Hydrogen influence on the electrical and optical properties of ZnO thin films grown under different atmospheres

Thin Solid Films 556 (2014) 18–22

Contents lists available at ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Hydrogen influence on the electrical and optical properties of ZnO thin films grown under different atmospheres I. Lorite ⁎, J. Wasik 1, T. Michalsky, R. Schmidt-Grund, P. Esquinazi Institut für experimentelle Physik II, Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnéstraße 5, 04103 Leipzig, Germany

a r t i c l e

i n f o

Article history: Received 17 April 2013 Received in revised form 17 December 2013 Accepted 17 December 2013 Available online 2 January 2014 Keywords: Zinc oxide Pulsed laser deposition Thin films Optical properties Electrical properties

a b s t r a c t In this work we studied the changes of the electrical and optical properties after hydrogen plasma treatment of polycrystalline ZnO thin films grown under different atmosphere conditions. The obtained results show that the gas used during the growth process plays an important role in the way hydrogen is incorporated in the films. The hydrogen doping can produce radiative and non-radiative defects that reduce the UV emission in ZnO films grown in oxygen atmosphere but it passivates defects created when the films are grown in nitrogen atmosphere. Impedance spectroscopy measurements show that these effects are related to regions where hydrogen is mostly located, either at the grain cores or boundaries. We discuss how hydrogen strongly influences the initial semiconducting behavior of the ZnO thin films. © 2014 Elsevier B.V. All rights reserved.

1. Introduction ZnO is a well known II–IV transparent semiconductor with a wide direct band gap of ~3.37 eV at room temperature and an exciton binding energy of 60 meV [1]. As grown, ZnO is usually an n-type semiconductor due to the presence of native donors such as oxygen vacancies and Zn interstitials [2]. Its electrical and optical properties make ZnO an interesting material for different applications such as short-wavelength light-emitter, transparent conducting electrode and piezoelectric material, varistors, transducers, gas sensors, and catalysts [3,4]. To tune the optical and electrical properties ZnO is intentionally doped with metals such as Al, In, and Ga [5,6]. Heavily doped n-type ZnO shows Fermi level degeneration and thus behaves metallic along with its high transparency [7]. Such transparent and conductive materials are outstanding candidates for application as transparent conducting electrodes in displays, and solar cells, as a relative inexpensive alternative to indium doped tin oxide, SnO [8,9]. Hydrogen related complexes have been studied as shallow donors in bulk ZnO and attributed as source of unintentionally n-type conductivity [10]. Exhaustive research has been developed to improve the properties of ZnO by adding hydrogen (H) in the ZnO structure. For this purpose different annealing treatment under H atmosphere [11] and ⁎ Corresponding author. Tel.: +49 314973256; fax: +49 3419732769. E-mail address: [email protected] (I. Lorite). 1 Erasmus Student on leave from Faculty of Applied Physics and Mathematics, Technical University of Gdańsk, Narutowicza 11/12, 80 952 Gdańsk, Poland. 0040-6090/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.12.057

hydrogen plasma, H+, treatments have been performed [12]. It has been observed that H-doped ZnO shows an improved electrical conductivity [11] and optical transparency caused by H incorporation as shallow donor and by interaction with defects forming complexes such as VZn–H2 [13,14]. Hydrogen incorporation near the surface can even trigger magnetic order in ZnO single crystals as observed by magnetization and transport measurements [11]. The reduction of the sample size up to the micro- and nano-scale can change considerably the properties of ZnO in comparison to its bulk counterparts. In particular thin films with polycrystalline structure are interesting due to the possible variation of properties related to interfaces or grain boundaries. The last one plays an important role for the electrical transport since a significant amount of ad-atoms can be accumulated directly at the grain boundary or within a range of a few atomic layers close to it [15], apart from lattice defects in the “bulk” of the crystallites. Due to the junction between the different grains, the so-called grain boundary, a double Schottky barrier is formed. Its height depends on the defects or dopants which lay mainly at the intergrain. It can provide different pathways of conduction from grain to grain boundary or a contribution of both of them [16]. For investigation of these effects and to clarify the possible impacts of defects on the conductivity properties, impedance spectroscopy is a useful technique because it can reveal the contribution of different electrical conduction paths in these structures, i.e. within the grains or at the grain boundaries, at secondary phases [17]. In the present work, we report on the changes of the optical and transport properties in H+ treated ZnO thin films. To study the role of

