Au Schottky contact to Ga-polarity GaN grown on Si (111) substrate

Journal of Alloys and Compounds 705 (2017) 782e787

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Influence of rapid thermal annealing on electrical and structural properties of Pd/Au Schottky contact to Ga-polarity GaN grown on Si (111) substrate Varun Singh Nirwal a, Koteswara Rao Peta a, *, V. Rajagopal Reddy b, Moon Deock Kim c a b c

Department of Electronic Science, University of Delhi South Campus, Benito Juarez Road, New Delhi 110-021, India Department of Physics, Sri Venkateswara University, Tirupati 517-502, India Department of Physics, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-765, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 December 2016 Received in revised form 1 February 2017 Accepted 14 February 2017 Available online 17 February 2017

We studied the effect of high temperature rapid thermal annealing on the electrical and structural properties of Pd/Au Schottky contact to Ga-polarity GaN grown by MBE on p-Si substrate. Currentvoltage (I-V), capacitance-voltage (C-V), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements are performed for the electrical and structural characterization of the Schottky diode. It has been observed that there is a significant improvement in barrier height and ideality factor with reduction in leakage current upon annealing. The estimated Schottky barrier height (4B0) for the as-deposited contact is 0.61eV (I-V) and 0.94 eV (C-V). While, the extracted barrier height for 700
C annealed contact is improved to 0.74 eV (I-V) and 1.09 eV (C-V). In addition, the surface state density is calculated using C-V and it is found that there is ten time reduction in surface state density for 700
C annealed Pd/Au Schottky contact compared to the as-deposited Schottky contact to semiconductor. X-ray photoelectron spectroscopy (XPS) depth profile results showed that there is out diffusion of Ga into metal film which may have formed metal-gallide phases for the annealed Schottky contacts that was confirmed by X-ray diffraction (XRD) results. It implies a reduction in nitrogen related vacancies and dangling bonds associated with GaN, which could be the reason for increase in the Schottky barrier height. Moreover, the surface morphology of the contacts is analysed by atomic force microscopy (AFM) and it is found that the surface roughness of Schottky contact does not degraded upon annealing. This indicates that the contacts were thermally stable during annealing. © 2017 Elsevier B.V. All rights reserved.

Keywords: III-V semiconductors Schottky barrier diode XPS XRD AFM

1. Introduction Recent advances in growing high quality gallium nitride (GaN) have opened a vast area of applications in electronic and optoelectronic devices. The electronic properties of GaN like wide band gap, high carrier mobility and large breakdown electric field make it a suitable choice for the devices such as metal semiconductor field effect transistors (MESFETs), high electron mobility transistors (HEMTs) and light emitting diodes (LEDs) [1e3]. All these devices require high quality ohmic as well as Schottky contacts. The quality of Schottky contacts relies on the surface properties of semiconductor and interfacial features between the metal and

* Corresponding author. E-mail addresses: [email protected], [email protected] (K.R. Peta). http://dx.doi.org/10.1016/j.jallcom.2017.02.162 0925-8388/© 2017 Elsevier B.V. All rights reserved.

semiconductor. GaN suffers from defects like dislocations, stacking faults and cracking due to unavailability of lattice matched substrates. These defects propagate to and relaxed at the surface. This leads to deterioration in surface quality of GaN. The electrically active surface states of GaN ultimately deteriorate the performance of Schottky contact while producing large leakage current. Schottky contacts to GaN grown on Si provide large range of applications because Si is a cheaper substrate than sapphire and it also allows large size wafer fabrication. However, the lattice mismatch is more for GaN grown on Si than on sapphire substrate, so a larger leakage current is expected. Thus, fabrication of high quality Schottky contact to GaN/Si with high Schottky barrier height and reduced leakage current is really a great challenge. Researchers reported many techniques to enhance the performance of GaN based Schottky contacts [4e7]. Lee et al. [7]

