- Email: [email protected]
Structural and optical properties of high quality ZnO thin film on Si with SiC buffer layer
Thin Solid Films 520 (2012) 6836–6840
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structural and optical properties of high quality ZnO thin film on Si with SiC buffer layer A. Osipov a, S.A. Kukushkin a, N.A. Feoktistov a, A. Osipova a, N. Venugopal b, G.D. Verma b, Bipin Kumar Gupta c, Anirban Mitra b,⁎ a b c
Institute of Problems of Mechanical Engineering, Russian Academy of Sciences V.O., Bolshoj pr., 61, St. Petersburg 199178, Russia Dept. of Physics, I.I.T., Roorkee 247667, Uttarakhand, India National Physical Laboratory (CSIR), Dr. K.S. Krishnan Road, New Delhi 110012, India
a r t i c l e
i n f o
Article history: Received 22 June 2011 Received in revised form 28 June 2012 Accepted 2 July 2012 Available online 27 July 2012 Keywords: Zinc oxide Thin films Silicon carbide Buffer layer Epitaxial film Sputtering
a b s t r a c t ZnO thin films are grown on Si substrates with SiC buffer layer using ion plasma high frequency magnetron sputtering. These substrates are fabricated using a technique of solid phase epitaxy. With this technique SiC layer of thickness 20–200 nm had been grown on Si substrates consisting pores of sizes 0.5–5 μm at SiC and Si interface. Due to mismatching in lattice constants as well as thermal expansion coefficients, elastic stresses have been developed in ZnO film. Pores at the interface of SiC and Si are acting as the elastic stress reliever of the ZnO films making them strain free epitaxial. ZnO film grown on this especially fabricated Si substrate with SiC buffer layer exhibits excellent crystalline quality as characterized using X-ray diffraction. Surface topography of the film has been characterized using Atomic Force Microscopy as well as Scanning Electron Microscopy. Chemical compositions of the films have been analyzed using Energy Dispersive X-ray Spectroscopy. Optical properties of the films are investigated using Photoluminescence Spectroscopy which also shows good optical quality. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently a lot of interest has grown among the researchers about wide band gap semiconductors because of their wide applications for fabrication of UV light sources and detectors. GaN and its family are among one of those semiconductors from which light emitting diodes and lasers have already been made [1]. Main problem of GaN lies in suitable lattice matching substrates and cost of growing epitaxial film with complex process like Molecular Beam Epitaxy [2]. ZnO is also another wide band gap semiconductor with a band gap of 3.36 eV same like that of GaN (3.44 eV). In addition to this ZnO also has large excitonic binding energy of 60 meV higher than that of GaN which is 25 meV. Owing to these properties and cost effectiveness of simple processes for growing thin films, it has the potential as an alternative semiconductor to substitute GaN in near future [3]. At this moment major challenge is to synthesize p type ZnO. Finding a suitable substrate for ZnO is also a major task. ZnO thin films have been grown on different substrates like α-Al2O3 [4], GaN [5], AlN [6], GaAs [7], ScAlMgO4 [8], 3C-SiC [9], 4H-SiC [10], 6H-SiC [11,12], etc. Among these substrates GaN and AlN have very close lattice matching with ZnO. But these substrates are expensive. 6H-SiC and 4H-SiC also have close lattice matching with ZnO along a-axis. Si is ⁎ Corresponding author at: Dept. of Physics, I.I.T., Roorkee 247667, Uttarakhand, India. Tel.: +91 1332 285652; fax: +91 1332 286662. E-mail address: [email protected] (A. Mitra). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.07.094
an important material in electronics industry because of its low cost and excellent properties for making electronic devices. Due to this reason researches are giving lot of efforts to grow thin film on Si substrates so that an integrated electronic component could be made. Unfortunately ZnO and Si have a very poor lattice matching in their lattice constants as well as thermal expansion coefficients which make it difficult to grow epitaxial ZnO thin film directly on Si. Researchers have been trying in different ways to eliminate this problem. It has been demonstrated that one of the ways to reduce this lattice mismatch is the deposition of ZnO on porous silicon [13,14]. Pores in the silicon can release the residual elastic stress developed due to the lattice mismatch between ZnO and Si. Another way to grow ZnO on Si substrates is with buffer layer of SiC on Si. There are several reports of growing ZnO on Si substrates with SiC buffer layer [15,16]. Z.D. Sha et al. had demonstrated that with SiC buffer layer lattice mismatching between ZnO and SiC becomes 5% [15]. As a consequence it shows an excellent crystal quality. J. Zhu et al. had demonstrated that using 3C-SiC buffer layer lattice mismatch will be tuned to almost attain the quality like single crystal as well as excellent optical properties [9]. Another advantage with 6H-SiC is that it is a naturally p type wide gap semiconductor with a band gap of 3.02 eV. There are several reports of fabrication of blue heterojunction Light Emitting Diodes and Diode Lasers with ZnO/p-SiC (4H) [17,18]. Though there is excellent lattice matching between SiC and ZnO along the a axis, some mismatch of the order of ~5% still exists. Due to this lattice mismatching a certain amount of elastic stress has been developed inside the ZnO
A. Osipov et al. / Thin Solid Films 520 (2012) 6836–6840
6837
thin film. One of the authors Kukushkin and his coworkers have developed a technique to remove this elastic stress [19]. They have grown SiC on Si (111) and Si(100) substrates using solid state epitaxy with carbon monoxide. These substrates have pores of sizes 0.5–5 μm at the interface of SiC/Si with a layer of SiC of thickness 20–200 nm on top. These pores serve the purpose as stress reliever of the SiC buffer layer due to its lattice mismatching with ZnO. Here in this paper we have presented some of the preliminary results of deposition of ZnO thin films on Si substrates with SiC buffer layer using ion plasma high frequency magnetron sputtering.
