- Email: [email protected]
Epitaxial growth of nonpolar and polar ZnO on γ-LiAlO2 (100) substrate by plasma-assisted molecular beam epitaxy
Journal of Crystal Growth 377 (2013) 82–87
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Epitaxial growth of nonpolar and polar ZnO on γ-LiAlO2 (100) substrate by plasma-assisted molecular beam epitaxy Y.-M. Chen a, T.-H. Huang a, T. Yan a, L. Chang a,n, M.M.C. Chou a, K.H. Ploog a, C.-M. Chiang b a Department of Materials and Optoelectronic Science/Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC b Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2012 Received in revised form 2 May 2013 Accepted 12 May 2013 Communicated by Dr. A. Brown Available online 22 May 2013
Low lattice mismatched γ-LiAlO2 (100) substrates were employed for the epitaxial growth of ZnO by plasma-assisted molecular beam epitaxy. The crystallographic orientations of the samples were determined by the X-ray diffraction (XRD) patterns. The surface morphology and roughness of ZnO films were investigated by scanning electronic microscopy (SEM) and atomic force microscopy (AFM). Results confirmed that both nonpolar (1010)- and polar (0001)-oriented ZnO epilayers can be grown, demonstrating the dual orientation-selection characteristic of the γ-LiAlO2 substrate. The growth temperature and grow rate are two major factor which affects the orientation competition. A two-step growth method was therefore designed to improve the quality of the polar ZnO film grown at high temperature. Room temperature photoluminescence spectra of both polar and nonpolar ZnO films showed a near band edge emission peak at around 377 nm and a negligible green band emission. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Scanning electronic microscopy A1. Atomic force microscopy A3. Molecular beam epitaxy B1. ZnO film B1. γ-LiAlO2
1. Introduction Zinc oxide has attracted much attention recently for possible applications in ultraviolet/blue semiconductor lasers and light emitting devices [1,2]. Analogous to GaN, epitaxial films of ZnO are conventionally grown on (0001) sapphire substrates. However, the large lattice mismatch between ZnO and sapphire induces a high density of threading dislocations in the epitaxial layer which deteriorates its electronic and optical properties [3–5]. Various low lattice-mismatched substrates have been employed to overcome the problem. For instance, the quality of ZnO grown on latticematched (0.09%) ScAlMgO4 substrate by laser MBE has been considerably improved in terms of surface morphology, crystallinity (a full width at half maximum (FWHM) of 39″ for (0002) rocking curve), and optical and electrical properties [6]. High quality ZnO has also been grown on GaN/c-Al2O3 substrate by metal organic chemical vapor deposition, showing a FWHM value of 182″ for the (0002) rocking curve and excellent optical properties without deep-level emission peak [7]. Recently, c-oriented ZnO has been successfully grown on a-plane sapphire substrate with a mismatch of 0.08% ðaZnO ==cAl2 O3 Þ and 2.4% mZnO ==mAl2 O3 , which exhibits better structural properties with small FWHM values of 272–470″ for the (0002) rocking curve [8–10]. In addition,
n
Corresponding author. Tel.: +886 7 5252000x4080; fax: +886 7 5254099. E-mail address: [email protected] (L. Chang).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.05.014
γ-LiAlO2 (LAO) was adopted for growth of nonpolar ZnO and Zn1−xMgxO by chemical vapor deposition. ZnO and Zn1−xMgxO epilayers with the FWHM values of 500–580″ [11,12] and 216″ [12], respectively, for the (1010) rocking curve were obtained. The LAO (100) substrate is an attractive alternative to sapphire for two reasons. First, LAO is suitable for growing nonpolar ZnO epilayers in 〈1010〉 direction. The problem of low electron–hole recombination probabilities due to the quantum confined Stark effect resulting from the spontaneous polarization fields (0.057 C/m2 [13]) along [0001]ZnO for ZnO heterostructures can thus be avoided. Second, the LAO substrate provides relatively low lattice mismatches of 0.7% in [010]LAO//[0001]ZnO and 3.5% in [001]LAO// [1210]ZnO directions as compared to the m-plane sapphire substrates. Indeed, m-plane ZnO and ZnMgO epilayers with low defect densities have been grown successfully on the LAO substrates by chemical vapor deposition [12]. However, also a quasi-hexagonal symmetry of the anions and cations is found on the LAO (100) surface. The (0001)ZnO and (100)LAO surfaces match less perfectly with a misfit value of ∼8.2% along [010]LAO which nevertheless can allow the (0001)-oriented ZnO crystals to nucleate epitaxially [14]. Consequently, it is possible to obtain either 〈1010〉- or 〈0001〉oriented ZnO epilayers on the LAO (100) substrates from the perspective of lattice matching. In literature, nonpolar (1010) ZnO epilayers have been grown on the LAO (100) substrate by chemical vapor deposition [11,15] and metal-organic chemical vapor deposition [16]. On the other hand, 〈0001〉 oriented ZnO on the same substrate has also been obtained using pulsed laser deposition [17]
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
and chemical vapor deposition, though the 〈1010〉 oriented crystals can still be found in the early stage of growth [14,18]. Results on the epitaxial growth of ZnO on LAO (100) substrates by molecular beam epitaxy (MBE) have not been reported yet. In the present study, attempts are made to study the orientation competition problem for ZnO epitaxial growth on LAO by using the kinetically dominated method of molecular beam epitaxy (MBE). The results indicate that ZnO epilayers of either 〈1010〉 or 〈0001〉 orientation can be obtained by manipulating the kinetic parameters of growth temperature and deposition rate.
