Growth of Bulky Single Crystalline Films of (Zn,Mg)O Alloy

Dec 30, 2008 - Jun Kobayashi, Hideyuki Sekiwa, Miyuki Miyamoto, Isao Sakaguchi, Yoshiki Wada, Takashi Sekiguchi, Yutaka Adachi, Hajime Haneda and ...
0 downloads 0 Views 1MB Size
Download PDF
Growth of Bulky Single Crystalline Films of (Zn,Mg)O Alloy Semiconductors by Liquid Phase Epitaxy Jun Kobayashi,†,‡ Hideyuki Sekiwa,†,‡ Miyuki Miyamoto,† Isao Sakaguchi,‡ Yoshiki Wada,‡ Takashi Sekiguchi,§ Yutaka Adachi,‡,⊥ Hajime Haneda,¶ and Naoki Ohashi*,‡,⊥

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 1219–1224

Tokyo Research Laboratory, Mitsubishi Gas Chemical Co. Ltd., 1-1, Niijyuku, 6-chome Katsushika-ku, Tokyo 125-0051, Japan, Optronic Materials Center, National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan, AdVanced Electronic Materials Center, National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan, Sensor Materials Center, National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan, and International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan ReceiVed October 28, 2008; ReVised Manuscript ReceiVed NoVember 20, 2008

ABSTRACT: Very thick (about 0.5 mm) single crystalline films of (Zn1-xMgx)O solid solution have been grown by a liquid phase epitaxy (LPE) technique. The source materials, ZnO and MgO, were dissolved in a molten PbO-Bi2O3 flux and deposited on ZnO substrates as epitaxial (Zn1-xMgx)O layers. The (Zn1-xMgx)O layers thus obtained showed high crystallinity, similar to that of the ZnO substrate, and exhibited n-type conductivity with relatively high Hall mobilities (>90 cm2 V-1 s-1 for x ) 0.1 at room temperature). Moreover, the activation energy of the mobile electrons was about 50 meV, and this value was independent of the MgO fraction. Since LPE is an appropriate technique for growing large area films, we examined the growth of (Zn,Mg)O thick films on 2 in. diameter ZnO substrates. The Mg concentration in the LPE grown layer was quite uniform, and the Li concentration was two or three orders lower than an ordinary ZnO substrate. Introduction Zinc oxide (ZnO) and related materials have been extensively studied because of their numerous potential applications.1 ZnO exhibits transparency to visible light and also shows n-type conductivity controlled by doping;2,3 therefore, many investigations have been carried out aimed at transparent electronics.4,5 In addition, its luminescence properties have also been investigated in many aspects, such as stimulated emission behavior,6,7 defect related luminescence,8,9 and luminescence from a multiquantum well.10 Most research efforts on ZnO-related science and technology have been focused on developing ZnO-based light emitting diodes (LEDs).11-13 In general, advances in multi-quantum-well structures (MQWs) and double-heterostructures are essential key technologies for the fabrication of LEDs with high efficiency and high performance.14 Since the wurtzite-type zinc magnesium oxide alloy, w-(Zn1-xMgx)O, possesses a wider energy band gap than ZnO,10 technologies for growth of high quality w-(Zn1-xMgx)O and control of its conductivity, as well as hole and electron doping levels, have to be developed to utilize ZnO for optoelectronic devices.15,16 Recently, it has been indicated that w-(Zn,Mg)O could potentially achieve a better luminescence efficiency that pure ZnO.17 In fact, the oscillator strength for excitonic transition in w-(Zn,Mg)O has been reported to be much higher than that of pure ZnO.18 Such new insights have attracted and motivated us to investigate w-(Zn,Mg)O. Epitaxial growth technology is very important for the fabrication of optoelectronic devices and structures, including * To whom correspondence should be addressed. E-mail: ohashi.naoki@ nims.go.jp. † Mitsubishi Gas Chemical Co. Ltd. ‡ Optronic Materials Center, National Institute for Materials Science. § Advanced Electronic Materials Center, National Institute for Materials Science. ⊥ International Center for Materials Nanoarchitectonics, National Institute for Materials Science. ¶ Sensor Materials Center, National Institute for Materials Science.