2. Experimental techniques SiC films are grown on Si through the reaction between crystalline silicon and gaseous carbon monoxide as the following: 2SiðcrÞ þ COðgasÞ ¼ SiCðcrÞ þ SiOðgasÞ↑: In this reaction, two Si atoms go into the formation of one SiC molecule, because one Si atom escapes from the system with gaseous SiO. As a result, a great number of vacancies and pores should form near the Si and SiC interface. The total number of voids should be approximately equal to the volume of the grown film. Following the above procedure 2.5 mm thick Si(111) and Si(100) substrates are exposed to carbon monoxide atmosphere at a pressure of p = 10–300 Pa in a vacuum furnace at T = 1100–1400 °C for 5–60 min. Within this time, a silicon carbide film of thickness 20–200 nm is formed on the silicon substrate's surface. The details of the procedure and modeling of the chemical kinetics are given in Ref. [19]. Schematic representation of the ZnO film on SiC/Si is shown in Fig. 1. Scanning Electron Microscope image of the same is shown in Fig. 2 illustrating the formation of voids at SiC/Si interface with SiC layer on its surface without ZnO film. Extensive use of X-ray, electron diffraction and luminescence analysis revealed that either hexagonal 4H polytype or cubic 3C polytype is formed more frequently and sometimes a mixture of them is grown. ZnO films are then deposited on this substrate of Si with SiC buffer layer using ion-plasma high frequency magnetron sputtering (Model Leybold Z-400) of a ceramic ZnO target under an ambient atmosphere of oxygen at a pressure of 3 Pa. 150 W power is introduced to the magnetron source and the substrate temperature is kept at 550 °C. Growth rate of the film is maintained at 0.3 μm per hour to grow 2–3 μm thick ZnO layer with 150–200 nm buffer layer of SiC. After the deposition ZnO thin film is cooled in pure oxygen at a rate of 3 °C per minute. Sizes of the pores in Si are varied from 0.5 to 5 μm. Pore size can easily be controlled by changing the synthesis parameters of the SiC film like temperature, pressure of CO gas and time duration of chemical reaction. For example if the film is synthesized
Fig. 2. SEM photograph of the cross sectional view of SiC layer on the Si substrate with pores at the interface of SiC and Si without ZnO.
at 1250 °C, for 20 min under the ambient gas pressure at 12 Pa of CO, the pore size will be approximately 1 μm. If the film is grown under CO pressure at 25 Pa while keeping the other parameters to remain unchanged, pore size becomes smaller roughly equal to 0.3 μm. By lowering the synthesis temperature up to 1200 °C, pore size can be further reduced to 0.15 μm. In Si(100) the pores are mostly of triangular shape, whereas in Si(111) they have indefinite complicated shape. The main role of the pores is to release the residual stress in the system due to the lattice mismatch between SiC and ZnO. We hope that the main part of the dislocations goes to Si and concentrates near pores in the bulk of Si. Crystal structure and orientations of the ZnO films have been characterized using X-ray diffractometer (XRD) (AXS BRUKER, Model: D8 Advance) with CuKα source in the Bragg–Brentano geometry (λ= 1.54 Ǻ). Morphology of the films has been characterized using Atomic Force Microscopy (AFM) (NT-MDT Ntegra, Prima) in semi-contact mode using a SiC probe with probe diameter of 6 nm and Scanning Electron Microscope (SEM) (Quanta 200F FEG & FEI, Netherlands) operating at 20 kV. Chemical compositions of the films have been analyzed using Energy Dispersive X-ray Spectroscopy (EDX). For EDX, X-FEG probe current was 0.4 nA in 0.31 nm spot with energy resolutionb 136 eV at Mn–K and 10 kcps with lithium drifted silicon detector. Optical properties of these films have been investigated using photoluminescence spectroscopy (PL). 3. Results and discussion X-ray diffraction pattern of ZnO thin film as shown in Fig. 3 exhibits a main peak attributed to (002) plane at 2θ = 34.58°. Others peaks belong to (100), (101), (102) and (103) planes at angles 2θ = 31.20°, 36.80°, 47.77° and 63.04° respectively. This indicates the existence of single phase ZnO with hexagonal wurtzite structure. Intensities of the peaks other than due to (002) plane, are very weak. Therefore we can say that the crystallites are highly oriented along the c-axis perpendicular to the substrate. The degree of orientation of the film can be measured from its XRD pattern by using a formula proposed by Lotgering [20]. The degree of preferred orientation fhkl of a particular plane (khl) can be written as f hkl ¼
P ðhklÞ−P 0 ðhklÞ 1−P 0 ðhklÞ I
Ihkl 0 hkl and P 0 ¼ . Here I0(hkl) is the (hkl) peak ∑I ðhklÞ ∑I 0 ðhklÞ intensity and ∑I 0 ðhklÞ is the sum of the intensities of all peaks recorded in the powder diffraction data of ZnO powder [21]. I(hkl)
where P hkl ¼
Fig. 1. Schematic diagram of the structure of ZnO thin film on the Si substrate with SiC buffer layer.
ð1Þ
6838
A. Osipov et al. / Thin Solid Films 520 (2012) 6836–6840
Fig. 3. X-ray diffraction pattern of the ZnO thin film on the Si substrate with SiC buffer layer.
is the (hkl) peak intensity and ∑I ðhklÞ is the sum of the intensities of all the peaks in the diffraction data of the ZnO thin film deposited on Si substrates with SiC buffer layer. The calculated degree of orientation of (002) peak is 0.9633. This is the natural tendency of wurtzite hexagonal phase of ZnO; to grow along the (002) plane which has minimum surface energy. It had been shown by Fujimara et al. [22] that the (002) plane has the lowest surface free energy. Thus minimization of surface energy favors (002) texture ZnO film [23–27]. Therefore in most of the cases, films grow with the (002) plane parallel to the surface of the substrate, thus, minimizing the surface energy of the film. Fragmented rings in the Electron diffraction pattern of the ZnO film also reveal that the ZnO film is polycrystalline. There is no amorphous phase. Considering XRD and electron diffraction pattern we can say that the ZnO films are polycrystalline with wurtzite hexagonal structure preferentially oriented along the c-axis. We believe that there are two ways in which one can get the benefits for growing good crystalline quality. One is due to SiC buffer layer (4H or 3C) that has good lattice matching with ZnO along a axis of around ~ 5% [9,15] and another way is the removal of the residual elastic stress due to the small lattice mismatching between ZnO and SiC through the pores inside the Si. The amount of residual stress σ in ZnO thin film deposited on SiC/Si substrate is derived according to the following equation [28]: σ ¼ −233
cfilm −cbulk ðGpaÞ cbulk
ð2Þ
where cfilm is the lattice parameter of the c axis for the ZnO films and cbulk is the lattice parameter of ZnO in the bulk (or powder), namely, the unstrained lattice parameter. The numerical values of the cfilm are calculated from XRD data according to the Bragg's equation: 2dhkl sin θ ¼ λ
ð3Þ
where dhkl is the lattice spacing of (hkl), λ is the wavelength of CuKα radiation (1.54 Å) and θ is the Bragg's angle (half of the peak position angle). At the same time ZnO has wurtzite hexagonal structure which follows the formula: 2
2
1 4 h þ hk þ k ¼ a2 d2hkl 3
!