2. Experimental Two-inch LAO wafers used throughout the experiments were cut and polished from a single crystal grown along its [100] direction using the Czochralski pulling technique. The wafer surface has a root mean square (rms) roughness of 2.0 nm, and the wafers were cut into 10 mm 10 mm substrate pieces for MBE growth. The substrates were cleaned in ultrasonic baths of acetone and isopropanol for 5 min followed by rinsing with ultra pure water. A Si wafer was glued subsequently to the substrate by indium to ensure a good thermal conductivity during growth. The plasma-assisted MBE growth was carried out in a CreaTec SY094 system equipped with two low temperature effusion cells and a radio frequency (RF) plasma source. Elemental Zn with purity of 99.99999 wt% and oxygen (purity
83
99.999%) were used as zinc and oxygen source, respectively. Prior to the epitaxial growth, the substrate was thermally cleaned by heating to 700 1C in H2 in the load-lock chamber for 30 min. Then the substrate was exposed to the Zn flux with a beam equivalent pressure (BEPZn) of 2.5–5.0 10−7 mbar. The oxygen flow rate was adjusted to produce a background pressure P O2 of 5–10 10−6 mbar with the RF power set to 300 W. The Zn/O flux ratio is expressed using the BEPzn to P O2 ratio accordingly. The ZnO epilayers were deposited at various substrate temperatures from 400 to 700 1C. The growth conditions and thickness of each sample are summarized in Table 1. Following the MBE growth, the orientation and the crystal quality of the ZnO films was determined by X-ray diffraction (XRD, Bruker D8, Cu Kα radiation). Surface morphology and roughness were investigated by scanning electron microscopy (SEM, JEOL JSM-6330TF & Zeiss Supra 55) and atomic force microscopy (AFM, DI Nanoscope IIIa) in the contact mode. Photoluminescence (PL) spectra were measured by exciting the sample with a 325 nm He–Cd laser at room temperature.
3. Results and discussion 3.1. Epitaxial orientation A series of x-ray ω–2θ diffraction patterns of the ZnO films grown at temperatures from 400 to 700 1C at the same BEPZn to
Table 1 Growth conditions, thickness and XRD results of each sample. Sample name
Temperature (1C)
Zn (mbar)
O2 (mbar)
BEPZn =P O2
Thickness (nm)
Growth rate (nm/h)
RC (1010) FWHM
RC (0002) FWHM
A B C D E F G H I J K L M N
700 600 500 400 700 600 500 400 400 700 700 700 700 700
5 10−7 5 10−7 5 10−7 5 10−7 2.5 10−7 2.5 10−7 2.5 10−7 2.5 10−7 2.5 10−7 4 10−7 2.5 10−7 2.5 10−7 3 10−7 3 10−7
1 10−5 1 10−5 1 10−5 1 10−5 5 10−6 5 10−6 5 10−6 5 10−6 1 10−5 5 10−6 1 10−5 5 10−6 1 10−5 3 10−6
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.025 0.08 0.025 0.05 0.03 0.1
1900 1920 1600 1560 240 420 390 240 540 180 170 120 N/A 360
630 640 530 520 40 70 66 80 180 60 54 40 N/A 120
0.601 0.561 0.881 / 1.151 0.851 0.631 / / 1.131 1.081 1.711 / /
2.91 3.61 2.11 3.51 / 1.41 4.01 6.01 1.51
/ 6.01 3.11
Fig. 1. XRD patterns of ZnO films grown at (a) BEPZn ¼ 5.0 10−7 mbar and P O2 ¼ 1:0 10−5 mbar (samples A–D) and (b) BEPZn ¼ 2.5 10−7 mbar and P O2 ¼ 5:0 10−6 mbar (samples E–H) at various temperatures from 400 to 700 1C.
84
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
P O2 ratio of 0.05 are shown in Fig. 1. Fig. 1a shows the diffraction patterns of the samples A–D grown at a high O2 pressure of 1 10−5 mbar. Each sample shows a very strong (200)LAO peak at 2θ¼34.681. For the epilayer grown at 400 1C, only a weak (0002)ZnO peak appears at the low-2θ shoulder of the (200)LAO peak. Upon increasing the growth temperature to 500 1C, the intensity of the (0002)ZnO peak increases profoundly and a very weak (1010)ZnO peak at 2θ¼31.751 is also observed. The intensity
of the (0002)ZnO peak reaches its maximum value at 600 1C and decreases slightly at 700 1C, whereas that of the (1010)ZnO peak keeps increasing with the growth temperature. However, the former is still much stronger than the latter one at temperatures as high as 700 1C. The diffraction patterns of the samples E–H grown at a low O2 pressure of 5 10−6 mbar are shown in Fig. 1b. Similarly, only a weak (0002)ZnO peak is observed at 400 1C. The intensity of the (0002)ZnO peak increases upon increasing the
Fig. 2. XRD patterns of ZnO films grown with different Zn flux and oxygen pressure at (a) 400 1C for samples D, H and I and (b) 700 1C for samples J, K and L.
Fig. 3. SEM images of ZnO films grown at (a) 400, (b) 500, (c) 600 and (d) 700 1C with BEPZn ¼2.5 10−7 mbar and P O2 ¼ 5:0 10−6 mbar.