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investigated the properties of Ni/Au contacts on (NH4)2Sx treated nGaN and reported that the surface state density was reduced. Other than such chemical methods, annealing of the Schottky contacts was also proved to be a very good technique for reducing the leakage current and to improve electrical parameters of Schottky contacts [4,8e10]. For example, Reddy et al. [8] reported the effect of annealing on electrical performance of Pd/Ru Schottky contact to GaN grown on sapphire by metal organic chemical vapour deposition (MOCVD). They reported that the electrical parameters of Schottky contact was improved upon annealing due to the formation of Ga-Ru and Ga-Pd interfacial phases at metal and GaN interface. Wang et al. [10] reported the effects of annealing on Schottky barrier height of the Pt/n-GaN Schottky contacts. They found that the Schottky barrier height (SBH) of Pt/n-GaN Schottky diode increased after annealing at 500
C and decreased after annealing above 600
C. These changes were due to the modified surface morphology of Pt films and variation of nonstoichiometric defects at the interface vicinity [10]. Kim et al. [4] studied post annealing effects on the Ni/AlGaN/GaN Schottky contacts and reported that the electrically active states reduced at metal and AlGaN interface for annealed diodes which led to decrease in the reverse leakage current density. Thus, thermal annealing is a good method to improve the quality of metal/semiconductor interface. In this work, our aim is to investigate the effect of high temperature rapid thermal annealing on the electrical performance and interfacial structural properties of Pd/Au Schottky contact to Gapolarity GaN grown by MBE on p-type Si substrate using currentvoltage (I-V), capacitance-voltage (C-V), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM) characterization at various annealing temperatures. 2. Experimental details Unintentionally doped Ga-polarity GaN films (1 mm of thickness) grown by plasma assisted molecular beam epitaxy (PAMBE) on p-type Si substrate was used in this study. Prior to the deposition, the Si(111) substrates were cleaned with HF solution for 1 min to remove the native oxides, and dried with N2 gas and immediately transferred to the growth chamber. Then, de-oxidation was
performed by heating the substrate at 830 C followed by Al soaking to avoid the formation of SiN nuclear layer and then 75 nm AlN buffer layer was grown sequentially at low (770
C) and higher temperature (830
C). Finally, GaN film of thickness 1 mm was grown at 780
C. The polarity of the GaN layer was examined by wet etching experiment using KOH aqueous solution (10 mol) at 90
C for 60 s. For Schottky barrier diode (SBDs) fabrication the GaN film was cut into three small pieces of size 0.5  0.5 cm2 which were cleaned by ultrasonication with trichloroethylene, acetone and methanol for 5 min each and dried using N2 gas. Then, the GaN films were immediately loaded in vacuum chamber for the ohmic contact formation. The ohmic metal scheme for n-GaN, Ti/Al (20 nm/100 nm) was formed on the portion of GaN sample using ebeam evaporation system (12A4D) in the vacuum pressure of 9  106 mbar. For good ohmic properties, the samples were annealed at 600
C for 2 min in N2 atmosphere. Finally, Pd/Au (40 nm/20 nm) Schottky metal contacts were deposited through a circular dot mask of 0.7 mm diameter. Annealing of Pd/Au Schottky contacts was done by using a rapid thermal annealing system in the presence of N2 ambient at the temperatures of 600
C and 700
C. IV measurement was done using Keithley source meter (model no. 2400) while C-V measurements were performed using a DLS-83D (SEMILAB) system operating at 1 MHz frequency on Schottky diodes. XPS and XRD measurements (Rigaku Ultima IV) were carried out to investigate interfacial reactions between the metal and GaN. The surface morphology of Schottky contacts was analysed using

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AFM (NT-MDT, solver NEXT). 3. Results and discussion The experimental semi-logarithmic I-V characteristics of fabricated Au/Pd/Ga-polarity GaN SBDs is shown in Fig. 1. For diode with as-deposited Schottky contact the reverse bias leakage current measured to be 6.84  105 A at -1 V, which reduced to 1.92  105A and 7.21  107 A at -1 V for 600
C and 700
C annealed samples respectively. It is found that the rectification ratio at 1 V is improved by almost two orders for 700
C annealed sample. The experimental I-V plot is used to calculate the electrical parameters of SBDs using thermionic emission theory. According to thermionic emission theory, the current flow in the forward bias region for V > 3kT/q is governed by the equation given as [11,12].

h n q o i ðV  IRs Þ  1 J ¼ Js exp nkT

(1)

where Js is the saturation current density which is given as

  q4BO Js ¼ A* T 2 exp kT

(2)

here k is Boltzmann constant and A* is effective Richardson constant (26.4 Acm2K2 at room temperature). Using equation (2), the SBH can be given as

