Aluminum-nitride codoped zinc oxide films prepared using a radio-frequency magnetron cosputtering system

JOURNAL OF APPLIED PHYSICS 102, 033516 2007 Aluminum-nitride codoped zinc oxide films prepared using a radio-frequency magnetron cosputtering system Day-Shan Liu a and Chia-Sheng Sheu Institute of Electro-Optical and Materials Science, National Formosa University, Huwei, Taiwan 63201, Republic of China Ching-Ting Lee Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, Republic of China Received 15 February 2007; accepted 27 June 2007; published online 13 August 2007 Al N codoped zinc oxide films were prepared using a radio-frequency magnetron cosputtering system at room temperature. AlN and ZnO materials were employed as the cosputtered targets. The as-deposited cosputtered films at various theoretical atomic ratios Al/ Al+Zn at. % showed n-type conductive behavior in spite of the N atoms exceeding that of the Al dopants, indicating that the N-related acceptors were still inactive. The crystalline structure was obviously correlated with the cosputtered AlN contents and eventually evolved into an amorphous structure for the Al N codoped ZnO film at a theoretical Al doping level reaching 60%. With an adequate postannealing treatment, the N-related acceptors were effectively activated and the p-type ZnO conductive behavior achieved. The appearance of the Zn 3 N 2 phase in the x-ray diffraction pattern of the annealed Al N codoped ZnO film provided evidence of the nitrification of zinc ions. The redshift of the shallow level transition and the apparent suppression of the oxygen-related deep level emission investigated from the photoluminescence spectrum measured at room temperature were concluded to be influenced by the activated N-related acceptors. In addition, the activation of the N acceptors denoted as N Zn bond and the chemical bond related to the Zn 3 N 2 crystalline structure were also observed from the associated x-ray photoelectron spectroscopy spectra. 2007 American Institute of Physics. DOI: 10.1063/1.2768010 I. INTRODUCTION Ultraviolet light-emitting diodes and laser diodes are required for the full-color displays and data storage systems in the optoelectronics industry. Zinc oxide ZnO material with wide and direct band gap of 3.37 ev similar to the commercial GaN material is a promising next generation semiconductor because of its large exciton binding energy of 60 mev, much greater than that of GaN 24 mev at room temperature. It becomes an attractive material for inducing stimulated emission at a low threshold voltage. 1 4 The fabrication of n- and p-type ZnO semiconductor using controllable extrinsic doping is required to realize ZnO-based optoelectronic devices. The n-type ZnO is achieved even without intentional doping. Unfortunately, the p-type ZnO is very difficult to prepare for the self-compensation effect originating from native defects as well as the limited solubility and inactivation of the acceptor dopants in the ZnO films. Recently, several researchers have put efforts into the exploration of p-zno film by doping group V elements as p-type dopants such as N, P, As, and Sb. 5 11 Among these acceptors, nitrogen dopants with a shallow acceptor level is a promising candidate to substitute for oxygen atoms as N O in the ZnO films due to their similar ionic radius. To date, N-doped ZnO films have been prepared using various deposition methods such as chemical vapor depositon, 5,12 14 a Author to whom correspondence should be addressed; FAX: 886-5- 6329257; electronic mail: dsliu@sunws.nfu.edu.tw pulsed-laser deposition, 15 implantation, 6 and sputtering technology using different nitrogen sources such as N 2,NH 3, N 2 O, Zn 3 N 2, and MMH y monomethyl hydrazine. 