Crystal Growth of Nonpolar m-Plane ZnO on a Lattice-Matched (100) γ-LiAlO2 Substrate Mitch M.C. Chou,* Liuwen Chang, Da-Ren Hang, Chenlong Chen, Da-Sin Chang, and Chu-An Li
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2073–2078
Department of Materials and Optoelectronic Science, National Sun Yat-Sen UniVersity, Kaohsiung, 80424, Taiwan, R.O.C ReceiVed April 1, 2008; ReVised Manuscript ReceiVed December 25, 2008
ABSTRACT: In this study, a nearly lattice-matched (100) γ-LiAlO2 single crystal was demonstrated as a potential substrate for the epitaxial growth of nonpolar ZnO materials. The adopted growth approach is a simple chemical vapor deposition (CVD) method. The orientation was identified as the [10-10] orientation (m-plane) by the X-ray diffraction pattern. Characterization of further structures and the defects of nonpolar ZnO films were performed by field emission transmission electron microscopy (FETEM). Room-temperature photoluminescence spectra of nonpolar ZnO showed a strong UV emission peak at around 377 nm and a negligible green band emission. The dependence of growth characteristics on the growth temperatures, deposition time, and chamber pressure were investigated.
1. Introduction The wide-band-gap semiconductor ZnO with a wurtzite structure is popular for application in ultraviolet (UV) lightemitting devices (LED).1-3 It is also a potential candidate for use in piezoelectric devices.4 Some optoelectronic applications of ZnO might overlap with GaN. However, ZnO has some advantages over GaN, such as the availability of high-quality ZnO single crystals, which results in a potentially lower cost for ZnO-based devices.5 Hexagonal ZnO grows preferentially along the [0001] growth direction. However, wurtzite-type materials, like GaN and ZnO, generate a spontaneous polarization effect along the [0001] direction, leading to internal electrical fields in the barriers of multiquantum well (MQW) structures.6 Besides the effect of the net dipole moment, piezoelectric fields also appear. The internal electrical field in MQW structures separates electrons and holes in real space, which leads to an undesirable red shift in the emission spectra of MQW structures designed for UV emission and lower quantum efficiencies for radiative transitions. This so-called quantum confined Stark effect (QCSE) is well established for GaN-based heterostructures.6-8 However, very limited reports are related to the ZnO-based structures free of electrostatic fields. Recently, this QCSE was found in ZnO/ZnMgO MQW structures.9 Morhain et al. performed time-resolved photoluminescence (PL) spectroscopy to demonstrate the presence of a large electrical field on (0001) ZnO quantum wells structure.10 Kalusniak et al. studied the polarization field of a (0001) ZnCdO single quantum well (SQW) structure grown by MBE.11 They also found that the polarization field is as big as that known for III-Nitride materials. It has been proposed to grow ZnO films along nonpolar directions, such as (11-20) (a-plane),12,13 (10-10) (m-plane),14,15 and (-102) (r-plane).16-18 Zuniga-Perez et al. used metal organic vapor phase epitaxy (MOVPE) to grow a-plane and m-plane ZnO on Al2O3 substrates and studied their polarity effects.12,15 Moriyama et al. also used MOVPE to obtain m-plane and a-plane nonpolar ZnO epitaxial film on m-plane, and r-plane Al2O3 substrates.16 They found that higher growth * To whom correspondence should be addressed. E-mail: mitch@ faculty.nsysu.edu.tw.
temperatures (500-800 °C) and higher mole fractions of oxygen to zinc precursors led to the growth of nonpolar ZnO films. When both the growth temperature and the VI/II mole fraction were decreased, (01-13) ZnO started to form. Matsui et al. reported ZnO homoepitaxy on (10-10) ZnO single-crystal substrates by molecular beam epitaxy (MBE).17,18 They found that the small islands that elongated along the {0001} direction were generated on two-dimensional (2D) growth surfaces above a critical thickness and then developed into [10-10] nanostripe arrays. Previous work indicates that the optical and electrical properties of the ZnO materials often depend on their crystallographic orientation. Being able to control the orientation of ZnO represents a significant step toward tuning its properties. The proper way to control the growth direction of ZnO films is to select a proper substrate. Lattice match is the primary criteria for determining the suitability of a substrate for the growth of nonpolar ZnO materials. Properties other than the lattice match including the structure, surface roughness, chemical, and thermal properties of the substrate are also important. The lattice mismatch between ZnO and Al2O3 is as large as 18.3%. Without the proper nucleation layers or surface treatments, the considerable misfit strains and dislocations cannot be removed. ZnO single crystals grown by the hydrothermal method are expensive, especially for the nonpolar ZnO orientation. γ-LiAlO2 has a tetragonal structure where the a-c (100) plane has the same atomic arrangement as the prismatic face (10-10) plane of the wurtzite structure. From the view point of lattice mismatch between ZnO and LiAlO2, [001]LiAlO2//[11-20] ZnO, cLiAlO2(6.278 Å) = 2aZnO(3.252 Å) with 3.47% mismatch. [010]LiAlO2//[0001]ZnO, aLiAlO2(5.167 Å) = cZnO(5.313 Å) with 2.71% mismatch. It can be clearly seen why the ZnO hexagonal cell is in the (10-10) or m-plane orientation, which places the polar c axis on the LiAlO2 wafer plane, Figure 1. This orientation will remove the influence of the electrostatic fields on ZnO and eliminate the problems of low electron-hole recombination probabilities due to QCSE. Besides this, it is a major advantage that LiAlO2 can be grown by the Czochralski pulling method. However, LiAlO2 has several drawbacks. It is difficult to grow high-quality LiAlO2 single crystals because Li atoms continue to evaporate during the growth and cooling process. This leads to the nonstoichiometric ratio of the
10.1021/cg800328g CCC: $40.75 2009 American Chemical Society Published on Web 03/09/2009
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Figure 1. Interface between LiAlO2 and ZnO, showing a good lattice matching.