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
growth temperature to 500 1C but then decreases when further increasing the temperature. At 700 1C, the (0002)ZnO peak is no longer observed. On the other hand, the intensity of the (1010)ZnO peak increases with temperature at a much higher rate compared to their high O2 pressure counterparts. In addition to the growth temperature and O2 pressure, the next issue to be explored is wether the Zn/O ratio (i.e., BEPZn/P O2 ) influences the crystal orientation. Fig. 2 shows the XRD patterns of ZnO films (samples D–H–I and J–K–L) grown at 400 and 700 1C, respectively, with different BEPZn/P O2 ratios. According to the XRD patterns in Fig. 2(a), only the nucleation of the 〈0001〉-oriented ZnO is allowed at 400 1C regardless the Zn/O ratio. On the other hand, pure 〈1010〉 oriented ZnO epilayers are obtained at 700 1C with BEPZn ¼2.5 10−7 and P O2 ¼ 5 10−6 mbar (sample L, having the same growth parameters as sample E in Fig. 1b) as described previously. Once increasing BEPZn to 4.0 10−7 mbar or P O2 to 1 10−5 mbar, which results in an increase in the growth rate, a weak (0001) ZnO peak appears as shown in Fig. 2(b). Ko et al. [19] have pointed out that the growth rate in molecular beam epitaxy of ZnO is strongly related to the Zn/O flux ratio and the O2 pressure (flux). Briefly speaking, the growth rate increases monotonically with increasing the Zn/O ratio till the stoichiometric value is reached. In addition, the growth rate increases with the O2 pressure (flux) at a constant Zn/O ratio. In the present case, the Zn/O flux ratios for all the growth conditions used here are not high enough to reach the stoichiometric value. The growth rate therefore should keep increasing with increasing the values of BEPZn and P O2 . In other words, all the
85
samples were grown under oxygen-rich conditions. The growth rate is indeed a very complicate function of the Zn/O flux ratio, sticking coefficient, temperature and surface structure (or crystal orientation) [20,21]. Since no report on the sticking coefficients of Zn and O at 400–700 1C on the LAO substrate is available at the moment, the growth rate shown in Table 1 represents an additive result of the sticking coefficient, dissociation rate of oxygen and flux. It is clearly shown in Table 1 that the increase of the P O2 from 5 10−6 to 1 10−5 mbar results in a 6–8 increase of the growth rate. The growth temperature, however, shows very limited effect on the growth rate. Moreover, the crystal orientation seems to play an important role. The growth rates are only 40–60 nm/h regardless the P O2 value for samples having pure (1010) orientation (E, J, K and L). The present results therefore indicate that not only the growth temperature, but also the growth rate plays an important role for the epitaxial growth of ZnO on LAO substrates. In a short summary, nonpolar ZnO tends to nucleate at high temperatures and at low growth rates. The nucleation of the nonpolar ZnO is completely suppressed at low temperature of 400 1C regardless the growth rate. A pure c-plane epilayer is thus obtained at the low growth temperature of 400 1C. As a consequence, the growth rate rather than the BEPZn =P O2 ratio is the major factor which affects the orientation competition. It is well known that the c-axis of ZnO is the easy direction of growth. This is attributed to the lowest surface energy of the (0001) basal plane [22,23]. Taking this into account, the polar ZnO can nucleate and grow at a low temperature as the formation of the mplane ZnO is prohibited by kinetics. However, the nonpolar m-plane
Fig. 4. AFM images of ZnO films grown at (a) 400, (b) 500, (c) 600 and (d) 700 1C with BEPZn ¼2.5 10−7 mbar and P O2 ¼ 5:0 10−6 mbar in the area of 3 3 μm2. (The scale bar is 0–150 nm).
86
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
ZnO is favored to be formed thermodynamically due to the low strain energy associated with its formation. As a result, a high growth temperature is essential for growing the m-plane ZnO epilayers. It should be noted that this is specific to LAO substrate. Nonpolar m-plane ZnO has been prepared on m-plane sapphire at temperatures of 400 1C or even lower by pulsed laser deposition [24,25].
these crystallites probably have a nonpolar orientation. For the sample grown at 700 1C (Fig. 4d), the typically anisotropic morphology of nonpolar ZnO (∼150 1000 nm2) is observed. The root mean square (rms) roughness of the films is 13 nm (400 1C), 9 nm (500 1C), 6 nm (600 1C) and 13 nm (700 1C) in a 3 3 μm2 area. Obviously, the sample grown at 600 1C shows the smoothest surface among them. A similar phenomenon has also been reported for the growth of nonpolar ZnO film on m-plane sapphire by MBE [27].
3.2. Epitaxial layer morphology Fig. 3 shows the SEM secondary electron images of ZnO samples E–H grown at a low O2 pressure of 5 10−6 mbar. It is apparent that the surface morphology of the epilayers depends strongly on the growth temperature. When the sample was grown at 400 1C (Fig. 3a), the surface is rough with wrinkles. As the growth temperature is increased to 500 1C (Fig. 3b), the surface is relatively flat with fine bumps. At 600 1C (Fig. 3c), some rectangular-shaped blocks are present while some of the areas are still similar to the morphology observed at 500 1C. For the nonpolar ZnO film grown at 700 1C (Fig. 3d), the surface is full of rectangular stripes elongated along the [010]LAO direction, most of which coalesce with each other. This stripe shape is a typical feature observed in nonpolar ZnO [26]. AFM images of the ZnO films were taken for further clarifying the surface morphology and roughness. Fig. 4a shows that the epilayer grown at 400 1C is composed of elliptical crystals having a size of 50–200 nm, indicating that the three-dimensional island growth is the dominant mode of epitaxy. The epilayers obtained at 500 1C and 600 1C are also composed of elongated crystallites as shown in Fig. 4b–c. However, now many crystallites have a square or rectangular shape and are oriented in the same direction, indicating that
3.3. Polar versus nonpolar ZnO epilayers grown at 700 1C In order to improve the surface morphology of the c-plane epilayer, a low temperature (LT) buffer layer was grown at 350 1C followed by the growth at higher temperature of 700 1C. The LT buffer layer is about 20 nm thick to ensure a complete coverage of the substrate by the buffer layer. Fig. 5 shows the SEM and AFM images of two samples grown at different oxygen pressures. The film (sample M) grown at a high O2 pressure (1 10−5 mbar) shows a surface (Fig. 5a) which is smoother than the one grown at 400 1C (Fig. 4a). The AFM image in Fig. 5a further verifies that the rms roughness of the sample is 10 nm. At a low O2 pressure of 3 10−6 mbar, the ZnO film (sample N) becomes even flatter with a corresponding rms roughness of 6 nm in an area of 3 3 μm2 (see Fig. 5b), which is obviously lower than that of the 400 1C grown sample. Fig. 6 shows the room temperature PL spectra of the polar and nonpolar ZnO epilayers grown at 700 1C. The polar sample (sample N) was prepared at a high BEPZn =P O2 ¼ 0.1 (BEPZn ¼3.0 10−7 mbar and P O2 ¼3 10−6 mbar) with a 20 nm LT buffer layer. The
Fig. 5. SEM and AFM images of polar ZnO grown at 700 1C with a LT buffer layer at (a) P O2 ¼ 1:0 10−5 mbar (sample M) and (b) P O2 ¼ 3:0 10−6 mbar (sample N) with BEPZn ¼3.0 10−7 mbar. (The scanning area is 3 3 μm2 with the same scale bar in Fig. 4).