9,16,17 However, the reliability and reproducibility in obtaining p-type ZnO:N is still controversial. Because of the chemical activity of O is higher than that of N, Zn is prone to combine with O rather than N, resulting in the N atoms being difficult to introduce into ZnO films. To solve this problem, some researches had diverted their concentration on the Zn 3 N 2 material properties, 18 and obtained p-type ZnO using thermal oxidation of Zn 3 N 2 films. 5,9,19 In recent years, p-type ZnO films were comprehensively achieved using the codoping method simultaneously using nitrogen acceptors and reactive III-group donors such as Ga N, In N, and Al N dopants to increase the solubility of N atoms in the ZnO films. 20 22 Compared with Ga and In atoms, Al is more suitable as reactive donors for their superior advantages such as low cost and near containment-free material as well as the superior stability for the strong Al N and Al O bonds. ZnO p-n homojunction devices were therefore successfully prepared with Al N codoped technology using the dc reactive magnetron sputtering system employing a specific Zn:Al alloy metal target under N 2 O O 2 reactive gas ambient. 23 However, the above-mentioned deposition method was reported to be deeply influenced by the Zn:Al mixture target and the partial pressure of reactive N 2 O gas, resulting in an unchanged atomic composition of zinc to aluminum in the deposition film. To simplify the deposition parameters and 0021-8979/2007/102 3 /033516/6/$23.00 102, 033516-1 2007 American Institute of Physics

033516-2 Liu, Sheu, and Lee J. Appl. Phys. 102, 033516 2007 TABLE I. Detailed cosputtered deposition conditions of the AlN ZnO cosputtered films. Deposition parameters Base pressure Cosputtering gas ambient Cosputtering pressure Size of target rf power supplied on target Deposition temperature Cosputtering film thickness Target-substrate distance Substrate holder rotation Conditions 2 10 4 Pa Argon gas 1.33 Pa 5 cm in diameter Zinc oxide target: 40, 46, 76, 137, 321, and 410 W Aluminum nitrogen target: 85 and 150 W Room temperature cooled down by cooling water 500 nm Target center to substrate holder center: 5 cm 72 rpm obtain Al N codoped films at various Al doping levels, we propose a controllable and well-configured rf magnetron cosputtering method using the hexagonal crystalline structures of the ZnO and AlN targets to prepare Al N codoped ZnO films. The doping concentration in the ZnO films was easily controlled and derived using the cosputtered rf power on each target. To activate the N-related acceptor dopants and achieve p-type ZnO films, Al N codoped ZnO films at a specific Al doping level were processed with an additive postannealing treatment. The related electrical and material properties of the cosputtered Al N codoped ZnO films at various theoretical Al atomic ratios Al/ Al+Zn at. % were investigated. The activation of the N-related acceptors in the p-type ZnO film was conducted from the crystalline structure, photoluminescence characteristics, and chemical bond nature compared to that of the undoped ZnO film. II. EXPERIMENT The rf magnetron cosputtering system used in this study is equipped with a dual rf power supply that generated two different rf powers with synchronized phases. The configuration of the rf magnetron cosputtering chamber is illustrated elesewhere. 24 ZnO purity of 99.99% and AlN purity of 99.99% materials were selected as the cosputtered targets. To deposit cosputtered films at various Al doping levels on glass and n-type Si 100 substrate at room temperature, the rf power supplied on AlN target was fixed at 85 W and the rf power supplied on ZnO target was controlled to vary from 46 to 410 W. The high AlN contents introduced into ZnO films was also prepared using the rf powers supplied on the AlN and ZnO targets at 150 and 40 W, respectively. The detailed cosputtered deposition conditions of the AlN ZnO cosputtered films are given in Table I. The theoretical Al atomic ratios Al/ Al+Zn at. % introduced into the ZnO films could be evaluated from the following expression similar to our previous descriptions in preparing the cosputtered indium tin oxide ITO -ZnO transparent and conductive films: 25 D 1 A d 1 : D 2 A d 2 = P:Q, 1 M 1 M 2 where D 1 and D 2 cm/min are the deposition rates evaluated from the thickness of the undoped AlN and ZnO films prepared at specific rf powers, respectively; A cm 2 are defined as the cross-section area of the substrate surface; d 1 and d 2 g/cm 3 are related to the density of the AlN 3.26 g/cm 3 and ZnO 5.66 g/cm 3 materials. In addition, M 1 and M 2 g/mol are the molecular weights of the AlN and ZnO materials; P and Q mole are the synthesized mole ratios of Al and Zn atoms in these cosputtered films. According to the deposition rates of the undoped ZnO and AlN films prepared at each rf power, shown in Table I, the theoretical Al atomic ratios Al/ Al+Zn at. % in the as-deposited cosputtered films were approximated to 5%, 10%, 20%, 30%, 40%, and 60%, respectively. With the aim of effectively activating the doping impurities, especially for the N-related acceptors, a postannealing treatment was carried out. The postannealing temperatures were varied in the range from 300 to 700 C for 30 min under nitrogen ambient. The undoped ZnO film deposited at an rf power of 321 W supplied on the ZnO target was also prepared as a standard reference. The film thickness of the cosputtered films and the undoped ZnO films before and after annealing treatments was measured using a surface profile system Veeco, Dektak 6M. The practical doping levels of Al and N in these as-deposited cosputtered films were examined using an energy dispersive x-ray spectroscopy EDS quantitative analysis attached to a scanning electron microscope JEOL, JSM-5410LV. Resistivity, carrier concentration, and Hall mobility were measured using the van der Pauw method with a Hall measurement system Ecopia, HMS-3000. The crystalline structures were examined using x-ray diffraction XRD patterns observed from a diffractometer Siemens, model D-500 using a Cu K radiation source. Photoluminescence PL spectra were measured at room temperature using a He Cd laser =325 nm pumping source. The chemical bonds were analyzed using x-ray photoelectron spectroscopy XPS with a monochromatic Al K source. III. RESULTS AND DISCUSSIONS The measured Al atomic ratios Al/ Zn+Al at. % in these cosputtered films decreased from 9.22 to 2.46 at. % with increasing rf power supplied on the ZnO target conducted from an energy dispersive x-ray spectroscope quantitative analysis were much smaller than the theoretical Al doping levels, indicating that the AlN contents were difficult to incorporate into the ZnO films. The true atomic concentration of nitrogen to aluminum N/Al in at. % in the AlN ZnO cosputtered films as a function of the rf cosputtered power on the ZnO target is shown in Fig. 1. The Al and N

033516-3 Liu, Sheu, and Lee J. Appl. Phys. 102, 033516 2007 FIG. 1. Atomic concentration of nitrogen to aluminum N/Al in the AlN ZnO cosputtered films as a function of the rf cosputtered power on ZnO target the theoretical Al atomic ratio Al/ Zn+Al at. % is shown in bracket. dopants were successfully introduced in the cosputtered films referred as Al N codoped ZnO films hereafter. The N atomic concentrations in these Al N codoped ZnO films were found to be higher than that of the Al atomic concentrations, especially for the films prepared at elevated cosputtered powers on the ZnO target. Since large amounts of native defects such as zinc interstitials and oxygen vacancies were reported as prone to be induced while depositing the ZnO films at higher rf powers, 26 N atoms ionized from cosputtering were likely to be attracted by those Zn-rich defects. This brought about more N impurities introduced into the ZnO films. However, because of the