double hetero structures and MQWs. Lattice mismatch between the substrate and epitaxial layers causes degradation of the crystallinity of the epitaxial layers. Considering the development of nitride semiconductors, significant efforts have been expended to obtain self-standing gallium nitride (GaN) substrates for homo-epitaxial growth of GaN.19 From this viewpoint, latticematched substrates appropriate for epitaxial growth of w(Zn,Mg)O are desirable for the development of ZnO-based devices.20,21 The use of self-standing w-(Zn,Mg)O wafers with high crystallinity is likely the right strategy for epitaxial growth of high quality n- and p-type w-(Zn,Mg)O layers. Thus, we were motivated to develop free-standing w-(Zn,Mg)O substrates for epitaxial wafer applications to contribute to the development of ZnO-based heterostructure devices. We employed a liquid phase epitaxy (LPE) method for growing the bulky w-(Zn,Mg)O layers.22 The LPE method can be regarded as a flux growth method using a substrate. Since ZnO decomposes to Zn and O2 vapor at high temperatures, the growth of w-(Zn,Mg)O by solidification of molten Zn-Mg-O is not easy. This is why most prior studies on growth of bulk ZnO were carried out using either a hydrothermal method, flux methods, or vapor transport methods.23 In general, the LPE technique has many advantages, including fast growth rates, capability for large wafer dimensions, and control of chemical composition of the growing layers. This technique has been used for growing epitaxial layers of many functional materials, for example, growth of garnet-type magneto-optical layers24 and compound semiconductors.25 The LPE technique, using an adequate flux having low eutectic temperature and sufficient solubility for ZnO and MgO, enables us to grow ZnO and w-(Zn,Mg)O layers at relatively low temperatures. A low eutectic temperature is desirable to reduce Zn evaporation. Thus, we selected the LPE technique as the route to obtain thick and large area w-(Zn,Mg)O layers. With regards to the flux material for growing ZnO, there are several prior studies that have explored the most appropriate

10.1021/cg801211m CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

1220 Crystal Growth & Design, Vol. 9, No. 2, 2009

Kobayashi et al.

Figure 2. Photograph of a w-(Zn,Mg)O/ZnO sample obtained in this study. The brownish material at the edge of the wafer is solidified flux attached to the side face.

Figure 1. Schematic illustration of the experimental setup used for LPE.

flux for ZnO growth. Lead fluoride (PbF2) or a mixture of lead oxide (PbO) and PbF2 have been used for the flux growth of ZnO.26 Although the PbF2-PbO flux is an appropriate one to grow ZnO, the use of PbF2 is a potential source of fluorine contamination in the resultant crystals. Since fluorine in ZnO acts as an donor impurity,27 fluorine contamination is not favorable. Thus, in the present study, we did not use a PbOPbF2 flux. On the other hand, bismuth oxide (Bi2O3) is known to be an oxide with a relatively low melting point (820 °C). Since Bi2O3 is an indispensable additive in ZnO varistor ceramics28 and enhances grain growth of ZnO during the sintering process for producing varistor ceramics,29 we examined Bi2O3 as a potential flux material for LPE growth of ZnO in the present study. In our previous letter,22 we briefly reported on LPE growth of bulky w-(Zn,Mg)O and their properties. In this paper, we provide full and updated information on LPE growth of w-(Zn,Mg)O crystals, as well as the electrical and optical properties of the w-(Zn,Mg)O crystals obtained, including the MgO fraction-dependent properties of the alloy semiconductor. Some of the properties, such as electron mobility, reported in this paper have been improved, compared to the results in our previous letter, by optimizing the crystal growth conditions. Experimental Section The thick (Zn,Mg)O layers were prepared using LPE. Figure 1 schematically shows the experimental setup used. We employed the PbO-Bi2O3 eutectic mixture as the solvent. First, ZnO, MgO, PbO, and Bi2O3 were mixed in powder form, which was then placed in a platinum crucible located in the furnace. The mixture was heated to 800-1100 °C to form a melt and then cooled down to a specific temperature for LPE growth. Precursor powders with various MgO/ZnO ratios were prepared to grow w-(Zn1-xMgx)O layers with various x values. Single crystal wafers of ZnO30 were used as substrates for the LPE growth. The surface of the wafer was chemo-mechanically polished, and the wafers were subsequently annealed at 1100 °C. The substrate, attached to the sample rod, was contacted to the melt to initiate LPE growth. It is to be noted that we examined LPE growth of the w-(Zn1-xMgx)O layers only on the (0001) face of ZnO. Results from prior work on hydrothermal growth of ZnO31 suggested that ZnO single crystals with lower defect concentrations can be obtained when the crystals were grown not on the (0001) face but rather on the (0001) face. On the basis of this, we examined LPE growth only on the (0001) face of the ZnO substrate in the present study. The growth temperature was varied from 750 to 850 °C to optimize the growth rate and crystallinity of the obtained crystals. Note that all growth processes were carried out in air. The LPE growth sequence was completed by separating the substrate from the melt after growth for 20 to 80 h. The wafers thus