2
þ
l c2
ð4Þ
where a and c are the lattice constants. The value of the cfilm, the lattice parameter of the c axis for the ZnO film calculated using Eqs. (3) and (4) is 0.518 nm. The lattice constant cbulk for the strain-free bulk ZnO is 0.5206 nm as taken from the JCPDS (#36-1451) data card. The calculated stress using Eq. (2) for ZnO thin film is 0.89 Gpa which is
quite low. Now we analyze the source of tensile stress in the ZnO film on the SiC buffer layer. There are three sources that affect the stress in an epitaxial film. The first one is the lattice mismatch between the film and the buffer layer. Lattice constants of ZnO wurtzite hexagonal structure are a = 3.252 Å and c = 5.206 Å and those for 3C-SiC (cubic) and 4H-SiC (hexagonal) are a = 4.36 Å; a = 3.07 Å, c = 10.0 Å as depicted in Table 1 with other properties. Although the crystal structure and the lattice constant of 3C-SiC are different from that of ZnO and Si, the mismatch between ZnO and Si can be reduced with the insertion of a 3C-SiC buffer layer because the atomic arrangement of the (111) plane of 3C-SiC is similar to that of the (0001) plane of 6H-SiC and ZnO. J. Zhu et al. proposed a structural model to explain the reason [9]. They have shown that in the ZnO direction (0001) plane, the shortest atom distance along the 1120 is 3.083 Å. The lattice mismatch is 3.252 Å, and that of SiC along 110 is only 5.3%, which is much between ZnO 1120 and SiC along 110 smaller than that between ZnO and Si (16.6%). There is an excellent lattice matching between 4H-SiC (a = 3.07 Å) and ZnO (3.249 Å) along the a axis which is around 5.4%. But along the c axis lattice parameter of 4H-SiC is almost double than that of ZnO. This can also generate stress in the ZnO film [10]. The second factor is the thermal mismatch. As the Si substrate is much thicker than the SiC buffer layer and the ZnO film, we will consider only the thermal mismatch between the ZnO film and Si substrate. Due to the fact that thermal expansion coefficient Si (2.6 × 10 6/K) and ZnO (2.9 × 10 6/K and 4.75 × 10 6/K) are nearly equal and as no annealing treatment has done, we can exclude the thermal mismatching in our ZnO/SiC/Si thin film. The last main mechanism is that coalescence of grain or islands could generate tensile stress in the film during the growth [29]. At the beginning during the quick growth of ZnO thick film, the atoms do not have much time to move laterally, thus there will be many individual islands. As the film continues to grow, these islands can grow both vertically and laterally until they coalesce. When the incremental decrease in surface energy produced by coalescence is greater than the incremental increase in elastic strain energy associated with the islands being under tension, coalescence of islands occurs. During this process, the tension is introduced into the film. In our ZnO/SiC/Si structure we exclude the factor of thermal mismatching and attribute the reason of tensile stress in the ZnO film to the coalescence of islands or grains during growth and lattice mismatching between ZnO and SiC. SEM pictures and EDX analysis of ZnO thin films are shown in Fig. 4(a) and (b) respectively. Table 2 shows that ratio of wt.% of Zn and O is 90:10 and at.% is 68:32. The unassigned peak in the EDX analysis is due to the gold coating required for capturing SEM photograph. Sizes of the particles are around 100 nm. AFM picture of the ZnO film shown in Fig. 5 has been taken across the area of 10 × 10 μm 2. The particles look like the hexagonal structure. Distributions of the particles show that most of the particles have the size of 100 nm. PL spectra of the ZnO samples have been measured using a Perkin-Elmer spectrometer (Model No. LS-55) selected at excitation wavelength of 373 nm of a Xenon lamp. We choose this wavelength
Table 1 Properties of ZnO, Si and SiC. Material
ZnO
Si
3C-SiC
4H-SiC
Eg (eV) Lattice constant (Å)
3.36 a = 3.25 c = 5.206 2.9 a axis 4.75 c axis 0.6
1.12 a = 5.43
2.3 a = 4.36
2.6
3.3
1.48
4.9
3.26 a = 3.07 c = 10 2 a axis 2.2 c axis 3.7
Coefficient of thermal expansion (10−6/K) Coefficient of thermal conductivity (W/cm K)
A. Osipov et al. / Thin Solid Films 520 (2012) 6836–6840
6839
1 µm
Fig. 5. AFM topographical image of the surface of the ZnO thin film on the Si substrate with SiC buffer layer.
of five intrinsic defects in ZnO film, such as zinc vacancy VZn, oxygen vacancy VO, interstitial zinc Zni, interstitial oxygen Oi, and antisite oxygen OZn. Sun calculated the energy levels of various intrinsic defects in ZnO by applying full-potential linear muffin-tin orbital method as shown in Fig. 7 [33]. According to Sun's calculation, the calculated energy interval from the upside of the valence band to interstitial zinc (Zni) level is 2.9 eV, which is well consistent with the violet emission peak centered at around 426 nm (or 2.921 eV) of the ZnO film. So, the violet emission of the ZnO film may be attributed to the energy transition of electrons from the upside of the valence band Fig. 4. (a) SEM image and (b) EDX spectra of the ZnO thin film on the Si substrate with SiC buffer layer.
using Photoluminescence Excitation (PLE) method by varying the wavelength of the source where the amount of emission from the sample is maximum. Then we fixed the excitation wavelength at 373 nm and observed a strong peak at 426 nm as shown in Fig. 6. Generally photoluminescence spectrum of ZnO thin films shows a UV emission with a sharp peak around 388 nm which attributed to near band edge emission and broad peaks in the visible range 450–730 nm related to deep level emission [30]. Both of them greatly depend upon preparation method and conditions. As our films are highly oriented along the (002) plane, absence of any peak in the UV region is unexpected. It may be due to the excitation wavelength at 373 nm which is just above the band gap. It is our experimental limitation that in the region well below the band gap of ZnO i.e. 366 nm (3.34 eV) power of the excitation source is very weak. UV emission could be observed if it is excited with laser at wavelength well below the band gap of ZnO where the absorption is very strong. D.H. Zhang et al. [31] have also observed that UV emission did not appear when excited with shorter wavelength light. Visible emissions are related to various intrinsic defects in ZnO [32]. There are reports
Fig. 6. PL spectrum of the ZnO thin film on the Si substrate with SiC buffer layer excited with a source at 373 nm.
Table 2 Composition of the elements of the ZnO film deposited on Si with SiC buffer layer as observed from the EDX. Element
wt.%
at.%
OK ZnK Matrix
10 90 Correction
32 68 ZAF
Fig. 7. Calculated energy defect levels by Sun [33] for the ZnO thin film.