soluble limitation of N in the ZnO films, the measured N atom concentrations in these cosputtered films were nearly unchanged in spite of the increased Al doping concentration. As a result, the Al atom doping concentrations in these cosputtered films were gradually close to that of N atoms at lower cosputtered powers supplied on the ZnO target. The electrical property evolutions of these as-deposited Al N codoped ZnO films at various theoretical Al doping levels, as well as that of an undoped ZnO film, are illustrated in Fig. 2. Although the N impurity atomic ratios in the as-deposited cosputtered films were found to be in excess of that of the Al contents. These as-deposited Al N codoped ZnO films still showed n-type conductive behavior. High electron carrier concentrations about 10 18 10 19 cm 3 were obtained from these cosputtered films at theoretical Al atomic ratios ranging from 10% to 40%, while that of the undoped ZnO film showed a much lower electron carrier concentration. This implied that the FIG. 2. Film resistivity, carrier concentration, and Hall mobility of the asdeposited Al N codoped and undoped ZnO films. FIG. 3. X-ray diffraction patterns of the as-deposited Al N codoped ZnO films as well as the undoped ZnO and AlN films. doping Al impurities were able to donate free electrons in the as-deposited cosputtered films whereas the N-related acceptors were electrically inactive. As a result, an apparent increase in the electron carrier concentration was obtained in spite of the high N to Al atomic ratios shown in Fig. 1 measured in the as-deposited Al N codoped ZnO films. Figure 3 shows the crystalline structures of the as-deposited cosputtered films as well as undoped ZnO and AlN films deposited on glass substrates. An apparent diffraction peak determined as ZnO 002 phase was observed from the diffraction pattern of the undoped ZnO film. A weak diffraction peak identified as AlN 100 appeared in the undoped AlN film. This indicated that the preferred growth orientation of the undoped ZnO film was distinct from the undoped AlN film in spite of their similar hexagonal crystalline structures. As a result, the crystalline structure was suggested to be disordered due to the cosputtered AlN contents introduced into the ZnO films. Indeed, the preferred orientations were gradually evolved from the ZnO 002 phase to ZnO 100 phase with increasing the AlN contents in the ZnO films. For the cosputtered film at a theoretical Al atomic ratio of 60%, ZnO phases were entirely absent in the diffraction pattern and it became an amorphous crystalline structure. The poor crystalline structure was the consequence of the inferior electrical property shown in Fig. 2. So far, we succeeded in realizing Al N codoped ZnO films from this rf magnetron cosputtering system using ZnO and AlN targets. The N to Al atomic doping levels in the ZnO films could be simply tuned via controlling the rf powers supplied on the ZnO and AlN targets. The related electrical and material properties were found to be deeply influenced by the AlN contents introduced into the ZnO films. However, from the view point of these cosputtered film conductive types, the incorporated N acceptor dopants in these Al N codoped films deposited at room temperature seemed to be inactive and resulted in the n-type conductive behavior for all the produced cosputtered films. Applications in the photoelectronic devices using ZnO homojunction structures were thus limited due to the monotonic conductive type performance of these as-deposited Al N codoped ZnO films. As a result, the N-related acceptor dopants should be properly activated to convert the cosputtered film conductive type into p-type conductive behavior. Under this consideration, a postannealing treatment was carried out to activate these N-related acceptor dopants in the cosputtered films. For p-type ZnO codoped films with reac-