4BO ¼

 * 2 kT A T ln q Js

(3)

The ideality factor (n) of Schottky diode is calculated using following equation

n¼

q kT



dV dðln JÞ

 (4)

The estimated electrical parameters of SBDs are given in Table 1. The SBH increased from 0.61 eV (as-deposited) to 0.74 eV (700
C) upon annealing and ideality factor decreased from 1.7 to 1.2 as

Fig. 1. Semi-logarithmiccurrent-voltage (I-V)characteristics of as-deposited and rapid thermal annealed Pd/Au Schottky contacts toGa-polarity GaN grown on p-type Si substrate.

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Table 1 The leakage current -1 V, Schottky barrier height (4), ideality factor (n), effective carrier density (Nd) and surface state density (Ns) for Au/Pd/Ga-polarity GaN/Si Schottky diode. Diode

Leakage current at 1 V (A)

4I-V (eV)

n

4Norde (eV)

4C-V (eV)

Nd (cm3)

Ns (cm2)

as-deposited 600
C 700
C

6.84  105 1.92  105 7.21  107

0.61 0.67 0.74

1.7 1.4 1.2

0.61 0.66 0.75

0.94 1.03 1.09

1.04  1015 8.89  1014 7.85  1014

4.11  1010 2.54  1010 8.92  109

given in Table 1. This indicates the electrical properties are improved after annealing. The calculated value of ideality factor deviated from unity which suggests that the current transport mechanism departs from ideal thermionic emission theory, which doesn't include current components like thermionic field emission, generation recombination and image force lowering [11,12]. The presence of interface states like surface states on GaN also modifies the current predicted by thermionic emission theory [13,14]. Norde method was also employed to calculate the SBH [15]. Using Norde method reliable value of SBH can be obtained even with the effect of series resistance, which affects the calculation of SBH by I-V method. Norde plots for the Schottky contact on as-deposited and annealed Schottky contacts is given in Fig. 2. Norde function F(V) is given by the relation as

FðVÞ ¼

  V kT IðVÞ  ln 2 q AA* T 2

(5)

Fig. 2 shows the plot of F(V) versus V for as-deposited and annealed Pd/Au Schottky contacts. The SBH is estimated using the following formula

4B0 ¼ FðVÞmin þ Vmin 

kT q

(6)

where Vmin is the minimum value of voltage corresponding to minimum value F(Vmin). The estimated values of SBH by Norde method is given in Table 1. The calculated values are in good agreement with those calculated from I-V method. The reverse bias C-V characteristics of as-deposited and

Fig. 2. The function F(V) obtained from I-V characteristics versus V plot of as-deposited and rapid thermal annealed Pd/Au Schottky contacts to Ga-polarity GaN.

annealed Au/Pd/GaN/Si Schottky diode is shown in Fig. 3. These measurements are performed at a frequency of 1 MHz. The C-V curves can be characterized using the relation as [11,12].

  d C 2 2 ¼ dV qεs A2 Nd

(7)

where єs is the permittivity of GaN and is given by єs ¼ 9.5є0, є0 is the permittivity in vacuum, k is Boltzmann constant, Nd is carrier concentration and A is area of Schottky contact. Using equation (7), the value of carrier concentration is calculated using slope of 1/C2 versus V plot. The calculated carrier concentration for the asdeposited Schottky diode is 1.04  1015cm3 which decreases slightly to 8.89  1014 cm3 and 7.85  1014 cm3 after annealing at 600
C and 700
C respectively. The value SBH can be estimated using C-V characteristics by the following relation

q4B0 ¼ qVbi þ qVn

(8)

where Vn¼(kT/q)ln (Nc/Nd), is the depth of Fermi level below conduction band. Nc is the effective density of states in conduction band given by the relation Nc ¼ 2 (2Pm*kBT/h2)3/2, where m* ¼ 0.22mo. The numerical value of Nc is 2.38  1018 cm3 [16] for GaN at room temperature. Vbi is built in potential calculated using the intercept at V axis in Fig. 3. The value of Vbi is 0.75 V, 0.82 V and 0.87 V for the as-deposited, 600
C and 700
C annealed Schottky contacts respectively, while the corresponding barrier heights are calculated as 0.94 eV, 1.03 eV and 1.09 eV. Thus, there is a

Fig. 3. Capacitance e voltage (C2versus V) characteristics of Au/Pd/Ga-polarity GaNSchottky diode for as-deposited and annealed samples.

