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ZnMgO multiple quantum wells in ZnMgO nanocolumns grown on Si (111)
Journal of Alloys and Compounds 717 (2017) 41e47
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optoelectronic properties of ZnO/ZnMgO multiple quantum wells in ZnMgO nanocolumns grown on Si (111) M.A. Pietrzyk a, *, E. Placzek-Popko b, K.M. Paradowska b, E. Zielony b, M. Stachowicz a, A. Reszka a, A. Kozanecki a a
Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL-02668, Warsaw, Poland Department of Quantum Technologies, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wybrzeze Wyspianskiego 27, 50-370, Wroclaw, Poland
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2017 Received in revised form 4 May 2017 Accepted 6 May 2017 Available online 10 May 2017
Superlattice structures formed using the II-VI family of semiconductors have attracted a great deal of attention due to some unique properties (chemical sensors, UV detectors, lasers, white LED). We present a comparison of optical and electrical measurements of 10-period ZnO/ZnMgO multiple quantum wells (MQWs). The structures have been fabricated on (111) Si by molecular beam epitaxy grown at very high temperatures. The optical properties were analyzed by photoluminescence and cathodoluminescence techniques. Cross-sectional SEM-CL mapping shows that the ZnO/ZnMgO multiple quantum wells are located in ZnMgO nanocolumns. Based on these structures, diodes were processed and characterized by current-voltage (I-V), capacitance-voltage and deep level transient spectroscopy techniques (DLTS). I-V measurements confirm that in both diodes, deep traps govern the conduction mechanisms at a forward bias. DLTS studies yield signatures of these traps as well as those related to the presence of QWs. In particular, DLTS results let us estimate the distance of 30 meV from the QW ground state to the barrier conduction band. © 2017 Elsevier B.V. All rights reserved.
Keywords: Oxide materials Optical properties Optical spectroscopy Electronic properties
1. Introduction Zinc oxide (ZnO) has attracted a lot of research interest due to its unique properties. The high thermal conductivity, high electron mobility, good transparency, a wide and direct band gap (3.37 eV) and large exciton binding energy (60 meV) makes ZnO a possible candidate for next generation of optoelectronic piezoelectric devices, lasers and chemical sensors [1,2]. ZnO is often considered to be a future alternative to GaN (exciton binding energy of 26 meV [3]) in device applications due to its low production costs and excellent optical properties. Mixing of ZnO with MgO, which has a band gap of Eg ¼ 7.7 eV, leads to an increase of the band gap of ZnO and thus allows for optoelectronic applications in the deep UV spectral range [4]. ZnMgO has an important advantage over other oxide materials because of its small lattice mismatch to ZnO (about 1%) in a wide composition range. Furthermore, ZnMgO can be used in single and
* Corresponding author. E-mail address: [email protected] (M.A. Pietrzyk). http://dx.doi.org/10.1016/j.jallcom.2017.05.090 0925-8388/© 2017 Elsevier B.V. All rights reserved.
multiple ZnMgO/ZnO/ZnMgO quantum well structures, where ZnMgO is a barrier layer. In ZnMgO/ZnO/ZnMgO quantum wells the binding energy of excitons increases up to 100 meV, which results in efficient electroluminescence in devices operating on excitonic transitions at room temperature and above. The 1-D ZnMgO nanocolumns are potentially useful while fabricating various nano-devices such as light emitting diodes (LEDs), chemical sensors, solar cells, and piezoelectric devices, because of their high aspect ratio and large surface area to volume ratio, which would ensure the high efficiency and sensitivity in these applications [5]. Although sapphire is frequently used as substrate for the growth of ZnMgO, a significant effort was focused on the growth of ZnMgO films on Si wafers. Si substrates offer low price as compared to sapphire and SiC, high crystalline perfection, availability of large size, both types of conductivity, high thermal conductivity (1.5 Wcm1), easier device fabrication and potential for multifunctional device integration with Si electronics. In this paper, luminescence and electrical properties of ZnMgO/ ZnO nanocolumns are investigated by different approaches for possible future applications of these nanostructures as light
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emitting diodes or lasers. The optical properties were analyzed by photo- (PL) and cathodoluminescence (CL) measurements. CL technique allow studying of light emission from the specific sites on the sample (cross sectional). In this work the electrical properties of ZnO/ZnMgO MQWs structures embedded in the depletion layer of Si/ZnO heterojunctions are also investigated using current-voltage (I-V) and capacitance-voltage (C-V) measurements in the 50e350 K temperature range. The basic parameters of the structures, such as ideality factor, series resistance, rectifying ratio and built-in voltage were determined confirming the rectifying properties of the investigated junctions. Temperature dependence of saturation current was also analyzed and it was found to be thermally activated. Studies of I-V characteristics allowed for preliminary characterization of electrical properties of investigated structures. Moreover they proved suitability of the samples for application of a deep level transient spectroscopy (DLTS) method to characterize traps related to defects present in these materials. Usual parameters, such as apparent activation energy Ea and capture cross section sn were determined from Arrhenius analysis of the DLTS peak positions as a function of rate window en. 2. Experiments The ZnMgO nanocolumns were grown on Si (111) substrates with low temperature buffer structures using MBE method. The typical structures consist of a 350 nm thick ZnO buffer grown at low temperature (450 C), followed by a 230 nm thick ZnMgO barrier and quantum structures grown at 800 C. A 10 period of ZnO/ ZnMgO MQWs were grown on top of these barriers. The thickness of the barrier layers was kept constant at about 15 and 2 nm, while that of the quantum wells ranged from 3 to 1.7 nm (sample A and sample B respectively). Conventional Knudsen-cells were used for the evaporation of Zn (6N) and Mg (5N) pure elements. Before the growth process, the Si (111) wafers were heated up to 950 C to remove native oxide which is confirmed by appearance of a reflection high-energy electron diffraction (RHEED) pattern reconstruction corresponding to bare Si surface. The growth temperature was controlled with a thermocouple placed behind the substrate. During the growth, the radio-frequency power of oxygen plasma was fixed at 350 W. The growth of the heterostructures was monitored in situ by RHEED. More information about the growth of the samples can be found in the paper [6]. The optical properties of QWs structures were investigated by PL using a 302.4 nm (20 mW) line of Arþ laser as the excitation source and CL technique. The cross-sectional scanning electron microscopy (SEM) imaging was performed using a Hitachi SU-70 microscope. To carry out electrical characterization of heterojunctions ohmic contacts of Ti/Au were deposited on the top of the ZnMgO layer and Ni/Au contacts on the bottom of Si substrate. Current-voltage characteristics were measured within the 50e350 K temperature range with the use of Keithley 2601 I-V sourceemeter and a Janis cryostat with Lake Shore temperature control system. The capacitance-voltage-temperature, C-V-T, and DLTS studies were performed using a lock-in DLTS system based on Boonton 7200 capacitance meter.