033516-4 Liu, Sheu, and Lee J. Appl. Phys. 102, 033516 2007 TABLE II. Electrical properties of the Al N codoped ZnO films theoretical Al atomic ratio of 10% deposited on silicon substrates annealed at various temperatures under nitrogen ambient for 30 min. Annealing temperature Carrier concentration cm 3 Mobility cm 2 V 1 s 1 Resistivity cm Carrier type As-deposited 8.99 10 18 0.96 1.97 n 300 C 1.49 10 19 0.83 1.38 n 400 C 5.04 10 18 2.35 0.527 p 500 C 1.04 10 18 3.64 1.65 p 600 C 1.53 10 17 5.03 1.34 10 1 p 700 C 5.88 10 15 6.58 1.62 10 2 p FIG. 4. X-ray diffraction patterns of the Al N codoped theoretical Al atomic ratio of 10% and undoped ZnO films deposited on silicon substrates annealed at a temperature of 400 C under nitrogen ambient for 30 min. tive donors and nitrogen acceptors, the reactive donors using the group III elements III=Al, Ga, or In and 2N was demonstrated by theoretical calculation. 27 As a result, a fine control on the N to Al atoms doping concentration and the suppression of the Al contents in the ZnO film was required to accomplish p-type ZnO film preparation. According to the measured N/Al atomic ratios shown in Fig. 1, the theoretical calculation of N/Al=2:1 indicated by the dashed line is located between the as-deposited cosputtered films at the theoretical Al atomic ratios of 10% and 20%. Because the measured N/Al atomic ratio in the cosputtered film at a theoretical Al doping level of 10% was about 2.69, the p-type conduction was expected to be achieved due to the enhanced quantity of Zn N bonds via complex N Al N shallow acceptors formation after thermal activation treatment. 28 Moreover, since the activation of the N O -double-donor complexes such as N O V O,N O Zn i, and N O Zn O were demonstrated to donate free electron carriers in the N-doped ZnO films, 29,30 the annealing temperature and atmosphere became critical issues to properly activate the N-related acceptors in the cosputtered films. As a result, they were annealed under nitrogen ambient to avoid the outdiffusion of N atoms that had been introduced into the ZnO films by cosputtering. P-type ZnO conductive behavior was obtained from the cosputtered film at a theoretical Al doping level of 10% processed with an additive postannealing treatment at temperatures ranging from 400 to 600 C under nitrogen ambient for 30 min. The carrier concentration, Hall mobility, and resistivity of the Al N codoped ZnO films at a theoretical Al doping level of 10% deposited onto silicon substrates after thermal annealing based on the Hall effect measurements at room temperature are summarized in Table II. At an annealing temperature of 300 C, the Al N codoped ZnO film performed n-type conduction with a slightly higher electron concentration than the as-deposited film. This indicated that the annealing temperature was too low to activate the N-related acceptors and more donors were generated. As the annealing temperature reached 400 C, the annealed Al N codoped ZnO film with a hole carrier concentration of 5.04 10 18 cm 3, mobility of 2.35 cm 2 V 1 s 1, and resistivity of 0.527 cm was obtained, whereas that of an undoped ZnO film annealed under the same conditions showed an electron carrier concentration of 2.57 10 17 cm 3 and Hall mobility of 5.47 cm 2 V 1 s 1. This implied that large amounts of N-related acceptors were effectively activated and predominated over the donors in the codoped film under this annealing treatment. As a result, free hole carriers were measured from the annealed Al N codoped ZnO films. By further increasing the annealing temperature, more donor-related defects such as V O and V N were prone to be produced, resulting in a decrease in the hole carrier concentration. X-ray diffraction patterns for Al N codoped and undoped ZnO film crystalline structures deposited on silicon substrates annealed at a