched in HF:H2O (1:10) solution for 60 s to remove surface native oxide, the following step was one more rinse in DI-water and dry in flowing N2. For realizing Ohmic contacts, Ti (20 nm)/Pt (30 nm)/Au (150 nm) were evaporated on bottom surface of the device at 8 × 10−6 Torr, the sample was then thermally annealed at 300 °C for 60 s under N2 atmosphere. After Ohmic contacts deposition, the thin MoO3 layer with diameter of 1 mm was grown on the p-InP substrate via thermal evaporation with the vacuum of 1 × 10− 6 Torr. Then, 100 nm thick aluminum (Al) circular dots rectifying contact with the diameter of 1 mm were deposited onto the MoO3 surface with the vacuum of 2 × 10− 6 Torr. MoO3 and Al dots were all defined by photolithography and formed by lift-off process. The Al/p-InP SBDs were fabricated with the same process of Al/MoO3/p-InP SBDs, except for the MoO3 deposition step. I-V and C-V measurements were performed by Agilent 4156C and Agilent E4980A, respectively. VDC1100B temperature controller was used to control the device temperature. 3. Result and discussion 3.1. I-V measurements The typical I-V characteristics of Al/MoO3/p-InP and Al/p-InP SBDs are presented in Fig. 1(a) and (b). The forward current all grow on an exponential curve with applied bias from 0.1 to 0.3 V, which increase with the increase of temperature. Such good rectifying behavior of these SBDs can be interpreted by employing TE theory. The forward current is defined as [27]

 qV −qV 1− exp I ¼ I 0 exp nkT kT

ð1Þ

where V is forward-bias voltage, q is electronic charge, k is Boltzmann's constant, T is absolute temperature and I0 is the reverse saturation current, which can be written as   qφap I0 ¼ AA T 2 exp − kT

Fig. 1. I-V characteristics of (a) Al/MoO3/p-InP and (b) Al/p-InP SBDs with temperature varying from 310 K to 400 K (inset is the structure of the SBDs).

increasing temperature for both Al/MoO3/p-InP and Al/p-InP SBDs, which is mainly due to barrier inhomogeneities. According to Biyikli et al. [28], the BH is dependent on the current transport mechanism across

ð2Þ

where A is SBDs area, φap is apparent or zero-bias BH, A* is the Richardson constant (60 A cm−2 K−2). φap can be given from Eq. (2) as φap ¼

kT AA T 2 ln I0 q

! ð3Þ

n is the ideality factor, which is expressed as n¼

q dV kT dð lnIÞ

ð4Þ

3.2. Variations of zero-bias BH φap and ideality factor n Fig. 2 shows the variations of φap and n vs. experimental temperature for the Al/MoO3/p-InP and Al/p-InP SBDs. An obvious enhanced φap and larger n can be found for Al/MoO3/p-InP SBDs compared with the Al/p-InP SBDs. In addition, φap increases and n decreases with

Fig. 2. The plot of zero-bias BH φap and ideality factor n vs. temperature for the Al/MoO3/pInP and Al/p-InP SBDs in the temperature range of 310–400 K (the closed triangles represent the SBDs with MoO3 layer, and the closed squares represent the SBDs without MoO3 layer.)

J. Chen et al. / Thin Solid Films 616 (2016) 145–150

147

the MS and MIS Schottky contacts. The current transport across the diode will flow preferentially through the lower barriers in the potential distribution. Electrons experience lower BHs at lower temperature, i.e., the dominant BHs for carriers transport are lower. As temperature increases, increasing carriers will have enough energy to overcome higher BHs. Thus, the dominant BHs will increase [29–30]. Our values of the zero-bias BH and ideality factor are close to the values obtained from the other metal/p-InP contacts [31] and metal/insulator/p-InP contacts [32]. Meanwhile, the temperature-dependent behavior of n can be interpreted using thermionic field emission [33]. The n of the Al/MoO3/p-InP SBDs in our experiment are larger than that of Al/p-InP SBDs, which is mainly attributed to the existence of the interfacial insulator layer (MoO3 layer) [34–36]. Due to barrier inhomogeneities, BH will increase and ideality factor will decrease with increasing temperature. Experimentally observed zero-bias BH φap vs. ideality factor n for Al/MoO3/p-InP and Al/p-InP SBDs are plotted in Fig. 3. It is found that zero-bias BH φap increases linearly with the decrease of ideality factor n, which is mainly due to the lateral BHs inhomogeneities at the interfaces in real SBDs [37–38]. Thus, the homogeneous value of BH can be obtained as 2.01 V with n = 1 for Al/MoO3/p-InP SBDs and 1.4753 V with n = 1 for Al/p-InP SBDs via an extrapolation of the experimental plot φap vs. n.