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considerable improvement in the Schottky barrier height upon annealing. The SBH calculated from C-V method is higher than those calculated by I-V method which is attributed to the different nature of these two measuring techniques. The current in I-V measurement is dominated by current flow through lower SBH patches, tunnelling current and generation recombination current paths are also responsible for lowering the SBH, while C-V measurement gives an arithmetic average of SBH [16e18]. The prevalence of Schottky barrier height inhomogeneity at metal semiconductor interface is responsible for the difference in barrier height calculated from I-V and C-V measurement. In addition, C-V measurement is less sensitive to the potential fluctuations on length scale of less than space charge width [16,17]. The SBH for a Schottky contact on n-type semiconductor is also given as

q4B0 ¼ q4m  q4s þ qVn

(9)

where q4m is the work function of Schottky metal, and q4s is the surface work function of n-type semiconductor given as

q4s ¼ qVn þ c þ qjVs j

(10)

where c is electron affinity and qjVsj is surface band bending energy. Substituting the value of q4s from equation (10)e(9), we get

q4B0 ¼ q4m  c  qjVs j

(11)

The work function of Pd, q4m ¼ 5.2eV and electron affinity,

c~4.1 eV for GaN [19], a simplified relation for surface bend bending in our case is given as

qjVs j ¼ 1:1  q4B0

(12)

The surface state density (Ns) is related to surface band bending energy by the relation given as [7].

qjVs j ¼

ðqNs Þ2 2εs Nd

(13)

The value of Ns calculated using equation (13) is 4.11  1010 cm2 for the as-deposited Schottky diode while it reduced to 2.54  1010 cm2 and 8.92  109 cm2 after annealing at 600
C and 700
C respectively. So, there is a reduction in the surface state density for the Schottky contact annealed at 700
C. The presence of surface states on GaN is related to point defects like nitrogen related vacancies, oxygen interstitials and Ga dangling bonds [20,21]. These surface states also lead to increase in the ideality factor of Schottky contacts on GaN [21]. Annealing is done in the presence of N2 gas, so that nitrogen dissociation should not take place on annealing. Annealing causes a disturbance in periodicity of crystal lattice and there will be a rearrangement of atoms in the semiconductor. This produces new periodicities on the surface of semiconductor and the interface quality with Schottky metal may differ for the three samples. The interfacial features of as-deposited and annealed Schottky contacts are investigated using XPS depth of profile analysis and XRD studies. To study the possible interfacial solid-state reactions that occurred between the metal atoms and semiconductor, X-ray photoelectron spectroscopy depth of profile was performed on the Au/Pd/Ga polarity GaN/Si structure before and after annealing at 600
C and 700
C temperatures. The as-deposited sample, Fig. 4 (a), exhibits relatively sharp interfaces attributed to no significant interfacial reactions. The depth of profile of sample annealed at 600
C shows that a small amount of Pd inter-diffuse and small amount of Ga out-diffuses towards metal layer (Fig. 4 (b)). While for the sample annealed at 700
C, a significant amount of Ga out-

Fig. 4. XPS depth of profile for (a) as-deposited (b) 600
C (c) 700
C annealedAu/Pd/ Ga-polrityGaNSchottky diode grown on Si substrate.

diffused into the metal film as shown in Fig. 4 (c). This may have caused the formation of metal-gallide phases at the interface. Further, it is noted that a small amount of gold inter-diffused for the

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sample annealed at 700
C, revealing that a possible reaction between gold and Ga. In addition, it is observed that there is no clear evidence for out-diffusion of Si into the metal layer. Further, XRD measurement is carried to find the possible

interfacial phases. The XRD spectra of as-deposited, 600
C and 700
C annealed Schottky contact is shown in Fig. 5. The XRD pattern of as-deposited Schottky contact (Fig. 5(a)) shows GaN (0 0 2) (0 0 4) peaks along with a peak of Ga2Pd5 (0 0 2). The XRD spectra of 600
C annealed sample (Fig. 5 (b)) shows additional peaks such as Ga3Pd5 (201), GaPd2 (112) and Au2Ga (511) peaks along with GaN characteristic peaks. However, there are extra gallide peaks observed in 700
C (Fig. 5(c)) annealed Schottky contact which were identified as Ga3Pd5 (2 0 1), Au7Ga2 (1 1 2),

Fig. 5. XRD spectra for Pd/Au Schottky contact (a) as deposited (b) 600
C (c) 700
C annealed samples.

