Structural Defects in Laterally Overgrown GaN Layers Grown on Non-polar Substrates
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Structural Defects in Laterally Overgrown GaN Layers Grown on Non-polar Substrates Zuzanna Liliental-Weber1, X. Ni2, and H. Morkoc2 1 Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, M/S 62R0203-8255, BERKELEY, CA, 94720-8139 2 Virginia Commonwealth University, Richmond, VA, 23284 ABSTRACT Transmission electron microscopy was used to study defects in lateral epitaxial layers of GaN which were overgrown on a template of a-plane (1120) GaN grown on (1102) rplane Al2O3. A high density of basal stacking faults is formed in these layers because the cplanes of wurtzite structure are arranged along the growth direction. Density of these faults is decreasing at least by two orders of magnitude lower in the wings compared to the seed areas. Prismatic stacking faults and threading dislocations are also observed, but their densities drastically decrease in the wings. The wings grow with opposite polarities and the Ga-wing width is at least 6 times larger than N-wing and coalescence is rather difficult. Some tilt and twist was detected using Large Angle Convergent Beam Electron Diffraction. INTRODUCTION The total polarization of nitride films used for devices is aligned along the polar [0001] axis of the wurtzite crystal structure. This direction coincides with the growth direction of the GaN-based layers. This, in turn, causes spontaneous and piezoelectric polarizations within the active layers, which influences the electrical and optical properties of devices [13]. One of the possible solutions to eliminate these undesirable effects is to grow GaNbased epilayers in nonpolar orientations. However, regardless of the growth direction (caxis or non-polar) a high defect density is usually observed for all GaN-based layers, since they are grown heteroepitaxially on foreign substrates (Al2O3 or SiC). These defects are detrimental for the application of these materials for laser diodes. An established method to decrease the defect density is lateral overgrowth. Two related methods; laterally epitaxial overgrowth (LEO) and pendeo-epitaxy have been developed. It was found that defects formed in GaN layers grown on polar and non-polar substrates are different. For the growth in polar direction the main defects are threading dislocations, which propagate along the growth direction. In case of growth in non-polar direction stacking faults are formed on cplanes that are along growth direction [4], since their formation energy on these planes is the lowest [5]. In this report we describe laterally overgrown GaN grown on (1102) r-plane of Al2O3 studied by transmission electron microscopy (TEM) in the cross-section configuration. The aim of this study was to investigate structural defects present in the layers and tilt/twist between wings. The JEOL 3010 microscope with 300 keV acceleration voltage was used for this work. EXPERIMENTAL
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The (1120) a-plane GaN layers were grown on the (1102) r-plane of Al2O3. A 1.5 µm thick GaN with a low-temperature nucleation layer was used as a template. A 10
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