3.3. Barrier inhomogeneities Fig. 4 shows the experimental φap and n− 1 − 1 vs. 1/2kT for the Al/MoO3 /p-InP and Al/p-InP SBDs. The decrease of φap can be interpreted by assuming a spatial BH Gaussian distribution with the zero-bias mean BH φb0 and zero-bias standard deviation σs0 , σs0 is usually too small to be neglected [39–40]. Furthermore, the linearity of φap vs. 1/2kT curve can be found in Fig. 4, which obeys the following expression [41]

φap ¼ φbo −

qσ 2s0 2kT

Fig. 4. Curves of ideality factor n−1 − 1 and zero-bias barrier height φap vs. 1/2kT for (a) Al/MoO3/p-InP and (b) Al/p-InP SBDs.

ð5Þ

The deviation from classical TE theory can be explained by a spatial fluctuation of the BH at interface. The plot of experimental zero-bias BH φap vs. 1/2kT in Fig. 4(a), (b) give σs0 = 167 mV, φb0 = 1.31 V for Al/MoO3/p-InP SBDs and σ0 = 113 mV, φb0 = 1.25 V for Al/p-InP SBDs, respectively. The value of σs0 (167, 113 mV) is not small compared to the mean value of φb0 (1.31, 1.25 V), verifying the BH Gaussian distribution for the two diodes [42].

The relationship between experimental ideality factor n and temperature for the Al/MoO3/p-InP SBDs can be expressed as [11]
 1 qρ −1 ¼ ρ2 − 3 2kT n

where n is the experimental ideality factor, ρ2 and ρ3 are temperaturedependent bias coefficients, which correspond to the voltage deformation of SBHs distribution. Based on Eq. (6), plot of experimental n−1 − 1 vs. 1/2kT should be a straight line, where ρ2 = − 0.40105 V and ρ3 = − 0.01029 V for Al/MoO3/p-InP SBDs and ρ2 = − 0.4035 V and ρ3 = −0.04585 V for Al/p-InP SBDs, respectively. In Fig. 4, good linear relationship between n−1 − 1 and 1/2kT can be observed, suggesting that n does indeed account for the voltage deviation of Gaussian distribution for the both SBHs [43]. According to the theory of SBHs Gaussian distribution, we can modify the Richardson plot with Eq. (2) and Eq. (5) as follows [36]
ln

Fig. 3. The plot of the zero-bias BH φap vs. ideality factor n for the Al/MoO3/p-InP and Al/p-InP SBDs.

ð6Þ


2 2 q σ s0 qφ − ¼ ln ðAA Þ− bo 2 kT T 2k T 2 I0

2

ð7Þ

the modified Richardson plots of ln(I0/T2) − (q2σ2s0/2k2T2) vs. 1/kT for the Al/MoO3/p-InP and Al/p-InP SBDs are showed in Fig. 5. Straight lines are the least-square fitting curve. The slope yields mean BH φb0 = 1.31 V (Al/MoO3/p-InP SBDs), 1.25 V (Al/p-InP SBDs) and the intercepts are proportional to the modified effective Richardson constant A*. A* is obtained as 66.16 A cm−2 K− 2 for Al/MoO3/p-InP SBDs, 59.07 A cm− 2 K−2 for Al/p-InP SBDs, respectively. It can be found that the value of φb0 = 1.31 and 1.25 V compared to the values

