Defect reduction in gallium nitride using cantilever epitaxy.

Abstract

Cantilever epitaxy (CE) has been developed to produce GaN on sapphire with low dislocation densities as needed for improved devices. The basic mechanism of seeding growth on sapphire mesas and lateral growth of cantilevers until they coalesce has been modified with an initial growth step at 950°C. This step produces a gable with ) 2 2 11 ( facets over the mesas, which turns threading dislocations from vertical to horizontal in order to reduce the local density above mesas. This technique has produced material with densities as low as 2-3x10 7 /cm 2 averaged across extended areas of GaN on sapphire, as determined with AFM, TEM and cathodoluminescence (CL). This density is about two orders of magnitude below that of conventional planar growths; these improvements suggest that locating wide-area devices across both cantilever and mesa regions is possible. However, the first implementation of this technique also produced a new defect: cracks at cantilever coalescences with associated arrays of lateral dislocations. These defects have been labeled "dark-block defects" because they are non-radiative and appear as dark rectangles in CL images. Material has been grown that does not have dark-block defects. Examination of the evolution of the cantilever films for many growths, both partial and complete, indicates that producing a film without these defects requires careful control of growth conditions and crystal morphology at multiple steps. Their elimination enhances optical emission and uniformity over large (mm) size areas. ACKNOWLEDGMENTS

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“…All this information together indicated that the problem was with the initial growth of cantilevers and with the way they are grown together at coalescence. We found that two modifications to our CE growths were able to eliminate dark-block defects [9]. First, the initial nucleation of GaN on the mesas needed to be fully dense.…”