Growth and Characterization of Nonpolar (1010) Zn1x

Jun 16, 2009 - ABSTRACT: γ-LiAlO2 (100) and sapphire (101j0) substrates have been employed to grow nonpolar (101j0) Zn1-xMgxO films using...
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Growth and Characterization of Nonpolar (101j0) Zn1-xMgxO (0 e x e 0.113) Epitaxial Films: A Comparison of γ-LiAlO2 (100) and Sapphire (101j0) Substrates Wan-Hsien Lin,† Jih-Jen Wu,*,† Mitch M. C. Chou,‡ and Liuwen Chang‡

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 3301–3306

Department of Chemical Engineering, National Cheng Kung UniVersity, Taiwan, and Department of Materials Science and Opto-electronic Engineering, National Sun Yat-Sen UniVersity, Taiwan ReceiVed January 21, 2009; ReVised Manuscript ReceiVed May 14, 2009

ABSTRACT: γ-LiAlO2 (100) and sapphire (101j0) substrates have been employed to grow nonpolar (101j0) Zn1-xMgxO films using metalorganic chemical vapor deposition. Zn1-xMgxO films with various Mg contents (0 e x e 0.113) are obtained by adjusting the partial pressure of the Mg metalorganic precursor in gas phase. Mg atoms incorporate within the films by means of substituting Zn. No segregated phase such as MgO or metal Mg is observed throughout the Zn1-xMgxO films. Structural characterization of the films indicates that γ-LiAlO2 (100) is a superior substrate to sapphire (101j0) for the growth of the nonpolar (101j0) Zn1-xMgxO films. The epitaxial (101j0) Zn1-xMgxO films are successfully grown on the γ-LiAlO2 substrates with the epitaxial relationship of [101j0]ZMO | [100]LAO and [12j10]ZMO | [001]LAO. On the other hand, the Zn1-xMgxO films with both (101j0) and (101j3) orientations are obtained on sapphire substrates although Zn1-xMgxO (101j0) becomes dominant with increasing Mg content. In addition, room-temperature cathodoluminescence spectra of the epitaxial (101j0) Zn1-xMgxO films show an obvious blue shift of the near-band-edge emission with increasing Mg content, demonstrating bandgap engineering in the epitaxial nonpolar (101j0) Zn1-xMgxO films on the γ-LiAlO2 substrates. Introduction Owing to its wide band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature, ZnO is believed to be a potential material for blue and UV optoelectronic applications.1-3 ZnO exhibits a wurtzite structure and grows preferentially along the [0001] direction. Therefore, c-plane ZnO films are conventionally obtained on most substrates. However, devices based on (0001)-oriented wurtzite GaN are known to present spontaneous and piezoelectric electrostatic fields. The existence of the internal electric field spatially separates electrons and holes in the active layers, resulting in a quantum-confined stark effect (QCSE) that not only causes a reduction of the device quantum efficiency but also leads to an undesirable red shift in the emission spectra. It has been demonstrated that the electrostatic field of the GaN is absent in the directions orthogonal to the [0001] polar axis.4 Research concerning the internal electric field in the ZnO/Zn1-xMgxO quantum wells has attracted much attention as well due to the promising photonic properties of ZnO crystal. Nevertheless, only few papers reported the growth of the nonpolar ZnO layers, such as (112j0) (a-plane)5-7 or (101j0) (m-plane),8-10 which is ascribed to the limitation of the available lattice-matched substrates. Lattice match between the films and substrates is one of the crucial criterions for epitaxial growth. Among all of the possible substrates, γ-LiAlO2 (100)10 and sapphire (101j0)11,12 (denoted as LAO and m-sapphire hereafter) have been employed to fabricate m-plane ZnO films. In the case of m-sapphire substrate, the lattice constant along the c-axis sapphire is 12.991 Å, which is nearly four times the lattice constant along the a-axis ZnO, 3.242 Å. The lattice mismatch is only -0.18%. Nevertheless, the other in-plane alignment between m-plane ZnO films and m-sapphire, [0001]ZnO | [112j0]sapphire, cZnO (5.194 Å) and asapphire (4.758 Å), possesses a larger lattice mismatch of 8.39%. * Contact address: [email protected]. † National Cheng Kung University. ‡ National Sun Yat-Sen University.