I. Lorite et al. / Thin Solid Films 556 (2014) 18–22

ZnO films were grown on 6 × 6 mm r-plane Al2O3 substrates by Pulsed Lased Deposition (PLD) using a KrF excimer laser and a sintered polycrystalline ZnO target. The purity of the ZnO powder used for the preparation of the PLD target was 5 N. The film thickness of ~100 nm was controlled by the number of laser pulses with an energy of ~ 1 J cm−2. The films were grown either in oxygen or nitrogen partial pressures of 1.25 × 10− 2 mbar, from a basal vacuum of 10−7 mbar, which samples are called hereafter as ZnO:O and ZnO:N, respectively. The crystal structure was characterized by X-ray diffraction (XRD) θ–2θ scans using a Cu–Kα source. After deposition the ZnO films were exposed to remote hydrogen DC plasma produced in a parallel-plate configuration for 1 h obtaining the samples ZnO:O:H and ZnO:N:H. The power to produce the H+-plasma was kept at ~7.5 W with a voltage drop of ~300 V between the two parallel and circular plates with a diameter of 2 cm. The samples were mounted on a vertical heater block placed ∼ 100 mm downstream from the plasma with a bias voltage of ~300 eV and held at a fixed temperature of 100 °C for 1 h. The applied temperature was chosen to ensure hydrogen diffusion of several micrometers [18]. The energy used for H+ implantation into the films is provided by a metallic Cu plate placed right behind the heater block where the sample was located. A total current of ∼60 μA through the sample was measured during the experiment. Taking into account the dimensions of the sample the amount of H+ implanted was ∼1016 atoms/cm2. The pressure in the hydrogen plasma chamber during the treatment was maintained at ∼ 5 × 105 Pa with a flow of gas. The gas used to obtain the plasma was an Ar/H2 90%/10% mixture. Impedance spectroscopy was carried out with an Agilent 4294A precision impedance analyser within the frequency range 40 Hz–2 MHz. ZView® software was employed to model the Nyquist diagrams and obtain the different contributions to the total resistance, i.e. grain and/or grain boundary. Electrical resistance measurements were performed using a Keithley 2182A nanovoltmeter and a Keithley 6221 current source. Electrical contacts were done with clenched indium directly to the ZnO thin film which provide ohmic contact. The resistance was measured in plane two-probe configuration. Photoluminescence at room temperature was excited by the 325 nm line of a He–Cd laser operating in a continuous-wave mode, spectrally dispersed by a 320 mm monochromator and detected by a Peltier-cooled CCD camera.

Intensity (Arb.Units)

2

ZnO:N

ZnO:O 30

40

50 60 2θ (°)

55.0

55.5

80

ZnO:O

56.0

56.5

57.0

57.5

58.0

2θ (°) Fig. 1. X ray diffraction pattern of ZnO:O and ZnO:N in a restricted angle region (110 reflection). The inset shows the diffraction peaks of ZnO (110) and r-Saphire.

grown films and also between both films. It is worthy to highlight the increase of the UV luminescence for the ZnO:N:H thin film, I′1/I′2 N 1, and the decrease, I′1/I′2 b 1, observed for ZnO:O:H after the H+ treatment. Hydrogen implantation can produce different effects: formation of non-radiative complexes by H/defect interaction, VZn–H2 [23], which produce an increase of UV emission, and/or formation of new radiative defects which can quench the UV emission [24]. If the proportion of Vzn is large the formation of VZn–H2 complexes can be larger than the formation of radiative defects. Therefore, the final effect is an increase of

(a)

ZnO:N ZnO:N:H

I1

I2

400

450

500

550

Wavelength (nm)

3. Results and discussion

ZnO:O ZnO:O:H

(b) Intensity (Arb. Units)

Fig. 1 shows the XRD pattern of the as grown ZnO:O and ZnO:N thin films. The thin films are polycrystalline and show wurtzite structure highly textured in the reflection direction (110) corresponding to the c-axis parallel to the substrate plane. The inset shows the r-sapphire diffraction peaks in addition to ZnO diffraction peaks are found in the range 25°–65°. Within experimental error no other peaks were observed, which let us rule out the existence of large amounts (N 0.1%) of secondary phases. The slight shift of the diffraction peak towards lower diffraction angles of the ZnO:N film with respect to that of the ZnO:O film can be related to the incorporation of nitrogen into the ZnO film [19]. Fig. 2 shows the room temperature photoluminescence of the asgrown and H+ treated thin films. Each film shows two distinct emission bands, one labeled I1 at ~375 nm attributed to the recombination of the free excitons [20,21], and one broad labeled I2 at 450–575 nm. This socalled green luminescence band is attributed to intra-band gap defect levels as Zn vacancies (Vzn) and/or O vacancies (VO) [22]. While the as-grown films show a similar relative intensity ratio of I1/I2, this differs considerably after hydrogen plasma treatment regarding that of the as-