Fig. 6. Atomic Force Microscopy (AFM) images for (a) as-deposited, (b) 600
C and (c) 700
C annealed Pd/Au Schottky contact on Ga-polarity GaN.

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GaPd2 (1 1 2), Ga2Pd5 (7 1 0) and GaPd2 (0 2 1). These extra peaks show the difference in interfacial features of Schottky contact on Ga-polarity GaN before and after annealing. The formation of extra gallide phases at metal/GaN at high temperature treatment of the GaN films might have led to the outflow of Ga atoms. Outflow of Ga atoms from GaN upon annealing was already reported [8]. Fig. 6 shows the surface morphology for as-deposited and annealed Pd/Au contact on Ga-polarity GaN/Si. The surface morphology of the as-deposited Pd/Au Schottky contact (Fig. 6(a)) is found to be smooth with a root-mean-square (rms) roughness 1.932 nm. The surface of Pd/Au Schottky contacts becomes smooth after annealing. It is found that the rms roughness for the 700
C annealed contact is 0.873 nm. The improvement in the electrical parameters such SBH and leakage current is also related to the formation of gallide phases upon annealing as evident from XRD measurement (Fig. 5). Gallium vacancies were induced near the interface due to the formation of gallide phases which lead to increase of barrier height upon annealing at 700
C. Akkaya et al. [9] and Reddy et al. [8] also attributed to the increase of barrier height upon annealing to the formation of gallide phases at metal/GaN interface. The reduction in the value of leakage current is due to the increase in barrier height upon annealing, as suggested by Wang et al. [10]. The reduction in surface state density is attributed to formation of Ga-M bonds to reduce nitrogen related vacancies and Ga dangling bonds on the surface of GaN. Thermal annealing of GaN also reduces dislocation density which also contributes reduction of surface states density at the interface [22]. A large number of surface states may accommodate additional charges resulting from intimate contact between semiconductor and metal without change in Fermi level of semiconductor leading to the problem of Fermi level pinning. The SBH value for Schottky contacts on annealed GaN is 1.09 eV which is close to the Schottky limit for Pd metal which suggests that the Fermi level pinning is reduced for annealed Schottky contact. 4. Conclusions I-V and C-V measurements are used to determine the electrical properties Pd/Au/Ga-polarity GaN/Si SBDs. To correlate the electrical properties with structural properties, XPS and XRD measurements were employed. There is significant improvement in electrical parameters (leakage current, SBH and ideality factor) upon rapid thermal annealing of diodes. In addition, it is identified that, there is significant reduction in the surface state density for 700
C annealed Pd/Au/Ga-polarity GaN structure. The enhanced electrical properties of SBDs is due to the formation of metallic gallide phases at interface of Pd/Au and Ga-polarity GaN and attributed to decrease in nitrogen related vacancies, as evident from the XRD and XPS results. AFM results showed fairly smooth surface of Pd/Au contact to Ga-polarity GaNwith rms roughness of 0.873 nm annealed at 700
C.