separated with 15 nm ZnMgO barriers. Spectra presented in Fig. 1 contain few peaks. Two of them show the most intense emission, with maxima placed at 3.381 eV and 3.60 eV, and these were ascribed to MQW structure and ZnMgO barrier, respectively. The low energy part of the spectra consists of not well resolved peaks which correspond to LO phonon replicas. As it can be noticed, the MQW structure peak changes its position with temperature. Firstly it shifts towards lower energies by 8 meV at 30 K and then it moves back towards higher energies to its original position at 150 K. Finally the position of the peak at room temperature is shifted again towards lower energies, by 35 meV, so it moves along S-curve typical for exciton localization effect. It describes the inhomogeneity on the interface, originating from the high density of defect. The presence of the defects was confirmed by the electrical measurements. The MQW peak situated at 3.381 eV is consistent with experimental results and theoretical calculations of excitation transition energies for a single ZnO/ZnMgO QW presented by Coli et al. [7]. This indicates that QWs in samples investigated in this paper are well separated and do have assumed widths, although the broadening of the peak indicates some Mg/Zn content fluctuations at the wells-barriers interfaces as previously mentioned. Room temperature panchromatic and monochromatic SEM-CL imaging was performed to confirm the origin of PL of the studied structures and the results are presented in Fig. 2. Decomposition of the panchromatic image to monochromatic one reveals a strong emission from theMQWs at 3.342 eV and from the ZnMgO barrier at 3.73 eV. Fig. 3 shows the MQWs peak shift of ZnO/ZnMgO MQWs emission as a function of temperature. This allows analyzing the alloy and interface fluctuations in the quantum structures. In investigated samples, the emission energy decreases monotonically with increasing temperature. Fig. 4 shows the PL spectra of ZnO/ZnMgO MQW structure containing 10 quantum wells each 1.7 nm wide with 2 nm ZnMgO inter-well barriers measured at various temperatures. A weak emission observed at 3.725 eV comes from the ZnMgO cap/barrier layers. On this basis the Mg content has been estimated at about 15%. A strong emission at 3.455 eV can be attributed to the radiative recombination of the localized exciton in the ZnMgO/ZnO/ZnMgO MQWs. Two transitions denoted as 1 LO and 2 LO are longitudinal optical (LO) phonon replicas, since their position and energy distances match the 71 meV intervals counted from the position of MQWs. A weak peak at 3.363 eV at 8 K can be attributed to the
3. Results and discussion 3.1. Optical measurements Full temperature range PL measurements were performed for ZnO/ZnMgO MQW structure with a 10 periods of QWs (3 nm)
Fig. 1. Temperature-dependent PL of ZnMgO/ZnO/ZnMgO MQWs.
M.A. Pietrzyk et al. / Journal of Alloys and Compounds 717 (2017) 41e47
43
Fig. 2. A cross-sectional CL-images of the 10-period MQW sample measured at room temperature (inside e CL spectra taken at 15 kV).
Fig. 3. A MQWs peak shift of ZnO/ZnMgO MQWs emission as a function of temperature.
ZnMgO interface and another one where the MQW structure is located. It seems that this emission is accidental result of coincidence of two different transitions related to the interface and surface states of the structure. We undertook an effort to perform a numerical evaluation with EpitaxyProject software allowing for simple evaluation of energy levels in arbitrarily defined quantum wells. The finite-element €dinger equation calculation resolves directly 1-dimensional Shro with effective mass approximation for electrons, heavy and light holes. For calculation we assumed the band offset DEc/DEv ¼ 4/1, while the effective masses were me* ¼ 0.28, mhh* ¼ 0.59, and mlh* ¼ 0.45 [8,9]. As it is shown in the Table 1, the calculated energy levels are in fair agreement with the experimental data, confirming correctness of the spectra peaks ascribing. It is worth mentioning that energies in the Table 1, which are for QWs, were obtained including excitonic binding energy correction of Eb~70 meV after Senger et al. [10]. In the case of 2 nm barrier in MQW structure calculated energy levels are generally higher than those observed in the experiment. It can be caused by electron wave function overlapping of neighboring QW, which has not been taken into account in performed simulations.
3.2. Electrical measurements
Fig. 4. PL spectra of ZnO/ZnMgO MQW taken at various temperatures (8e300 K).
emission of excitons bound to neutral donors (D0X) from the ZnO low temperature buffer. Fig. 5 shows cross-sectional CL-images of the 10-period MQW sample measured at room temperature. A panchromatic CL image can be decomposed to monochromatic images and it can be noticed that indeed the emission line occurring at 3.65 eV comes from the ZnMgO buffer while the other one at 3.42 eV corresponds to MQWs. Emission at 3.31 eV originates from two areas e one close to ZnO/
Before analyzing the electrical measurements it has to be emphasized that the p-n heterojunction (HJ) is formed between the Si substrate and ZnO buffer layer. The presence of other layers contributes to the series resistance of the HJs. In Fig. 6 room temperature I-V curves for both structures are shown for comparison. Both characteristics confirm rather poor rectifying properties of the heterojunctions. Fundamental parameters of the diodes based on Fig. 6 are collected in Table 2. The ideality factor is larger than 1 for both junctions, suggesting that beyond the thermionic emission there are several mechanisms responsible for the current transport. These may be due to the inhomogeneity in the barrier height or the presence of interface states. In the ideal case, the current e voltage characteristics of a heterojunction diode due to the thermionic emission is given by equation (1):
qðV IRÞ 1 ; I ¼ I0 exp nkT
(1)
where k is Boltzmann constant, T e temperature, n e ideality factor and R e series resistance. Assuming that the saturation current I0 is thermally activated, with activation energy Ea, it can be expressed as follows:
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Fig. 5. A cross-sectional CL-images of the 10-period MQW sample measured at room temperature.