obtained with the (Zn,Mg)O layer on top were polished to remove the solidified flux remaining on the surface, yielding a clean and flat surface appropriate for characterization. The thickness of the (Zn,Mg)O layer after polishing was within 0.5-300 µm. The chemical composition of the LPE-grown layer was analyzed using energy dispersive X-ray microanalysis (EDS) installed in a scanning electron microscope (SEM), atomic emission spectroscopy using inductively coupled plasma emission (ICP-AES), and secondary ion mass spectroscopy (SIMS). For the SIMS and ICP-AES analyses, we used standard samples for quantitative analysis.32 The crystallinity of the LPE grown layers were characterized by X-ray diffraction (XRD) measurements, including the θ-2θ mode, ω-scan rocking curve mode, and reciprocal space mapping (RSM) mode using an X’pert system (Panalytical, Tokyo, Japan). All XRD measurements were carried out at room temperature. The surface morphology was examined using an atomic force microscope to obtain the degree of surface flatness after the polishing treatment. The electrical properties were characterized by Hall effect measurements at 80-300 K. We employed the four-probe Van der Pauw method for the electrical measurements. The applied magnetic field for the Hall measurements was 0.5 T. Ohmic electrodes for the electrical measurements were formed by vacuum evaporation of aluminum or indium metal. The optical properties were characterized by photoluminescence (PL) and cathodoluminescence (CL) measurements. A He-Cd laser (λ ) 325 nm) or the fourth harmonic generation of a neodymiumdoped yttrium vanadate (Nd:YVO4) laser was used for the PL excitation, and we examined temperature dependence of the PL spectra in the temperature range of 10-300 K. We also evaluated the position dependence of the PL spectra (PL mapping) on the surface of the grown crystals to examine the homogeneity of the LPE grown layers. The CL measurements were carried out using a homemade CL instrument33 consisting of an SEM with an electron field emitter and an optical spectrometer. We observed the position dependence of the CL spectra on samples to examine variations in the luminescence properties along the growth direction and the optical properties at the interface between the LPE-grown layer and the ZnO substrate. The CL spectra were recorded under excitation with an electron beam at 5 keV.

Result and Discussion Figure 2 shows a photograph of a typical w-(Zn,Mg)O/ZnO sample obtained in the present study. This photograph was taken after removal of the solidified flux remaining on the LPE grown layer. The w-(Zn,Mg)O/ZnO exhibited very high transparency. In this photograph, brownish material can be observed at the edge of the wafer. These are not defects in the w-(Zn,Mg)O layer but rather are solidified flux attached to the side face. The results of both EDS and SIMS analyses indicated homogeneous distribution of Mg concentration in the w(Zn,Mg)O layers. Typical results of EDS analysis were presented in our previous letter,22 and a typical SIMS depth profile of the Mg concentration is shown in Figure 3. Thus, we can safely conclude that the LPE grown w-(Zn,Mg)O layer was very homogeneous in terms of x in (Zn1-xMgx)O. The homogeneity

LPE Growth of (Zn,Mg)O Alloy Semiconductors

Crystal Growth & Design, Vol. 9, No. 2, 2009 1221

Figure 3. SIMS depth profile of the concentration of Mg and unintentional impurities (Al and Li) in w-(Zn0.9Mg0.1)O.

