ZnSe Heterocrystalline Junctions Based on Zinc Blende-Wurtzite

J. Phys. Chem. C 2010, 114, 1411–1415

1411

ZnSe Heterocrystalline Junctions Based on Zinc Blende-Wurtzite Polytypism Lei Jin,†,‡,§ Jianbo Wang,*,†,‡ Shuangfeng Jia,†,‡ Qike Jiang,†,‡ Xue Yan,† Ping Lu,† Yao Cai,† Liangzi Deng,† and Wallace C. H. Choy*,§ Department of Physics and Center for Electron Microscopy, and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan 430072, China, and Department of Electrical and Electronic Engineering, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, China ReceiVed: June 11, 2009; ReVised Manuscript ReceiVed: December 11, 2009

ZnSe zinc blende-wurtzite (ZB-WZ) heterocrystalline junctions were successfully fabricated via pressure variation during the thermal evaporation process and characterized using electron microscopy techniques. Two types of ZB-WZ junction configurations were observed, and the orientation relationships, denoted as [111j]ZB//[0001]WZ (type I) and ([111]ZB//[0001]WZ (type II), respectively, were determined in detail by systematic parallel beam selected area electron diffraction combined with stereographic projections. The relatively weak polarities on both sides of such junctions were also detected using convergent beam electron diffraction combined with simulation. Different from the ideal models suggested by the theoretical studies on the (111)ZB-(0001)WZ interface of ZnSe (even other II-VI semiconductors), the present results indicate the existence of the polarity reversal across the junctions, which may result in charge accumulations around the interfaces. Therefore, it offers an interesting real system for theoretical investigation on such polytypic interface and also an appropriate experimental platform for further electrical transport measurement across these junctions. Introduction Heterostructures have been widely investigated on the nanometer scale nowadays since they possess enormous fundamental physics and also vast potential in nanodevice fabrications.1-5 Heterostructures are generally constructed by introducing variations in material composition2 or electrostatic potential (in the case of doping superlattice).3 Besides, there are still different ways of making a single material act as an inhomogeneous structure, by changing the crystal orientations or crystal structures to form a twinning superlattice/junction4,5 or polytypic heterocrystalline structure,5 respectively. The interfacial states, such as diffusion of chemical compositions, charge accumulation, strain, defects, and polarity reversal (if existent), have significant influence on the transport properties of the heterostructure and therefore have attracted extraordinary attention.1 Polytypism is one of the most fascinating properties in compound semiconductors. More than 100 crystallographic modifications (polytypes) are known for ZnS.6 Polytypism and polymorphism are also characteristics for ZnSe, ZnTe, and CdSe, etc.,7 though to a lower extent. In particular, the two most extreme polytypes are zinc blende (ZB) with pure cubic stacking of ...ABCABC... along the [111] direction and wurtzite (WZ) with pure hexagonal stacking of ...ABAB... along the [0001] axis. Other forms combine these two basic stacking sequences. Calculations for SiC with structural relaxation have shown that the different polytypes match perfectly along the [111]ZB/ [0001]WZ direction with mismatch smaller than 0.1%.8,9 For this reason, heterocrystalline structure with different polytypes is expected to be free of stress and/or dangling bonds and thus * Corresponding authors. E-mail: [email protected] (J.W.); chchoy@ eee.hku.hk (W.C.H.C.). † Department of Physics and Center for Electron Microscopy, Wuhan University. ‡ Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University. § Department of Electrical and Electronic Engineering, The University of Hong Kong.