6840
A. Osipov et al. / Thin Solid Films 520 (2012) 6836–6840
to the interstitial zinc level (Zni). Main origin of the interstitial zinc defect in the ZnO thin films could come from two aspects. One is the influence of ambient oxygen pressure for deposition of the ZnO thin film. In present experiment, the very low oxygen pressure (O2 pressure is 3 Pa) plays a crucial role for the formation of interstitial zinc (Zni) defects. Low oxygen pressure means shortage of oxygen. The deficiency of oxygen instigates the formation of interstitial zinc defects. Second aspect is the annealing temperature. If the annealing temperature is higher than the melting temperature (419 °C) of the metal zinc then oxidation carries out at an unstable liquid state. It can be considered that the concentration of the interstitial zinc defects in liquid zinc could be increased with increasing annealing temperature. However, after being Zn oxidized to form ZnO, because the annealing temperature compared with the melting temperature (1973 °C) of ZnO became very low, the diffusion velocity of Zn atoms in ZnO should be slower. Therefore, the interstitial zinc defects which previously existed in liquid Zn may be greatly prevailed in the ZnO films. As a result, a lot of interstitial zinc defects cause strong violet emission which could be observed in PL spectra if the annealing temperature is higher than the melting point of zinc. However, too much high temperature promotes the evaporation of Zn atoms from the ZnO thin film which in turn decrease the concentration of interstitial Zn defects. As a consequence a certain drop in the intensity level of violet emission could be observed. However this effect can be excluded in our case as we have not annealed our samples. Further study is required to find the effect of oxygen pressure as well as annealing temperature on the violet emission peak for our samples. In fact it is possible to obtain ZnO thin films with different photoluminescence properties by varying its structure through different preparation methods. B.J. Jin et al. and Z.D. Shah et al. [30,16] also had observed the same kind of peak around 413 nm from the ZnO film on Si with buffer layer of SiC. In their case with increasing annealing temperature the interface between ZnO and SiC is becoming more and more impacted and complicated which affect the interface trap existing in depletion regions located at the grain boundaries and result in enhancement of PL intensity. As the crystal quality in our case is good it is very unlikely that visible emission in our case at 426 nm is happening due to interface trapping. 4. Conclusions In conclusion we have deposited ZnO thin films on SiC/Si substrates with pores in the interface of SiC and Si using high frequency ion plasma magnetron sputtering. These pores help to reduce the residual elastic stress developed between ZnO and SiC due to their lattice mismatching. Characterization of ZnO films using XRD, SEM, AFM and PL spectroscopy showed promising results. As SiC is a naturally p type semiconductor it may be used in the future with naturally n type
ZnO to fabricate p-SiC/n-ZnO heterojunction Light Emitting Diodes or Diode Lasers.
Acknowledgments This work is supported jointly by the Russian Foundation for Basic Research (RFBR) and the Department of Science and Technology (DST), India (grant no. DST-464-PHY).
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997.. [2] S.A. Kukushkin, A.V. Osipov, V.N. Vbessolov, B.K. Medvedev, V.K. Nevolin, K.A. Tcarik, Rev. Adv. Matter. Sci. 17 (2008) 1. [3] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [4] P. Fons, K. Iwata, S. Niki, A. Yamada, K. Matsubara, J. Cryst. Growth 201–202 (1999) 627. [5] H.J. Ko, S. Hong, Y. Chen, T. Yao, Thin Solid Films 409 (2002) 153. [6] A. Tiwari, M. Park, C. Jin, H. Wang, D. Kumar, J. Narayan, J. Mater. Res. 17 (2002) 2480. [7] W. Shih, M. Wu, Journal of Crystal Growth 137 (1994) 319. [8] A. Ohtomo, K. Tamura, K. Saikusa, K. Takashashi, T. Makino, Y. Segawa, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 2635. [9] Junjie Zhu, Bixia Lin, Xiankai Sun, Ran Yao, Chaoshu Shi, Fu. Zhuxi, Thin Solid Films 478 (2005) 218. [10] Ji-Hong Kim, Dae-Hyung Cho, Wonyong Lee, Byung-Moo Moon, Wook Bahng, Sang-Cheol Kim, Nam-Kyun Kim, Sang-Mo Koo, J. Alloys Compd. 489 (2010) 179. [11] B.M. Ataev, Ya.I. Alivov, E.V. Kalinina, V.V. Mamedov, G.A. Onushkin, S.Sh. Makhmudov, A.K. Omaev, Journal of Crystal Growth 275 (2005) e2471. [12] A.B.M.A. Ashrafi, B.-P. Zhang, N.T. Binh, K. Wakatsuki, Y. Segawa, Journal of Crystal Growth 275 (2005) e2439. [13] Hong Kai, J. Phys. Chem. Solids 70 (2009) 967. [14] R. Prabakaran, M. Peres, J. Non-Cryst. Solids 354 (2008) 2181. [15] Z.D. Sha, J. Wang, Z.C. Chen, A.J. Chen, Z.Y. Zhou, X.M. Wu, L.J. Zhuge, Physics Educ. 33 (2006) 263. [16] Z.D. Sha, X.M. Wu, L.J. Zhuge, Phys. Lett. A 355 (2006) 228. [17] S. Eunice, P. Leong, S.F. Yu, Adv. Mater. 18 (2006) 1685. [18] Clement Yuen, S.F. Yu, S.P. Lau, Rusli, T.P. Chen, Appl. Phys. Lett. 86 (2005) 241111. [19] S.A. Kukushkin, A.V. Osipov, Phys. Solid State 7 (2008) 1238. [20] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113. [21] JCPDS No. 36-1451 [22] N. Fujimara, T. Nishibara, S. Goto, Journal of Crystal Growth 130 (1993) 269. [23] Y. Zhang, H. Zheng, J. Su, B. Lin, Z. Fu, J. Lumin. 124 (2007) 252. [24] H.D. Li, S.F. Yu, A.P. Abiyasa, C. Yuen, S.P. Lau, H.Y. Yang, E.S.P. Leong, Appl. Phys. Lett. 86 (2005) 261111. [25] Y. Lee, Y.J. Kim, J. Mater. Res. 13 (1998) 1260. [26] J. Myong, W. Yoon, D. Lee, I. Yun, S. Bea, Jpn. J. Appl. Phys. 41 (2002) 28. [27] D. Bao, H. Gun, A. Kuang, Thin Solid Films 312 (1998) 37. [28] D. Dimova-Malinovska, H. Nichev, O. Angelov, Phys Sat. Sol. (c) 5 (2008) 3353. [29] E.V. Etzcorn, D.R. Clarke, J. Appl. Phys. 89 (2001) 1025. [30] B.J. Jin, S. Im, S.Y. Lee, Thin Solid Films 366 (2000) 107. [31] D.H. Zhang, Q.P. Wang, Z.Y. Xue, Appl. Surf. Sci. 207 (2003) 20. [32] Zhao Lei, Lian Jian-She, Liu Yu-Hua, Jiang Quing, Trans. Nonferrous Met. Soc. China 18 (2008) 145. [33] Y.M. Sun, Ph.D thesis, University of Science and Technology of China, July, 2000.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structural and optical properties of high quality ZnO thin film on Si with SiC buffer layer A. Osipov a, S.A. Kukushkin a, N.A. Feoktistov a, A. Osipova a, N. Venugopal b, G.D. Verma b, Bipin Kumar Gupta c, Anirban Mitra b,⁎ a b c