Table 1 Theoretical and experimental energy values for quantum wells and ZnMgO barrier. Sample barrier thick.
QWs trans. [eV]
ZnMgO barrier [eV]
Exp. 15 nm barrier Calc. 15 nm barrier Exp. 2 nm barrier Calc. 2 nm barrier
3.381 3.354 ± 0.056 3.455 3.412 ± 0.089
3.599 3.676 ± 0.106 3.742 3.725 ± 0.064
both junctions are presented. From the plot the value of the activation energy Ea y0:1eV for both diodes was calculated. This value is much smaller than the bandgap energy of ZnO. This supports the hypothesis that the diffusion and recombination current in the depletion region are not the main mechanisms causing the low forward bias current, since the activation energies for these two mechanisms are on the order of the bandgap energy, Eg or Eg/2, respectively [11] (much larger than those determined by us). In order to get insight into possible current transport mechanisms, in Fig. 7 the forward I-V characteristics are presented in a double logarithmic scale. At a low voltage bias (<0.1 V for the sample A and 0.03 V for the sample B) labeled as region I, the slopes of the plots are close to 1, indicating the Ohmic region, due to the tunneling current. In the region labeled II, the log-log plots obey the relationship characteristic for recombination-tunneling, multisteptunneling or multitunneling capture e emission current (MTCE) [12]. Taking into account the dependence of saturation current density on temperature (inset in Fig. 6) it may be assumed that this is the MTCE process which governs the current transport in both diodes in the low forward bias [12], because only in this case saturation current is temperature sensitive. In the region III space charge limited current (SCLC) dominates. For such mechanism the power law I ~ Vm is observed. The slope of the log-log plot m > 2 for both diodes indicates the SCLC with contribution of traps [13]. It has to be emphasized that the concentration of traps in both samples is large, because neither of the curves reach the voltage trap filled limit, VTFL [13]. If it has occurred,
Fig. 6. Room temperature characteristics for the studied diodes. In the inset the Richardson plots of the saturation current for both diodes are shown. Solid lines are the best least square fits to the experimental data.
Table 2 Parameters of the studied diodes: ideality factor n, saturation current, I0, series resistance, R, rectification ratio, RR and built-in voltage, Vb. Parameter units Sample A Sample B
n e 2.7 4.2
Io A 7.4 109 2.7 106
R (I-V)
U 1109 22
RR (IV) e 10 10
Ea ; I0 ¼ Aexp kT
Vb V 1.66 2.72
(2)
with A ¼ const. The Richardson plot of lnI0 ¼ f
1 T
yields energy
activation Ea . In the inset of Fig. 6 the Richardson plots of the saturation currents determined from the I-V-T characteristics for
Fig. 7. I-V curves for the sample A and sample B, shown in a double logarithmic scale.
M.A. Pietrzyk et al. / Journal of Alloys and Compounds 717 (2017) 41e47
the I ~ V2 power law would have been observed. In the case of the sample A region IV can be distinguished with a very high value of m, indicating exponentially distributed traps [13]. The SCLC is due to the different offsets of the conduction and valence band at the n-ZnO/p-Si interface. Based on the Anderson model [14] it can be shown that in the n-ZnO/p-Si HJ valence band offset (2.55 eV) is much larger than the conduction band offset (0.4 eV) [15,16]. So at a forward bias the current is dominated by the flow of electrons from the ZnO buffer layer to the Si substrate because the barrier is much higher for the holes than for the electrons. It may be concluded that in both diodes, deep traps govern the conduction mechanisms at a forward bias, because at low forward bias the dominant mechanism of current transport is the multitunneling capture-emission current, while for larger bias space charge limited current prevails but again with the traps contribution. The trap concentration is larger for the diode (sample B). In order to shed light on the deep traps present in the diodes, DLTS measurements were performed. The DLTS studies were preceded by the C-V characteristics measurements. In Fig. 8 the room temperature C-V plots for the diode are shown. The shape of the CV curve near 0 V bias shows a mountain shape for the nonohmic contacts. Carrier concentration profile determined from the measured C-V characteristics, shown for one of the diodes (sample B) in Fig. 9, exhibits a single accumulation peak. The accumulation peak means the existence of the quantum structure just like a QW. Based on the C-V characteristics the DLTS measurements' settings such as a reverse voltage bias and filling pulse heights were chosen for each diode. The DLTS signal for the diode of sample A, visible in Fig. 10, contains two well resolved maxima (P1 and P2) within measured temperature range whereas the high temperature maximum (labeled as P2) shown in Fig. 11 presenting DLTS signal for the other diode is evidently composed of two overlapping peaks. Arrhenius plots ln Ten2 ¼ f T1 corresponding to the DLTS maxima shown in Fig. 10 and in Fig. 11 are presented in Fig. 12. Here en stands for the emission rate which is related to the lock-in frequency en ¼ 2:17f . Parameters of the traps obtained from the Arrhenius plots are collected in Table 3. The signature for the shallow level P1 could not be determined from the measured spectra, because for the sample A capacitance significantly drops down below 50 K. Maxima labeled as P1 in both samples are presumably related to MQWs. In the case of the sample B there is one carrier accumulation peak in its carrier profile (cf. Fig. 9), and the accumulation peak results from the existence of a quantum structure, just like a QW. DLTS can find only ground energy-state of a QW
45
Fig. 9. Carrier concentration profile for a sample B.
Fig. 10. Temperature dependence of the DLTS signal for the sample A measured at different lock-in frequencies. Measurements' settings: reverse bias 2.5 V, filling pulse height 0 V, filling pulse width 0.1 ms.
Fig. 11. Temperature dependence of the DLTS signal for the sample B, measured at several lock-in frequencies. Measurements' settings: reverse bias 2.5 V, filling pulse height 0.5 V, filling pulse width 0.1 ms. Fig. 8. Room temperature C-V characteristics for both studied diodes.