of the crystals was also confirmed by line profiling of the CL spectra and PL mapping as described below. It is also worth noting that a linear relationship between the fraction of MgO in the source material and in the grown crystals was obtained. In other words, the MgO fraction in the LPE grown layer could be controlled by changing the fraction of MgO added into the melt. The highest MgO fraction achieved under the present growth conditions was 13% [x ) 0.13 in (Zn1-xMgx)O]. w-(Zn,Mg)O layers with an MgO fraction more than 13% were not achieved in the present study, although we put more MgO into the melt. Thus, we suppose that 13% is close to the solubility limit of MgO in ZnO at the growth temperature examined in the present study. The solubility limit of MgO in ZnO has been reported to be about 15% at 1000 °C.34 The present results, indicating that MgO fraction is limited at about 13% around 850 °C, is consistent with previous reports. It is also indicated that the growth condition in the present study was close to the equilibrium state. Figure 3 also shows typical results of the SIMS depth profile for minor and unintentional impurities. We found that the LPE grown layer contained Al and Li as unintentional impurities. The concentrations of these impurities varied from sample to sample. Typical concentrations of the Al- and Li-impurity were on the order of 1017 cm-3 and 1013-1015 cm-3, respectively. A relatively low Al concentration, in the range of 1016 cm-3, was found in some LPE grown (Zn,Mg)O layers; however, most of the crystals contained Al impurity on the order of 1017 cm-3. We suppose that the likely source of this Al and Li contamination is the furnace, and the concentrations of these impurities are affected by the usage history of the furnace. The incorporation of Pb and Bi into the LPE grown layers can be considered, since the (Zn,Mg)O layers were grown in a molten Pb-Bi-O liquid. The concentration of Pb was determined to be a few ppm by CIP-AES. This Pb contamination level was close to the Pb concentration in commercial ZnO powders that we typically use. The concentration of Bi in the obtained (Zn,Mg)O layers could not be determined at the present stage because the Bi concentration was less than the detection limit of our ICP-AES instrument. Such low levels of Bi and Pb contamination may be due to the relatively larger ionic radii of these elements35 for occupying substitutional Zn sites or interstitial sites in the ZnO crystal lattice. Figure 4 shows typical results of XRD measurements. As seen in this figure, the LPE grown films showed only 00l diffraction of w-(Zn,Mg)O in the θ-2θ mode scan profile and no trace of inclusion were found. Moreover, the [102] pole figure patterns for the LPE grown w-(Zn,Mg)O layer indicated that there was no trace of rotation domains. These results confirmed

Figure 4. X-ray diffraction (XRD) pattern of the LPE grown w-(Zn0.9Mg0.1)O layer. The inset shows the XRD pole figure pattern for the [102] pole of the w-(Zn0.9Mg0.1)O layer.

Figure 5. Reciprocal space mapping of X-ray diffraction for a 114 diffraction spot of w-(Zn,Mg)O layer grown by the LPE.

that the LPE grown layers were actually single crystalline epitaxial layers. The full-width-at-half-maximum (fwhm) of the 002 ω-scan profile of the w-(Zn,Mg)O layer was as narrow as about 30 arcsec regardless of the MgO fraction, when the film thickness was on the order of 10 µm. As the fwhm values of the LPE grown layer were nearly the same as those of the ZnO substrate used for LPE growth, we can conclude that no degradation of crystallinity occurred when the LPE-grown layer was as thin as several tens of microns. On the other hand, an increase in the fwhm value in the ω-scan curves was observed when the film thickness reached the order of 100 µm. This broadening of the XRD profile is attributed to strain induced by the lattice mismatch between the LPE-grown w-(Zn,Mg)O layer and the ZnO substrate. A typical result of the XRD-RSM measurements for 114 diffraction of the w-(Zn,Mg)O layer is shown in Figure 5. This was obtained for a 30 µm thick as-grown (Zn1-xMgx)O layer with x ) 0.10. The size of the diffraction spot for the LPE grown layer was very small, and its size and shape were very similar to that for the ZnO substrate, indicating that the LPE grown layer has very high crystallinity similar to the ZnO substrate

1222 Crystal Growth & Design, Vol. 9, No. 2, 2009

Kobayashi et al.

Figure 6. MgO fraction dependence of the lattice parameters in the w-(Zn1-xMgx)O layer grown on the ZnO substrate. The open symbols are for thin samples less than 100 µm in thickness, and the closed symbols are for thick films more than 100 µm in thickness.