Institute of Problems of Mechanical Engineering, Russian Academy of Sciences V.O., Bolshoj pr., 61, St. Petersburg 199178, Russia Dept. of Physics, I.I.T., Roorkee 247667, Uttarakhand, India National Physical Laboratory (CSIR), Dr. K.S. Krishnan Road, New Delhi 110012, India
a r t i c l e
i n f o
Article history: Received 22 June 2011 Received in revised form 28 June 2012 Accepted 2 July 2012 Available online 27 July 2012 Keywords: Zinc oxide Thin films Silicon carbide Buffer layer Epitaxial film Sputtering
a b s t r a c t ZnO thin films are grown on Si substrates with SiC buffer layer using ion plasma high frequency magnetron sputtering. These substrates are fabricated using a technique of solid phase epitaxy. With this technique SiC layer of thickness 20–200 nm had been grown on Si substrates consisting pores of sizes 0.5–5 μm at SiC and Si interface. Due to mismatching in lattice constants as well as thermal expansion coefficients, elastic stresses have been developed in ZnO film. Pores at the interface of SiC and Si are acting as the elastic stress reliever of the ZnO films making them strain free epitaxial. ZnO film grown on this especially fabricated Si substrate with SiC buffer layer exhibits excellent crystalline quality as characterized using X-ray diffraction. Surface topography of the film has been characterized using Atomic Force Microscopy as well as Scanning Electron Microscopy. Chemical compositions of the films have been analyzed using Energy Dispersive X-ray Spectroscopy. Optical properties of the films are investigated using Photoluminescence Spectroscopy which also shows good optical quality. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently a lot of interest has grown among the researchers about wide band gap semiconductors because of their wide applications for fabrication of UV light sources and detectors. GaN and its family are among one of those semiconductors from which light emitting diodes and lasers have already been made [1]. Main problem of GaN lies in suitable lattice matching substrates and cost of growing epitaxial film with complex process like Molecular Beam Epitaxy [2]. ZnO is also another wide band gap semiconductor with a band gap of 3.36 eV same like that of GaN (3.44 eV). In addition to this ZnO also has large excitonic binding energy of 60 meV higher than that of GaN which is 25 meV. Owing to these properties and cost effectiveness of simple processes for growing thin films, it has the potential as an alternative semiconductor to substitute GaN in near future [3]. At this moment major challenge is to synthesize p type ZnO. Finding a suitable substrate for ZnO is also a major task. ZnO thin films have been grown on different substrates like α-Al2O3 [4], GaN [5], AlN [6], GaAs [7], ScAlMgO4 [8], 3C-SiC [9], 4H-SiC [10], 6H-SiC [11,12], etc. Among these substrates GaN and AlN have very close lattice matching with ZnO. But these substrates are expensive. 6H-SiC and 4H-SiC also have close lattice matching with ZnO along a-axis. Si is ⁎ Corresponding author at: Dept. of Physics, I.I.T., Roorkee 247667, Uttarakhand, India. Tel.: +91 1332 285652; fax: +91 1332 286662. E-mail address: [email protected] (A. Mitra). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.07.094
an important material in electronics industry because of its low cost and excellent properties for making electronic devices. Due to this reason researches are giving lot of efforts to grow thin film on Si substrates so that an integrated electronic component could be made. Unfortunately ZnO and Si have a very poor lattice matching in their lattice constants as well as thermal expansion coefficients which make it difficult to grow epitaxial ZnO thin film directly on Si. Researchers have been trying in different ways to eliminate this problem. It has been demonstrated that one of the ways to reduce this lattice mismatch is the deposition of ZnO on porous silicon [13,14]. Pores in the silicon can release the residual elastic stress developed due to the lattice mismatch between ZnO and Si. Another way to grow ZnO on Si substrates is with buffer layer of SiC on Si. There are several reports of growing ZnO on Si substrates with SiC buffer layer [15,16]. Z.D. Sha et al. had demonstrated that with SiC buffer layer lattice mismatching between ZnO and SiC becomes 5% [15]. As a consequence it shows an excellent crystal quality. J. Zhu et al. had demonstrated that using 3C-SiC buffer layer lattice mismatch will be tuned to almost attain the quality like single crystal as well as excellent optical properties [9]. Another advantage with 6H-SiC is that it is a naturally p type wide gap semiconductor with a band gap of 3.02 eV. There are several reports of fabrication of blue heterojunction Light Emitting Diodes and Diode Lasers with ZnO/p-SiC (4H) [17,18]. Though there is excellent lattice matching between SiC and ZnO along the a axis, some mismatch of the order of ~5% still exists. Due to this lattice mismatching a certain amount of elastic stress has been developed inside the ZnO
A. Osipov et al. / Thin Solid Films 520 (2012) 6836–6840
6837
thin film. One of the authors Kukushkin and his coworkers have developed a technique to remove this elastic stress [19]. They have grown SiC on Si (111) and Si(100) substrates using solid state epitaxy with carbon monoxide. These substrates have pores of sizes 0.5–5 μm at the interface of SiC/Si with a layer of SiC of thickness 20–200 nm on top. These pores serve the purpose as stress reliever of the SiC buffer layer due to its lattice mismatching with ZnO. Here in this paper we have presented some of the preliminary results of deposition of ZnO thin films on Si substrates with SiC buffer layer using ion plasma high frequency magnetron sputtering.