Moriyama et al. reported on ZnO films grown on m-sapphire using metalorganic vapor phase epitaxy.11 They demonstrated that both ZnO (101j0) and (011j3) orientations were observed on the m-sapphire at temperatures lower than 700 °C. The m-plane ZnO became dominant only when the growth temperature was increased to 800 °C.11 On the other hand, Chou et al. have demonstrated the growth of the nonpolar m-plane ZnO epitaxial film on the LAO single crystal substrate via thermal chemical vapor deposition.10 LAO is considered to be a promising substrate for m-plane ZnO growth since the LAO (100) plane exhibits a relatively small lattice mismatch to m-plane ZnO, namely, [0001]ZnO | [010]LAO with ∼0.52% and [112j0]ZnO | [001]LAO with ∼3.18%. Apart from the small lattice mismatch, other advantages, such as low melting point (1750 °C), easy processing due to its low hardness, and ability to be removed by chemical etching after ZnO growth,13 all make LAO an appropriate substrate for the fabrication of the m-plane ZnO films. Recently, incorporation of Mg into ZnO has been extensively investigated to widen the bandgap of the ZnO based materials by forming a Zn1-xMgxO alloy without changing its wurtzite structure.14-21 Among this research, very few papers demonstrated the growth of the nonpolar a-plane19-21 and m-plane22 Zn1-xMgxO. To the best of our knowledge, there is no paper reporting the formation of the nonpolar (101j 0) epitaxial Zn1-xMgxO films. Due to the aforementioned importance of the nonpolar ZnO/Zn1-xMgxO quantum wells, in this present work, the growth of the nonpolar m-plane Zn1-xMgxO films was examined using both m-sapphire and LAO substrates by a simple metalorganic chemical vapor deposition (MOCVD) method. Structural characterization of the films indicates that LAO is a superior substrate to m-sapphire for the growth of the m-plane Zn1-xMgxO films. The epitaxial m-plane Zn1-xMgxO films are successfully grown on the LAO substrates in the absence of the buffer layer. A blue shift of the near-band-edge emission with increasing Mg content is observed, demonstrating bandgap

10.1021/cg900071z CCC: $40.75  2009 American Chemical Society Published on Web 06/16/2009

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Figure 1. XPS investigation of the Zn1-xMgxO film on the LAO substrate. High-resolution scans take on the surface and Gaussian fitting results of (a) Zn 2p3/2 and (b) Mg 2p. Gaussian fitting results of the depth-profile scans of (c) Zn 2p3/2 and (d) Mg 2p. (e) Mg contents from film surface to the interface with substrate. The red line indicates the average x value to be 0.113. The concentration of Mg in the films was investigated by X-ray engineering in the epitaxial m-plane Zn1-xMgxO films on the photoelectron spectroscopy (XPS, JEOL JPS-9010 MX) using Mg KR LAO substrates. (hν ) 1253.6 eV) radiation. The Zn1-xMgxO films were etched layer by layer using Ar ion sputtering for depth-profile XPS analyses. The Experimental Section average etching rate was estimated to be about 50 nm/min, and the etching time between each layer was 0.5 min. The morphology of Zn1-xMgxO films were grown on the m-sapphire (MTI Co., USA) the Zn1-xMgxO films was examined using field-emission scanning and LAO single-crystal substrates by MOCVD in a three-temperatureelectron microscopy (FESEM, JEOL JSM-6700F). The crystal structures zone furnace. The LAO single-crystal substrates were obtained from of the Zn1-xMgxO films were investigated by X-ray diffraction (XRD, the 2 in. high-quality LAO (100) crystals grown using the Czochralski Rigaku D/MAX-2000 and Rigaku Rint-2000) and transmittance electron pulling technique.13 The substrates were cleaned in an ultrasonic bath microscopy (TEM, JEM-2100F). The rocking curves of ZnO and of acetone for 20 min followed by drying in N2 gas before being loaded Zn1-xMgxO were also carried out by Rigaku Rint-2000. Optical into the quartz tube. Zinc acetylacetonate hydrate [Zn(C5H7O2)2 · xH2O, properties of the Zn1-xMgxO films were characterized by cathodoluAlfa Aesar, 98%] and magnesium acetylacetonate [Mg(C5H7O2)2, minescence (CL, Gatan Mono-CL3, attached to the FESEM, JEOL Aldrich, 98%] used as the Zn and Mg sources were placed in two Pyrex JSM-7000F). glass containers and loaded into the two low-temperature zones of the furnace. Various Mg contents of the Zn1-xMgxO films were tailored by tuning the partial pressures of the Mg sources in gas phase via Results and Discussion adjusting the vaporizing temperatures of the Mg sources as well as the diameters of the Mg source containers. The vaporizing temperature of Figure 1 shows the XPS results of the Zn1-xMgxO films grown the Zn sources was controlled at 121 °C while the Mg sources were on the LAO substrates at a Mg source temperature of 215 °C. adjusted in the range of 180-215 °C. Pyrex glass containers of the Zn The high-resolution XPS scans of Zn 2p3/2 and Mg 2p taken on and Mg sources with diameters of 7.5-13.5 mm were used. The vapors the surface of the Zn1-xMgxO film are displayed in Figures 1(a) were carried by a 700 sccm O2 flow into the high-temperature zone of and 1(b), respectively. Both peaks show essentially symmetric the furnace, where the substrates were located at a total pressure of peak shapes and are able to be fitted by one Gaussian peak. 200 Torr. The growth temperature was maintained at 650 °C.