crystal grown by the hydrothermal method. This figure also indicates that the a-axis lattice parameter (a0) of the LPE grown w-(Zn,Mg)O layer was the same as that of the ZnO substrate, although the c-axis lattice parameters (c0) were different for the two. The lattice parameters of the w-(Zn,Mg)O layers evaluated from XRD-RSM are shown as a function of the MgO fraction in Figure 6. When the layer thickness was less than 100 µm, the parameter c0 of the LPE grown layers monotonically decreased with increasing MgO fraction, but the parameter a0 remained constant. This means, as mentioned above, that the very strong structural restriction inhibited structural relaxation of the w-(Zn,Mg)O layers to expand the a-axis. On the other hand, structural relaxation to expand the a-axis of the w(Zn,Mg)O layer occurred when the thickness of the LPE grown (Zn,Mg)O layer was much more than 100 µm. This suggests that the structural relaxation occurred when the film thickness was sufficiently large. The unit cell volume of the relaxed w-(Zn,Mg)O lattice in the very thick film was close to that of the unrelaxed w-(Zn,Mg)O in the relatively thin film. Thus, the unit cell volume always abides by Vegard’s law, and we can estimate the MgO fraction in the w-(Zn,Mg)O layer from the unit cell volume using the relationship between the unit cell volume and the MgO fraction indicated in this study. For utilizing ZnO in opto-electronic applications, the permanent polarization and polarization induced by piezoelectricity are important issues,36 and the residual strain in the w-ZnO layer is a cause of piezoelectric polarization. Thus, we need to investigate the lattice relaxation behavior and the crystal structure of the strained lattice in more detail in the future. The results of PL measurements at room temperature are shown in Figure 7. It is obvious that the near-band-edge luminescence peak showed a blue shift with an increase in the MgO fraction. The sample with the highest MgO fraction of 13% showed a relatively high visible luminescence intensity, but the intensity of this visible emission was much less than that of the near-band-edge luminescence peak. The uniformity of the obtained layers was characterized by mapping the distribution of the near-band-edge luminescence peak position on the LPE grown w-(Zn,Mg)O surface. The inset in Figure 7 shows the result of PL mapping on the LPE grown layer with 2 in. diameter. It is seen that the peak position of the nearband-edge luminescence peak, namely, the MgO distribution, in the sample was very uniform over the entire 2 in. diameter. The homogeneity of the film along the growth direction was also confirmed by CL measurements. We obtained line profiles of the CL spectra, as shown in Figure 8. These results indicated that the peak position in the near-band-edge region was unchanged along the growth direction. Moreover, the abrupt

Figure 7. PL of the w-(Zn1-xMgx)O layer at room temperature. The inset shows PL peak wavelength mapping on the surface of the LPEgrown w-(Zn1-xMgx)O layer. (Peaks at round 1.6-1.7 eV are the second order peaks at the near-band-edge region, and the sharp peaks at 2.33 and 1.55 eV are the second and third order of the beam (λ ) 266 nm) used for excitation.)

Figure 8. Line profiling of the CL spectra along the growth direction, across the film/substrate interface, measured at room temperature.

interface between the LPE grown w-(Zn,Mg)O layer and the ZnO substrate was found in the line profile. This abrupt interface is derived from growth at a relatively low temperature, where diffusion of Mg from the LPE grown layer to substrate was less enhanced. Actually, enhanced diffusion of Mg at elevated temperature results in the graded interface in terms of the Mg distribution.37 Summarizing the results of PL mapping in Figure 7 and CL mapping in Figure 8, we can safely conclude that the thick w-(Zn,Mg)O films obtained by the LPE technique are quite uniform from the viewpoint of the MgO fraction and the nearband-edge luminescence peak. The results of Hall measurements indicated that all the LPEgrown layers showed n-type conductivity. Here, we assumed that only the LPE-grown layers were conductive and that the substrates were kept semi-insulating even after the LPE growth process. Actually, high electrical resistivity was recorded when we placed the electrodes not on the LPE grown layer but rather on the substrate at the backside. As shown in Figure 9, the electron concentration in the LPE grown layer was not affected

LPE Growth of (Zn,Mg)O Alloy Semiconductors

Crystal Growth & Design, Vol. 9, No. 2, 2009 1223

Figure 10. PL spectra of ZnO and w-(Zn,Mg)O layer grown by the LPE technique measured at low temperature. The arrow indicates a peak of the free-exciton luminescence at 5 K.

Figure 9. Temperature and MgO fraction dependence of the electron concentration and mobility in the LPE-grown w-(Zn1-xMgx)O layer.