2. Experimental techniques SiC films are grown on Si through the reaction between crystalline silicon and gaseous carbon monoxide as the following: 2SiðcrÞ þ COðgasÞ ¼ SiCðcrÞ þ SiOðgasÞ↑: In this reaction, two Si atoms go into the formation of one SiC molecule, because one Si atom escapes from the system with gaseous SiO. As a result, a great number of vacancies and pores should form near the Si and SiC interface. The total number of voids should be approximately equal to the volume of the grown film. Following the above procedure 2.5 mm thick Si(111) and Si(100) substrates are exposed to carbon monoxide atmosphere at a pressure of p = 10–300 Pa in a vacuum furnace at T = 1100–1400 °C for 5–60 min. Within this time, a silicon carbide film of thickness 20–200 nm is formed on the silicon substrate's surface. The details of the procedure and modeling of the chemical kinetics are given in Ref. [19]. Schematic representation of the ZnO film on SiC/Si is shown in Fig. 1. Scanning Electron Microscope image of the same is shown in Fig. 2 illustrating the formation of voids at SiC/Si interface with SiC layer on its surface without ZnO film. Extensive use of X-ray, electron diffraction and luminescence analysis revealed that either hexagonal 4H polytype or cubic 3C polytype is formed more frequently and sometimes a mixture of them is grown. ZnO films are then deposited on this substrate of Si with SiC buffer layer using ion-plasma high frequency magnetron sputtering (Model Leybold Z-400) of a ceramic ZnO target under an ambient atmosphere of oxygen at a pressure of 3 Pa. 150 W power is introduced to the magnetron source and the substrate temperature is kept at 550 °C. Growth rate of the film is maintained at 0.3 μm per hour to grow 2–3 μm thick ZnO layer with 150–200 nm buffer layer of SiC. After the deposition ZnO thin film is cooled in pure oxygen at a rate of 3 °C per minute. Sizes of the pores in Si are varied from 0.5 to 5 μm. Pore size can easily be controlled by changing the synthesis parameters of the SiC film like temperature, pressure of CO gas and time duration of chemical reaction. For example if the film is synthesized
Fig. 2. SEM photograph of the cross sectional view of SiC layer on the Si substrate with pores at the interface of SiC and Si without ZnO.
at 1250 °C, for 20 min under the ambient gas pressure at 12 Pa of CO, the pore size will be approximately 1 μm. If the film is grown under CO pressure at 25 Pa while keeping the other parameters to remain unchanged, pore size becomes smaller roughly equal to 0.3 μm. By lowering the synthesis temperature up to 1200 °C, pore size can be further reduced to 0.15 μm. In Si(100) the pores are mostly of triangular shape, whereas in Si(111) they have indefinite complicated shape. The main role of the pores is to release the residual stress in the system due to the lattice mismatch between SiC and ZnO. We hope that the main part of the dislocations goes to Si and concentrates near pores in the bulk of Si. Crystal structure and orientations of the ZnO films have been characterized using X-ray diffractometer (XRD) (AXS BRUKER, Model: D8 Advance) with CuKα source in the Bragg–Brentano geometry (λ= 1.54 Ǻ). Morphology of the films has been characterized using Atomic Force Microscopy (AFM) (NT-MDT Ntegra, Prima) in semi-contact mode using a SiC probe with probe diameter of 6 nm and Scanning Electron Microscope (SEM) (Quanta 200F FEG & FEI, Netherlands) operating at 20 kV. Chemical compositions of the films have been analyzed using Energy Dispersive X-ray Spectroscopy (EDX). For EDX, X-FEG probe current was 0.4 nA in 0.31 nm spot with energy resolutionb 136 eV at Mn–K and 10 kcps with lithium drifted silicon detector. Optical properties of these films have been investigated using photoluminescence spectroscopy (PL). 3. Results and discussion X-ray diffraction pattern of ZnO thin film as shown in Fig. 3 exhibits a main peak attributed to (002) plane at 2θ = 34.58°. Others peaks belong to (100), (101), (102) and (103) planes at angles 2θ = 31.20°, 36.80°, 47.77° and 63.04° respectively. This indicates the existence of single phase ZnO with hexagonal wurtzite structure. Intensities of the peaks other than due to (002) plane, are very weak. Therefore we can say that the crystallites are highly oriented along the c-axis perpendicular to the substrate. The degree of orientation of the film can be measured from its XRD pattern by using a formula proposed by Lotgering [20]. The degree of preferred orientation fhkl of a particular plane (khl) can be written as f hkl ¼
P ðhklÞ−P 0 ðhklÞ 1−P 0 ðhklÞ I
Ihkl 0 hkl and P 0 ¼ . Here I0(hkl) is the (hkl) peak ∑I ðhklÞ ∑I 0 ðhklÞ intensity and ∑I 0 ðhklÞ is the sum of the intensities of all peaks recorded in the powder diffraction data of ZnO powder [21]. I(hkl)
where P hkl ¼
Fig. 1. Schematic diagram of the structure of ZnO thin film on the Si substrate with SiC buffer layer.
ð1Þ
6838
A. Osipov et al. / Thin Solid Films 520 (2012) 6836–6840
Fig. 3. X-ray diffraction pattern of the ZnO thin film on the Si substrate with SiC buffer layer.
is the (hkl) peak intensity and ∑I ðhklÞ is the sum of the intensities of all the peaks in the diffraction data of the ZnO thin film deposited on Si substrates with SiC buffer layer. The calculated degree of orientation of (002) peak is 0.9633. This is the natural tendency of wurtzite hexagonal phase of ZnO; to grow along the (002) plane which has minimum surface energy. It had been shown by Fujimara et al. [22] that the (002) plane has the lowest surface free energy. Thus minimization of surface energy favors (002) texture ZnO film [23–27]. Therefore in most of the cases, films grow with the (002) plane parallel to the surface of the substrate, thus, minimizing the surface energy of the film. Fragmented rings in the Electron diffraction pattern of the ZnO film also reveal that the ZnO film is polycrystalline. There is no amorphous phase. Considering XRD and electron diffraction pattern we can say that the ZnO films are polycrystalline with wurtzite hexagonal structure preferentially oriented along the c-axis. We believe that there are two ways in which one can get the benefits for growing good crystalline quality. One is due to SiC buffer layer (4H or 3C) that has good lattice matching with ZnO along a axis of around ~ 5% [9,15] and another way is the removal of the residual elastic stress due to the small lattice mismatching between ZnO and SiC through the pores inside the Si. The amount of residual stress σ in ZnO thin film deposited on SiC/Si substrate is derived according to the following equation [28]: σ ¼ −233
cfilm −cbulk ðGpaÞ cbulk
ð2Þ
where cfilm is the lattice parameter of the c axis for the ZnO films and cbulk is the lattice parameter of ZnO in the bulk (or powder), namely, the unstrained lattice parameter. The numerical values of the cfilm are calculated from XRD data according to the Bragg's equation: 2dhkl sin θ ¼ λ
ð3Þ
where dhkl is the lattice spacing of (hkl), λ is the wavelength of CuKα radiation (1.54 Å) and θ is the Bragg's angle (half of the peak position angle). At the same time ZnO has wurtzite hexagonal structure which follows the formula: 2
2
1 4 h þ hk þ k ¼ a2 d2hkl 3
!
