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T10.01/AA11.01 - Large Area Semipolar GaN Heterostructures Grown on Patterned Sapphire Wafers 
December 4, 2014   10:30am - 11:00am

Strain-induced piezoelectric fields within GaInN quantum wells grown on polar GaN are blamed as a potential reason for the fairly low efficiency of green LEDs. Therefore, non- and semipolar growth directions gained special attention over the recent years leading to reduced or even completely absent internal electric fields in such structures. Besides using semipolar GaN substrates cut from thick c-plane wafers, which are still very small in size and very expensive, foreign wafers like sapphire cut in non-c directions can be taken to achieve this goal. However, such less polar structures typically contain large dislocation and stacking fault densities. Therefore, we have concentrated on an approach, where metalorganic vapour phase epitaxial (MOVPE) growth is initiated by nucleating on inclined c-plane-like side-facets prepared by etching grooves into adequately oriented sapphire wafers. By this approach, we are still driving the growth mainly in the conventional c-direction. After coalescence of such initially striped nitride structures, they form large area planar semipolar surfaces on which GaInN quantum well structures can be grown. Following approach, several semipolar planes including (10-11), (11-22), and (20-21) can be produced on n-plane (11-23), r-plane (10?12), and s-plane (22-43) sapphire wafers, respectively. By carefully optimizing the growth conditions and applying various defect reduction methods, we were able to decrease the dislocation and stacking fault densities substantially. Excellent structural properties have been obtained, being evident from very narrow X-ray diffraction peaks in the range of 200 arcsec and photoluminescence spectra dominated by the excitonic peaks. Only very weak stacking fault related signals were visible, probably due to the lateral overgrowth of defect-rich areas in our stripes from the neighbour stripes. Overgrowth by hydride vapour phase epitaxy typically leads to further improved layer qualities. GaInN quantum well structures grown on such semipolar (10-11) and (11-22) layers show strong luminescence at about 500 nm indicating that also on such planes large amounts of In can be incorporated. It should be noticed that even more In is needed for such semipolar quantum wells as compared to polar c-plane structures, because of the significantly reduced quantum confined Stark effect. First LED test structures show fair electroluminescence, although the doping profile of these structures is not yet optimized. In this contribution, we will mainly address the defect reduction mechanisms which we observed. Moreover, we have analysed the anisotropic strain and the resulting bow of our layers as a consequence of the growth conditions. Thermally induced strain may be helpful for the self-separation of thick HVPE-grown layers, whereas a too large bow would be a problem for later surface polishing.

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