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
87
Acknowledgments This work is financially supported by the National Science Council of Taiwan under Grant no. NSC100-2221-E-110-055-MY3, by the Aim for the Top University Project as well as the Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University.
References
Fig. 6. Room temperature PL spectra of polar (sample N) and nonpolar (sample E) ZnO films grown at 700 1C.
nonpolar one (sample E) is, however, grown at BEPZn =P O2 ¼ 0.05 (BEPZn ¼2.5 10−7 mbar and P O2 ¼ 5 10−6 mbar). Both samples show a strong near band-edge emission (NBE) at around 3.3 eV with extremely weak deep level emission. The NBE peak intensity of the nonpolar sample is slightly lower than that of the polar one. It is worth noting that the polar sample shows a very large value (3.11) of the full width at half maximum (FWHM) of its (0002)ZnO rocking curve, whereas that of the nonpolar (1010)ZnO rocking curve is 1.11 (see Table 1). On the other hand, the rocking curve widths of the asymmetrical (1011) ZnO reflection of these two samples show the same value of 1.41, indicating that the large FWHM value of the polar sample is probably a result of heavy tilting existing between the crystallites rather than high defect densities. This feature was also observed in polar ZnO epilayer on LAO prepared by chemical vapor deposition [18]. 4. Conclusion The orientation competition problem for molecular beam epitaxy of ZnO on the γ-LiAlO2 (100) substrate has been addressed in present work. We have shown that nonpolar (1010) ZnO epilayers can be grown at 700 1C at a relatively low growth rate. On the contrary, polar (0001) ZnO epilayers can be obtained at a temperature of 400 1C or lower. Moreover, polar ZnO epilayers can be grown at high growth temperatures by introducing a low temperature buffer layer for proper nucleation. The room temperature PL spectrum of both the polar and nonpolar ZnO epilayers showed a strong UV emission at around 377 nm and a negligible green band emission.
[1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Applied Physics Letters 70 (1997) 2230. [2] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nature Materials 4 (2005) 42. [3] F. Vigué, P. Vennegues, S. Vezian, M. Laugt, J.-P. Faurie, Applied Physics Letters 79 (2001) 194. [4] T. Hino, S. Tomiya, T. Miyajimam, S. Hashimoto, M. Ikeda, Applied Physics Letters 76 (2000) 3421. [5] J.Y. Shi, L.P. Yu, Y.Z. Wang, H. Zhang, Applied Physics Letters 80 (2002) 2293. [6] K. Tamura, A. Ohtomo, K. Saikusa, Y. Osaka, T. Makino, Y. Segawa, M. Sumiya, S. Fuke, H. Koinuma, M. Kawasaki, Journal of Crystal Growth 214/215 (2000) 59. [7] J. Dai, H. Liu, W. Fang, L. Wang, Y. Pu, Y. Chen, F. Jiang, Journal of Crystal Growth 283 (2005) 93. [8] H. Kato, M. Sano, K. Miyamoto, T. Yao, Journal of Crystal Growth 237–239 (2002) 538. [9] Y. Xie, M. Madel, T. Zoberbier, A. Reiser, W. Jie, B. Neuschl, J. Biskupek, U. Kaiser, M. Feneberg, K. Thonke, Applied Physics Letters 100 (2012) 182101. [10] X. Pan, P. Ding, H. He, J. Huang, B. Liu, H. Zhang, Z. Ye, Optics Communications 285 (2012) 4431. [11] M.M.C. Chou, L. Chang, H.Y. Chung, T.H. Huang, J.J. Wu, C.W. Chen, Journal of Crystal Growth 308 (2007) 412. [12] T.-H. Huang, W.-H. Lin, J.-J. Wu, M.M.C. Chou, T. Yan, L. Chang, ECS Transactions 45 (2012) 63. [13] F. Bernardini, V. Fiorentini, D. Vanderbilt, Physical Review B 56 (1997) R10024. [14] T.H. Huang, L. Chang, M.M.C. Chou, U. Jahn, Journal of the Electrochemical Society 158 (2011) H38. [15] M.M.C. Chou, L. Chang, D.R. Hang, C. Chen, D.S. Chang, C.A. Li, Crystal Growth and Design 9 (2009) 2073. [16] H. Lin, S. Zhou, T. Huang, H. Teng, X. Liu, S. Gu, S. Zhu, Z. Xie, P. Han, R. Zhang, Journal of Alloys and Compounds 467 (2009) L8. [17] J. Zou, S. Zhou, C. Xia, Y. Hang, J. Xu, S. Gu, R. Zhang, Journal of Crystal Growth 280 (2005) 185. [18] L. Chang, M.M.C. Chou, T.H. Hwang, C.W. Chen, Physica Status Solidi A 206 (2009) 215. [19] H.-J. Ko, T. Yao, Y. Chen, S.-K. Hong, Journal of Applied Physics 92 (2002) 4354. [20] B.A. Joyce, Reports on Progress in Physics 48 (1985) 1637. [21] Ü. Özgür, V. Avrutin, H. Morkoc, Molecular Beam Epitaxy: from research to mass production, in: M. Henini (Ed.), Elsevier, Oxford, 2013, p. 369, chapter 16. [22] S.S. Kim, B.T. Lee, Thin Solid Films 446 (2004) 307. [23] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, Journal of Crystal Growth 130 (1993) 269. [24] J.-H. Kim, S.K. Han, S.I. Hong, S.-K. Hong, J.W. Lee, J.Y. Lee, J.-H. Song, J.S. Park, and T. Yao, Journal of Vacuum Science and Technology B 27 (2009) 1625. [25] Y. Li, Y. Zhang, H. He, Z. Ye, J. Jiang, J. Luo, J. Huang, Materials Research Bulletin 47 (2012) 2235. [26] P. Ding, X.H. Pan, J.Y. Huang, H.P. He, B. Lu, H.H. Zhang, Z.Z. Ye, Journal of Crystal Growth 331 (2011) 15. [27] J.H. Kim, S.K. Han, S. Hong, S.K. Hong, J.W. Lee, J.Y. Lee, J.H. Song, J.S. Park, T. Yao, Journal of Vacuum Science and Technology B 27 (3) (2009) 1625.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Epitaxial growth of nonpolar and polar ZnO on γ-LiAlO2 (100) substrate by plasma-assisted molecular beam epitaxy Y.-M. Chen a, T.-H. Huang a, T. Yan a, L. Chang a,n, M.M.C. Chou a, K.H. Ploog a, C.-M. Chiang b a Department of Materials and Optoelectronic Science/Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC b Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2012 Received in revised form 2 May 2013 Accepted 12 May 2013 Communicated by Dr. A. Brown Available online 22 May 2013