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Figure 2. SEM images of the Zn1-xMgxO with various x grown on the m-plane sapphire substrates. (a) x ) 0, (b) a sketch demonstrating the directional growth of c-plane ZnO on the m-plane sapphire substrate in (a), (c) x ) 0.012, (d) x ) 0.068, and (e) x ) 0.113.

Figure 3. Typical SEM morphologies of the Zn1-xMgxO films grown on the LAO substrates. (a), (b) top-view and (c) cross-sectional view images for 1 h deposition; (d), (e) top-view and (f) cross-sectional view images for 6 h deposition.

After calibration by the adventitious C 1s peak at 285.2 eV,23 the binding energies of Zn 2p3/2 and Mg 2p on the surface of the Zn1-xMgxO film are centered at 1021.9 and 49.5 eV, respectively. The depth-profile analyses of the Zn 2p3/2 and Mg 2p scans of the Zn1-xMgxO films were also performed to examine the uniformities of the composition throughout the film. As shown in Figures 1(c) and 1(d), the center positions of the calibrated Zn 2p3/2 and Mg 2p peaks at different depths are identical, at 1021.9 and 49.5 eV, respectively, indicating the superior chemical-state uniformity throughout the Zn1-xMgxO films. In addition, Mg contents estimated from the peak areas of Mg 2p to Zn 2p3/223 at layers from film surface to the interface with LAO substrate, as shown in Figure 1(e), are almost the same with an average of x ) 0.113. This result indicates that the Mg content is quite uniform throughout the Zn1-xMgxO film. In the present work, the x values of the Zn1-xMgxO films are determined by the average of the x values obtained from depthprofile XPS measurements. It reveals that the Mg contents (x) of the Zn1-xMgxO films are tunable from 0 to 0.113 by adjusting the partial pressures of the Mg sources in gas phase. SEM images of the Zn1-xMgxO grown on the m-sapphire and LAO substrates are shown in Figures 2 and 3, respectively. As shown in Figure 2(a), nanorods tilted at an angle about 30° to the normal of the m-sapphire substrate are obtained when the Mg source is absent. It has been reported that single crystalline ZnO nanowires could be grown vertically on the a-plane sapphire by taking advantage of the good epitaxial interface between the c-plane of ZnO and the a-plane of sapphire.24 Since the angle between the m-plane and a-plane of sapphire is 30°, as shown in Figure 2(b), it is suggested that the growth direction of the ZnO nanorods shown in Figure 2(a) is still along the c-axis. With an increase of the Mg content, Figures 2(c)-(e) show that the density of the nanorods decreases on the m-sapphire substrates. The rectangle-like blocks dominate the