by the MgO fraction and exhibited similar temperature dependence. This implies that the activation energy of the carriers (electrons) was not affected by alloying, and the concentration of the relatively shallow donor injecting electrons was nearly constant regardless of the MgO fraction. At the present stage, we attribute the origin of the shallow donor to be the unintentional Al-impurity. As mentioned above, Al contamination analyzed by means of SIMS was in the order of 1017 cm-3 in the (Zn,Mg)O layers. These results are consistent with our previous photoemission study,38 which indicated that the Fermi level in the w-(Zn,Mg)O thin films was pinned at 0.04-0.08 eV below the bottom of the conduction band regardless of the MgO fraction. We speculate that the activation energy for shallow donors in ZnO is not significantly affected by alloying with MgO. In contrast to the electron concentration, the electron mobility monotonically decreased with increasing MgO fraction as also shown in Figure 9. Since the crystallinity characterized by XRD was not degraded by alloying, we attribute this reduction in the electron mobility to an alloying effect. Namely, the spatial discontinuity of the Zn 4s band caused by substitution of Mg for Zn may be the cause of the lowered electron mobility in the (Zn,Mg)O alloy. The electron mobility in the (Zn0.9,Mg0.1)O alloy film at room temperature was reported to be about 60 cm2 s-1 V-1 in our previous report, but was found to be about 90 cm2 s-1 V-1 in Figure 9 in the present work. This improvement was achieved by optimization of the growth conditions. Thus, we can expect further improvements in the electron mobility in the (Zn,Mg)O alloy by improving the crystal quality. Figure 10 shows PL spectra at low temperatures. Here, the PL spectra of (Zn0.9Mg0.1)O and nominally undoped ZnO, both grown by the LPE method in this study, are compared. For the nominally undoped ZnO, both bound-exciton and free-exciton luminescence peaks were found at low temperature, for example, 5 K. Fine structures of the near-band-edge luminescence peak of the undoped film could not be resolved in this figure; however, there are several peaks in the near-band-edge region. Two of those peaks could be assigned to luminescence of excitons bound to the ionized donor and are identical to the I0 and I2 peaks in ref 39. The other peak with a relatively wide peak width was seen to be composed of neutral donor bound exciton peaks, such as I4 and I6 in ref 40. These identifications are consistent with the other experimental results for the SIMS

and Hall measurements, indicating the presence of an unintentional Al-impurity causing a shallow donor state. In contrast to the nominally pure ZnO, the near-band-edge luminescence peak of w-(Zn,Mg)O was very broad, and we could therefore not get to the details of the near band edge luminescence peak for the LPE-grown w-(Zn,Mg)O films. Such broadening of the near band edge luminescence peak of w-(Zn,Mg)O has been commonly observed in prior studies, for example, ref 17. It should be also noted that the sample surface for the PL measurements was prepared by a conventional mechanicalchemical polishing technique after ordinary mechanical lapping. Since there is the possibility that frictional heating during the polishing caused degradation of the crystalline quality or diffusion of impurities from the abrading agent, we need to continue efforts to obtain real bulk properties of w-(Zn,Mg)O layers. Shibata et al.17 have indicated that the quantum efficiency of the near-band-edge PL in w-(Zn,Mg)O was higher than that for the nominally pure ZnO. We wanted to determine whether the same phenomena could be seen in our samples. However, because of the polishing issue, we are not confident regarding the absolute luminescence intensity. Thus, we will not discuss details of the luminescence properties at the present stage. However, it is worth noting that the integrated luminescence intensity of the LPE-grown (Zn,Mg)O alloy was actually high because of its very wide PL peak width. The results for the LPE-grown ZnO indicated that the properties of the crystals are affected by unintentional impurities. Taking the results of SIMS analysis into account, the carrier concentration and the luminescence profile at low temperature are governed by contamination with Al. Although details of the luminescence properties of the (Zn,Mg)O alloy have not been elucidated because of the very broad PL peak, we speculate that the electrical and optical properties of the alloy films were also affected by the Al-impurity. Thus, we need to continue efforts to eliminate contamination in the LPE grown crystals to obtain (Zn,Mg)O alloy layers that truly exhibit intrinsic properties. As mentioned above, the electron mobility in the films shown in this paper is higher than that reported in our previous letter, and this improvement was due to optimization of the growth process. This means there are possibilities for further improvements in the crystallinity and purity of the LPE-grown crystals. Although Al-contamination, the cause of unintentional residual carriers, is a problem that inhibits obtaining (Zn,Mg)O layers showing intrinsic properties, the current results, that is, the Al causing shallow donors even in the (Zn,Mg)O alloy, are good news for researchers seeking materials that exhibit both