Low lattice mismatched γ-LiAlO2 (100) substrates were employed for the epitaxial growth of ZnO by plasma-assisted molecular beam epitaxy. The crystallographic orientations of the samples were determined by the X-ray diffraction (XRD) patterns. The surface morphology and roughness of ZnO films were investigated by scanning electronic microscopy (SEM) and atomic force microscopy (AFM). Results confirmed that both nonpolar (1010)- and polar (0001)-oriented ZnO epilayers can be grown, demonstrating the dual orientation-selection characteristic of the γ-LiAlO2 substrate. The growth temperature and grow rate are two major factor which affects the orientation competition. A two-step growth method was therefore designed to improve the quality of the polar ZnO film grown at high temperature. Room temperature photoluminescence spectra of both polar and nonpolar ZnO films showed a near band edge emission peak at around 377 nm and a negligible green band emission. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Scanning electronic microscopy A1. Atomic force microscopy A3. Molecular beam epitaxy B1. ZnO film B1. γ-LiAlO2
1. Introduction Zinc oxide has attracted much attention recently for possible applications in ultraviolet/blue semiconductor lasers and light emitting devices [1,2]. Analogous to GaN, epitaxial films of ZnO are conventionally grown on (0001) sapphire substrates. However, the large lattice mismatch between ZnO and sapphire induces a high density of threading dislocations in the epitaxial layer which deteriorates its electronic and optical properties [3–5]. Various low lattice-mismatched substrates have been employed to overcome the problem. For instance, the quality of ZnO grown on latticematched (0.09%) ScAlMgO4 substrate by laser MBE has been considerably improved in terms of surface morphology, crystallinity (a full width at half maximum (FWHM) of 39″ for (0002) rocking curve), and optical and electrical properties [6]. High quality ZnO has also been grown on GaN/c-Al2O3 substrate by metal organic chemical vapor deposition, showing a FWHM value of 182″ for the (0002) rocking curve and excellent optical properties without deep-level emission peak [7]. Recently, c-oriented ZnO has been successfully grown on a-plane sapphire substrate with a mismatch of 0.08% ðaZnO ==cAl2 O3 Þ and 2.4% mZnO ==mAl2 O3 , which exhibits better structural properties with small FWHM values of 272–470″ for the (0002) rocking curve [8–10]. In addition,
n
Corresponding author. Tel.: +886 7 5252000x4080; fax: +886 7 5254099. E-mail address: [email protected] (L. Chang).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.05.014
γ-LiAlO2 (LAO) was adopted for growth of nonpolar ZnO and Zn1−xMgxO by chemical vapor deposition. ZnO and Zn1−xMgxO epilayers with the FWHM values of 500–580″ [11,12] and 216″ [12], respectively, for the (1010) rocking curve were obtained. The LAO (100) substrate is an attractive alternative to sapphire for two reasons. First, LAO is suitable for growing nonpolar ZnO epilayers in 〈1010〉 direction. The problem of low electron–hole recombination probabilities due to the quantum confined Stark effect resulting from the spontaneous polarization fields (0.057 C/m2 [13]) along [0001]ZnO for ZnO heterostructures can thus be avoided. Second, the LAO substrate provides relatively low lattice mismatches of 0.7% in [010]LAO//[0001]ZnO and 3.5% in [001]LAO// [1210]ZnO directions as compared to the m-plane sapphire substrates. Indeed, m-plane ZnO and ZnMgO epilayers with low defect densities have been grown successfully on the LAO substrates by chemical vapor deposition [12]. However, also a quasi-hexagonal symmetry of the anions and cations is found on the LAO (100) surface. The (0001)ZnO and (100)LAO surfaces match less perfectly with a misfit value of ∼8.2% along [010]LAO which nevertheless can allow the (0001)-oriented ZnO crystals to nucleate epitaxially [14]. Consequently, it is possible to obtain either 〈1010〉- or 〈0001〉oriented ZnO epilayers on the LAO (100) substrates from the perspective of lattice matching. In literature, nonpolar (1010) ZnO epilayers have been grown on the LAO (100) substrate by chemical vapor deposition [11,15] and metal-organic chemical vapor deposition [16]. On the other hand, 〈0001〉 oriented ZnO on the same substrate has also been obtained using pulsed laser deposition [17]
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
and chemical vapor deposition, though the 〈1010〉 oriented crystals can still be found in the early stage of growth [14,18]. Results on the epitaxial growth of ZnO on LAO (100) substrates by molecular beam epitaxy (MBE) have not been reported yet. In the present study, attempts are made to study the orientation competition problem for ZnO epitaxial growth on LAO by using the kinetically dominated method of molecular beam epitaxy (MBE). The results indicate that ZnO epilayers of either 〈1010〉 or 〈0001〉 orientation can be obtained by manipulating the kinetic parameters of growth temperature and deposition rate.