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Acknowledgement This work is supported by funding from Research and Development Grants 2013e2014 & 2014e2015, University of Delhi (DRCH/R&D/2013-14/4155 & RC/2014/6820). We also acknowledge the UGC, for supporting the fellowship. References [1] P. Mukhopadhyay, U. Banerjee, A. Bag, S. Ghosh, D. Biswas, Influence of growth morphology on electrical and thermal modeling of AlGaN/GaN HEMT on sapphire and silicon, Solid-State Electron. 104 (2015) 101e108. [2] C. Lee, W. Lu, E. Piner, I. Adesida, DC and microwave performance of recessedgate GaN MESFETs using ICP-RIE, Solid-State Electron. 46 (2002) 743e746. [3] S.P. DenBaars, D. Feezell, K. Kelchner, S. Pimputkar, C.-C. Pan, C.-C. Yen, S. Tanaka, Y. Zhao, N. Pfaff, R. Farrell, Development of gallium-nitride-based light-emitting diodes (LEDs) and laser diodes for energy-efficient lighting and displays, Acta Mater. 61 (2013) 945e951. [4] H. Kim, M. Schuette, H. Jung, J. Song, J. Lee, W. Lu, J.C. Mabon, Passivation effects in Ni/AlGaN/GaN Schottky diodes by annealing, Appl. Phys. Lett. 89 (2006) 3516. [5] A. Kumar, T. Singh, M. Kumar, R. Singh, Sulphide passivation of GaN based Schottky diodes, Curr. Appl. Phys. 14 (2014) 491e495. [6] F. Iucolano, F. Roccaforte, F. Giannazzo, V. Raineri, Influence of hightemperature GaN annealed surface on the electrical properties of Ni/GaN Schottky contacts, J. Appl. Phys. 104 (2008), 093706e093706-093707. [7] C.-T. Lee, Y.-J. Lin, D.-S. Liu, Schottky barrier height and surface state density of Ni/Au contacts to (NH4) 2Sx-treated n-type GaN, Appl. Phys. Lett. 79 (2001) 2573. [8] N.N.K. Reddy, V.R. Reddy, C.-J. Choi, Influence of rapid thermal annealing effect on electrical and structural properties of Pd/Ru Schottky contacts to n-type GaN, Mater. Chem. Phys. 130 (2011) 1000e1006. [9] A. Akkaya, L. Esmer, B.B. Kantar, H. Çetin, E. Ayyıldız, Effect of thermal annealing on electrical and structural properties of Ni/Au/n-GaN Schottky contacts, Microelectron. Eng. 130 (2014) 62e68. [10] J. Wang, D. Zhao, Y. Sun, L. Duan, Y. Wang, S. Zhang, H. Yang, S. Zhou, M. Wu, Thermal annealing behaviour of Pt on n-GaN Schottky contacts, J. Phys. D Appl. Phys. 36 (2003) 1018. [11] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, John Wiley & Sons, 2006. [12] E.H. Rhoderick, R. Williams, Metal-semiconductor Contacts, Clarendon Press, Oxford, 1988. [13] H. Card, E. Rhoderick, Studies of tunnel MOS diodes I. Interface effects in silicon Schottky diodes, J. Phys. D Appl. Phys. 4 (1971) 1589. [14] N. Varun Singh, P. Koteswara Rao, Behavior of temperature dependent electrical properties of Pd/Au Schottky contact to GaN grown on Si substrate by MBE, Mater. Res. Express 3 (2016) 125901. [15] H. Norde, A modified forward I-V plot for Schottky diodes with high series resistance, J. Appl. Phys. 50 (1979) 5052e5053. [16] N. Yıldırım, K. Ejderha, A. Turut, On temperature-dependent experimental IV and CV data of Ni/n-GaN Schottky contacts, J. Appl. Phys. 108 (2010), 114506e114501. [17] J. Sullivan, R. Tung, M. Pinto, W. Graham, Electron transport of inhomogeneous Schottky barriers: a numerical study, J. Appl. Phys. 70 (1991) 7403e7424. [18] M. Sawada, T. Sawada, Y. Yamagata, K. Imai, H. Kimura, M. Yoshino, K. Iizuka, H. Tomozawa, Electrical characterization of n-GaN Schottky and PCVD-SiO 2/ n-GaN interfaces, J. Cryst. growth 189 (1998) 706e710. [19] J. Pankove, H. Schade, Photoemission from GaN, Appl. Phys. Lett. 25 (1974) 53e55. [20] F.-H. Wang, P. Krüger, J. Pollmann, Electronic structure of 1 1 GaN (0001) and GaN (0001) surfaces, Phys. Rev. B 64 (2001) 035305. [21] S. Jung, S.-N. Lee, H. Kim, Surface states and carrier transport properties at semipolar (11e22) n-type GaN planes, Appl. Phys. Lett. 102 (2013) 151603. [22] W. Li, A. Li, Post growth thermal annealing of GaN grown by RF plasma MBE, J. Cryst. growth 227 (2001) 415e419.