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detected in both diodes can be related either with the Si or ZnO side of the junction. However due to the lattice mismatch, different crystal structures (diamond in Si and wurtzite in ZnO) and type II HJ it may be assumed that the dislocated region is created at the interface between ZnO, mainly on the n-side of the HJ. Ci-Hui et al. [28] have investigated the defects level position in ZnO thin film by means of DLTS and PL measurements. From this study they suggested the presence of two defect levels, localized at E1 ¼ 0.13eV and E2 ¼ 0.43eV below the bottom of conduction band Ec. The energy of the level E1 is close to the activation energy of the saturation currents obtained from the present work as well as to the activation energy corresponding to the traps P2 in the sample B. It may be concluded that these traps are responsible for the observed multitunneling capture-emission mechanisms of current transport and are detected by DLTS as the trap P2 in the sample B. 4. Conclusion
Fig. 12. Arrhenius plots of the traps present in the DLTS spectra shown in Fig. 10 and in Fig. 11.
Table 3 Activation energy and apparent capture cross section calculated from the Arrhenius plots (shown in Fig. 12). Trap
Ea (eV)
sn (cm2)
Sample AeP1 Sample AeP2 Sample BeP2
0.032 ± 0.003 0.32 ± 0.04 0.17 ± 0.04
~1020 ~1019 ~1012
[17]. Thus the trap P1 can be assigned to the thermal activation energy from the ground state of a QW to the ZnMgO barrier. Unfortunately, it was impossible to determine the signature of this trap for this sample because of the tunneling effect occurring at low temperatures (cf. Fig. 12). The latter can be explained by the fact that in the case of this sample the barrier width was very narrow e 2 nm with comparison to the 15 nm for the other sample. The signature of the trap P1 was calculated for the sample A. Thus the distance from the QW ground state to the barrier conduction band equals 32 meV. The maximum P2 in the sample A may be related to the Zni [18]. The maximum labeled as P2 in the sample B may be assigned to the interface states because it is a very broad feature. The activation energy is close to that determined from the Richardson plot shown in Fig. 2 and thus it may be responsible for the afore mentioned MTCE current. Recently there were several reports on the presence of interface states in the n-ZnO/p-Si heterojunctions realized with different technology methods [19e28]. The authors found that the ideality factor determined from the I-V curves was much bigger than 1 and they assigned it to the presence of interface trap levels. The result which is very close to that obtained by us was reported by Zebbar et al. [19]. It was found that for the n-ZnO/p-Si heterojunction prepared by ultrasonic spray at low bias the forward conduction is dominated by multi-step tunneling current with a main contribution of the defect level located at 0.14 eV below the minimum of the conduction band edge, close to the values 0.11 and 0.15 eV determined by us. The acceptor density in p-Si of the 0.1Ucm resistivity equals Na~3 1017 cm3. The donor density in Zn0.9Mg0.1O equals 1017 1018 cm3 [29]. Similar values were obtained from the C-V measurements. Thus it may be assumed that the depletion region extends on both sides of the p-Si/n-ZnO HJ. It means that the traps
In summary, the catalyst-free growth of self-assembled ZnMgO nanocolumns with LT buffer layer on Si (111) by PA-MBE was achieved. We have grown a series of ZnO/ZnMgO MQWs with various well and barrier widths under well-controlled MBE conditions. PL analysis suggests that the MQWs show excitonic near-band edge emission at low temperature. CL measurements at room temperature confirm PL results. On top and bottom of the HJs the ohmic contacts were processed. I-V measurements performed for these diodes reveal dominant current mechanisms. At a low forward bias it is multitunneling capture-emission current while for the larger one e space charge limited current. The I-V curves confirm the presence of trap states at the n-ZnO/p-Si interface. C-V measurements yield charge accumulation at QWs for the diode B. DLTS studies yield signatures of these traps as well as those related to the presence of QWs. Acknowledgments The project was supported by the Polish National Science Centre (NCN) based on the decision No: DEC-2013/09/D/ST5/03881. References [1] Z.L. Wang, J. Phys. Condens. Matter 16 (2004) R829. [2] Z.P. Wei, Y.M. Lu, D.Z. Shen, Z.Z. Zhang, B. Yao, B.H. Li, J.Y. Zhang, D.X. Zhao, X.W. Fan, Z.K. Tang, Appl. Phys. Lett. 90 (2007) 042113. [3] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237. [4] E. Mollwo, in: O. Madelung, M. Schulz, H. Weiss (Eds.), Physics of II-VI and I-VII Compounds, Semimagnetic Semiconductors vol. 17, Springer, Berlin, 1982, p. 35. [5] X. Du, Y. Sun, Q. Wang, N. Ma, Z. Yuan, J. Optoelectron. Adv. M. 13 (2011) 631. [6] M.A. Pietrzyk, M. Stachowicz, A. Wierzbicka, P. Dluzewski, D. Jarosz, E. Przezdziecka, A. Kozanecki, J. Cryst. Growth 408 (2014) 102. [7] G. Coli, K.K. Bajaj, Appl. Phys. Lett. 78 (2001) 2861. [8] K.J. Button, D.R. Cohn, M. von Ortenbert, B. Lax, E. Mollwo, R. Helbig, Phys. Rev. Lett. 28 (1972) 1637. [9] K. Hummer, Phys. Stat. Sol. b Basic Res. 56 (1973) 249. [10] R.T. Senger, K.K. Bajaj, Phys. Rev. B 68 (2003) 205314. [11] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [12] H. Matsuura, T. Okuno, H. Okushi, K. Tanaka, J. Appl. Phys. 55 (1984) 1012. [13] M.A. Lampert, P. Mark, Current Injections in Solids, 1978. New York, London. [14] R.L. Anderson, IBM J. Res. Dev. 4 (1960) 283. [15] J.D. Ye, S.L. Gu, S.M. Zhu, W. Liu, S.M. Liu, R. Zhang, Y. Shi, Y.D. Zheng, Appl. Phys. Lett. 88 (2006) 182112. [16] J.H. He, C.H. Ho, Appl. Phys. Lett. 91 (2007) 233105. [17] P. Blood, J.W. Orton, in: The Electrical Characterization of Semiconductors: Majority Carriers and Electron States, Academic Press, NY, 1992. [18] M. Trunk, V. Venkatachalapathy, A. Galeckas, A. Yu. Kuznetsov, Appl. Phys. Lett. 97 (2010) 211901. [19] N. Zebbar, Y. Kheireddine, K. Mokeddem, A. Hafdallah, M. Kechouane, M.S. Aida, Mat. Sci. Semicond. Proc. 14 (2011) 229. [20] Y.S. Ocak, J. Alloys Compd. 513 (2012) 130. [21] F. Yakuphanoglu, Y. Caglar, M. Caglar, S. Ilican, Mat. Sci. Semicond. Proc. 13
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optoelectronic properties of ZnO/ZnMgO multiple quantum wells in ZnMgO nanocolumns grown on Si (111) M.A. Pietrzyk a, *, E. Placzek-Popko b, K.M. Paradowska b, E. Zielony b, M. Stachowicz a, A. Reszka a, A. Kozanecki a a
Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL-02668, Warsaw, Poland Department of Quantum Technologies, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wybrzeze Wyspianskiego 27, 50-370, Wroclaw, Poland
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2017 Received in revised form 4 May 2017 Accepted 6 May 2017 Available online 10 May 2017
Superlattice structures formed using the II-VI family of semiconductors have attracted a great deal of attention due to some unique properties (chemical sensors, UV detectors, lasers, white LED). We present a comparison of optical and electrical measurements of 10-period ZnO/ZnMgO multiple quantum wells (MQWs). The structures have been fabricated on (111) Si by molecular beam epitaxy grown at very high temperatures. The optical properties were analyzed by photoluminescence and cathodoluminescence techniques. Cross-sectional SEM-CL mapping shows that the ZnO/ZnMgO multiple quantum wells are located in ZnMgO nanocolumns. Based on these structures, diodes were processed and characterized by current-voltage (I-V), capacitance-voltage and deep level transient spectroscopy techniques (DLTS). I-V measurements confirm that in both diodes, deep traps govern the conduction mechanisms at a forward bias. DLTS studies yield signatures of these traps as well as those related to the presence of QWs. In particular, DLTS results let us estimate the distance of 30 meV from the QW ground state to the barrier conduction band. © 2017 Elsevier B.V. All rights reserved.