2. Experimental Two-inch LAO wafers used throughout the experiments were cut and polished from a single crystal grown along its [100] direction using the Czochralski pulling technique. The wafer surface has a root mean square (rms) roughness of 2.0 nm, and the wafers were cut into 10 mm 10 mm substrate pieces for MBE growth. The substrates were cleaned in ultrasonic baths of acetone and isopropanol for 5 min followed by rinsing with ultra pure water. A Si wafer was glued subsequently to the substrate by indium to ensure a good thermal conductivity during growth. The plasma-assisted MBE growth was carried out in a CreaTec SY094 system equipped with two low temperature effusion cells and a radio frequency (RF) plasma source. Elemental Zn with purity of 99.99999 wt% and oxygen (purity
83
99.999%) were used as zinc and oxygen source, respectively. Prior to the epitaxial growth, the substrate was thermally cleaned by heating to 700 1C in H2 in the load-lock chamber for 30 min. Then the substrate was exposed to the Zn flux with a beam equivalent pressure (BEPZn) of 2.5–5.0 10−7 mbar. The oxygen flow rate was adjusted to produce a background pressure P O2 of 5–10 10−6 mbar with the RF power set to 300 W. The Zn/O flux ratio is expressed using the BEPzn to P O2 ratio accordingly. The ZnO epilayers were deposited at various substrate temperatures from 400 to 700 1C. The growth conditions and thickness of each sample are summarized in Table 1. Following the MBE growth, the orientation and the crystal quality of the ZnO films was determined by X-ray diffraction (XRD, Bruker D8, Cu Kα radiation). Surface morphology and roughness were investigated by scanning electron microscopy (SEM, JEOL JSM-6330TF & Zeiss Supra 55) and atomic force microscopy (AFM, DI Nanoscope IIIa) in the contact mode. Photoluminescence (PL) spectra were measured by exciting the sample with a 325 nm He–Cd laser at room temperature.
3. Results and discussion 3.1. Epitaxial orientation A series of x-ray ω–2θ diffraction patterns of the ZnO films grown at temperatures from 400 to 700 1C at the same BEPZn to
Table 1 Growth conditions, thickness and XRD results of each sample. Sample name
Temperature (1C)
Zn (mbar)
O2 (mbar)
BEPZn =P O2
Thickness (nm)
Growth rate (nm/h)
RC (1010) FWHM
RC (0002) FWHM
A B C D E F G H I J K L M N
700 600 500 400 700 600 500 400 400 700 700 700 700 700
5 10−7 5 10−7 5 10−7 5 10−7 2.5 10−7 2.5 10−7 2.5 10−7 2.5 10−7 2.5 10−7 4 10−7 2.5 10−7 2.5 10−7 3 10−7 3 10−7
1 10−5 1 10−5 1 10−5 1 10−5 5 10−6 5 10−6 5 10−6 5 10−6 1 10−5 5 10−6 1 10−5 5 10−6 1 10−5 3 10−6
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.025 0.08 0.025 0.05 0.03 0.1
1900 1920 1600 1560 240 420 390 240 540 180 170 120 N/A 360
630 640 530 520 40 70 66 80 180 60 54 40 N/A 120
0.601 0.561 0.881 / 1.151 0.851 0.631 / / 1.131 1.081 1.711 / /
2.91 3.61 2.11 3.51 / 1.41 4.01 6.01 1.51
/ 6.01 3.11
Fig. 1. XRD patterns of ZnO films grown at (a) BEPZn ¼ 5.0 10−7 mbar and P O2 ¼ 1:0 10−5 mbar (samples A–D) and (b) BEPZn ¼ 2.5 10−7 mbar and P O2 ¼ 5:0 10−6 mbar (samples E–H) at various temperatures from 400 to 700 1C.
84
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
P O2 ratio of 0.05 are shown in Fig. 1. Fig. 1a shows the diffraction patterns of the samples A–D grown at a high O2 pressure of 1 10−5 mbar. Each sample shows a very strong (200)LAO peak at 2θ¼34.681. For the epilayer grown at 400 1C, only a weak (0002)ZnO peak appears at the low-2θ shoulder of the (200)LAO peak. Upon increasing the growth temperature to 500 1C, the intensity of the (0002)ZnO peak increases profoundly and a very weak (1010)ZnO peak at 2θ¼31.751 is also observed. The intensity
of the (0002)ZnO peak reaches its maximum value at 600 1C and decreases slightly at 700 1C, whereas that of the (1010)ZnO peak keeps increasing with the growth temperature. However, the former is still much stronger than the latter one at temperatures as high as 700 1C. The diffraction patterns of the samples E–H grown at a low O2 pressure of 5 10−6 mbar are shown in Fig. 1b. Similarly, only a weak (0002)ZnO peak is observed at 400 1C. The intensity of the (0002)ZnO peak increases upon increasing the
Fig. 2. XRD patterns of ZnO films grown with different Zn flux and oxygen pressure at (a) 400 1C for samples D, H and I and (b) 700 1C for samples J, K and L.
Fig. 3. SEM images of ZnO films grown at (a) 400, (b) 500, (c) 600 and (d) 700 1C with BEPZn ¼2.5 10−7 mbar and P O2 ¼ 5:0 10−6 mbar.
