Effect of AlGaAs Buffer Layer on Defect Distribution in Cubic GaN Grown on GaAs (001) by MOVPE

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1 Chiang Mai J. Sci. 2013; 40(6) 971 Chiang Mai J. Sci. 2013; 40(6) : Contributed Paper Effect of AlGaAs Buffer Layer on Defect Distribution in Cubic GaN Grown on GaAs (001) by MOVPE Jamreonta Parinyataramas [a], Sakuntam Sanorpim*[a,b], Chanchana Thanachayanont [c] and Kentaro Onabe [d] [a] Nanoscience and Technology, Graduate school, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand. [b] Department of Physics, Faculty of Science, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand. [c] National Metal and Materials Technology Center, 114 Thailand Science Park, Klong Luang, Pathumthani 12120, Thailand. [d] Department of Advanced Materials Science, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba, , Japan. *Author for correspondence; sakuntam.s@chula.ac.th Received: 11 April 2012 Accepted: 4 September 2012 ABSTRACT Transmission electron microscopy (TEM) was applied to verify defect-structures in cubic GaN layer on GaAs (001) substrate grown by metalorganic vapor phase epitaxy. As an expectation, with the use of AlGaAs as a buffer layer, the GaN/AlGaAs/GaAs interfaces were successfully protected from thermal decomposition at a relatively high growth temperature (960 C). The best quality of cubic GaN layer with a certain amount of the hexagonal phase inclusion was confirmed using electron diffraction patterns. Two different types of structural defects, which are stacking faults and threading dislocations, were clearly observed. Cross-sectional TEM image taken along the [110] zone-axis showed a high-density of planar defects generated from the interface of GaN/AlGaAs. On the other hand, planview TEM image showed an anisotropic distribution of the planar defects, which is elongated along the [110] direction. The cross-sectional TEM image taken along the [1-10] zone-axis confirmed a nearly planar-defects free layer. Keywords: cubic GaN, hexagonal phase, buffer layer, TEM, stacking faults 1. INTRODUCTION Cubic gallium nitride (cubic GaN) has potential applications in optoelectronic devices, such as green-, blue- and white-light emitting diodes (LEDs) [1, 2] and blue laser diodes for the next generation DVD player [2]. It is known that cubic GaN, which exhibits a higher crystallographic symmetry, is theoretically predicted to have superior electrical and optical properties, such as a lower phonon scattering and lower electron effective mass, over hexagonal GaN [3-5]. However, the cubic GaN crystal without

2 972 Chiang Mai J. Sci. 2013; 40(6) incorporation of hexagonal GaN is difficult to obtain due to its metastable nature [6, 7]. It is well known that the cubic GaN film with high cubic-phase purity can be grown on the (001) oriented GaAs substrate surface by both molecular beam epitaxy (MBE) [3, 6] and metalorganic vapor phase epitaxy (MOVPE) [8]. Despite the fact that it has the large lattice-mismatch between cubic GaN and GaAs as high as 20 % [7] and thermal decomposition of the GaAs substrate surface during the growth at relatively high temperature (>900 C) [9, 10]. These problems were experimentally solved using a buffer layer, which is expected to control the phase structures of cubic GaN top layer and to protect the GaAs substrate surface from thermal decomposition during the growth. However, there are only few reports on the growth of high-purity cubic GaN layers on GaAs (100) substrates with AlGaAs buffer layer by RF-plasma assisted MBE [4-5]. In the present work, AlGaAs is used as a buffer layer to protect the GaAs substrate surface from thermal decomposition at high growth temperature, since the bond dissociation energy of Al-As is higher than that of Ga-As in the Al 1-x Ga x As layer [11]. Transmission electron microscopy (TEM) analysis of cubic GaN layer grown on (001) oriented GaAs substrate surface with the AlGaAs buffer layer were done to elucidate the effects of the AlGaAs buffer layer on the generation of stacking faults (SFs) and threading dislocation (TDs), which results in an anisotropic distribution between [110] and [1-10] zone-axes. Cross-sectional and planview TEM image were used to verify type and arrangement of defect s distribution in cubic GaN grown film. 2. MATERIALS AND METHODS All the samples used in this study were fabricated by MOVPE at the University of Tokyo, Japan, using H 2 as a carrier gas. Trimethylgallium (TMGa), 1, 1- dimethylhydrazine (DMHy), trimethylgallium (TMAl) and AsH 3 were used as precursors of Ga, N, Al and As, respectively. For the growth procedure, cubic GaN films were grown by a two-stage growth method, which involved the growth of GaN at two substrate temperatures, i.e., the initial growth of GaN at low-temperature (600 C) and subsequent growth of a GaN top film at higher temperature (960 C). The total growth pressure was maintained at 160 Torr. Before the initial growth of GaN, a 300-nm thick AlGaAs buffer layer with Al content of 20% were grown on the GaAs (001) substrate at 700 C. TEM specimens were prepared using mechanical polishing, and ion polishing down to an electron transparency. A TEM experiment was performed in JEOL JEM 2010 operating at 200 kev. To verify type of defects and their distribution, cross-sectional TEM specimens along the [110] and [1-10] zone-axes as well as plan-view TEM specimens were prepared. 3. RESULTS AND DISCUSSION It is observed in the cross-sectional TEM image taken along the [110] zone-axis (Figure 3) that the AlGaAs buffer layer protected the GaAs (001) substrate surface from the thermal decomposition during the growth at relatively high temperature of 960 C. There was no void at the surface of GaAs (001) substrate; however, the AlGaAs buffer layer affected on structural phase-transition in cubic GaN layer, which is transformed from cubic-phase to a mixed cubic/hexagonal phases associated with a generation of planar defects, such as SFs and twins. It is known that such planar defects are constructed in the form of pyramid-like structures, which are likely generated from the interface between cubic