has great potential in band-structure engineering.6,10 Moreover, theoretical investigations show that polytypic heterocrystalline structures do not only represent new polytypes with averaged electronic properties and energy band gaps but also exhibit different spatial distribution of electrons and holes, indicating the low charge transfer across the heterocrystalline interface.6,10 Aside from the theoretical motivation to study the polytypism and polytype heterostructures, an anomalous photovoltaic effect (APE) has also been observed representatively in the ZnS crystals which contains various polytypes and stacking faults.11,12 Studies indicate that the internal fields, originated from different polytypes, are of great importance to APE.11,12 Similar phenomenon was reported as well in ZnSe crystals and thin films.13,14 To date, renewed interests have been focused on the heterocrystalline nanostructures due to their unique properties and potential applications in band-structure engineering.5,15,16 For example, Jiang et al. reported the first polytype-modulated ZnS nanostructure, which showed an obvious blue shift in photoluminescence compared to its bulk counterpart.15 Bao et al. developed a technique using transmission electron microscopy (TEM) and microphotoluminescence to directly correlate structural and optical properties of rotationally twinned ZB InP nanowires.16 However, such structures are rarely obtained in ZnSe, a key material in short-wavelength optoelectronic devices with the direct wide band gap of 2.68 eV at room temperature,17 because of the difficulty in growing the metastable WZ phase,18-23 and thus a much bigger challenge exists to create the ZB-WZ polytypic interface which needs a high capacity of controlling the growth conditions. Inspired by our previous studies on phase-controlled growth of ZnSe nanostructures,21-25 we successfully fabricated the ZnSe heterocrystalline junctions through a pressurecontrolled two-stage growth process. Temperature variation control was not used for its severe lagging effect. Two types of transition were observed, i.e., the [0001]WZ axis is parallel

10.1021/jp909182e  2010 American Chemical Society Published on Web 01/04/2010

1412

J. Phys. Chem. C, Vol. 114, No. 3, 2010

Figure 1. (a) and (b) SEM images and (c) schematic illustration of the as-grown products showing the growth process through wire-spiralbelt transition.

Jin et al.

Figure 2. (a) BF image of a representative type I spiral-belt transition. (b)-(d) Corresponding SAED patterns from (b) the spiral, (c) the interfacial, and (d) the belt region, respectively.

to the [111j ]ZB (type I) and ([111]ZB (type II) directions, respectively. Besides the phase transition, the polarity reverse may exist across the interface, which requires more theoretical consideration of the real system. Experimental Section The growth was carried out in a single zone tube furnace. The experimental details have been described previously25 and are summarized briefly as follows. About 0.5 g of ZnSe powder (99.99% Sigma Aldrich) was loaded into the furnace and then heated to 950 °C under an argon (premixed with 5% hydrogen) atmosphere. In order to obtain the heterocrystalline junctions, a two-stage fabrication process was adopted during which the reaction pressure varied from 100 to 1.5 torr. After the growth, the furnace was cooled naturally to room temperature, and yellowish products were obtained. As overall information, a LEO-1530 field emission gun scanning electron microscope (FE-SEM) was utilized for morphological analysis. Then, the products were scratched off the substrate, sonicated in ethanol, and dropped onto the holey carbon-coated copper grid for TEM observation. TEM techniques, including bright field (BF) and dark field (DF) imaging, selected area electron diffraction (SAED), and convergent beam electron diffraction (CBED), were performed using a JEOL JEM-2010(HT) electron microscope, with the acceleration voltage of 200 kV. Results and Discussion SEM investigation of the as-fabricated products clearly exhibits the morphological transformation through a wire-spiralbelt process during the growth, as shown in Figures 1a-c. At the first stage, the wires of ZB structure form.23 Then the reaction pressure decreases, the wires of the first stage would act directly as substrates, and the spiral forms from the tip of the wire as observed in Figure 1a. According to the Ostwald’s step rule, the lower reaction pressure facilitates the formation of metastable wurtzite phase.21,23 However, the spirals still possess ZB structure,25 which may be related to the ZB structure of the substrate wires and which can be regarded as the buffer products to undergo the ZB-WZ transition further. In order to observe the ZB-WZ transition, our research will focus on the spiralbelt region. The belts are not only at the end of the spiral (denoted as type I) but also at the lateral sides (denoted as type II), which have been summarized in Figure 1. Electron diffraction technique is utilized to further confirm the ZB-WZ structural transitions. SAED patterns along 〈110〉ZB