70

ZnO:N

Intensity (Arb. Units)

2. Experimental details

Intensity (Arb.Units)

the polycrystallinity and/or of the introduced defects in thin films, we have prepared films at different ambient conditions during growth with subsequent H+ treatments.

19

´

I2

´

I1 400

450

500

550

Wavelenght (nm) Fig. 2. Photoluminescence of ZnO grown in: a) Nitrogen (ZnO:N and ZnO:N:H), b) Oxygen (ZnO:O and ZnO:O:H).

20

I. Lorite et al. / Thin Solid Films 556 (2014) 18–22

the UV emission as observed for ZnO:N:H. The decrease of the UV emission observed for the ZnO:O:H film, however must be related to a larger creation of radiative defects [25] than that of VZn–H complex, likely due a smaller proportion of VZn than in Zn:N:H. This is in agreement with a larger formation and stabilization of VZn in N condition [26]. Fig. 3 shows the Nyquist diagram of the impedance spectroscopy analysis of the different thin films at room temperature. Here the Brick layer model was used. Such a model is normally employed for polycrystalline structures [15]. In general, two different resistance/capacitance (R/C) contributions in series must be considered: the grain core and the grain boundary. The general equation for the equivalent circuit, due to the different contributions, is written as:  −1  −1 ′ ″ Z
¼ Z þ iZ ¼ R0 þ 1=Rgb þ iωCgb þ 1=Rg þ iωCg where R0 is the total resistance due to external component as wiring or double layer due to the metallic contacts and the thin film, Rgb is the grain boundary resistance, and Cgb is the grain boundary capacitance, which dominates in the low frequency regime. Finally, Rg and Cg are the resistance and capacitance, respectively, from the grain interior contribution that dominates at higher frequencies. Fig. 3(a) and (b) show the Nyquist diagram for the as-grown ZnO thin films. The results can be well modeled by two overlapping semicircles (two R/C circuits in series) with different values of resistance and capacitance, see Table 1. Those two semi-arcs are due to two different contributions, grain and grain boundary. Those processes are given at different frequencies, t = wC, thus their convolution produces a depression in the observed arc instead being a perfect semiarc related to a single contribution. After the H+ treatment, there are clear changes in the impedance response of the ZnO thin films, see Fig. 3(c) and (d). The Nyquist diagrams are well modeled with the contribution of one R/C and two R/C equivalent circuits in series for the ZnO:N:H and ZnO: O:H, respectively.

Table 1 Values of Cg, Cgb, Rg, and Rgb for the equivalent circuits. ZnO:N(1), ZnO:N(2), ZnO:O(1), and ZnO:O(2) are the contributions to the measured resistance of different grains in the as-grown thin films. Thin film

Rgb (kΩ)

Cgb (nF)

(1)

ZnO:O ZnO:O(2) ZnO:O:H ZnO:N(1) ZnO:N(2) ZnO:N:H

4.5

92

1.2

2

Rg (kΩ)

Cg (pF)

480 137 5 77 8 –

4 0.3 8 4 0.8 –

Table 1 summarizes the values of the different resistance and capacitance contributions. The different conduction processes are related to the relaxation time τ = RC and thus directly related to the capacitance value. Typical ranges of capacitances are within ranges of 10− 11–10 − 12 F and 10− 9 –10− 10 F for grain core (bulk) and grain boundaries contributions, respectively [27]. The values obtained for the capacitances, of the equivalent circuit used for the non-treated thin films, indicate electrical transport mainly through the grain cores. The two different equivalent circuit contributions of the same order of magnitude used to fit the data of the untreated thin films point out the existence of different types of grains, e.g. different grain sizes. After H+ treatment, there is a clear change in the Nyquist diagram characteristics. The ZnO:O:H thin film presents two arcs. The related capacitances ~ 10− 10 F, at lower frequencies, and ~ 10− 12 F, at higher frequencies, indicate the contribution of the grain boundary and grain core, see Fig. 3(d) and Table 1. It is common that the resistance from the grain boundaries has a lower contribution to the final resistance than that of the grain core. This difference in the resistance has been previously attributed to the trapping of electrons by the localized states within or at the grain boundaries. These states can come from defects, dopants or impurities of atoms trapped at the interfaces, which can act as a donor and 10