1224 Crystal Growth & Design, Vol. 9, No. 2, 2009

high electrical conductivity and high transparency to near-UV light. The present results in fact suggest that intentional Aldoping into (Zn,Mg)O alloy may achieve high conductivity. Summary We have examined the applicability of a LPE technique using a Bi2O3-PbO2 flux to the growth of thick w-(Zn,Mg)O films on ZnO substrates. It was confirmed that this flux works well, and the growth of thick films, for example with 0.5 mm thickness, was achieved. The thick w-(Zn,Mg)O films thus obtained showed very high crystallinity, comparable to that of nominally undoped ZnO single crystals grown by a hydrothermal method. It has been shown that the growth of very thick and homogeneous w-(Zn,Mg)O wafers with a large area, such as 2 in. diameter wafers, is possible using this LPE technique. The LPE technology developed in the present study is a promising one to utilize the w-(Zn,Mg)O alloy wafers for optoelectronic applications in the visible and near-UV regions, and may enable us to obtain self-standing w-(Zn,Mg)O wafers for optoelectronic applications. The technique for removing the substrate to obtain self-standing w-(Zn,Mg)O layers will be developed in the near future. Acknowledgment. Part of this study was carried out at the International Center for Materials Nanoarchitectonics (MANA), NIMS, supported by a Grant-in-Aid for Construction of the World Premier Research Institute from the Ministry of Education, Culture, Sports, Science and Technology, Japan. This study has been also supported by a Grant-in-Aid for Scientific Research from the Japan Society for Promotion of Science. We also acknowledge the contribution of Mrs. Naoko Tateyama and Mrs. Hiroko Ishii of NIMS for research administration.

References (1) Look, D. C. Mater. Sci. Eng., B 2001, 80 (Special Issue), 383. (2) Kolb, D. M.; Schulz, H.-J. Current Topics in Materials Science; Kaldis, E., Ed.; North-Holland: Amsterdam, 1981; Vol. 7, pp 226-268. (3) Minami, T. MRS Bull. 2000, 25 (8), 38. (4) F. Carcia, R. S.; McLean, M. H. Reilly; Nunes, G. Appl. Phys. Lett. 2003, 82, 1117. (5) Ohtomo, A.; Takagi, S.; Tamura, K.; Makino, T.; Segawa, Y.; Koinuma, H.; Kawasaki, M. Jpn. J. Appl. Phys. 2006, 45, L694. (6) Wang, Y.-G.; Ohashi, N.; Wada, Y.; Sakaguchi, I.; Ohgaki, T.; Haneda, H. J. Appl. Phys. 2006, 100, 023524. (7) Ohashi, N.; Wang, Y.-G.; Ishigaki, T.; Wada, Y.; Taguchi, H.; Sakaguchi, I.; Ohgaki, T.; Adachi, Y.; Haneda, H. J. Cryst. Growth 2007, 306, 316. (8) Ohashi, N.; Ebisawa, N.; Sekiguchi, T.; Sakaguchi, I.; Wada, Y.; Takenaka, T.; Haneda, H. Appl. Phys. Lett. 2005, 86, 091902. (9) Leiter, F. H.; Alves, H. R.; Hofstaetter, A.; Hofmann, D. M.; Meyer, B. K. Phys. Status Solidi B 2001, 226, R4. (10) Sun, H. D.; Makino, T.; Segawa, Y.; Kawasaki, M.; Ohtomo, A.; Tamura, K.; Koinuma, H. Appl. Phys. Lett. 2001, 78, 3385. (11) Tsukazaki, A.; Kubota, M.; Ohtomo, A.; Onuma, T.; Ohtani, K.; Ohno, H.; Chichibu, S.-F.; Kawasaki, M. Jpn. J. Appl. Phys. 2005, 44, L643. (12) Jiao, S.-J.; Zhang, Z.-Z.; Lu, Y.-M.; Shen, D.-Z.; Yao, B.; Zhang, J.-Y.; Li, B.-H.; Zhao, D.-X.; Fan, X.-W.; Tang, Z.-K. Appl. Phys. Lett. 2006, 88, 031911.

