Homo-Epitaxial Growth on Misoriented GaN Substrates by MOCVD
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ABSTRACT The N-side of GaN single crystals with off-angle orientations of 0°, 2°, and 4° towards the [10 1 0] direction was used as a substrate for homo-epitaxial MOCVD growth. The highest misorientation resulted in a reduction of the density of grown hillocks by almost two orders of magnitude as compared with homo-epitaxial films grown on the exact (000 1 ) surface. The features still found on the 4° misoriented sample after growth can be explained by a model involving the interaction of steps, introduced by the misorientation and the hexagonal hillocks during the growth process. INTRODUCTION Metalorganic Chemical Vapour Deposition (MOCVD) growth of Gallium Nitride (GaN) in the [000 1 ] direction is associated with the formation of surface defects, such as hexagonal hillocks [1,2,3]. The inversion domain, located at the centre of the point-topped pyramids, apparently causes hillock formation as a result of the higher growth rate of this defect compared with the growth rate of the surrounding matrix [3]. Also for homoepitaxial growth on the N-side of GaN substrates formation of hexagonal pyramids is observed [4,5]. Homo-epitaxial growth in the [000 1 ] direction has the advantage that the N-side of GaN single crystals can be mechano-chemically polished to obtain epi-ready substrates [6], while the Ga-side (for growth in the [0001] direction) can only be mechanically polished, so that reactive ion etching is needed to prepare epi-ready substrates [7]. For device applications smooth surfaces are required, therefore the formation of hillocks should be avoided. A common way to avoid growth features on the surface is the use of misoriented substrates [8]. In the present work the surface morphology of homoepitaxial GaN layers is studied for different off-angle orientations from the exact [000 1 ] direction. It is found that the formation of hexagonal pyramids can be strongly suppressed by the use of a sufficiently large misorientation, resulting in much smoother layers. EXPERIMENTAL GaN single crystals [9] were used as substrates for MOCVD growth. The (000 1 ) substrate surfaces were mechano-chemically polished [6] to obtain off-angle orientations
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of 0°, 2°, and 4° towards the [10 1 0] direction. The misorientation was confirmed by Xray diffraction analysis. Homo-epitaxial GaN growth was performed at a temperature of 1040°C and a pressure of 50 mbar using trimethylgallium (TMG) and ammonia (NH3) as precursors, and hydrogen (H2) as carrier gas. The substrate crystals were heated to growth temperature under a NH3 gas flow diluted in nitrogen (N2). Growth was performed with a V/III molar ratio of about 1700 with a total flow of about 5 slm. The surfaces of the homo-epitaxial GaN layers were investigated by optical differential interference contrast microscopy (DICM), a technique very sensitive for detecting local differences in surface slope, which are visualised by different shades of grey in the figures. RESULTS AND DISCUSSION Optical examination with DICM of layers grown on (000 1 ) substrates without misorien
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