3

(a)

(b)

2

-Z´´ (x103 Ω)

-Z´´ (x104 Ω)

8

2MHz 1

6

2MHz

4 2

40Hz

0 0

1

2

3

0 0 ,0

4

40Hz 0 ,2

0 ,4

R (x104 Ω)

0 ,8

1 ,0

1 ,2

1 ,4

R (x103 Ω)

5

6

(c) -Z´´ (x103 Ω)

3 2 1

40Hz

0 1

4 3

2MHz 2 1

2MHz 0

(d)

5

4

-Z´´ (x105 Ω)

0 ,6

2

3

4

R (x105 Ω)

5

6

40Hz

0 7

0

2

4

6

8

R (x103 Ω)

Fig. 3. Impedance spectroscopy Cole–Cole plots of: a) ZnO:N, b) ZnO:O, c) ZnO:N:H and d) ZnO:O:H thin films. Red lines are the fit to the experimental data (black point) following the equivalent circuits given in the insets.

I. Lorite et al. / Thin Solid Films 556 (2014) 18–22

increase the conductance [28]. In the H+ treated ZnO:O:H thin film the grain boundary and grain core have similar contributions. It can be due to a large contribution of the radiative and non-radiative defects, observed by PL, produced by the hydrogen implantation mainly in the grain core. On the other hand, the ZnO:N:H thin film presents a single arc related to the grain boundary contribution, see Fig. 3(c) and Table 1. Thus, for the ZnO:N:H thin film the Hrelated defects have a larger contribution to the grain boundaries than for the ZnO:O:H thin film. It can be related to the neutralization of the nitrogen by hydrogen to form neutral defect complexes in the ZnO grain cores [25]. This reduces the carrier density in the grain core. The decrease in the carrier density makes the grain core provide a negligible contribution to the conductance, to obtain finally a single contribution from the grain boundary as conduction process. Fig. 4 shows the temperature dependence of the resistance for the different ZnO thin films after and before H+ treatment within the range of 40 K–280 K. The as-grown ZnO thin films present semiconducting like behavior with negative temperature coefficient of the resistance (dR/dT b 0), see Fig. 4(a) and (b). The film ZnO:N has lower resistance than that ZnO:O. In spite of the lower quantity of defects observed by PL, there must be an enhancement of the carrier density due to the nitrogen doping and the existence of Zn vacancies [24,29], which reduces the resistivity in agreement with published results [30]. On the other hand, H+ treated thin films present a further, drastic reduction of the resistivity of several orders of magnitude; see Fig. 4(c) and (d). This change of the resistance has been previously assigned to the H incorporation to ZnO acting as n-type donor [31]. Two different temperature dependent behaviors have been obtained. The ZnO:O:H thin film shows a nearly linear behavior with negative slope, see Fig. 4(d), a behavior that might be related to the increase of inter grain defects [32]. On the other hand, the ZnO:N:H thin film shows a positive resistance coefficient at T N 120 K, indicating a metallic like behavior, but a negative one at lower temperatures, see Fig. 4(c). Such a value of the temperature for the minimum in resistance has also

been reported in doped ZnO [33] and InGaN alloys [34]. We note, however, that hydrogen-treated ZnO single crystals also showed a minimum in R(T) at T ~ 220 K, which depends on the amount of implanted hydrogen [12]. In that work the temperature dependence of the resistance and in particular the observed minimum could be well understood assuming the contribution in series of two different regions because the implanted H+ ions were not distributed in the whole ZnO single crystalline samples due to the limited diffusion of H. In this paper, it is claimed that the hydrogen-rich regions would behave metallic like with Rm(T) ~ T, embedded in a doped semiconducting matrix with resistance Rs(T) ~ exp(ΔE / 2kBT) and an activation energy ΔE. The metallic region contributes mainly at high temperatures and therefore it was unnecessary to assume a more complicated T-dependence that may be applicable at temperatures T b 100 K. However, in the sample ZnO:N:H the minimum resistance is observed at much lower temperatures and there is no clear evidence from impedance spectroscopy for the existence of two regions, one H richer than the other one. Due to the thickness of the film, it is therefore possible that the hydrogen diffuses in the whole sample and has an homogeneous influence in the thin film ZnO:N:H and thus a different interpretation must be given. The estimated carrier density is n = 2.6 × 1019 cm−3 and the calculated Fermi wavelength (λF = 2π / (3π2n)1/3) and the electronic mean free path, Λ = h/ρne2λF, i.e. Λ = 22 nm and λF = 6.8 nm are comparable. This fact makes possible and interpretation of the thin film behavior take into account quantum corrections and the equation describing the process is given by [35–37]    1=2 RðTÞ ¼ 1= c0 þ mT ;