temperature of 400 C are shown in Fig. 4. It can be seen that both the codoped and undoped ZnO films annealed under nitrogen ambient exhibited polycrystalline structures with the dominated diffraction peaks of ZnO 002 and 101 phases in the associated diffraction patterns. Due to the influence of Al and N impurities, the full width at half maximum FWHM of the ZnO-related diffraction peaks in the diffraction pattern of the annealed Al N codoped ZnO film was broader than that of the undoped ZnO film. According to Scherrer s formula, the mean crystallite size of the annealed Al N codoped ZnO film evaluated from the FWHM of the ZnO 002 phase 9.7 nm was apparently smaller than that of the undoped ZnO film 17.3 nm and as a consequence of the lower carrier mobility due to the grain boundary scattering. In addition, except for the ZnO-related diffraction peaks, a weak diffraction peak determined as Zn 3 N 2 222 was observed from the annealed Al N codoped ZnO film diffraction pattern. The appearance of the zinc nitride phase was believed to be the nitrification reaction of the excess Zn and N atoms in the codoped films, indicating the excitation of the N ions after thermal annealing. The room temperature PL spectra of the Al N codoped and undoped ZnO films deposited on silicon substrates annealed at a temperature of 400 C are illustrated in Fig. 5. The main feature for the PL spectrum of the undoped ZnO film could be divided into three categories: the near band edge emission 3.24 ev, the low energy tail extending from the near band edge emission 3.07 ev, and the deep level emission 2.11 ev. According to the previous reports, the low energy tail extending from the near band edge emission was related to the Zn-related shallow level transitions, whereas the oxygen-related deep level transitions were responsible for the broad green-yellow emission. 31 33 The PL spectrum of the annealed Al N codoped ZnO film was obviously different from that of the annealed undoped ZnO film in both emission peak structure and intensity. The FWHM of the ultraviolet emission was broader and dominated by the

033516-5 Liu, Sheu, and Lee J. Appl. Phys. 102, 033516 2007 FIG. 5. Room temperature PL spectra of the Al N codoped theoretical Al atomic ratio of 10% and undoped ZnO films deposited on silicon substrates annealed at a temperature of 400 C under nitrogen ambient for 30 min. shallow defect transition with an apparent redshift of about 60 mev. The oxygen-related deep emission was suppressed and a broad blue-green emission at about 2.62 ev emerged from the PL spectrum. The blue-green emission was regarded as the dopant-induced defects that also had been observed from related research in the preparation of the Al N codoped ZnO films. 33,34 Except for the defect-related emission induced from the codoping process, the activated N-related acceptors in the oxygen sites N O was responsible for the redshift on the shallow level transition due to the fact that N O was recognized as shallow acceptors and sequentially suppressed the formation of the oxygen-related deep level emission. 35 As a consequence, the annealed Al N codoped ZnO film performed with p-type conductive behavior. Figures 6 a and 6 b show the typical XPS spectra of Zn 2p 3/2 andn1s core level obtain from the Al N codoped and undoped ZnO films deposited on silicon substrates annealed at a temperature of 400 C. The binding energy peak of the Zn 2p 3/2 for the Al N codoped ZnO film 1021.6 ev was higher than that of the undoped ZnO film 1021.3 ev. The related FWHM of the Al N codoped ZnO film 2.0 ev was also broader than that of the undoped ZnO film 1.8 ev. For the Al N codoped ZnO film, the crystalline structure was inferior to that of the undoped ZnO film due to the introduced N and Al dopants, as shown in Fig. 4. As a result, the electrical cloud around the Zn atoms in the Al N codoped ZnO film were prone to be asymmetrical, resulting in the increase in the binding energy of the Zn 2p 3/2 core level. 