Figure 3. (a) BF image of another type I spiral-belt transformation and (inset) the low-magnified BF image. (d) SAED pattern recorded from the circular area in (a). (b) and (c) DF images using (011j0) and (111) beams of (d) as illuminating sources. (e) and (f) Corresponding SAED patterns from the WZ and ZB regions, respectively.

and 〈2j110〉WZ axes characteristic of ZB and WZ structures respectively are chosen for phase identification hereafter. The BF image in Figure 2a shows a representative type I spiral-belt transformation, which corresponds to the SEM image in Figure 1a. Corresponding SAED patterns shown in Figures 2b-d are recorded from the spiral, interfacial, and belt regions, respectively. The spiral displays a twinning-like diffraction pattern with the sharing [111] directions pointing to the center of the spiral and a high density of planar defects (as indicated by the streaks in Figure 2b). The in-plane bending originates from the presence of numerous Lomer-Cottrell sessile dislocations, which has been discussed in our previous study.25 The pure WZ phase is observed at the belt region without any further tilting, and the corresponding SAED pattern (Figure 2d) reveals the crystallographic relations between the WZ and ZB polytypes as [2j110]WZ//[1j10]ZB, [0001]WZ//[111j]ZB, and [011j0]WZ//[1j1j2j]ZB, respectively. These relations are directly confirmed by the SAED pattern in Figure 2c, which has been fully indexed as a superimposition of two sets of ZB-structured (three indexes) and a set of WZ-structured patterns (four indexes). The sharp diffraction spots (Figure 2d) further indicate that the WZ polytype has high crystal quality.

Zinc Blende-Wurtzite Polytypism

J. Phys. Chem. C, Vol. 114, No. 3, 2010 1413

Figure 5. (a) BF image and (b)-(d) corresponding SAED patterns recorded from the (b) B, (c) C, and (d) D selected areas, respectively. The inset shows the schematic illustration of the structural transformation. Figure 4. (a) BF image and (b)-(e′) corresponding SAED patterns recorded from the (b) B, (c) C, (d) D, (e) E, and (e′) E′ areas, respectively. (f) (011j 0) DF image showing the WZ polytypes. (g) Schematic illustration of type II transition from both sides of the spiral.

Imaging techniques are used further for the type I spiral-belt transformation. The corresponding SAED pattern (Figure 3d) from the circular region of Figure 3a can be attributed to a superimposition of two sets of SAED patterns along [11j0]ZB and [21j1j0]WZ zone axes. The spots marked by the rectangular frame originate from the [21j1j0]WZ axis, and those marked by the rhombus come from the [11j0]ZB axis, respectively. The crystallographic relations between WZ and ZB polytypes with [2j110]WZ//[1j10]ZB, [0001]WZ//[111j]ZB, and [011j0]WZ//[1j1j2j]ZB are confirmed. The interfacial structures between the WZ and ZB polytypes have been clearly demonstrated in the DF images in Figures 3b and c by using (011j0) and (111) diffraction beams in Figure 3d, respectively. At the interfacial region sandwiched by the dashed lines, planar defects are observed which suggest a turbulent state during the phase transition process in the nanostructure growth. The corresponding SAED patterns recorded from the belt and spiral regions are also presented in Figures 3e and f, respectively. Slightly different from that in Figure 2b, the SAED pattern in Figure 3f indicates the single crystalline nature as pure ZB structure without twinning at the end part of the spiral region. Besides the type I transition, two other kinds of type II transition with the crystallographic relations of [2j110]WZ// [1j10]ZB, [0001]WZ//([111]ZB, and [011j0]WZ//([1j1j2]ZB are also obtained. Such transition may occur at either both sides or only the outer side of the spirals, which are consistent with the observation in Figure 1b. Figure 4a represents the type II transition from both sides of spirals. Further SAED investigations using the selected areas denoted by B, C, D, E, and E′ (Figure 4a) are carried out respectively in Figures 4b-e′ with the same tilting angle. Region B is pure ZB without twinnings (Figure 4b). However, the contribution of ZB twinnings and WZ polytypes becomes increasingly significant in the SAED patterns (Figures 4c and d) as the selected areas shifting from B to D, which further

Figure 6. Stereographic projections of (a) type I and (b) type II transitions, respectively.