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ln(I0/T2) vs. 1000/T is usually employed to determine the values of χ1/2 σ, and χ1/2σ is obtained as 11.82 from the intercept of leastsquare fitting curve in Fig. 6, which can be expressed as
ln

I0

T2



¼ ln ðAA Þ−

qφbo −χ 1=2 δ kT

ð9Þ

According to Eq. (8), the temperature-dependent flat-band BH φbf for the Al/MoO3/p-InP SBDs can be obtained at various temperature in Fig. 7(a). For Al/p-InP MS SBDs, according to Werner et al. [45], the flat-band BH φbf can be calculated from the experimental ideality factor and zero-bias apparent BH using Eq. (10)

 kT NV φbf ¼ nφap −ðn−1Þ Ln NA q

Fig. 5. Modified Richardson plots of ln(I0/T2)−(q2σ2s0/2k2T2) vs. 1/kT for the Al/MoO3/p-InP and Al/p-InP SBDs.

of φb0 = 1.31 and 1.25 V obtained by plotting φap vs. 1/2kT for the two diodes, respectively. Meanwhile, the extracted A* = 66.16 A cm−2 K−2 and 59.07 A cm− 2 K− 2 are both comparable to the theoretical value (60 A cm−2 K−2) for p-InP.

ð10Þ

where NA is the concentration of acceptor (1.2 × 1018 cm−3 for p-InP at 310 K), NV is the state density on valence-band edge, NV is usually taken as 1.08 × 1019 cm−3 for p-InP at 310 K [31], and the results are showed in Fig. 7(b). Unlike the behavior of φap, the φbf for Al/MoO3/p-InP MIS and Al/p-InP MS SBDs decrease with increasing temperature due to

3.4. Flat-band BH Flat-band BH φbf is usually known for reflecting the true quantity of the Al/MoO3/p-InP SBDs, which is obtained on flat-band condition. In this case, due to no electric field across the contacts, the semiconductor bands are flat, which eliminated the effect of image force lowering that would affect the I-V characteristics and removed the influence of lateral inhomogeneity. For Al/MoO3/p-InP MIS SBDs, according to Altındal et al. [44], the temperature-dependent ideality factor n should also be taken into conclusion for the saturation current. Thus, the modified reverse saturation current is described as
   qφbf I 0 ¼ AA T 2 exp −χ 1=2 δ exp − nðT ÞkT

ð8Þ

where φbf is the flat-band BH, n(T) is temperature-dependent ideality factor and χ1/2σ is hole tunneling factor. The Richardson plot of

Fig. 6. Richardson plot ln(I0/T2) vs. 1000/T for the Al/MoO3/p-InP SBDs.

Fig. 7. The plot of zero-bias BH φap (triangles), flat-band BH φbf (squares) and capacitance BH φb(C−V) (circles) vs. temperature for (a) Al/MoO3/p-InP and (b) Al/p-InP SBDs. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

J. Chen et al. / Thin Solid Films 616 (2016) 145–150

the negative temperature coefficient of InP band gap [45], and φbf can be described as φbf ¼ φbf ðT ¼ 0 KÞ−αT

ð11Þ

where φbf ðT ¼ 0 KÞ is the value of flat-band BH at 0 K, α is temperature coefficient. φbf ðT ¼ 0 KÞ = 1.31 and 1.48 V and α = 5.48 × 10−4 and 1.11 × 10−3 V/K for Al/MoO3/p-InP MIS and Al/p-InP MS SBDs are determined from the intercept and slope of the fitting line in Fig. 7. 3.5. C-V measurements The plot of experimental reverse capacitance C−2 with applied reverse bias V over the temperature range 310–390 K for Al/MoO3/p-InP and Al/p-InP SBDs are shown in Fig. 8. These data were measured under the frequency of 300 kHz. According to Altındal [44], we can express the capacitance in Schottky contacts as 1 C

2

¼

!