Figure 2. Setup of chemical vapor deposition for the growth of nonpolar ZnO on LiAlO2 substrate.
materials. Since LiAlO2 is sensitive to most polishing solutions, it is difficult to obtain an atomic-flat surface. To date, only Zou et al.19 reported the growth of ZnO on LiAlO2 substrate by using pulse laser deposition, but the orientation of ZnO was identified as the normal (0001) orientation. In this report, a chemical vapor deposition (CVD) approach was presented to grow nonpolar ZnO with (10-10) orientation (m-plane) on a lattice-matched γ-LiAlO2 single-crystal substrate. The results are comparable to both MBE and MOCVD.
2. Experimental Section A single-crystal LiAlO2 was successfully grown by the Czochralski pulling technique. Epi-ready (100) γ-LiAlO2 substrates with rms roughness of 0.24-0.32 nm were used for all of experiments. More details about the crystal growth and polishing methods can be found in other articles.20 Figure 2 schematically shows the CVD setup for the growth of nonpolar ZnO epitaxial film. LiAlO2 substrate was located at the center of quartz tube in a CVD furnace. Zinc acetylacetonate hydrate (Zn(C5H7O2)2 · xH2O, Lancaster) was used as the zinc source, which was vaporized at a low temperature of 130-140 °C. The vapor was carried by a mixture of N2/O2 gas flow into the high-temperature zone, where the substrate was located. At first, the pressure of the chamber was pumped to 8 × 10-3 Torr and then kept at 50-200 Torr. The flow rates of both NH3 and N2 were 500 sccm. Without any buffer layer, ZnO epitaxial films were directly grown on (100) LiAlO2 singlecrystal substrate at a temperature of 550-650 °C. The overall reaction was
Zn(C5H7O2)2 + 12O2 f ZnO + 10CO2 + 7H2O (1) Following the CVD growth, the surface morphologies and roughness of ZnO epitaxial film were investigated by scanning electron microscopy (SEM, JEOL JSM-6330TF) and atomic force microscopy (AFM, Digital Instrument: NanoMan NS4+D3100). The orientation and structure were identified by the X-ray diffraction pattern (XRD, Simens D5000). Further structural characterization and defect analysis of the nonpolar ZnO epilayer was performed using a field emission gun transmission
Figure 3. (a) Surface SEM image of nonpolar ZnO on (100) γ-LiAlO2 substrate at a temperature of 650 °C for 180 min. (b) XRD diffraction patterns showing ZnO is in the (10-10) orientation, and the corresponding Rocking curve with fwhm 580 acrsec. electron microscope [(FE-TEM), FEI E.O Tecnai F20 G2] operated at 120 kV. The PL spectra were obtained by exciting the samples using a continuous wave (CW) He-Cd laser (325 nm), and the emission spectra were analyzed with a Jobin-Yvon TRIAX 550 monochromator with a 0.025 nm resolution. The samples were placed in a variabletemperature closed-cycle helium cryostat. The dependences of growth characteristics on the growth temperature, pressure of the chamber (75-200 Torr), and deposition time (30-180 min) were also investigated.