where c0 is related to the residual resistivity R0(c0 = 1 / R0); mT1/2 describes the electron–electron interaction.

(a)

(b)

2

Resistance (x107 Ω)

Resistance (x105 Ω)

21

1

1

0.1 50

100

150

200

250

50

100

T (K)

200

250

200

250

T (K)

1.2

1.8

(c)

1.7

1.19

Resistance (x104 Ω)

Resistance (x103 Ω)

150

1.18 1.17 1.16

(d)

1.6 1.5 1.4 1.3 1.2

1.15 50

100

150

T (K)

200

250

50

100

150

T (K)

Fig. 4. Temperature dependence of the resistance of ZnO grown in: a) Nitrogen, b) Oxygen, c) ZnO grown in nitrogen and H+ treated and d) ZnO grown in oxygen and H+ treated.

22

I. Lorite et al. / Thin Solid Films 556 (2014) 18–22

An additional term was taken into account on the high temperature scattering, T2, [38] thus the final fitting equation is given by:   −1 1=2 2 RðTÞ ¼ 1= c0 þ mT þ bT : This equation appears to fit very well with the experimental data in the shown temperature range, see Fig. 4(c), with the following fittings parameters c 0 = (8.2 ± 0.1) × 10 − 4 Ω − 1 , m = (0.52 ± 0.01) × 10 − 5 Ω − 1 K − 1/2 and b = (1.21 ± 0.01) × 10− 3 Ω K− 2. Thus, the different growth conditions change the interaction between sample and H to provide a different behavior of the resistance as a function of temperature and provide the metallic response at high temperature. Further studies of the growth condition and H+ treatments have to be addressed for a complete understanding of the different behaviors observed due to the H+ incorporation. 4. Conclusions In summary, crystalline ZnO films were deposited on r-Al2O3 by PLD at different atmosphere conditions. The crystal size estimated through the c-axis parameter was found to be similar for both of the films. After hydrogen treatment a variation of the defect related emission was observed by photoluminescence. With the help of impedance spectroscopy we interpret the decreased UV emission of ZnO:O:H by radiative and non-radiative defects likely created in the grain core due to the hydrogen implantation. The increased UV emission of ZnO:N:H we relate to the interaction of the intra-band defects with hydrogen to become non-radiative. The H+ treatment causes a reduction of the film resistance due to the incorporation of hydrogen showing a metallic to semiconductor like transition at 120 K which depends on the growth parameters. References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [2] P.D. Yang, H.Q. Yan, S. Mao, R. Russo, J. Jonson, R. Saykally, Adv. Funct. Mater. 12 (2002) 323. [3] C. Klingshirn, ChemPhysChem 8 (2007) 782. [4] D. Barreca, D. Bekermann, E. Comini, A. Devi, R.A. Fischer, A. Gasparotto, C. Maccato, C. Sada, G. Sberveglieri, E. Tondello, Cryst. Eng. Comm. 12 (2010) 3419.