2 2

ε s qN A A

V bi −

kT −V q

149

measurements is called capacitance BH φb(C ‐ V), which can be given as φb(C ‐ V) = Vbi + Vp, where Vp = kT/q ln (NV/NA). Similar to the behavior of φbf, φb(C ‐ V) decreases with increasing temperature. And φb(C ‐ V) can be described as φb ðC‐VÞ ¼ φb ðC‐VÞðT ¼ 0 KÞ−βT, where φb ðC‐VÞ ðT ¼ 0 KÞ is the value of the φb(C − V) at 0 K, β is temperature coefficient. The red straight lines in Fig. 7 are the least-square fitting curve of the calculated data, yielding φb ðC−VÞðT ¼ 0 KÞ = 1.71 and 1.67 V and β = 1.79 × 10−3 and 1.95 × 10−3 V/K. As clearly observed in Fig. 7, φb(C ‐ V) is higher than φap at the same temperature for these two diodes. This is mainly attributed to the effect of interfacial charges distribution, image force lowering and Schottky barrier inhomogeneity in IV measurement, which will reduce BH compared to those data obtained from C-V measurement [46–47]. Based on previous reports [48–49], capacitance C is just dependent on the band bending, thus, φb(C ‐ V) is equivalent to φb0 and can be given as follows φb ðC−V Þ ¼ φb0

ð13Þ

so, we get φb(C ‐ V) − φap from Eq. (5) and (12) as

 ð12Þ

where εS is the permittivity for InP (εS = 12.5ε0), Vbi is built-in voltage, which can be determined by extrapolating the plot of C−2-V to x-axis if the values of εsqNAA2 and kT/q are given. The BH calculated from C−2-V

φb ðC−V Þ−φap ¼

qσ 2 ðT Þ qσ 2s0 qσ σ þ ≈ 2kT k 2kT

ð14Þ

according to Eq. (14), the plot of φb(C ‐ V) − φap vs. 1/2kT for the Al/ MoO3/p-InP and Al/p-InP SBDs must show two straight lines, which can be observed in Fig. 9. The straight lines are the least-square fitting curve whose intercept and slope are proportional to the standard deviation σ2s0 and σσ. σσ represents the temperature-dependence of σs0. σs0 and σσ are extracted to be 255 mV and 1.461 × 10− 4 V2 K− 1 for Al/ MoO3/p-InP SBDs, 266 mV and 1.598 × 10−4 V2 K−1 for Al/p-InP SBDs, respectively. σs0 = 255 and 266 mV are both close to the value of σs0 (167, 113 mV) obtained by plotting φap vs. 1/2kT and also cannot be neglected, indicating the spatial barrier inhomogeneities at the interfaces of the Schottky contacts. [48–49]. 4. Conclusion The fabricated I-V and C-V characteristics of the Al/MoO3/p-InP enhanced Schottky barrier SBDs and Al/p-InP SBDs have been measured in the temperature of 310–400 K, and analyzed by the model of Gaussian distribution based on TE theory. Both φap and n are temperaturedependent, while φap increases and n decreases with the increase of temperature, which is mainly due to barrier inhomogeneities at the interfaces. Furthermore, the modified Richardson plot considering

Fig. 8. C−2-V characteristics (300 kHz) of (a) Al/MoO3/p-InP and (b) Al/p-InP SBDs at various temperatures.

Fig. 9. The plot of experimental φb(C‐V)−φb0 vs. 1/2kT for the Al/MoO3/p-InP and Al/pInP SBDs.

