Transmission Electron Microscopy Study of Nonpolar a-Plane GaN Grown by Pendeo-Epitaxy on (11 2 0) 4H-SiC.
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Transmission Electron Microscopy Study of Nonpolar a-Plane GaN Grown by Pendeo-Epitaxy on (1120) 4H-SiC. D.N. Zakharov1, Z. Liliental-Weber1, B. Wagner2, Z.J. Reitmeier2, E.A. Preble2, and R.F. Davis2 1 2
Lawrence Berkeley National Laboratory, MS 62-203, Berkeley, CA 94720 North Carolina State University, Raleigh, NC 27695
ABSTRACT Pendeo-epitaxy has been applied to nonpolar a-plane GaN layers in order to observe if such process will lead to defect reduction in comparison with direct growth on this plane. Uncoalesced and coalesced a-plane GaN layers with thicknesses 2µm and 12µm, respectively, have been studied by conventional and high resolution electron microscopy. The following structural defects have been observed in pendeo-epitaxial layers: (1) basal stacking faults, (2) threading dislocations, and (3) prismatic stacking faults. A drastic decrease in the density of threading dislocations and stacking faults was observed in ‘wing’ areas with respect to ‘seed’ areas. Cross-section images reveal cracks and voids at the areas where two coalesced wings meet each other. High resolution electron microscopy shows that the majority of stacking faults are low-energy planar defects of the types I1, I2 and I3. The I3 type basal stacking fault, predicted theoretically, was observed experimentally for the first time. INTRODUCTION Recent studies have shown that spontaneous polarization and the piezoelectric effect can be eliminated in GaN grown along non-polar directions, such as [1120] and [1100] [1,2]. These two effects were observed in GaN layers grown along [0001] direction and led to high interface charge densities and spatial separations of electron and holes wave functions in GaN-based quantum well structures [3]. Transmission electron microscopy investigations performed on (1100) (m-plane) and (1120) (a-plane) GaN layers grown by heteroepitaxial methods on different substrates revealed, however, a high density of basal stacking faults (BSFs) and, associated with them, threading dislocations [4-6]. Since Pendeo-epitaxy (PE) was proven to be effective method for defects reduction in GaN [7], we applied this method for a-plane GaN grown by metalorganic vapor phase epitaxy on (1120) 4H-SiC substrates with AlN buffer layers. Uncoalesced and coalesced samples have been produced. These samples were studied by transmission electron microscopy (TEM) in cross-section and plan-view configuration in order to investigate the structural defects present in the layers and look for possible reduction in defect density. EXPERIMENTAL The epitaxial AlN buffer layers, the subsequently deposited epitaxial GaN seed films, and the pendeo-epitaxial (PE) GaN layers were grown in-situ in a cold-wall, vertical,
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pancake-style metalorganic vapor phase epitaxy (MOVPE) system. These PE layers were grown on 4H-SiC (1120) substrates previously etched in a 10% HF solution in order to remove the native oxide. The 100 nm thick AlN buffer layers and the 500 nm thick GaN seed layers were grown at 20 Torr and temperatures of 1100°C and
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