14,18,30,36 In addition, the broad FWHM also suggested to be contributed by the Zn N bonds. However, since the Zn 2p 3/2 peak was almost completely dominated by the Zn O chemical bond, it was quite difficult to deconvolve the Zn N chemical bond. Therefore, the N 1s core level signal was a preferable tool to investigate the activation of N impurities. As can be seen in Fig. 6 b, no obvious peak corresponding to N was observed from the undoped ZnO film, whereas that of the Al N codoped film showed two peaks at around 395.7 and 399.7 ev. The peak at a binding energy of 395.7 ev was in agreement with XPS spectra reported on Zn 3 N 2 thin films, 18,37 manifesting the crystalline structure shown in Fig. 4. In addition, the peak at a high binding energy of 399.7 ev was broad and asymmetrical with a shoulder observed on the low binding energy side. This implied that this binding energy signal was composed of at least two chemical bonds. According to the previous works on nitrogen-doped and Al N codoped ZnO films, 5,14,30,38 40 the lower binding energy tail was attributed to the N ions substituted for O ions recognized as N Zn chemical bond indicated by N O at about 398.3 ev in the ZnO lattice. The high binding energy side was denoted as the chemical state emerged from the neutrally charged N ions 400 ev. The appearance of the N Zn chemical bond was directly related to the activation of N acceptors and a consequence of achieving p-type ZnO conductive behavior. IV. CONCLUSIONS FIG. 6. Typical XPS spectra of a Zn 2p 3/2 and b N1s core level for the Al N codoped and undoped ZnO films deposited on silicon substrates annealed at a temperature of 400 C under nitrogen ambient for 30 min. In summary, Al N codoped ZnO films had been successfully prepared using an rf magnetron cosputtering system at room temperature with AlN and ZnO targets. Because of the oxygen atoms deficiency in the ZnO films, the nitrogen atomic concentrations were in excess of the Al contents in the as-deposited cosputtered films, especially at low Al doping levels. The conductive behavior of these as-deposited codoped Al N films all showed n-type conduction. Regarding the crystalline structure evolutions, the preferred ZnO 002 c-axis diffraction peak appeared in the x-ray diffraction pattern of the undoped ZnO film and gradually vanished due to AlN content introduction and domination by the ZnO 100 crystalline phase with increasing the Al doping impurities. As the theoretical Al doping level Al/ Al +Zn at. % reached 60%, the crystalline structure of the cosputtered film evolved into an amorphous structure. The N-related acceptors in the Al N codoped ZnO film at a theoretical Al atomic ratio of 10% were found to be sufficiently activated with an adequate postannealing treatment. A hole carrier concentration of 5.04 10 18 cm 3, Hall mobility of 2.35 cm 2 V 1 s 1, and film resistivity of 0.527 cm was

033516-6 Liu, Sheu, and Lee J. Appl. Phys. 102, 033516 2007 achieved for the Al N codoped ZnO films annealed at a temperature of 400 C under nitrogen ambient for 30 min. The Zn 3 N 2 phase that appeared in the x-ray diffraction pattern was attributed to the nitrification of the excess zinc ions in the codoped films after thermal annealing. In addition, the redshift of the shallow level transition and oxygen-related deep level emission suppression in PL spectrum were related to the activated N-related acceptors. The chemical bond ascribed to O atoms replaced by N impurities and the chemical nature recognized as the Zn 3 N 2 crystalline structure observed from the N 1s core level in the XPS spectra of the annealed Al N codoped ZnO film also provided a direct evidence of the N dopant activation of and sequentially performed p-type ZnO conductive behavior. The controllable Al N codoped ZnO film obtained from this rf magnetron cosputtering system using AlN and ZnO targets at room temperature and the conversion of p-type ZnO conductive behavior with an additive postannealing treatment will greatly benefit the development of ZnO-based homostructural optoelectronic devices. ACKNOWLEDGMENT This work was supported by the National Science Council of the Republic of China under NSC94-2218-E150-004. 