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
growth temperature to 500 1C but then decreases when further increasing the temperature. At 700 1C, the (0002)ZnO peak is no longer observed. On the other hand, the intensity of the (1010)ZnO peak increases with temperature at a much higher rate compared to their high O2 pressure counterparts. In addition to the growth temperature and O2 pressure, the next issue to be explored is wether the Zn/O ratio (i.e., BEPZn/P O2 ) influences the crystal orientation. Fig. 2 shows the XRD patterns of ZnO films (samples D–H–I and J–K–L) grown at 400 and 700 1C, respectively, with different BEPZn/P O2 ratios. According to the XRD patterns in Fig. 2(a), only the nucleation of the 〈0001〉-oriented ZnO is allowed at 400 1C regardless the Zn/O ratio. On the other hand, pure 〈1010〉 oriented ZnO epilayers are obtained at 700 1C with BEPZn ¼2.5 10−7 and P O2 ¼ 5 10−6 mbar (sample L, having the same growth parameters as sample E in Fig. 1b) as described previously. Once increasing BEPZn to 4.0 10−7 mbar or P O2 to 1 10−5 mbar, which results in an increase in the growth rate, a weak (0001) ZnO peak appears as shown in Fig. 2(b). Ko et al. [19] have pointed out that the growth rate in molecular beam epitaxy of ZnO is strongly related to the Zn/O flux ratio and the O2 pressure (flux). Briefly speaking, the growth rate increases monotonically with increasing the Zn/O ratio till the stoichiometric value is reached. In addition, the growth rate increases with the O2 pressure (flux) at a constant Zn/O ratio. In the present case, the Zn/O flux ratios for all the growth conditions used here are not high enough to reach the stoichiometric value. The growth rate therefore should keep increasing with increasing the values of BEPZn and P O2 . In other words, all the
85
samples were grown under oxygen-rich conditions. The growth rate is indeed a very complicate function of the Zn/O flux ratio, sticking coefficient, temperature and surface structure (or crystal orientation) [20,21]. Since no report on the sticking coefficients of Zn and O at 400–700 1C on the LAO substrate is available at the moment, the growth rate shown in Table 1 represents an additive result of the sticking coefficient, dissociation rate of oxygen and flux. It is clearly shown in Table 1 that the increase of the P O2 from 5 10−6 to 1 10−5 mbar results in a 6–8 increase of the growth rate. The growth temperature, however, shows very limited effect on the growth rate. Moreover, the crystal orientation seems to play an important role. The growth rates are only 40–60 nm/h regardless the P O2 value for samples having pure (1010) orientation (E, J, K and L). The present results therefore indicate that not only the growth temperature, but also the growth rate plays an important role for the epitaxial growth of ZnO on LAO substrates. In a short summary, nonpolar ZnO tends to nucleate at high temperatures and at low growth rates. The nucleation of the nonpolar ZnO is completely suppressed at low temperature of 400 1C regardless the growth rate. A pure c-plane epilayer is thus obtained at the low growth temperature of 400 1C. As a consequence, the growth rate rather than the BEPZn =P O2 ratio is the major factor which affects the orientation competition. It is well known that the c-axis of ZnO is the easy direction of growth. This is attributed to the lowest surface energy of the (0001) basal plane [22,23]. Taking this into account, the polar ZnO can nucleate and grow at a low temperature as the formation of the mplane ZnO is prohibited by kinetics. However, the nonpolar m-plane
Fig. 4. AFM images of ZnO films grown at (a) 400, (b) 500, (c) 600 and (d) 700 1C with BEPZn ¼2.5 10−7 mbar and P O2 ¼ 5:0 10−6 mbar in the area of 3 3 μm2. (The scale bar is 0–150 nm).
86
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
ZnO is favored to be formed thermodynamically due to the low strain energy associated with its formation. As a result, a high growth temperature is essential for growing the m-plane ZnO epilayers. It should be noted that this is specific to LAO substrate. Nonpolar m-plane ZnO has been prepared on m-plane sapphire at temperatures of 400 1C or even lower by pulsed laser deposition [24,25].
these crystallites probably have a nonpolar orientation. For the sample grown at 700 1C (Fig. 4d), the typically anisotropic morphology of nonpolar ZnO (∼150 1000 nm2) is observed. The root mean square (rms) roughness of the films is 13 nm (400 1C), 9 nm (500 1C), 6 nm (600 1C) and 13 nm (700 1C) in a 3 3 μm2 area. Obviously, the sample grown at 600 1C shows the smoothest surface among them. A similar phenomenon has also been reported for the growth of nonpolar ZnO film on m-plane sapphire by MBE [27].
3.2. Epitaxial layer morphology Fig. 3 shows the SEM secondary electron images of ZnO samples E–H grown at a low O2 pressure of 5 10−6 mbar. It is apparent that the surface morphology of the epilayers depends strongly on the growth temperature. When the sample was grown at 400 1C (Fig. 3a), the surface is rough with wrinkles. As the growth temperature is increased to 500 1C (Fig. 3b), the surface is relatively flat with fine bumps. At 600 1C (Fig. 3c), some rectangular-shaped blocks are present while some of the areas are still similar to the morphology observed at 500 1C. For the nonpolar ZnO film grown at 700 1C (Fig. 3d), the surface is full of rectangular stripes elongated along the [010]LAO direction, most of which coalesce with each other. This stripe shape is a typical feature observed in nonpolar ZnO [26]. AFM images of the ZnO films were taken for further clarifying the surface morphology and roughness. Fig. 4a shows that the epilayer grown at 400 1C is composed of elliptical crystals having a size of 50–200 nm, indicating that the three-dimensional island growth is the dominant mode of epitaxy. The epilayers obtained at 500 1C and 600 1C are also composed of elongated crystallites as shown in Fig. 4b–c. However, now many crystallites have a square or rectangular shape and are oriented in the same direction, indicating that
3.3. Polar versus nonpolar ZnO epilayers grown at 700 1C In order to improve the surface morphology of the c-plane epilayer, a low temperature (LT) buffer layer was grown at 350 1C followed by the growth at higher temperature of 700 1C. The LT buffer layer is about 20 nm thick to ensure a complete coverage of the substrate by the buffer layer. Fig. 5 shows the SEM and AFM images of two samples grown at different oxygen pressures. The film (sample M) grown at a high O2 pressure (1 10−5 mbar) shows a surface (Fig. 5a) which is smoother than the one grown at 400 1C (Fig. 4a). The AFM image in Fig. 5a further verifies that the rms roughness of the sample is 10 nm. At a low O2 pressure of 3 10−6 mbar, the ZnO film (sample N) becomes even flatter with a corresponding rms roughness of 6 nm in an area of 3 3 μm2 (see Fig. 5b), which is obviously lower than that of the 400 1C grown sample. Fig. 6 shows the room temperature PL spectra of the polar and nonpolar ZnO epilayers grown at 700 1C. The polar sample (sample N) was prepared at a high BEPZn =P O2 ¼ 0.1 (BEPZn ¼3.0 10−7 mbar and P O2 ¼3 10−6 mbar) with a 20 nm LT buffer layer. The
Fig. 5. SEM and AFM images of polar ZnO grown at 700 1C with a LT buffer layer at (a) P O2 ¼ 1:0 10−5 mbar (sample M) and (b) P O2 ¼ 3:0 10−6 mbar (sample N) with BEPZn ¼3.0 10−7 mbar. (The scanning area is 3 3 μm2 with the same scale bar in Fig. 4).
Y.-M. Chen et al. / Journal of Crystal Growth 377 (2013) 82–87
87
Acknowledgments This work is financially supported by the National Science Council of Taiwan under Grant no. NSC100-2221-E-110-055-MY3, by the Aim for the Top University Project as well as the Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University.
