Freestanding GaN crystals were fabricated by hydride vapor phase epitaxy using a random-islands facet-initiated epitaxial overgrowth technique. In this method, small micrometer sized GaN islands were firstly deposited on a TiC buffer layer on a sapphire substrate. Successive three-dimensional growth of GaN was controlled to a thickness of a few hundred micrometers on the buffer layer. Finally, a thick GaN layer was grown and high quality freestanding GaN crystals (dislocation density: <3×106 cm−2, radius of curvature: >5 m) were obtained by self-separation from the sapphire substrate. It was found that the dislocation density was drastically reduced in the initial growth stage of this method by the appearance of sidewall facets. Depth profiles of the residual strain in the freestanding GaN substrates have been successfully measured by a novel method employing cross-sectional micro-reflectance spectroscopy. It was found that the intrinsic strain, the driving force of wafer bending, can be greatly reduced by the introduction of three-dimensional growth in the initial growth stage.
Source: Journal of Crystal Growth