Zn1-xMgxO film with x ) 0.113, however, the formation of a smooth Zn1-xMgxO film on the m-sapphire substrate is not achieved in the present work. Moreover, although the m-sapphire has been demonstrated to be a promising substrate for the formation of the m-plane ZnO at 800 °C,11 in the present work, Zn1-xMgxO film with obvious hexagons was obtained at this temperature as shown in Figure S1 in the Supporting Information. In contrast to the m-sapphire substrates, the epitaxial m-plane ZnO films have been grown on the LAO substrates at a temperature of 650 °C.10 In the case of Mg addition, the morphology of the Zn1-xMgxO films deposited on the LAO substrates is similar to that of the m-plane ZnO films. As shown in Figures 3(a) and 3(b), typical SEM micrographs of the Zn1-xMgxO films with low and high magnifications, Zn1-xMgxO blocks were arranged continuously side by side to each other. The periodical islands shown in Figure 3(a) are formed following the topography of the LAO substrates (Figure S2 in the Supporting Information). Figure 3 (c) shows a typical crosssection image of the Zn1-xMgxO films grown on the LAO substrates for 1 h. The film thickness is estimated to be about 180 nm. When prolonging the deposition time to 6 h, the Zn1-xMgxO blocks tend to merge together and form a continuous film as shown in Figures 3 (d) and 3(e). The thickness of the Zn1-xMgxO films grown on the LAO substrates for 6 h increases to ∼850 nm as indicated in Figure 3(f). Although the Zn1-xMgxO blocks grow along the topography of the LAO substrates at the beginning, continuous Zn1-xMgxO films are potentially able to be obtained when the film is thicker than 850 nm. Figure 4 shows the XRD patterns of the Zn1-xMgxO grown on the m-sapphire and LAO substrates with various Mg concentrations. The XRD patterns of those grown on the m-sapphire substrates shown in Figure 4(a) are normalized by the intensity of the m-sapphire (101j0) diffraction peak. In addition to the sapphire (101j0) peak, the ZnO (101j0) and (101j3)

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Lin et al. Table 1. Intensity Ratios of XRD Diffraction Peaks (101j0) and (101j3) of the Zn1-xMgxO Grown on the m-Sapphire Compared to That in the JCPDS Card for ZnO

Figure 4. (a) XRD patterns of the Zn1-xMgxO with various x grown on the m-sapphire substrates. Pattern (I) x ) 0, (II) x ) 0.012, (III) x ) 0.068, (IV) x ) 0.113. The inset is the XRD pattern of the m-sapphire substrates. (b) XRD patterns of the m-plane Zn1-xMgxO films grown on LAO substrates and the Mg-content dependence of the a-axis lattice constant (inset). (c) Rocking curves of the pure ZnO and Zn0.905Mg0.095O films on the LAO substrates.

peaks are observed in the pattern of the pure ZnO deposit, as illustrated in pattern (I). The other two extremely small peaks denoted by the arrows are assigned to come from the m-sapphire substrate by comparison to the pattern of the bare substrate shown in the inset. The intensity ratio of the ZnO (101j0) and (101j3) peaks in pattern (I) is much lower than that in the powder diffraction pattern (JCPDS card 75-0576) as listed in Table 1. It is consistent with the morphology of the ZnO nanorods shown in Figure 2(a) since the angle of 30° between ZnO (0001) and (101j3) matches well with that between the nanorod and the normal of the substrate, confirming the [0001] growth direction of the ZnO nanorods illustrated in Figure 2(b). On the other hand, the intensity ratio of the (101j0) and (101j3) peaks gradually increases with the increment of Mg contents as shown in patterns

Mg %

intensity ratio of (101j0)/(101j3)