Kobayashi et al. (13) Sun, J.-C.; Liang, H.-W.; Zhao, J.-Z.; Bian, J.-M.; Feng, Q.-J.; Hu, L.-Z.; Zhang, H.-Q.; Liang, X.-P.; Luo, Y.-M.; Du, G.-T. Chem. Phys. Lett. 2008, 460, 548. (14) Sze S. M. Physics of Semiconductor DeVices; John Wiley & Sons: New York, 1981; p 689. (15) Koike, K.; Hama, K.; Nakashima, I.; Takada, G.; Ogata, K.; Sasa, S.; Inoue, M.; Yano, M. J. Cryst. Growth 2005, 278, 288. (16) Kamada, Y.; Kawaharamura, T.; Nishinaka, H.; Fujita, S. Jpn. J. Appl. Phys. 2006, 45, L857. (17) Shibata, H.; Tampo, H.; Matsubara, K.; Yamada, A.; Sakurai, K.; Ishizuka, S.; Niki, S.; Sakai, M. Appl. Phys. Lett. 2007, 90, 124104. (18) Tampo, H.; Shibata, H.; Maejima, K.; Yamada, A.; Matsubara, K.; Fons, P.; Niki, S.; Tainaka, T.; Chiba, Y.; Kanie, H. Appl. Phys. Lett. 2007, 91, 261907. (19) Nagahama, S.; Iwasa, N.; Senoh, M.; Matsushita, T.; Sugimoto, Y.; Kiyoku, H.; Kozaki, T.; Sano, M.; Matsumura, H.; Umemoto, H.; Chocho, K.; Mukai, T. Jpn. J. Appl. Phys. 2000, 39, L647. (20) Tsukazaki, A.; Yuji, H.; Akasaka, S.; Tamura, K.; Nakahara, K.; Tanabe, T.; Takasu, H.; Ohtomo, A.; Kawasaki, M. Appl. Phys. Express 2008, 1, 055004. (21) Tsukazaki, A.; Ohtomo, A.; Kawasaki, M. Appl. Phys. Lett. 2006, 88, 152106. (22) Kobayashi, J.; Sekiwa, H.; Miyamoto, M.; Ohashi, N.; Sakaguchi, I.; Wada, Y.; Adachi, Y.; Haneda, H. Appl. Phys. Express 2008, 1, 071201. (23) Krebs, K.; Littbarski, R. In Current Topics in Materials Science; Kaldis, E., Ed.; North-Holland: Amsterdam, 1981; Vol. 7, pp 169198. (24) Aichele, T.; Lorenz, A.; Hergt, R.; Gornert, P. Cryst. Res. Tech. 2003, 38, 575. (25) Sugai, M.; Kochiya, T.; Oyama, Y.; Nizhizawa, J. J. Phys. Chem. Solids 2008, 69, 411. (26) Ohashi, N.; Tanaka, J.; Ohgaki, T.; Haneda, H.; Ozawa, M.; Tsurumi, T. J. Mater. Res. 2002, 17, 1529. (27) Sanchez-Juareza, A.; Tiburcio-Silverb, A.; Ortizc, A.; Zironid, E. P.; Rickardsd, J. Thin Solid Films 1998, 333, 196. (28) Hwang, J. H.; Mason, T. O.; Dravid, V. P. J. Am. Ceram. Soc. 1994, 77, 1499. (29) Kuo, S. T.; Tuan, W. H.; Lao, Y. W.; Wen, C. K.; Chen, H. R. J. Am. Ceram. Soc. 2008, 91, 1572. (30) Maeda, K.; Sato, M.; Niikura, I.; Fukuda, T. Semicond. Sci. Technol. 2005, 20, S49. (31) Ohashi, N.; Ohgaki, T.; Sugimura, S.; Maeda, K.; Sakaguchi, I.; Ryoken, H.; Niikura, I.; Sato, M.; Haneda, H. Mater. Res. Soc. Symp. Proc. 2004, 744, 255. (32) Sakaguchi, I.; Hishita, S. Surf. Interface Anal. 2004, 36, 645. (33) Dierre, B.; Yuan, X. L.; Ohashi, N.; Sekiguchi, T. J. Appl. Phys. 2008, 103, 083551. (34) Ryoken, H.; Ohashi, N.; Sakaguchi, I.; Adachi, Y.; Hishita, S.; Haneda, H. J. Cryst. Growth 2006, 287, 134. (35) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (36) Tsukazaki, A.; Ohtomo, A.; Kita, T.; Ohno, Y.; Ohno, H.; Kawasaki, M. Science 2007, 315, 1388. (37) Ryoken, H.; Sakaguchi, I.; Ohgaki, T.; Adachi, Y.; Takenaka, T.; Ohashi, N.; Haneda, H. J. Jpn. Soc. Powder Powder Metall. 2004, 51, 852. (38) Ohsawa, T.; Ohashi, N.; Adachi, Y.; Sakaguchi, I.; Ryoken, H.; Matsumoto, K.; Haneda, H.; Ueda, S.; Yoshikawa, H.; Kobayashi, K. Appl. Phys. Lett. 2008, 92, 232108. (39) Meyer, B. K.; Sann, J.; Lautenschla¨ger, S.; Wagner, M. R.; Hoffmann, A. Phys. ReV. B 2007, 76, 184120. (40) Meyer, B. K.; Sann, J.; Hofmann, D. M.; Neumann, C.; Zenner, A. Semicond. Sci. Technol. 2005, 20, S62.

CG801211M