[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

[19] [20] [21] [22] [23] [24] [25] [26]

[27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

[37] [38]

K.T.R. Reddy, R.W. Miles, J. Mater. Sci. Lett. 17 (1998) 279. M. Chen, Z.L. Pei, X. Wang, C. Sun, L.S. Wen, J. Vac. Sci. Technol. A 19 (2001) 963. Q. Wan, J. Huang, A. Lu, T. Wang, Appl. Phys. Lett. 93 (2008) 103109. T. Minami, Semicond. Sci. Technol. 20 (2005) S35. Y. Liu, Y. Li, H. Zeng, J. Nanomater. (2013) 9. M.D. McCluskey, S.J. Jokela, Phys. B Condens. Matter (2007) 355. A. Prasad, A. Pandey, V.K. Kunapuli, P.L. Bergstrom, Y.K. Yap, J. Phys. Chem. C 116 (2012) 8210. M. Khalid, P. Esquinazi, Phys. Rev. B 85 (2012) 134424. A. Janotti, C.G. van de Walle, Nat. Mater. 6 (2007) 44. E.V. Lavrov, J. Weber, F. Borrnert, C.G. van de Walle, R. Helbig, Phys. Rev. B 66 (2002) 165205. C.W. Nan, S. Holten, R. Birringer, H. Gao, H. Kliem, H. Gleiter, Phys. Status Solidi A 164 (1997) R1. I. Lorite, L. Villaseca, P. Díaz-Carrasco, M. Gabás, J.L. Costa-Krämer, Thin Solid Films 548 (2013) 657. In: J.R. MacDonald (Ed.), Impedance Spectroscopy, Emphasizing Solid Materials and Systems, John Wiley & Sons, New York, 1987. K. Ipa, M.E. Overberg, Y.W. Heo, D.P. Norton, S.J. Pearton, C.E. Stutz, S.O. Kucheyev, C. Jagadish, J.S. Williams, B. Luo, F. Ren, D.C. Look, J.M. Zavada, Solid State Electron. 47 (2003) 2255. W.W. Liu, B. Yao, Z.Z. Zhang, Y.F. Li, B.H. Li, C.X. Shan, J.Y. Zhang, D.Z. Shen, X.W. Fan, J. Appl. Phys. 109 (2011) 093518. K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. I. Lorite, F. Rubio-Marcos, J.J. Romero, J.F. Fernández, Mater. Lett. 63 (2009) 212. U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005) 041301. J.K. Dangbégnon, K. Talla, J.R. Botha, Opt. Mater. 34 (2012) 920. J.-K. Lee, M. Natasi, D.W. Hamby, D.A. Luca, Appl. Phys. Lett. 86 (2005) 171102. H. Song, J.-H. Kim, E. Kim, H. Jaehwan, J. Hong, H. Chu, C. Lee, J. Korean Phys. Soc. 53 (2008) 2540. M. Khalid, M. Ziese, A. Setzer, P. Esquinazi, M. Lorenz, H. Hochmuth, M. Grundmann, D. Spemann, T. Butz, G. Brauer, W. Anwand, G. Fischer, W.A. Adeagbo, W. Hergert, A. Ernst, Phys. Rev. B 80 (2009) 035331. E. Chinarro, J.R. Jurado, F.M. Figueiredo, J.R. Frade, Solid State Ionics 160 (2003) 161. G. Blatter, F. Greuter, Phys. Rev. B 33 (6) (1986) 3952. Hui Chen, Shulin Gu, Kun Tang, Shunmin Zhu, Zhenbang Zhu, Jiandong Ye, Rong Zhang, Youdou Zheng, J. Lumin. 131 (2011) 1189–1192. K. Mahmood, S.B. Park, J. Cryst. Growth 347 (2012) 104. Chris G. Van de Walle, Phys. Rev. Lett. 85 (2000). P.J. Cote, L.V. Meisel, Phys. Rev. Lett. 39 (1977) 102. S.R. Meher, R.M. Naidu, K.P. Biju, A. Subrahmanyam, M.K. Jain, Appl. Phys. Lett. 99 (2011) 082112. V. Bhosle, A. Tiwari, J. Narayan, J. Appl. Phys. 100 (2006) 033713. W. Noun, B. Berini, Y. Dumont, P.R. Dahoo, N. Keller, J. Appl. Phys. 102 (2007) 063709. B.L. Altshuler, A.G. Aronov, in: A.L. Efros, M. Pollak (Eds.), Electron–Electron Interaction in Disordered Conductors, ed. by A. L. Efros and M. Pollak, North-Holland, Amsterdam, 1985. R.V. Muniswami-Naidu, A. Subrahmanyam, A. Verger, M.K. Jain, B. Rao, S. V. N. S. N. Jha, D. M. Phase, Electron. Mater. Lett., 8 (2012) p. 457. M. Nistor, F. Gherendi, N.B. Mandache, C. Hebert, J. Perrière, W. Seiler, J. Appl. Phys. 106 (2009) 103710.