3 Chiang Mai J. Sci. 2013; 40(6) 973 GaN and AlGaAs buffer layer as reported in our previous work for the cubic GaN on GaAs [12]. It is due to the roughness of AlGaAs surface, exhibiting the (111) surface steps and leading to a high density of planar defects generated along the (111) oriented cubic GaN surface. To verify a distribution of extended structural defects, scanning electron microscopy (SEM) and atomic force microscopy (AFM) of (001) oriented cubic GaN/AlGaAs/GaAs surface were investigated as shown in Figure 1(a) and Figure 1(b), respectively. Surface morphology of the cubic GaN film showed the oriented rectangular morphology along the [110] direction, which corresponds to an alignment of SFs in plan-view TEM image as shown in Figure 2. In Figure 2, plan-view TEM image was used to investigate a characteristic of SFs in cubic GaN film that elongated along the [110] direction. It is surprising that a generation of SFs were not observed along the [1-10] direction. Also, electron diffraction (ED) pattern taken along the [001] zone-axis demonstrated the single phase of cubic GaN. In general, type, density and arrangement of SFs with low density in thin film can use a plan-view TEM image to analyze. But a high density SFs can be analyzed by a crosssectional TEM image due to the overlapping of SFs contrasts in plan-view image [13]. The plan-view TEM image (Figure 2(a)) showed only contrasts along the [110] direction corresponding with previous observation that found high density of planar defects taken along the [110] zone-axis from cross-sectional TEM image. A model of defect distribution in cubic GaN layer with the AlGaAs buffer layer showed in Figure 2(b). The cross-sectional TEM image in [1-10] zone-axis is expected to have less SFs and the horizontal line resulting from SFs in [110] zone-axis. Due to the cross-sectional TEM image taken along the [110] direction showed a high density of SFs parallel to (111) facets of cubic GaN and the top-surface made from the top of facet parallel to (111) of cubic GaN. Therefore, the anisotropic of planar defect between [110] and [1-10] directions can be expected from plan-view TEM image. Figure 1. Surface morphologies of cubic GaN layer on AlGaAs/GaAs (001) substrate observed by (a) SEM and (b) AFM.

4 974 Chiang Mai J. Sci. 2013; 40(6) Figure 2. (a) Plan-view TEM image of cubic GaN grown on AlGaAs /GaAs (001) substrate showing line contrasts associated to SFs along the [110] direction. Inset shows electron diffraction pattern taken from the same sample and (b) the possible model of defect structure in cubic GaN grown on AlGaAs /GaAs (001) substrate. Figure 3 shows cross-sectional TEM image taken along the [1-10] zone-axis, which focused at the interface of cubic GaN and AlGaAs buffer layer/gaas (001) substrate. The interface of GaN and AlGaAs/ GaAs (001) substrate was fairly flat, uniform; moreover, there was no void at the surfaces of AlGaAs as well as GaAs. The surface of AlGaAs buffer layer was smooth, leading to defect less in GaN layer. Planar defects e.g. SFs and twins were not found in cubic GaN layer and a few threading dislocation created from the interface of cubic GaN and AlGaAs buffer layer. SFs were anisotropic between two [110] and [1-10] zone-axes because anisotropy of surface morphology that showed (111) surface steps on the AlGaAs buffer layer at [110] zone-axis leading to a formation of hexagonal as a seed of SFs. Figure 3. Cross-sectional TEM image of cubic GaN grown on AlGaAs /GaAs (001) substrate taken along the [1-10] zone-axis.