2
þ
l c2
ð4Þ
where a and c are the lattice constants. The value of the cfilm, the lattice parameter of the c axis for the ZnO film calculated using Eqs. (3) and (4) is 0.518 nm. The lattice constant cbulk for the strain-free bulk ZnO is 0.5206 nm as taken from the JCPDS (#36-1451) data card. The calculated stress using Eq. (2) for ZnO thin film is 0.89 Gpa which is
quite low. Now we analyze the source of tensile stress in the ZnO film on the SiC buffer layer. There are three sources that affect the stress in an epitaxial film. The first one is the lattice mismatch between the film and the buffer layer. Lattice constants of ZnO wurtzite hexagonal structure are a = 3.252 Å and c = 5.206 Å and those for 3C-SiC (cubic) and 4H-SiC (hexagonal) are a = 4.36 Å; a = 3.07 Å, c = 10.0 Å as depicted in Table 1 with other properties. Although the crystal structure and the lattice constant of 3C-SiC are different from that of ZnO and Si, the mismatch between ZnO and Si can be reduced with the insertion of a 3C-SiC buffer layer because the atomic arrangement of the (111) plane of 3C-SiC is similar to that of the (0001) plane of 6H-SiC and ZnO. J. Zhu et al. proposed a structural model to explain the reason [9]. They have shown that in the ZnO direction (0001) plane, the shortest atom distance along the 1120 is 3.083 Å. The lattice mismatch is 3.252 Å, and that of SiC along 110 is only 5.3%, which is much between ZnO 1120 and SiC along 110 smaller than that between ZnO and Si (16.6%). There is an excellent lattice matching between 4H-SiC (a = 3.07 Å) and ZnO (3.249 Å) along the a axis which is around 5.4%. But along the c axis lattice parameter of 4H-SiC is almost double than that of ZnO. This can also generate stress in the ZnO film [10]. The second factor is the thermal mismatch. As the Si substrate is much thicker than the SiC buffer layer and the ZnO film, we will consider only the thermal mismatch between the ZnO film and Si substrate. Due to the fact that thermal expansion coefficient Si (2.6 × 10 6/K) and ZnO (2.9 × 10 6/K and 4.75 × 10 6/K) are nearly equal and as no annealing treatment has done, we can exclude the thermal mismatching in our ZnO/SiC/Si thin film. The last main mechanism is that coalescence of grain or islands could generate tensile stress in the film during the growth [29]. At the beginning during the quick growth of ZnO thick film, the atoms do not have much time to move laterally, thus there will be many individual islands. As the film continues to grow, these islands can grow both vertically and laterally until they coalesce. When the incremental decrease in surface energy produced by coalescence is greater than the incremental increase in elastic strain energy associated with the islands being under tension, coalescence of islands occurs. During this process, the tension is introduced into the film. In our ZnO/SiC/Si structure we exclude the factor of thermal mismatching and attribute the reason of tensile stress in the ZnO film to the coalescence of islands or grains during growth and lattice mismatching between ZnO and SiC. SEM pictures and EDX analysis of ZnO thin films are shown in Fig. 4(a) and (b) respectively. Table 2 shows that ratio of wt.% of Zn and O is 90:10 and at.% is 68:32. The unassigned peak in the EDX analysis is due to the gold coating required for capturing SEM photograph. Sizes of the particles are around 100 nm. AFM picture of the ZnO film shown in Fig. 5 has been taken across the area of 10 × 10 μm 2. The particles look like the hexagonal structure. Distributions of the particles show that most of the particles have the size of 100 nm. PL spectra of the ZnO samples have been measured using a Perkin-Elmer spectrometer (Model No. LS-55) selected at excitation wavelength of 373 nm of a Xenon lamp. We choose this wavelength
Table 1 Properties of ZnO, Si and SiC. Material
ZnO
Si
3C-SiC
4H-SiC
Eg (eV) Lattice constant (Å)
3.36 a = 3.25 c = 5.206 2.9 a axis 4.75 c axis 0.6
1.12 a = 5.43
2.3 a = 4.36
2.6
3.3
1.48
4.9
3.26 a = 3.07 c = 10 2 a axis 2.2 c axis 3.7
Coefficient of thermal expansion (10−6/K) Coefficient of thermal conductivity (W/cm K)
A. Osipov et al. / Thin Solid Films 520 (2012) 6836–6840
6839
1 µm
Fig. 5. AFM topographical image of the surface of the ZnO thin film on the Si substrate with SiC buffer layer.
of five intrinsic defects in ZnO film, such as zinc vacancy VZn, oxygen vacancy VO, interstitial zinc Zni, interstitial oxygen Oi, and antisite oxygen OZn. Sun calculated the energy levels of various intrinsic defects in ZnO by applying full-potential linear muffin-tin orbital method as shown in Fig. 7 [33]. According to Sun's calculation, the calculated energy interval from the upside of the valence band to interstitial zinc (Zni) level is 2.9 eV, which is well consistent with the violet emission peak centered at around 426 nm (or 2.921 eV) of the ZnO film. So, the violet emission of the ZnO film may be attributed to the energy transition of electrons from the upside of the valence band Fig. 4. (a) SEM image and (b) EDX spectra of the ZnO thin film on the Si substrate with SiC buffer layer.
using Photoluminescence Excitation (PLE) method by varying the wavelength of the source where the amount of emission from the sample is maximum. Then we fixed the excitation wavelength at 373 nm and observed a strong peak at 426 nm as shown in Fig. 6. Generally photoluminescence spectrum of ZnO thin films shows a UV emission with a sharp peak around 388 nm which attributed to near band edge emission and broad peaks in the visible range 450–730 nm related to deep level emission [30]. Both of them greatly depend upon preparation method and conditions. As our films are highly oriented along the (002) plane, absence of any peak in the UV region is unexpected. It may be due to the excitation wavelength at 373 nm which is just above the band gap. It is our experimental limitation that in the region well below the band gap of ZnO i.e. 366 nm (3.34 eV) power of the excitation source is very weak. UV emission could be observed if it is excited with laser at wavelength well below the band gap of ZnO where the absorption is very strong. D.H. Zhang et al. [31] have also observed that UV emission did not appear when excited with shorter wavelength light. Visible emissions are related to various intrinsic defects in ZnO [32]. There are reports
Fig. 6. PL spectrum of the ZnO thin film on the Si substrate with SiC buffer layer excited with a source at 373 nm.
Table 2 Composition of the elements of the ZnO film deposited on Si with SiC buffer layer as observed from the EDX. Element
wt.%
at.%
OK ZnK Matrix
10 90 Correction
32 68 ZAF
Fig. 7. Calculated energy defect levels by Sun [33] for the ZnO thin film.