Keywords: Oxide materials Optical properties Optical spectroscopy Electronic properties
1. Introduction Zinc oxide (ZnO) has attracted a lot of research interest due to its unique properties. The high thermal conductivity, high electron mobility, good transparency, a wide and direct band gap (3.37 eV) and large exciton binding energy (60 meV) makes ZnO a possible candidate for next generation of optoelectronic piezoelectric devices, lasers and chemical sensors [1,2]. ZnO is often considered to be a future alternative to GaN (exciton binding energy of 26 meV [3]) in device applications due to its low production costs and excellent optical properties. Mixing of ZnO with MgO, which has a band gap of Eg ¼ 7.7 eV, leads to an increase of the band gap of ZnO and thus allows for optoelectronic applications in the deep UV spectral range [4]. ZnMgO has an important advantage over other oxide materials because of its small lattice mismatch to ZnO (about 1%) in a wide composition range. Furthermore, ZnMgO can be used in single and
* Corresponding author. E-mail address: [email protected] (M.A. Pietrzyk). http://dx.doi.org/10.1016/j.jallcom.2017.05.090 0925-8388/© 2017 Elsevier B.V. All rights reserved.
multiple ZnMgO/ZnO/ZnMgO quantum well structures, where ZnMgO is a barrier layer. In ZnMgO/ZnO/ZnMgO quantum wells the binding energy of excitons increases up to 100 meV, which results in efficient electroluminescence in devices operating on excitonic transitions at room temperature and above. The 1-D ZnMgO nanocolumns are potentially useful while fabricating various nano-devices such as light emitting diodes (LEDs), chemical sensors, solar cells, and piezoelectric devices, because of their high aspect ratio and large surface area to volume ratio, which would ensure the high efficiency and sensitivity in these applications [5]. Although sapphire is frequently used as substrate for the growth of ZnMgO, a significant effort was focused on the growth of ZnMgO films on Si wafers. Si substrates offer low price as compared to sapphire and SiC, high crystalline perfection, availability of large size, both types of conductivity, high thermal conductivity (1.5 Wcm1), easier device fabrication and potential for multifunctional device integration with Si electronics. In this paper, luminescence and electrical properties of ZnMgO/ ZnO nanocolumns are investigated by different approaches for possible future applications of these nanostructures as light
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emitting diodes or lasers. The optical properties were analyzed by photo- (PL) and cathodoluminescence (CL) measurements. CL technique allow studying of light emission from the specific sites on the sample (cross sectional). In this work the electrical properties of ZnO/ZnMgO MQWs structures embedded in the depletion layer of Si/ZnO heterojunctions are also investigated using current-voltage (I-V) and capacitance-voltage (C-V) measurements in the 50e350 K temperature range. The basic parameters of the structures, such as ideality factor, series resistance, rectifying ratio and built-in voltage were determined confirming the rectifying properties of the investigated junctions. Temperature dependence of saturation current was also analyzed and it was found to be thermally activated. Studies of I-V characteristics allowed for preliminary characterization of electrical properties of investigated structures. Moreover they proved suitability of the samples for application of a deep level transient spectroscopy (DLTS) method to characterize traps related to defects present in these materials. Usual parameters, such as apparent activation energy Ea and capture cross section sn were determined from Arrhenius analysis of the DLTS peak positions as a function of rate window en. 2. Experiments The ZnMgO nanocolumns were grown on Si (111) substrates with low temperature buffer structures using MBE method. The typical structures consist of a 350 nm thick ZnO buffer grown at low temperature (450 C), followed by a 230 nm thick ZnMgO barrier and quantum structures grown at 800 C. A 10 period of ZnO/ ZnMgO MQWs were grown on top of these barriers. The thickness of the barrier layers was kept constant at about 15 and 2 nm, while that of the quantum wells ranged from 3 to 1.7 nm (sample A and sample B respectively). Conventional Knudsen-cells were used for the evaporation of Zn (6N) and Mg (5N) pure elements. Before the growth process, the Si (111) wafers were heated up to 950 C to remove native oxide which is confirmed by appearance of a reflection high-energy electron diffraction (RHEED) pattern reconstruction corresponding to bare Si surface. The growth temperature was controlled with a thermocouple placed behind the substrate. During the growth, the radio-frequency power of oxygen plasma was fixed at 350 W. The growth of the heterostructures was monitored in situ by RHEED. More information about the growth of the samples can be found in the paper [6]. The optical properties of QWs structures were investigated by PL using a 302.4 nm (20 mW) line of Arþ laser as the excitation source and CL technique. The cross-sectional scanning electron microscopy (SEM) imaging was performed using a Hitachi SU-70 microscope. To carry out electrical characterization of heterojunctions ohmic contacts of Ti/Au were deposited on the top of the ZnMgO layer and Ni/Au contacts on the bottom of Si substrate. Current-voltage characteristics were measured within the 50e350 K temperature range with the use of Keithley 2601 I-V sourceemeter and a Janis cryostat with Lake Shore temperature control system. The capacitance-voltage-temperature, C-V-T, and DLTS studies were performed using a lock-in DLTS system based on Boonton 7200 capacitance meter.