JCPDS card (75-0576) for ZnO 0 0.012 0.068 0.113

2.06 0.48 4.42 8.63 9.33

(II)-(IV) of Figure 4(a) and listed in Table 1. This result is also consistent with the SEM images shown in Figure 2 that the rectangle-like blocks of m-plane Zn1-xMgxO become dominant with increasing x, indicating the encouragement of the formation of the m-plane Zn1-xMgxO film on m-sapphire substrates as Mg concentration is large. It has been reported that, in the wurtzite Zn1-xMgxO crystal, the lattice constant a gradually increases whereas the lattice constant c decreases with increasing Mg content as a result of the substitution of Mg atoms for Zn atoms.16,25 For the growth of the m-plane ZnO epitaxial film on the m-sapphire substrate, the lattice mismatch along [112j0]ZnO and [0001]sapphire is only -0.18% (tension) while that of [0001]ZnO and [112j 0]sapphire is as large as 8.39% (compression). When Mg atoms are substituted for Zn atoms, the increase of the lattice constant a will make the strain between the film and substrate from tension to compression which only changes the mismatch slightly. On the other hand, the decrease of the lattice constant c may reduce the mismatch along [0001]ZnO | [112j0]sapphire, which is relatively more significant than the influence along [112j0]ZnO | [0001]sapphire while growing the m-plane ZnO on m-sapphire substrates. Therefore, we speculate that the substitution of Mg atoms for Zn atoms in wurtzite ZnO structures may partially eliminate the lattice mismatch between the ZnO and m-sapphire substrates along [0001]ZnO | [112j0]sapphire, resulting in the (101j0) preferentially oriented Zn1-xMgxO film on the m-sapphire substrates. However, the growth of large-area epitaxial m-plane ZnO films on the m-sapphire substrates is not achievable in the present work, which is indicated by the presence of the (101j3) diffraction peak in the XRD pattern. Figure 4(b) presents the XRD patterns of the Zn1-xMgxO films grown on the LAO substrate with various Mg contents. All patterns shown in this figure are calibrated by the position of the LAO (100) peak. In contrast to those grown on m-sapphire substrates, only the diffraction peaks of Zn1-xMgxO (101j0) and LAO (100) appear in the XRD patterns of the Zn1-xMgxO films grown on the LAO substrates. It indicates that the Zn1-xMgxO films are grown preferentially oriented in the m-axis direction on the LAO substrates in a Mg content range of 0 to 0.113. In addition, there is no diffraction peak of MgO or metal Mg presenting in the XRD patterns, suggesting that no MgO or Mg clusters exist throughout the as-grown Zn1-xMgxO films. The Mg-content dependence of the a-axis lattice constants (d(101j0) value) is shown in the inset of Figure 4(b). It reveals that the a-axis lattice constant increases with the Mg concentrations, which is consistent with the previous reports.16,25 The absence of the diffraction peaks of MgO as well as Mg phases in the XRD patterns and the systematical dependence of the Mg content with the lattice constant both suggest the Mg incorporated within the m-plane ZnO films by means of substituting Zn. Furthermore, Figure 4(c) shows the rocking curves of the ZnO and Zn0.905Mg0.095O (101j0) films grown on the LAO substrates. The peaks are sharp and symmetrical with FWHM of 0.361° and 0.365° for ZnO and Zn0.905Mg0.095O, respectively. This result demonstrates the high crystalline qualities of the ZnO films grown on the LAO substrates even after Mg incorporation.

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Figure 6. Room-temperature CL spectra of the Zn1-xMgxO films with various x grown on the LAO substrates (solid line, from right to left, x ) 0, 0.012, 0.040, 0.068, 0.095, 0.104, 0.113; dashed line, LAO substrate) and the dependence of the CL emission energy as a function of the Mg content (inset). Figure 5. (a) Cross-sectional bright-field TEM image and (b) HRTEM image of the Zn1-xMgxO film grown on the LAO substrate. SAED patterns taken from (c) the Zn1-xMgxO films far from the interface and (d) the interfacial region of the Zn1-xMgxO film on the LAO substrate.

The Zn1-xMgxO films grown on the LAO substrates show superior m-plane characteristics compared to those on msapphire substrates. Consequently, it is worth addressing that LAO is a promising substrate for the growth of the nonpolar m-plane ZnO and Zn1-xMgxO films. Further structure characterization of the nonpolar Zn1-xMgxO (ZMO) films grown on the LAO substrate was performed using TEM. Figure 5(a) shows a cross-sectional bright-field image of the nonpolar Zn0.932Mg0.068O film on the LAO (100) substrate. The image was taken near the [0001]ZMO zone axis with g ) 12j10. A clear-cut interface between the nonpolar Zn0.932Mg0.068O film and LAO substrate is observed due to the high contrast. The thickness of the Zn0.932Mg0.068O film measured here is about 185 nm, which is close to that shown in Figure 3(c). The highresolution (HR) TEM micrograph of the interfacial region of the Zn1-xMgxO film and LAO substrate is shown in Figure 5(b). The incident beam is parallel to [0001]ZMO and [010]LAO. The HRTEM image demonstrates an apparent interface between the Zn1-xMgxO film and LAO substrate. The lattice spacing of the planes perpendicular to the substrate is estimated to be 0.28 nm. It matches well with the literature value for (101j0) planes in wurtzite ZnO structure, confirming that the Zn1-xMgxO film grows along the [101j0] direction. Figure 5(c) shows the corresponding selected-area electron diffraction (SAED) pattern taken from the Zn1-xMgxO film portion. Only sharp and clear diffraction spots that belong to ZnO {101j0} are observed. No other ambiguous diffraction spot appearing provides that no Mg or MgO clusters forms in the Zn1-xMgxO films. This consequence is consistent with aforementioned XRD results. SAED was also taken from the interface region between the Zn1-xMgxO film and LAO substrates, as shown in Figure 5(d). Extra diffraction spots in addition to those contributed from the γ-LAO phase and the wurtzite ZMO phase are observed in Figure 5(d). These spots very possibly result from R-LAO phase transformed from the γ-phase on irradiation of high energy electron beam.26 The distorted lattice image of LAO shown in Figure 5(b) also gives evidence of the irradiation-induced transformation. According to Figure 5(d), the epitaxial relationship between Zn1-xMgxO and LAO is determined as [101j0]ZMO | [100]LAO and [12j10]ZMO | [001]LAO. Both the HRTEM image and