1414

J. Phys. Chem. C, Vol. 114, No. 3, 2010

Figure 7. (a) BF image of a type II junction. (b) Experimental and (c) simulated CBED patterns with thickness of 210 nm along [11j00]WZ projection. No obvious intensity difference between ((0002) disks is detected.

indicates the turbulent state during the formation of the ZB-WZ junction. Crystallographic relations of [2j 110]WZ// [1j 10]ZB, [0001]WZ//[1j 1j 1j ]ZB, [011j 0]WZ//[1j 1j 2]ZB, and [2j 110]WZ// [1j 10]ZB, [0001]WZ//[111]ZB, [011j 0]WZ//[112j ]ZB are applicable to the outer and inner sides, respectively. Moreover, both E and E′ areas belong to the WZ structure confirmed by the same diffraction pattern as Figures 4e and e′ and the same contrast shown in the DF image (Figure 4f) illuminated by (011j 0) diffraction of Figure 4e. A schematic illustration is given in Figure 4g for better understanding this transition. Transition only from the outer side of the spiral has also been observed representatively in Figure 5a. Corresponding SAED patterns from the indicated B, C, and D areas indicate the formation of a heterocrystalline junction and confirm the above-mentioned crystallographic relations as [2j110]WZ//[1j10]ZB, [0001]WZ//[1j1j1j]ZB, and [011j0]WZ//[1j1j2]ZB. The schematic illustration in the inset of Figure 5a further demonstrates the structural transitions along the [0001]WZ direction in combination with the SAED results shown in Figures 5b-d. The experimentally observed crystallographic relations [2j 110]WZ//[1j 10]ZB, [0001]WZ//[111j ]ZB, [011j 0]WZ//[1j 1j 2j ]ZB (for type I) and [2j 110]WZ//[1j 10]ZB, [0001]WZ//[1j 1j 1j ]ZB, [011j 0]WZ// [1j 1j 2]ZB (for type II) are consequently summarized into the stereographic projections in Figure 6, based on which all the orientation relations for both type I and II transitions can be fully elucidated. The commonly reported [0001]WZ//[111]ZB relations will not be further discussed here. Now we will focus on the polarity perspective across the interfacial structures between the ZB and WZ polytypes, according to the crystallographic relations discussed in Figures