separated with 15 nm ZnMgO barriers. Spectra presented in Fig. 1 contain few peaks. Two of them show the most intense emission, with maxima placed at 3.381 eV and 3.60 eV, and these were ascribed to MQW structure and ZnMgO barrier, respectively. The low energy part of the spectra consists of not well resolved peaks which correspond to LO phonon replicas. As it can be noticed, the MQW structure peak changes its position with temperature. Firstly it shifts towards lower energies by 8 meV at 30 K and then it moves back towards higher energies to its original position at 150 K. Finally the position of the peak at room temperature is shifted again towards lower energies, by 35 meV, so it moves along S-curve typical for exciton localization effect. It describes the inhomogeneity on the interface, originating from the high density of defect. The presence of the defects was confirmed by the electrical measurements. The MQW peak situated at 3.381 eV is consistent with experimental results and theoretical calculations of excitation transition energies for a single ZnO/ZnMgO QW presented by Coli et al. [7]. This indicates that QWs in samples investigated in this paper are well separated and do have assumed widths, although the broadening of the peak indicates some Mg/Zn content fluctuations at the wells-barriers interfaces as previously mentioned. Room temperature panchromatic and monochromatic SEM-CL imaging was performed to confirm the origin of PL of the studied structures and the results are presented in Fig. 2. Decomposition of the panchromatic image to monochromatic one reveals a strong emission from theMQWs at 3.342 eV and from the ZnMgO barrier at 3.73 eV. Fig. 3 shows the MQWs peak shift of ZnO/ZnMgO MQWs emission as a function of temperature. This allows analyzing the alloy and interface fluctuations in the quantum structures. In investigated samples, the emission energy decreases monotonically with increasing temperature. Fig. 4 shows the PL spectra of ZnO/ZnMgO MQW structure containing 10 quantum wells each 1.7 nm wide with 2 nm ZnMgO inter-well barriers measured at various temperatures. A weak emission observed at 3.725 eV comes from the ZnMgO cap/barrier layers. On this basis the Mg content has been estimated at about 15%. A strong emission at 3.455 eV can be attributed to the radiative recombination of the localized exciton in the ZnMgO/ZnO/ZnMgO MQWs. Two transitions denoted as 1 LO and 2 LO are longitudinal optical (LO) phonon replicas, since their position and energy distances match the 71 meV intervals counted from the position of MQWs. A weak peak at 3.363 eV at 8 K can be attributed to the
3. Results and discussion 3.1. Optical measurements Full temperature range PL measurements were performed for ZnO/ZnMgO MQW structure with a 10 periods of QWs (3 nm)
Fig. 1. Temperature-dependent PL of ZnMgO/ZnO/ZnMgO MQWs.
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Fig. 2. A cross-sectional CL-images of the 10-period MQW sample measured at room temperature (inside e CL spectra taken at 15 kV).
Fig. 3. A MQWs peak shift of ZnO/ZnMgO MQWs emission as a function of temperature.
ZnMgO interface and another one where the MQW structure is located. It seems that this emission is accidental result of coincidence of two different transitions related to the interface and surface states of the structure. We undertook an effort to perform a numerical evaluation with EpitaxyProject software allowing for simple evaluation of energy levels in arbitrarily defined quantum wells. The finite-element €dinger equation calculation resolves directly 1-dimensional Shro with effective mass approximation for electrons, heavy and light holes. For calculation we assumed the band offset DEc/DEv ¼ 4/1, while the effective masses were me* ¼ 0.28, mhh* ¼ 0.59, and mlh* ¼ 0.45 [8,9]. As it is shown in the Table 1, the calculated energy levels are in fair agreement with the experimental data, confirming correctness of the spectra peaks ascribing. It is worth mentioning that energies in the Table 1, which are for QWs, were obtained including excitonic binding energy correction of Eb~70 meV after Senger et al. [10]. In the case of 2 nm barrier in MQW structure calculated energy levels are generally higher than those observed in the experiment. It can be caused by electron wave function overlapping of neighboring QW, which has not been taken into account in performed simulations.
3.2. Electrical measurements
Fig. 4. PL spectra of ZnO/ZnMgO MQW taken at various temperatures (8e300 K).
emission of excitons bound to neutral donors (D0X) from the ZnO low temperature buffer. Fig. 5 shows cross-sectional CL-images of the 10-period MQW sample measured at room temperature. A panchromatic CL image can be decomposed to monochromatic images and it can be noticed that indeed the emission line occurring at 3.65 eV comes from the ZnMgO buffer while the other one at 3.42 eV corresponds to MQWs. Emission at 3.31 eV originates from two areas e one close to ZnO/
Before analyzing the electrical measurements it has to be emphasized that the p-n heterojunction (HJ) is formed between the Si substrate and ZnO buffer layer. The presence of other layers contributes to the series resistance of the HJs. In Fig. 6 room temperature I-V curves for both structures are shown for comparison. Both characteristics confirm rather poor rectifying properties of the heterojunctions. Fundamental parameters of the diodes based on Fig. 6 are collected in Table 2. The ideality factor is larger than 1 for both junctions, suggesting that beyond the thermionic emission there are several mechanisms responsible for the current transport. These may be due to the inhomogeneity in the barrier height or the presence of interface states. In the ideal case, the current e voltage characteristics of a heterojunction diode due to the thermionic emission is given by equation (1):
qðV IRÞ 1 ; I ¼ I0 exp nkT
(1)
where k is Boltzmann constant, T e temperature, n e ideality factor and R e series resistance. Assuming that the saturation current I0 is thermally activated, with activation energy Ea, it can be expressed as follows:
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Fig. 5. A cross-sectional CL-images of the 10-period MQW sample measured at room temperature.
Table 1 Theoretical and experimental energy values for quantum wells and ZnMgO barrier. Sample barrier thick.