5 Chiang Mai J. Sci. 2013; 40(6) 975 Cross-sectional TEM image taken along the [1-10] zone-axis of other regions and selected area diffraction (SAD) pattern showed in Figure 4. Different defect types, which were SFs and TDs, were found in this [1-10] direction. The low density of SFs generated from the cubic GaN/AlGaAs interface that decreased toward the sample surface. TDs were found over SFs that generated from the middle to the top layer of cubic GaN, parallel to the [001] direction. The SAD pattern obtained from the top of c-gan where content the TDs showed single crystal of cubic GaN. The TDs can be produced by a coalescence of pyramid-shaped grains in the [110] direction. Figure 4. Cross-sectional TEM image of cubic GaN layer grown on AlGaAs/GaAs (001) substrate taken along the [1-10] zone-axis at the c-gan layer and their selected area electron diffraction pattern (inset). The evolution of crystal structure in GaN layer taken along [1-10] zone axis as shown in Figure 5 that investigated by their corresponding SAD patterns. The SAD pattern showed cubic/hexagonal mixed phase that taken from area near GaN/ AlGaAs interface, but SAD pattern taken from middle and top layer area showed the single crystal of cubic GaN. Although the area near interface was mixed phases between cubic GaN and hexagonal GaN, the area above mixed phases was wide area of single cubic GaN structure. Therefore, the best quality of cubic GaN with less hexagonal phase was achieved from the smooth surface and fairly flat of AlGaAs buffer surface. In addition, the cubic GaN (111) and hexagonal GaN (0002) diffraction spots were overlapped, so the epitaxial relationship between cubic and hexagonal GaN were {0002} h-gan //{111} c-gan and [11-20] h-gan // [1-10] c-gan. These results confirm the planview TEM image that showed the planar defect was elongated along only the [110] direction. Due to the cubic GaN layer at the [1-10] direction was nearly free planar defect but the [110] direction showed high density of planar defect. Thus, this anisotropic in SF distribution is likely to be associated with the different atomic structure of surface step along the [110] and [1-10] crystallographic directions on the (001) oriented AlGaAs surface. Furthermore, interfaces between cubic GaN and AlGaAs were surprisingly

6 976 Chiang Mai J. Sci. 2013; 40(6) abrupt for the cross-section parallel to the (110) plane as compared to the cross-section parallel to the (1-10) plane. It is demonstrated that formation of seeds for SFs are often associated with the atomic (111) steps at the cubic GaN/AlGaAs interface. Figure 5. Cross-sectional TEM image of cubic GaN layer grown on AlGaAs/GaAs (001) substrate taken along the [1-10] zone axis and their corresponding selected area electron diffraction pattern, showing an evolution of crystal structure thought the GaN layer. 4. CONCLUSIONS The effect of the AlGaAs buffer layer inserted between cubic GaN film and GaAs (001) substrate, which was successfully to protect the GaAs surface from thermal decomposition, was investigated by TEM. It was clearly seen that no voids were observed at the cubic GaN/AlGaAs buffer layer. Moreover, TEM analysis demonstrates that the AlGaAs buffer layer induced the anisotropic distribution of extended defects, which were SFs and TDs. This anisotropic distribution of SFs was likely to be associated with the different atomic structure of the surface step along the [110] and [1-10] crystallographic direction on the (001) oriented AlGaAs surface. A high density of SFs were found in the [110] direction that associated with the atomic (111) steps at the cubic GaN/ AlGaAs interface, leading to form a seed of SFs. The best quality of cubic GaN layer with nearly free planar defect was achieved from [1-10] direction because of the smooth and fairly flat of AlGaAs surface. These results indicated the pure cubic GaN structure without planar defect can be growth by inserted AlGaAs buffer layer to protect the GaAs substrate surface from thermal decomposition. However, the growth condition of AlGaAs buffer layer will improve to decrease the surface roughness showing (111) step. ACKNOWLEDGEMENTS This work was partly supported by Thailand Research Fund (Contract No. RSA ) and the Thai Government Stimulus Package 2 (TKK2555), under the Project for Establishment of Comprehensive Center for Innovative Food, Health Products and Agriculture. One of the authors (J. P.) acknowledges the Development and Promotion of Science and Technology Talents Project (DPST), and the 90 th Anniversary of Chulalongkorn University Fund.

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