Jin et al. 2-5. Experimentally, direct determination of such polarity is extremely difficult, although the CBED technique (Figure 7) has been attempted in combination with the theoretical simulations (Figures S1-S4, Supporting Information). To ZB ZnSe, the CBED technique is effective, which allows us to successfully measure the polarity along the 〈111〉 direction and therefore determine the twinning types in the {113} twinned ZnSe bicrystal nanobelts filled with 〈111〉 twinnings.24 However, the existence of planar defects in the ZB region may reduce the measurement accuracy. To WZ ZnSe, it is rather difficult to determine the polarity due to the thickness limitation in the asfabricated products. Detailed discussions are listed below, based on the simulated CBED patterns in Figures S1-S5. (i) The presence of ((0001) secondary diffractions (d0001 ) 0.6506 nm) along [21j1j0]WZ projection (see Figures 2-5) inevitably leads to the severe overlapping of diffraction disks and therefore prohibits further investigations. (ii) The diffraction intensities within ((0002) disks along [19 118j0]WZ (Figure S1), [54j1j0]WZ (Figure S2), [11 101j0]WZ (Figure S3) (randomly selected), and [11j00]WZ (Figure S4) axes, which are 5.21°, 19.11°, 25.29°, and 30° away from the [21j1j0]WZ axis, respectively, are nearly the same under most of the thicknesses, which results in a hard polarity measurement, i.e., for the as-fabricated products with limited thickness. Notably, the intensity difference at thicknesses less than 30 nm (Figures S1-S4) is almost undetectable, which is completely opposite to that in WZ ZnO26 (Figure S5) and which reveals a relatively weak polarity in WZ ZnSe. Such weak polarity can be attributed to the weaker electronegativity of Se than O27 and may facilitate their potential applications in electronic devices. (iii) Actually, both experimental CBED patterns (Figure 7b) taken from a type II junction (Figure 7a) and the corresponding simulated pattern based on dynamical electron diffraction theory (Figure 7c) exhibit an undetectable intensity difference between ((0002) diffraction disks. The thickness is about 210 nm based on the simulated result in Figure 7c. (iv) As a direct comparison, CBED simulations for ZnO with the same parameters used for Figure S4 have also been performed, and as shown in Figure S5, the intensity difference between ((0002) disks is quite remarkable in almost all thicknesses. Meanwhile, the (0002) disk has a higher intensity

Figure 8. (a) Schematic illustration showing the interfacial structures from the polarity perspective and indicating the polarity reversal across the ZB-WZ interface. (b)-(e) Rudimentary atomic models illustrating the polarity reversal: (b) Se-terminated, (c) Se-terminated, (d) Zn-terminated, and (e) Zn-terminated, respectively. Red arrowheads indicate the positive polarity directions with Zn pointing to Se.

Zinc Blende-Wurtzite Polytypism than (0002j) at the thickness of ∼10 nm, which is consistent with the experimental result in ref 26. Logically, the asymmetric growth of sawlike structures is commonly observed at the ZB region (Figures 2-4), which is generally explained by the spontaneous polarization mechanism28,29 combined with the chemical activity of the Znterminated (111)ZB polar surfaces, as illustrated by Zn(ZB) in Figure 8a. Correspondingly, the (1j1j1j) and (111j) surfaces are Se-terminated (near the dashed line). On the other hand, in order to keep a fast growth to form the WZ nanobelt (Figure 8a), the Zn-terminated (0001)WZ surfaces at the WZ regions as illustrated by Zn(WZ) are assumed to be maintained. Therefore, the polarity reversal (see red arrowheads in Figure 8a) probably occurs at the ZB-WZ interface unlike the theoretical calculations in refs 6 and 30. Such polarity reversal has been experimentally observed in the {113} twinned bicrystal nanobelts even with ZB structure24 and inevitably results in charge accumulations around the interface, which is expected to affect the performance of heterocrystalline junctions.1c-e Even though an opposite result with [1j1j1j] pointing to sawlike structures is applied, the polarity reversal may also occur at the inner side of the spiral. For better understanding this reversal, four other types of rudimentary atomic models are proposed as Figures 8b-e, which have taken into account the Se- (Figures 8b and c) and Zn(Figures 8d and e) terminated surfaces between the ZB and WZ polytypes, respectively. Meanwhile, it is worth noting that minor displacement31 either parallel or perpendicular to the interface may also exist; therefore, further calculations with structural relaxations on the interfacial energy and the band structure are in progress. Future research work, such as considering the electrostatic potential, charges, and strain effect, is deserved.32 Conclusions ZB-WZ heterocrystalline junctions have been successfully fabricated. Two types of ZB-WZ junctions are observed and systematically characterized using TEM. Rudimentary atomic models of the ZB-WZ interface have also been proposed. Unlike the previous theoretical studies on the ZnSe (111)ZB-(0001)WZ interface, our results suggest that the asgrown heterocrystalline junctions form accompanied with polarity reversal across the interfaces, which may provide not only an interesting model system for further theoretical investigations but also a good platform to investigate the electrical transport properties. Acknowledgment. This work was financially supported by the Program for New Century Excellent Talents in University (NCET-07-0640), Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20090141110059), National Fund for Talent Training in Basic Science (Grant No. J0830310), and National University Students Training in Scientific Research Program of MOE (Grant No. 081048611), China. W.C.H.C. is thankful for the support by the University Development Fund (UDF) and the seed funding of the University of Hong Kong. Supporting Information Available: Simulated CBED patterns for WZ ZnSe along [19 118j0]WZ, [54j1j0]WZ, [11 101j0]WZ, and [11j00]WZ axes (Figures S1-S4, respectively). As a direct comparison, simulated CBED patterns for WZ ZnO with the same parameters used for Figure S4 are also presented in Figure