QWs trans. [eV]
ZnMgO barrier [eV]
Exp. 15 nm barrier Calc. 15 nm barrier Exp. 2 nm barrier Calc. 2 nm barrier
3.381 3.354 ± 0.056 3.455 3.412 ± 0.089
3.599 3.676 ± 0.106 3.742 3.725 ± 0.064
both junctions are presented. From the plot the value of the activation energy Ea y0:1eV for both diodes was calculated. This value is much smaller than the bandgap energy of ZnO. This supports the hypothesis that the diffusion and recombination current in the depletion region are not the main mechanisms causing the low forward bias current, since the activation energies for these two mechanisms are on the order of the bandgap energy, Eg or Eg/2, respectively [11] (much larger than those determined by us). In order to get insight into possible current transport mechanisms, in Fig. 7 the forward I-V characteristics are presented in a double logarithmic scale. At a low voltage bias (<0.1 V for the sample A and 0.03 V for the sample B) labeled as region I, the slopes of the plots are close to 1, indicating the Ohmic region, due to the tunneling current. In the region labeled II, the log-log plots obey the relationship characteristic for recombination-tunneling, multisteptunneling or multitunneling capture e emission current (MTCE) [12]. Taking into account the dependence of saturation current density on temperature (inset in Fig. 6) it may be assumed that this is the MTCE process which governs the current transport in both diodes in the low forward bias [12], because only in this case saturation current is temperature sensitive. In the region III space charge limited current (SCLC) dominates. For such mechanism the power law I ~ Vm is observed. The slope of the log-log plot m > 2 for both diodes indicates the SCLC with contribution of traps [13]. It has to be emphasized that the concentration of traps in both samples is large, because neither of the curves reach the voltage trap filled limit, VTFL [13]. If it has occurred,
Fig. 6. Room temperature characteristics for the studied diodes. In the inset the Richardson plots of the saturation current for both diodes are shown. Solid lines are the best least square fits to the experimental data.
Table 2 Parameters of the studied diodes: ideality factor n, saturation current, I0, series resistance, R, rectification ratio, RR and built-in voltage, Vb. Parameter units Sample A Sample B
n e 2.7 4.2
Io A 7.4 109 2.7 106
R (I-V)
U 1109 22
RR (IV) e 10 10
Ea ; I0 ¼ Aexp kT
Vb V 1.66 2.72
(2)
with A ¼ const. The Richardson plot of lnI0 ¼ f
1 T
yields energy
activation Ea . In the inset of Fig. 6 the Richardson plots of the saturation currents determined from the I-V-T characteristics for
Fig. 7. I-V curves for the sample A and sample B, shown in a double logarithmic scale.
M.A. Pietrzyk et al. / Journal of Alloys and Compounds 717 (2017) 41e47
the I ~ V2 power law would have been observed. In the case of the sample A region IV can be distinguished with a very high value of m, indicating exponentially distributed traps [13]. The SCLC is due to the different offsets of the conduction and valence band at the n-ZnO/p-Si interface. Based on the Anderson model [14] it can be shown that in the n-ZnO/p-Si HJ valence band offset (2.55 eV) is much larger than the conduction band offset (0.4 eV) [15,16]. So at a forward bias the current is dominated by the flow of electrons from the ZnO buffer layer to the Si substrate because the barrier is much higher for the holes than for the electrons. It may be concluded that in both diodes, deep traps govern the conduction mechanisms at a forward bias, because at low forward bias the dominant mechanism of current transport is the multitunneling capture-emission current, while for larger bias space charge limited current prevails but again with the traps contribution. The trap concentration is larger for the diode (sample B). In order to shed light on the deep traps present in the diodes, DLTS measurements were performed. The DLTS studies were preceded by the C-V characteristics measurements. In Fig. 8 the room temperature C-V plots for the diode are shown. The shape of the CV curve near 0 V bias shows a mountain shape for the nonohmic contacts. Carrier concentration profile determined from the measured C-V characteristics, shown for one of the diodes (sample B) in Fig. 9, exhibits a single accumulation peak. The accumulation peak means the existence of the quantum structure just like a QW. Based on the C-V characteristics the DLTS measurements' settings such as a reverse voltage bias and filling pulse heights were chosen for each diode. The DLTS signal for the diode of sample A, visible in Fig. 10, contains two well resolved maxima (P1 and P2) within measured temperature range whereas the high temperature maximum (labeled as P2) shown in Fig. 11 presenting DLTS signal for the other diode is evidently composed of two overlapping peaks. Arrhenius plots ln Ten2 ¼ f T1 corresponding to the DLTS maxima shown in Fig. 10 and in Fig. 11 are presented in Fig. 12. Here en stands for the emission rate which is related to the lock-in frequency en ¼ 2:17f . Parameters of the traps obtained from the Arrhenius plots are collected in Table 3. The signature for the shallow level P1 could not be determined from the measured spectra, because for the sample A capacitance significantly drops down below 50 K. Maxima labeled as P1 in both samples are presumably related to MQWs. In the case of the sample B there is one carrier accumulation peak in its carrier profile (cf. Fig. 9), and the accumulation peak results from the existence of a quantum structure, just like a QW. DLTS can find only ground energy-state of a QW
45
Fig. 9. Carrier concentration profile for a sample B.
Fig. 10. Temperature dependence of the DLTS signal for the sample A measured at different lock-in frequencies. Measurements' settings: reverse bias 2.5 V, filling pulse height 0 V, filling pulse width 0.1 ms.
Fig. 11. Temperature dependence of the DLTS signal for the sample B, measured at several lock-in frequencies. Measurements' settings: reverse bias 2.5 V, filling pulse height 0.5 V, filling pulse width 0.1 ms. Fig. 8. Room temperature C-V characteristics for both studied diodes.
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detected in both diodes can be related either with the Si or ZnO side of the junction. However due to the lattice mismatch, different crystal structures (diamond in Si and wurtzite in ZnO) and type II HJ it may be assumed that the dislocated region is created at the interface between ZnO, mainly on the n-side of the HJ. Ci-Hui et al. [28] have investigated the defects level position in ZnO thin film by means of DLTS and PL measurements. From this study they suggested the presence of two defect levels, localized at E1 ¼ 0.13eV and E2 ¼ 0.43eV below the bottom of conduction band Ec. The energy of the level E1 is close to the activation energy of the saturation currents obtained from the present work as well as to the activation energy corresponding to the traps P2 in the sample B. It may be concluded that these traps are responsible for the observed multitunneling capture-emission mechanisms of current transport and are detected by DLTS as the trap P2 in the sample B. 4. Conclusion
Fig. 12. Arrhenius plots of the traps present in the DLTS spectra shown in Fig. 10 and in Fig. 11.