References
Fig. 6. Room temperature PL spectra of polar (sample N) and nonpolar (sample E) ZnO films grown at 700 1C.
nonpolar one (sample E) is, however, grown at BEPZn =P O2 ¼ 0.05 (BEPZn ¼2.5 10−7 mbar and P O2 ¼ 5 10−6 mbar). Both samples show a strong near band-edge emission (NBE) at around 3.3 eV with extremely weak deep level emission. The NBE peak intensity of the nonpolar sample is slightly lower than that of the polar one. It is worth noting that the polar sample shows a very large value (3.11) of the full width at half maximum (FWHM) of its (0002)ZnO rocking curve, whereas that of the nonpolar (1010)ZnO rocking curve is 1.11 (see Table 1). On the other hand, the rocking curve widths of the asymmetrical (1011) ZnO reflection of these two samples show the same value of 1.41, indicating that the large FWHM value of the polar sample is probably a result of heavy tilting existing between the crystallites rather than high defect densities. This feature was also observed in polar ZnO epilayer on LAO prepared by chemical vapor deposition [18]. 4. Conclusion The orientation competition problem for molecular beam epitaxy of ZnO on the γ-LiAlO2 (100) substrate has been addressed in present work. We have shown that nonpolar (1010) ZnO epilayers can be grown at 700 1C at a relatively low growth rate. On the contrary, polar (0001) ZnO epilayers can be obtained at a temperature of 400 1C or lower. Moreover, polar ZnO epilayers can be grown at high growth temperatures by introducing a low temperature buffer layer for proper nucleation. The room temperature PL spectrum of both the polar and nonpolar ZnO epilayers showed a strong UV emission at around 377 nm and a negligible green band emission.
[1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Applied Physics Letters 70 (1997) 2230. [2] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nature Materials 4 (2005) 42. [3] F. Vigué, P. Vennegues, S. Vezian, M. Laugt, J.-P. Faurie, Applied Physics Letters 79 (2001) 194. [4] T. Hino, S. Tomiya, T. Miyajimam, S. Hashimoto, M. Ikeda, Applied Physics Letters 76 (2000) 3421. [5] J.Y. Shi, L.P. Yu, Y.Z. Wang, H. Zhang, Applied Physics Letters 80 (2002) 2293. [6] K. Tamura, A. Ohtomo, K. Saikusa, Y. Osaka, T. Makino, Y. Segawa, M. Sumiya, S. Fuke, H. Koinuma, M. Kawasaki, Journal of Crystal Growth 214/215 (2000) 59. [7] J. Dai, H. Liu, W. Fang, L. Wang, Y. Pu, Y. Chen, F. Jiang, Journal of Crystal Growth 283 (2005) 93. [8] H. Kato, M. Sano, K. Miyamoto, T. Yao, Journal of Crystal Growth 237–239 (2002) 538. [9] Y. Xie, M. Madel, T. Zoberbier, A. Reiser, W. Jie, B. Neuschl, J. Biskupek, U. Kaiser, M. Feneberg, K. Thonke, Applied Physics Letters 100 (2012) 182101. [10] X. Pan, P. Ding, H. He, J. Huang, B. Liu, H. Zhang, Z. Ye, Optics Communications 285 (2012) 4431. [11] M.M.C. Chou, L. Chang, H.Y. Chung, T.H. Huang, J.J. Wu, C.W. Chen, Journal of Crystal Growth 308 (2007) 412. [12] T.-H. Huang, W.-H. Lin, J.-J. Wu, M.M.C. Chou, T. Yan, L. Chang, ECS Transactions 45 (2012) 63. [13] F. Bernardini, V. Fiorentini, D. Vanderbilt, Physical Review B 56 (1997) R10024. [14] T.H. Huang, L. Chang, M.M.C. Chou, U. Jahn, Journal of the Electrochemical Society 158 (2011) H38. [15] M.M.C. Chou, L. Chang, D.R. Hang, C. Chen, D.S. Chang, C.A. Li, Crystal Growth and Design 9 (2009) 2073. [16] H. Lin, S. Zhou, T. Huang, H. Teng, X. Liu, S. Gu, S. Zhu, Z. Xie, P. Han, R. Zhang, Journal of Alloys and Compounds 467 (2009) L8. [17] J. Zou, S. Zhou, C. Xia, Y. Hang, J. Xu, S. Gu, R. Zhang, Journal of Crystal Growth 280 (2005) 185. [18] L. Chang, M.M.C. Chou, T.H. Hwang, C.W. Chen, Physica Status Solidi A 206 (2009) 215. [19] H.-J. Ko, T. Yao, Y. Chen, S.-K. Hong, Journal of Applied Physics 92 (2002) 4354. [20] B.A. Joyce, Reports on Progress in Physics 48 (1985) 1637. [21] Ü. Özgür, V. Avrutin, H. Morkoc, Molecular Beam Epitaxy: from research to mass production, in: M. Henini (Ed.), Elsevier, Oxford, 2013, p. 369, chapter 16. [22] S.S. Kim, B.T. Lee, Thin Solid Films 446 (2004) 307. [23] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, Journal of Crystal Growth 130 (1993) 269. [24] J.-H. Kim, S.K. Han, S.I. Hong, S.-K. Hong, J.W. Lee, J.Y. Lee, J.-H. Song, J.S. Park, and T. Yao, Journal of Vacuum Science and Technology B 27 (2009) 1625. [25] Y. Li, Y. Zhang, H. He, Z. Ye, J. Jiang, J. Luo, J. Huang, Materials Research Bulletin 47 (2012) 2235. [26] P. Ding, X.H. Pan, J.Y. Huang, H.P. He, B. Lu, H.H. Zhang, Z.Z. Ye, Journal of Crystal Growth 331 (2011) 15. [27] J.H. Kim, S.K. Han, S. Hong, S.K. Hong, J.W. Lee, J.Y. Lee, J.H. Song, J.S. Park, T. Yao, Journal of Vacuum Science and Technology B 27 (3) (2009) 1625.