3. Results and Discussion 3.1. SEM and Structural Analysis of Nonpolar ZnO Epitaxial Films. Figure 3a is the top view SEM image of ZnO film grown on (100) LiAlO2 substrate at a temperature of 650 °C for 180 min by CVD. A crack-free surface with a few hexagonal pits was observed. The average thickness of the film was around 200 nm. Figure 3b shows XRD patterns of ZnO films grown on (100) γ-LiAlO2 substrates. The two peaks are indexed as {10-10} and {20-20} of the wurtzite structure of ZnO, indicating that the ZnO films are oriented in the nonpolar m-plane direction. The inset of Figure 3b is the {10*10} ZnO rocking curve with full width at half-maximum (fwhm) of 580 acrsec. Compared to the results of the nonpolar a-plane (11-20) ZnO film grown on r-plane sapphire by MBE,21,22 the fwhm of the X-ray rocking curve was 1100-1296 arcsec. It was proven that the nonpolar ZnO grown on (100) LiAlO2 crystal had a better quality. The detailed microstructures and orientations of the nonpolar ZnO film were performed using FE-TEM. Cross-sectioned TEM
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Figure 5. Room-temperature photoluminescence spectrum of nonpolar ZnO film on LiAlO2 substrate. A strong UV emission at 377 nm without a clear green band is observed. The inset shows the near-band-edge PL spectrum taken at T ) 15 K.
Figure 4. (a) Cross-sectional bright-field TEM image, and the corresponding diffraction patterns. (b) High-resolution image of the nonpolar ZnO grown on LiAlO2 substrate at a growing temperature of 650 °C.
samples were prepared using the focus ion beam (SMI 3050) lift-out method.23 A Pt layer was predeposited on the sample to prevent charging. LiAlO2 is very sensitive to the irradiation of high-energy electrons. Severe damage, such as phase transformation, amorphization, and evaporation, can be observed after the sample is exposed to electron beam irradiation for 10 s. A low acceleration voltage of 120 kV was used accordingly. Figure 4a shows a cross-sectional bright-field image of the nonpolar ZnO grown on (100) LiAlO2 substrate at a growing temperature of 650 °C. The ZnO film/LiAlO2 substrate interface is clear and smooth. No clear stain contrast resulting from threading dislocations were found. A unique cross-hatched pattern of LiAlO2 was found. These patterns reveal a superlattice structure. The atomic diameters and bonding lengths of Li and Al atoms are very different, so their atomic sites can be exchanged in this organized way to relieve the stress. These site exchanges result in the phase shift which causes the interference pattern.24 It was also observed that a semicircle LiAlO2 area starts to get vaporized. The insets of Figure 4a are the corresponding selected-area electron diffraction (SAED) patterns taken from ZnO and LiAlO2. The zone axes are [-2110]ZnO and [001]LiAlO2. They clearly showed [10-10]ZnO// [001]LiAlO2 and [11-20]ZnO//[001]LiAlO2 and also provided direct evidence to the structural relationship of ZnO and LiAlO2 material, Figure 1. The calculated interplane spacing was c )
5.2 Å and d10-10 ) 2.8 Å. Figure 4b is a high-resolution TEM image of the ZnO. The phase contrast at the interface indicated the stress due to the small lattice mismatching between ZnO and LiAlO2. The {0002} ZnO planes parallel to the growth direction are clearly observable. 3.2. Photoluminescence Analysis of Nonpolar ZnO Epitaxial Films. Figure 5 is the RT-PL spectra of the (10-10) m-plane ZnO grown on LiAlO2 substrate at a growth temperature of 650 °C. A strong UV emission at around 377 nm was observed. This UV emission is related to the direct recombination of photon-generated charge carriers. The intensity of the wide green band (475-575nm) related to the defects is very small and can be ignored. This implied that the ZnO film was good quality with few intrinsic defects and oxygen vacancy.25 To reveal more optical characteristics, the near-band-edge PL spectrum taken at T ) 15 K is shown in the inset of Figure 5. The main emission band at 15 K peaks at 3.360 eV and has a fwhm of 20 meV. A clear shoulder at 3.32 eV can be seen on the low-energy side of the main emission band. The dominant emission at 3.360 eV is attributed to the neutral shallow donor bound exciton (DBE) emissions because of the presence of donors due to unintentional impurities.26 As a characteristic of the neutral DBE transition, the emission band at 3.32 eV may be due to the excited state associated with the intense neutral DBE at 3.360 eV. This is consistent with the previous report by Teke et al.26 Thus, the emission mechanisms at low temperatures are closely related to the bound exciton effects in our ZnO film. 3.3. Characterization of ZnO Epitaxial Films at Various Growth Temperatures and Deposition Times. The surface morphology of ZnO films depends strongly on the growth temperature. Figure 6a is the SEM image of ZnO films grown on (100) LiAlO2 substrate at the temperature of 550 °C. Most of the areas are occupied by rectangular-like spots, and only a small portion have hexagons. The average sizes of the rectangular spots is 250 nm × 100 nm, and the hexagons have an edge size of 400 nm. Their orientations were identified respectively as (10-10) m-direction and (0001) c-direction ZnO by electron backscattering diffraction (EBSD). At a high growth temperature of 600 °C, the ZnO starts to grow sideways and form the rectangular-like blocks, Figure 6b. At a growth temperature of 650 °C, the lateral growth speeds up to form
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Figure 6. SEM images of ZnO grown on LiAlO2 substrate at different growth temperatures: (a) 550 and (b) 600 °C.