Table 3 Activation energy and apparent capture cross section calculated from the Arrhenius plots (shown in Fig. 12). Trap
Ea (eV)
sn (cm2)
Sample AeP1 Sample AeP2 Sample BeP2
0.032 ± 0.003 0.32 ± 0.04 0.17 ± 0.04
~1020 ~1019 ~1012
[17]. Thus the trap P1 can be assigned to the thermal activation energy from the ground state of a QW to the ZnMgO barrier. Unfortunately, it was impossible to determine the signature of this trap for this sample because of the tunneling effect occurring at low temperatures (cf. Fig. 12). The latter can be explained by the fact that in the case of this sample the barrier width was very narrow e 2 nm with comparison to the 15 nm for the other sample. The signature of the trap P1 was calculated for the sample A. Thus the distance from the QW ground state to the barrier conduction band equals 32 meV. The maximum P2 in the sample A may be related to the Zni [18]. The maximum labeled as P2 in the sample B may be assigned to the interface states because it is a very broad feature. The activation energy is close to that determined from the Richardson plot shown in Fig. 2 and thus it may be responsible for the afore mentioned MTCE current. Recently there were several reports on the presence of interface states in the n-ZnO/p-Si heterojunctions realized with different technology methods [19e28]. The authors found that the ideality factor determined from the I-V curves was much bigger than 1 and they assigned it to the presence of interface trap levels. The result which is very close to that obtained by us was reported by Zebbar et al. [19]. It was found that for the n-ZnO/p-Si heterojunction prepared by ultrasonic spray at low bias the forward conduction is dominated by multi-step tunneling current with a main contribution of the defect level located at 0.14 eV below the minimum of the conduction band edge, close to the values 0.11 and 0.15 eV determined by us. The acceptor density in p-Si of the 0.1Ucm resistivity equals Na~3 1017 cm3. The donor density in Zn0.9Mg0.1O equals 1017 1018 cm3 [29]. Similar values were obtained from the C-V measurements. Thus it may be assumed that the depletion region extends on both sides of the p-Si/n-ZnO HJ. It means that the traps
In summary, the catalyst-free growth of self-assembled ZnMgO nanocolumns with LT buffer layer on Si (111) by PA-MBE was achieved. We have grown a series of ZnO/ZnMgO MQWs with various well and barrier widths under well-controlled MBE conditions. PL analysis suggests that the MQWs show excitonic near-band edge emission at low temperature. CL measurements at room temperature confirm PL results. On top and bottom of the HJs the ohmic contacts were processed. I-V measurements performed for these diodes reveal dominant current mechanisms. At a low forward bias it is multitunneling capture-emission current while for the larger one e space charge limited current. The I-V curves confirm the presence of trap states at the n-ZnO/p-Si interface. C-V measurements yield charge accumulation at QWs for the diode B. DLTS studies yield signatures of these traps as well as those related to the presence of QWs. Acknowledgments The project was supported by the Polish National Science Centre (NCN) based on the decision No: DEC-2013/09/D/ST5/03881. References [1] Z.L. Wang, J. Phys. Condens. Matter 16 (2004) R829. [2] Z.P. Wei, Y.M. Lu, D.Z. Shen, Z.Z. Zhang, B. Yao, B.H. Li, J.Y. Zhang, D.X. Zhao, X.W. Fan, Z.K. Tang, Appl. Phys. Lett. 90 (2007) 042113. [3] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237. [4] E. Mollwo, in: O. Madelung, M. Schulz, H. Weiss (Eds.), Physics of II-VI and I-VII Compounds, Semimagnetic Semiconductors vol. 17, Springer, Berlin, 1982, p. 35. [5] X. Du, Y. Sun, Q. Wang, N. Ma, Z. Yuan, J. Optoelectron. Adv. M. 13 (2011) 631. [6] M.A. Pietrzyk, M. Stachowicz, A. Wierzbicka, P. Dluzewski, D. Jarosz, E. Przezdziecka, A. Kozanecki, J. Cryst. Growth 408 (2014) 102. [7] G. Coli, K.K. Bajaj, Appl. Phys. Lett. 78 (2001) 2861. [8] K.J. Button, D.R. Cohn, M. von Ortenbert, B. Lax, E. Mollwo, R. Helbig, Phys. Rev. Lett. 28 (1972) 1637. [9] K. Hummer, Phys. Stat. Sol. b Basic Res. 56 (1973) 249. [10] R.T. Senger, K.K. Bajaj, Phys. Rev. B 68 (2003) 205314. [11] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [12] H. Matsuura, T. Okuno, H. Okushi, K. Tanaka, J. Appl. Phys. 55 (1984) 1012. [13] M.A. Lampert, P. Mark, Current Injections in Solids, 1978. New York, London. [14] R.L. Anderson, IBM J. Res. Dev. 4 (1960) 283. [15] J.D. Ye, S.L. Gu, S.M. Zhu, W. Liu, S.M. Liu, R. Zhang, Y. Shi, Y.D. Zheng, Appl. Phys. Lett. 88 (2006) 182112. [16] J.H. He, C.H. Ho, Appl. Phys. Lett. 91 (2007) 233105. [17] P. Blood, J.W. Orton, in: The Electrical Characterization of Semiconductors: Majority Carriers and Electron States, Academic Press, NY, 1992. [18] M. Trunk, V. Venkatachalapathy, A. Galeckas, A. Yu. Kuznetsov, Appl. Phys. Lett. 97 (2010) 211901. [19] N. Zebbar, Y. Kheireddine, K. Mokeddem, A. Hafdallah, M. Kechouane, M.S. Aida, Mat. Sci. Semicond. Proc. 14 (2011) 229. [20] Y.S. Ocak, J. Alloys Compd. 513 (2012) 130. [21] F. Yakuphanoglu, Y. Caglar, M. Caglar, S. Ilican, Mat. Sci. Semicond. Proc. 13
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