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Gaussian distribution of SBDs gives the Richardson constant of 66.16 and 59.07 A cm−2 K−2 for Al/MoO3/p-InP MIS and Al/p-InP MS SBDs, respectively, which are close to the theoretical value for p-InP (60 A cm−2 K−2). Acknowledgment This project is partially supported by the National Natural Science Foundation of China (No. 61307044), the Natural Science Foundation of Jiangsu Province of China (No. BK20130321), the open project of Key Laboratory of Infrared Imaging Materials and Detectors, Chinese Academy of Sciences (No. IIMDKFJJ-15-06), the Research Fund for the Doctoral Program of Higher Education of China (No. 20133201120009), Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China and the Research Innovation Program for College Graduates of Jiangsu Province (SJLX15-0600). References [1] A. Thankappan, S. Divya, A.K. Augustine, C.P. Girijavallaban, P. Radhakrishnan, S. Thomas, V.P.N. Nampoori, Highly efficient betanin dye based ZnO and ZnO/Au Schottky barrier solar cell, Thin Solid Films 583 (2015) 102–107. [2] L.M. Cai, L.J. Wang, J. Huang, B.L. Yao, K. Tang, J.J. Zhang, K.F. Qin, J.H. Min, Y.B. Xia, Preparation of polycrystalline CdZnTe thick film Schottky diode for ultraviolet detectors, Vacuum 88 (2013) 28–31. [3] D. Korucu, S. Duman, Current-voltage-temperature characteristics of Au/p-InP Schottky barrier diode, Thin Solid Films 531 (2013) 436–441. [4] E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, second ed. Clarendon Press, Oxford, 1988. [5] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [6] T.S. Shafai, Schottky barrier characterization of lead phthalocyanine/aluminium interfaces, Thin Solid Films 3 (2008) 1200–1203. [7] K.R. Rajesh, C.S. Menon, Study on the device characteristics of FePc and FePcCl organic thin film Schottky diodes: influence of oxygen and post deposition annealing, J. Non Cryst. Solids 4 (2007) 398–404. [8] O. Pakma, N. Serin, T. Serin, S. Altindal, The double Gaussian distribution of barrier heights in Al/TiO2/p-Si (metal-insulator-semiconductor) structures at low temperatures, J. Appl. Phys. 104 (2008) 014501-1–014501-6. [9] T. Sugino, H. Ito, J. Shirafui, Barrier height enhancement of InP Schottky junctions by treatment with photo-decomposed PH3, Electron. Lett. 26 (1990) 1750–1751. [10] J. Dow, R. Allen, Surface defects and Fermi-level pinning in InP, J. Vac. Sci. Technol. 20 (1982) 659–661. [11] J.H. Werner, H.H. Guttler, Barrier inhomogeneities at Schottky contacts, J. Appl. Phys. 69 (1991) 1522–1533. [12] A.M. Cowley, S.M. Sze, Surface states and barrier height of metal-semiconductor systems, J. Appl. Phys. 36 (1965) 3212–3220. [13] T. Kampen, A. Schuller, D.R.T. Zahn, B. Biel, J.E. Ortega, R. Pérez, F. Florès, Schottky contacts on passivated GaAs(100) surfaces: Barrier height and reactivity, Appl. Surf. Sci. 234 (2004) 341–348. [14] A.R.V. Roberts, D.A. Evans, Modification of GaAs Schottky diodes by thin organic interlayers, Appl. Phys. Lett. 86 (2005) 072105-1–072105-3. [15] A. Bolognesi, A.D. Carlo, P. Lugli, T. Kampen, D.R.T. Zahn, Experimental investigation and simulation of hybrid organic/inorganic Schottky diodes, J. Phys. Condens. Matter 15 (2003) S2719–S2728. [16] Ö. Güllü, S. Aydoğan, A. Türüt, Electronic parameters of high barrier Au/Rhodamine101/n-Inp Schottky diode with organic interlayer, Thin Solid Films 520 (2012) 1944–1948. [17] D.T. Quan, H. Hbib, High barrier height Au/n-type InP Schottky contacts with a POxNyHz interfacial layer, Solid State Electron. 36 (1993) 339–344. [18] M. Soylu, B. Abay, Y. Onganer, The effects of annealing on Au/pyronine-B/MD n-InP Schottky structure, J. Phys. Chem. Solid 71 (2010) 1398–1403. [19] A. Umapathi, V.R. Reddy, Effect of annealing on the electrical and interface properties of Au/PVC/n-InP organic-on-inorganic structures, Microelectron. Eng. 114 (2014) 31–37. [20] N. Miyata, T. Suzuki, R. Ohyama, Physical properties of evaporated molybdenum oxide films, Thin Solid Films 282 (1996) 218–222. [21] T. Matsushima, Y. Kinoshita, H. Murata, Formation of Ohmic hole injection by inserting an ultrathin layer of molybdenum trioxide between indium tin oxide and organic hole-transporting layers, Appl. Phys. Lett. 91 (2007) 2535041–253504-3.

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