J. Phys. Chem. C, Vol. 114, No. 3, 2010 1415 S5. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Agarwal, R. Small 2008, 4, 1872. (b) Zhang, X.; Kulik, J.; Dickey, E. C. J. Nanosci. Nanotechnol. 2007, 7, 717. (c) Lu, W.; Xiang, J.; Timko, B. P.; Wu, Y.; Lieber, C. M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10046. (d) Polyakov, V. M.; Schwierz, F. J. Appl. Phys. 2005, 98, 023709. (e) Li, L. Y.; Ketharanathan, S.; Drucker, J.; McCartney, M. R. Appl. Phys. Lett. 2009, 94, 232108. (f) Bykhovski, A.; Gelmont, B.; Shur, M. J. Appl. Phys. 1993, 74, 6734. (g) An, S. J.; Park, W. I.; Yi, G. C.; Kim, Y. J.; Kang, H. B.; Kim, M. Appl. Phys. Lett. 2004, 84, 3612. (h) Shindo, D.; Park, Y. G.; Kim, B. J.; Wang, J. F.; Isshiki, M. J. Phys.: Condens. Matter 2002, 14, 13305. (i) Wang, X.; Ren, X.; Kahen, K.; Hahn, M. A.; Rajeswaran, M.; Maccagnano-Zacher, S.; Silcox, J.; Cragg, G. E.; Efros, A. L.; Krauss, T. D. Nature 2009, 459, 686. (2) (a) Bjo¨rk, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Appl. Phys. Lett. 2002, 80, 1058. (b) Yan, J.; Fang, X. S.; Zhang, L. D.; Bando, Y.; Gautam, U. K.; Dierre, B.; Sekiguchi, T.; Golberg, D. Nano Lett. 2008, 8, 2794. (3) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (4) (a) Algra, R. E.; Verheijen, M. A.; Borgstro¨m, M. T.; Feiner, L. F.; Immink, G.; van Enckevort, W. J. P.; Vlieg, E.; Bakkers, E. P. A. M. Nature 2008, 456, 369. (b) Shan, X. D.; Zhang, X. Z.; Gao, J. Y.; You, L. P.; Xu, H. J.; Xu, J.; Yu, D. P.; Ye, H. Q. doi: 10.1021/jp9024206. (5) Caroff, P.; Dick, K. A.; Johansson, J.; Messing, M. E.; Deppert, K.; Samuelson, L. Nat. Nanotechnol. 2009, 4, 50. (6) Bandic´, Z. Z.; Ikonic´, Z. Phys. ReV. B 1995, 51, 9806. (7) Verma, A.; Krishna, P. Polymorphism and Polytypism in Crystals; Wiley: New York, London, Sydney, 1966. (8) Ka¨ckell, P.; Wenzien, B.; Bechstedt, F. Phys. ReV. B 1994, 50, 17037. (9) Cheng, C.; Heine, V.; Needs, R. J. J. Phys.: Condens. Matter 1990, 2, 5115. (10) Bechstedt, F.; Ka¨ckell, P. Phys. ReV. Lett. 1995, 75, 2180. (11) Shachar, G.; Brada, Y.; Alexander, E.; Yacobt, Y. J. Appl. Phys. 1970, 41, 723. (12) Shachar, G.; Brada, Y. J. Appl. Phys. 1970, 41, 3127. (13) Cutter, J. R.; Woods, J. J. Phys. D: Appl. Phys. 1975, 8, 314. (14) El-Shazly, A. A.; Kenawy, M. A.; El-Shair, H. T.; El-Nahass, M. M.; Soliman, H. S. J. Phys. D: Appl. Phys. 1983, 16, 1967. (15) Jiang, Y.; Meng, X. M.; Liu, J.; Hong, Z. R.; Lee, C. S.; Lee, S. T. AdV. Mater. 