Figure 7. SEM images of ZnO grown on LiAlO2 substrate at a growth temperature of 650 °C for the different deposition times of (a) 30, (b) 60, and (c) 120 min. (d) X-ray rocking curve of nonpolar ZnO grown at 650 °C for deposition times of 30, 60, 120, and 180 min.
thin ZnO films, Figure 3a. At the lower growth temperature, such as 500 °C, it is difficult to deposit ZnO on the LiAlO2 substrate. At 700 °C, the ZnO surface becomes very rough. This suggested that pure m-plane ZnO films could be grown at a temperature of 650 °C. At a growth temperature of 650 °C, the growth characteristics of ZnO vs the deposition time were studied. Figure 7a is the SEM image of ZnO film grown for 30 min. Two clear areas corresponding to the polarity of LiAlO2 substrate along (100) and (-100) were observed. Fortunately, it was found that the
polarity of LiAlO2 does not affect the growth direction of ZnO materials. The details about the polarity of LiAlO2 will be discussed in a separate report. If the deposition time is increased to 60 min, Figure 7b, the boundaries like in Figure 7a become vague. However, there are still some holes on the surface. Figure 7c is the SEM image for a deposition time of 120 min. It showed a nanostripe array structure with a lateral periodicity of 0.2 µm running along the (010) LiAlO2 direction. The smooth nonpolar ZnO surface can be achieved by increasing the deposition time to 180 min, Figure 3a. To check the quality of ZnO films, X-ray
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Figure 8. SEM images and XRD of ZnO grown on LiAlO2 substrate at different chamber pressures of (a) 50, (b) 75, and (c) 200 Torr. (d) X-ray diffraction pattern of ZnO grown at different chamber pressures of 50-200 Torr.
rocking curves were performed on all of the as-grown samples. Figure 7d is the comparison of rocking curves of ZnO (10-10) planes grown at 650 °C for 30, 60, 120, and 180 min. Apparently, the longer deposition time, like 180 min, would lead to a better quality of nonpolar ZnO film. 3.4. Characterization of ZnO Epitaxial Films with Different Chamber Pressures. The surface morphology of nonpolar ZnO films was found to depend strongly on the chamber’s pressure. Figure 8a is the top-view SEM image of ZnO film grown on (100) LiAlO2 substrate at a chamber pressure of 50 Torr. Many ZnO hexagonal pits were found. The density of ZnO hexagons can be greatly reduced by increasing the pressure to 75 Torr, Figure 8b. Figure 8c showed that the small hexagonal islands along the (0001) direction will disappear on the layer’s surface by increasing the chamber’s pressure to 200 Torr, and then the nanostripe arrays will be developed. The lateral periodicity was around 0.2 µm. Figure 8d is their corresponding XRD. The small peak next to (200) LiAlO2 was identified as (0002) ZnO at 50 Torr. The (0002) ZnO peak becomes smaller and finally disappears with increasing chamber pressure. Conclusions This is the first report which demonstrated that nonpolar ZnO epitaxial film with [10-10] orientation (m-plane) was successfully grown on (100) γ-LiAlO2 single-crystal substrate via
thermal chemical vapor deposition. FE-TEM images showed direct evidence that a flat interface provides the lattice-matched relationship of nonpolar ZnO and LiAlO2 substrate. No clear stain contrast resulting from threading dislocations were found. A unique cross-hatched pattern of LiAlO2 due to the site exchanges of the Li and Al atoms was also found. The RT-PL spectrum of the (10-10) m-plane ZnO shows a strong UV emission peaking at around 377 nm, while the wide green band (475-575 nm) can be ignored. In addition, the PL spectrum taken at low temperature reveals the importance of bound excitons to the optical emission mechanism. The dependence of growth characteristics at various growth temperatures (500-700 °C) and deposition times (30-180 min) was investigated under the proper gas flux. It was found that the nonpolar ZnO films with a uniform surface can be achieved at a growth temperature of 650 °C and deposition time of 180 min. The impacts of various chamber pressures (50-200 Torr) to the growth behavior were also investigated. The c-plane ZnO will nucleate easier at lower chamber pressures. Once the pressure is increased to 200 Torr, the pure m-plane ZnO can be obtained. We believe that LiAlO2 crystals have potential to be the substrate of nonpolar ZnO materials. Detailed investigations such as the strain state are certainly worth pursuing. Acknowledgment. This work was supported by the National Science Council of Taiwan, ACORC, Center for
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