6840
A. Osipov et al. / Thin Solid Films 520 (2012) 6836–6840
to the interstitial zinc level (Zni). Main origin of the interstitial zinc defect in the ZnO thin films could come from two aspects. One is the influence of ambient oxygen pressure for deposition of the ZnO thin film. In present experiment, the very low oxygen pressure (O2 pressure is 3 Pa) plays a crucial role for the formation of interstitial zinc (Zni) defects. Low oxygen pressure means shortage of oxygen. The deficiency of oxygen instigates the formation of interstitial zinc defects. Second aspect is the annealing temperature. If the annealing temperature is higher than the melting temperature (419 °C) of the metal zinc then oxidation carries out at an unstable liquid state. It can be considered that the concentration of the interstitial zinc defects in liquid zinc could be increased with increasing annealing temperature. However, after being Zn oxidized to form ZnO, because the annealing temperature compared with the melting temperature (1973 °C) of ZnO became very low, the diffusion velocity of Zn atoms in ZnO should be slower. Therefore, the interstitial zinc defects which previously existed in liquid Zn may be greatly prevailed in the ZnO films. As a result, a lot of interstitial zinc defects cause strong violet emission which could be observed in PL spectra if the annealing temperature is higher than the melting point of zinc. However, too much high temperature promotes the evaporation of Zn atoms from the ZnO thin film which in turn decrease the concentration of interstitial Zn defects. As a consequence a certain drop in the intensity level of violet emission could be observed. However this effect can be excluded in our case as we have not annealed our samples. Further study is required to find the effect of oxygen pressure as well as annealing temperature on the violet emission peak for our samples. In fact it is possible to obtain ZnO thin films with different photoluminescence properties by varying its structure through different preparation methods. B.J. Jin et al. and Z.D. Shah et al. [30,16] also had observed the same kind of peak around 413 nm from the ZnO film on Si with buffer layer of SiC. In their case with increasing annealing temperature the interface between ZnO and SiC is becoming more and more impacted and complicated which affect the interface trap existing in depletion regions located at the grain boundaries and result in enhancement of PL intensity. As the crystal quality in our case is good it is very unlikely that visible emission in our case at 426 nm is happening due to interface trapping. 4. Conclusions In conclusion we have deposited ZnO thin films on SiC/Si substrates with pores in the interface of SiC and Si using high frequency ion plasma magnetron sputtering. These pores help to reduce the residual elastic stress developed between ZnO and SiC due to their lattice mismatching. Characterization of ZnO films using XRD, SEM, AFM and PL spectroscopy showed promising results. As SiC is a naturally p type semiconductor it may be used in the future with naturally n type
ZnO to fabricate p-SiC/n-ZnO heterojunction Light Emitting Diodes or Diode Lasers.
Acknowledgments This work is supported jointly by the Russian Foundation for Basic Research (RFBR) and the Department of Science and Technology (DST), India (grant no. DST-464-PHY).
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997.. [2] S.A. Kukushkin, A.V. Osipov, V.N. Vbessolov, B.K. Medvedev, V.K. Nevolin, K.A. Tcarik, Rev. Adv. Matter. Sci. 17 (2008) 1. [3] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [4] P. Fons, K. Iwata, S. Niki, A. Yamada, K. Matsubara, J. Cryst. Growth 201–202 (1999) 627. [5] H.J. Ko, S. Hong, Y. Chen, T. Yao, Thin Solid Films 409 (2002) 153. [6] A. Tiwari, M. Park, C. Jin, H. Wang, D. Kumar, J. Narayan, J. Mater. Res. 17 (2002) 2480. [7] W. Shih, M. Wu, Journal of Crystal Growth 137 (1994) 319. [8] A. Ohtomo, K. Tamura, K. Saikusa, K. Takashashi, T. Makino, Y. Segawa, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 2635. [9] Junjie Zhu, Bixia Lin, Xiankai Sun, Ran Yao, Chaoshu Shi, Fu. Zhuxi, Thin Solid Films 478 (2005) 218. [10] Ji-Hong Kim, Dae-Hyung Cho, Wonyong Lee, Byung-Moo Moon, Wook Bahng, Sang-Cheol Kim, Nam-Kyun Kim, Sang-Mo Koo, J. Alloys Compd. 489 (2010) 179. [11] B.M. Ataev, Ya.I. Alivov, E.V. Kalinina, V.V. Mamedov, G.A. Onushkin, S.Sh. Makhmudov, A.K. Omaev, Journal of Crystal Growth 275 (2005) e2471. [12] A.B.M.A. Ashrafi, B.-P. Zhang, N.T. Binh, K. Wakatsuki, Y. Segawa, Journal of Crystal Growth 275 (2005) e2439. [13] Hong Kai, J. Phys. Chem. Solids 70 (2009) 967. [14] R. Prabakaran, M. Peres, J. Non-Cryst. Solids 354 (2008) 2181. [15] Z.D. Sha, J. Wang, Z.C. Chen, A.J. Chen, Z.Y. Zhou, X.M. Wu, L.J. Zhuge, Physics Educ. 33 (2006) 263. [16] Z.D. Sha, X.M. Wu, L.J. Zhuge, Phys. Lett. A 355 (2006) 228. [17] S. Eunice, P. Leong, S.F. Yu, Adv. Mater. 18 (2006) 1685. [18] Clement Yuen, S.F. Yu, S.P. Lau, Rusli, T.P. Chen, Appl. Phys. Lett. 86 (2005) 241111. [19] S.A. Kukushkin, A.V. Osipov, Phys. Solid State 7 (2008) 1238. [20] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113. [21] JCPDS No. 36-1451 [22] N. Fujimara, T. Nishibara, S. Goto, Journal of Crystal Growth 130 (1993) 269. [23] Y. Zhang, H. Zheng, J. Su, B. Lin, Z. Fu, J. Lumin. 124 (2007) 252. [24] H.D. Li, S.F. Yu, A.P. Abiyasa, C. Yuen, S.P. Lau, H.Y. Yang, E.S.P. Leong, Appl. Phys. Lett. 86 (2005) 261111. [25] Y. Lee, Y.J. Kim, J. Mater. Res. 13 (1998) 1260. [26] J. Myong, W. Yoon, D. Lee, I. Yun, S. Bea, Jpn. J. Appl. Phys. 41 (2002) 28. [27] D. Bao, H. Gun, A. Kuang, Thin Solid Films 312 (1998) 37. [28] D. Dimova-Malinovska, H. Nichev, O. Angelov, Phys Sat. Sol. (c) 5 (2008) 3353. [29] E.V. Etzcorn, D.R. Clarke, J. Appl. Phys. 89 (2001) 1025. [30] B.J. Jin, S. Im, S.Y. Lee, Thin Solid Films 366 (2000) 107. [31] D.H. Zhang, Q.P. Wang, Z.Y. Xue, Appl. Surf. Sci. 207 (2003) 20. [32] Zhao Lei, Lian Jian-She, Liu Yu-Hua, Jiang Quing, Trans. Nonferrous Met. Soc. China 18 (2008) 145. [33] Y.M. Sun, Ph.D thesis, University of Science and Technology of China, July, 2000.