1 Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, Appl. Phys. Lett. 72, 3270 1998. 2 A. Mitra and R. K. Thareja, J. Appl. Phys. 89, 2025 2001. 3 Y. I. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev, A. K. Omaev, M. V. Chukichev, and D. M. Bagnall, Appl. Phys. Lett. 83, 4719 2003. 4 W. Liu et al., Appl. Phys. Lett. 88, 092101 2006. 5 B. S. Li et al., J. Mater. Res. 18, 8 2003. 6 C. C. Lin, S. Y. Chen, S. Y. Cheng, and H. Y. Lee, Appl. Phys. Lett. 84, 5040 2004. 7 D. C. Look, G. M. Renlund, R. H. Burgener II, and J. R. Sizelove, Appl. Phys. Lett. 85, 5269 2004. 8 V. Vaithianathan, B. T. Lee, and S. S. Kim, J. Appl. Phys. 98, 043519 2005. 9 Y. Nakano, T. Morikawa, T. Ohwaki, and Y. Taga, Appl. Phys. Lett. 88, 172103 2006. 10 F. X. Xiu, Z. Yang, L. J. Mandalapu, J. L. Liu, and W. P. Beyermann, Appl. Phys. Lett. 88, 052106 2006. 11 Y. J. Zeng et al., Appl. Phys. Lett. 88, 262103 2006. 12 G. Du, Y. Ma, Y. Zhang, and T. Yang, Appl. Phys. Lett. 87, 213103 2005. 13 J. L. Zhao, X. M. Li, J. M. Bian, W. D. Yu, and C. Y. Zhang, J. Cryst. Growth 280, 495 2005. 14 Z. Xiao, Y. Liu, J. Zhang, D. Zhao, Y. Lu, D. Shen, and X. Fan, Semicond. Sci. Technol. 20, 796 2005. 15 J. G. Lu, Y. Z. Zhang, Z. Z. Ye, L. P. Zhu, L. Wang, B. H. Zhao, and Q. L. Liang, Appl. Phys. Lett. 88, 222114 2006. 16 C. Wang, Z. Ji, K. Liu, Y. Xiang, and Z. Ye, J. Cryst. Growth 259, 279 2003. 17 M. L. Tu, Y. K. Su, and C. Y. Ma, J. Appl. Phys. 100, 053705 2006. 18 M. Futsuhara, K. Yoshioka, and O. Takai, Thin Solid Films 322, 274 1998. 19 E. Kaminska et al., Phys. Status Solidi C 2, 1119 2005. 20 A. V. Singh, R. M. Mehra, A. Wakahara, and A. Yoshida, J. Appl. Phys. 93, 396 2003. 21 J. M. Bian, X. M. Li, X. D. Gao, W. D. Yu, and L. D. Chen, Appl. Phys. Lett. 84, 541 2004. 22 F. Zhuge et al., J. Cryst. Growth 268, 163 2004. 23 F. Zhuge et al., Appl. Phys. Lett. 87, 092103 2005. 24 D. S. Liu, C. C. Wu, and C. T. Lee, Jpn. J. Appl. Phys., Part 1 44, 5119 2005. 25 D. S. Liu, C. H. Lin, F. C. Tsai, and C. C. Wu, J. Vac. Sci. Technol. A 24, 694 2006. 26 C. R. Aita, R. J. Lad, and T. C. Tisone, J. Appl. Phys. 51, 6405 1980. 27 T. Yamamoto and H. K. Yoshida, Jpn. J. Appl. Phys., Part 2 38, L166 1999. 28 G. W. Cong et al., Appl. Phys. Lett. 88, 062110 2006. 29 Y. M. Lu, W. S. Hwang, W. Y. Liu, and J. S. Yang, Mater. Chem. Phys. 72, 269 2001. 30 E. C. Lee, Y. S. Kim, Y. G. Jin, and K. J. Chang, Phys. Rev. B 64, 085120 2001. 31 X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong, Appl. Phys. Lett. 78, 2285 2001. 32 B. Lin, Z. Fu, Y. Jia, and G. Liao, J. Electrochem. Soc. 148, G110 2001. 33 Y. G. Wang, S. P. Lau, H. W. Lee, S. F. Yu, B. K. Tay, X. H. Zhang, and H. H. Hng, J. Appl. Phys. 94, 354 2003. 34 J. G. Lu, L. P. Zhu, Z. Z. Ye, F. Zhuge, B. H. Zhao, J. Y. Huang, L. Wang, and J. Yuan, J. Cryst. Growth 283, 413 2005. 35 U. Ozgur et al., J. Appl. Phys. 98, 041301 2005. 36 Y. M. Chung, C. S. Moon, M. J. Jung, and J. G. Han, Surf. Coat. Technol. 200, 936 2005. 37 F. Zong, H. Ma, C. Xue, H. Zhuang, X. Zhang, H. Xiao, J. Ma, and F. Ji, Solid State Commun. 132, 521 2004. 38 H. Maki, I. Sakaguchi, N. Ohashi, S. Sekiguchi, H. Haneda, J. Tanaka, and N. Ichinose, Jpn. J. Appl. Phys., Part 1 42, 75 2003. 39 Y. F. Mei, R. K. Y. Fu, G. G. Siu, K. W. Wong, P. K. Chu, R. S. Wang, and H. C. Ong, Appl. Surf. Sci. 252, 8131 2006. 40 S. U. Yuldashev, G. N. Panin, T. W. Kang, R. A. Nusretov, and I. V. Khvan, J. Appl. Phys. 100, 013704 2006.