corresponding SAED reveal the successful formation of the epitaxial Zn1-xMgxO (101j0) films on LAO substrates. Figure 6 exhibits the room-temperature CL spectra of the m-plane Zn1-xMgxO films grown on LAO substrates. The CL spectrum of the LAO substrate is also shown in this figure. The CL spectra have been normalized for comparison. A strong UV emission ascribed to the near-band-edge emission of the wide bandgap Zn1-xMgxO is observed in the CL spectra. In addition to the strong UV emission, another UV emission (around 300-350 nm) and a visible emission (475-525 nm) are also present in the spectra of the Zn1-xMgxO film. The former is attributed to the CL emission of the LAO substrate, and the latter might be caused by the oxygen vacancies in the Zn1-xMgxO films. Strong UV emission peaks at 382, 377, 367, 362, 359, 357, and 352 nm are shown in the CL spectra of the Zn1-xMgxO films with x ) 0, 0.012, 0.040, 0.068, 0.095, 0.104 and 0.113, respectively. An obvious blue shift of the near-bandedge emission without significant change of FWHM is observed with increasing Mg content in the films. This result indicates that the near-band-edge emission energies of the Zn1-xMgxO films measured at room temperature increase monotonically with the Mg contents as shown in the inset of Figure 6. Conclusion MOCVD growth of the nonpolar (101j0) Zn1-xMgxO films has been examined on the LAO (100) and m-sapphire substrates in the present work. XPS analyses reveal that the Zn1-xMgxO films possess a good uniformity of composition. The Mg contents of the Zn1-xMgxO films are tunable from 0 to 0.113 by adjusting the partial pressures of the Mg sources in the gas phase. The Zn1-xMgxO (101j0) and (101j3) diffraction peaks are observed in the XRD patterns of the films grown on m-sapphire substrates although the intensity ratio of the (101j0) and (101j3) peaks gradually increases with the increment of Mg contents. On the other hand, only the diffraction peaks of Zn1-xMgxO (101j0) and LAO (100) appear in the XRD patterns of the Zn1-xMgxO films grown on the LAO substrates. No Mg metal or MgO exists throughout the Zn1-xMgxO films. TEM analyses demonstrate the successful formation of the epitaxial Zn1-xMgxO (101j0) films on LAO substrates. The HRTEM image reveals an apparent interface between the Zn1-xMgxO film and LAO substrate. The epitaxial relationship between Zn1-xMgxO and LAO is determined as [101j0]ZMO | [100]LAO and [12j10]ZMO | [001]LAO. Both XRD and TEM results indicate that γ-LiAlO2

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(100) is a superior substrate to sapphire (101j0) for the growth of the nonpolar (101j0) epitaxial Zn1-xMgxO films. In addition, room-temperature CL spectra of the epitaxial (101j0) Zn1-xMgxO show that the near-band-edge emission energies of the Zn1-xMgxO films increase monotonically with the Mg contents, demonstrating bandgap engineering in the epitaxial nonpolar (101j0) Zn1-xMgxO films on the γ-LiAlO2 substrates. Acknowledgment. The authors thank the National Science Council in Taiwan for the financial support of this work under Contract No. NSC-97-2221-E006-121-MY2. Supporting Information Available: SEM image and XRD pattern of the Zn0.887Mg0.113O grown on the m-sapphire substrate at 800 °C as well as SEM image of the LAO substrate. This material is available free of charge via the Internet at http://pubs.acs.org.

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