2003, 15, 1195. (16) Bao, J. M.; Bell, D. C.; Capasso, F.; Wagner, J. B. Nano Lett. 2008, 3, 836. (17) Rujkorakarn, R.; Nelson, A. J. J. Appl. Phys. 2000, 87, 8557. (18) Ohtake, A.; Nakamura, J.; Terauchi, M.; Sato, F.; Tanaka, M.; Kimura, K.; Yao, T. Phys. ReV. B 2001, 63, 195325. (19) Xi, B. J.; Xiong, S. L.; Xu, D. C.; Li, J. F.; Zhou, H. Y.; Pan, J.; Li, J. Y.; Qian, Y. T. Chem. Eur. J. 2008, 14, 9786. (20) Zhai, T. Y.; Zhong, H. Z.; Gu, Z. J.; Peng, A. D.; Fu, H. B.; Ma, Y.; Li, Y. F.; Yao, J. N. J. Phys. Chem. C 2007, 111, 2980. (21) Leung, Y. P.; Choy, W. C. H.; Markov, I.; Pang, G. K. H.; Ong, H. C.; Yuk, T. I. Appl. Phys. Lett. 2006, 88, 183110. (22) Liu, S. Y.; Choy, W. C. H.; Jin, L.; Leung, Y. P.; Zheng, G. P.; Wang, J. B.; Soh, A. K. J. Phys. Chem. C 2007, 111, 9055. (23) Jin, L.; Choy, W. C. H.; Leung, Y. P.; Yuk, T. I.; Ong, H. C.; Wang, J. B. J. Appl. Phys. 2007, 102, 044302. (24) Jin, L.; Wang, J. B.; Cao, G. Y.; Xu, Z. L.; Jia, S. F.; Choy, W. C. H.; Leung, Y. P.; Tuk, T. I. J. Phys. Chem. C 2008, 112, 4903. (25) Jin, L.; Wang, J. B.; Choy, W. C. H. Cryst. Growth Des. 2008, 8, 3829. (26) Wessler, B.; Steinecker, A.; Mader, W. J. Cryst. Growth 2002, 242, 283. (27) Allred, A. L. J. Inorg. Nucl. Chem. 1961, 17, 215. (28) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett. 2003, 91, 185502. (29) Ding, Y.; Wang, X. D.; Wang, Z. L. Chem. Phys. Lett. 2004, 398, 32. (30) Northrup, J. E.; Ihm, J.; Cohen, M. L. Phys. ReV. B 1980, 22, 2060. (31) (a) Lim, S. H.; Shindo, D.; Yonenaga, I.; Brown, P. D.; Humphreys, C. J. Phys. ReV. Lett. 1998, 81, 5350. (b) Li, L. Y.; Wang, J. B.; Wang, R. H.; Liu, H. J.; Jia, C. L.; Ma, L. L.; Yu, Y. Appl. Phys. Lett. 2006, 89, 113109. (32) Zhang, L. N.; He, J.; Zhang, J.; Liu, F.; Fu, Y.; Song, Y.; Zhang, X. IEEE Trans. Electron DeVices 2008, 55, 2907.

JP909182E