GaN substrates with low dislocation density are key material for the commercial production of violet lasers. Hydride vapor phase epitaxy (HVPE) is a practical growth method for the production of GaN substrates. In this report a newly-improved method for the reduction of dislocations in GaN crystals by HVPE is described.

In the past, we developed a method for the dislocation reduction named dislocation elimination by the epitaxial-growth with inverse-pyramidal pits (DEEP) [1]. The typical process of the DEEP is as follows. Using HVPE a GaN layer starts to be grown epitaxially on a foreign substrate with a high dislocation density generated at the interface. The thick GaN layer grows with numerous large hexagonal inverse-pyramidal pits maintained on the surface. The pit is constructed by facet planes such as {11-22}. As the growth proceeds, dislocations in the GaN crystal are concentrated to the center of the pit through {11-22} facet plane. As a result a wide area with low dislocation density is formed within the pit except the center area with high dislocation density. These pits are randomly positioned.

By the DEEP process a GaN crystal with pits about 100 μm in diameter was obtained and then flattend. The dislocation density of this substrate was in the range of 105cm-2 or lower at a low density area and in the range of 10⁸cm^-2 at a high density area. A GaN crystal with pits about 500μm in diameter was also obtained and examined. Although the dislocation density was in the range of 10⁵cm^-2 or lower at the low density area, the area with the dislocation density over 1x10⁷cm^-2 was extended around the center. These pits positioned randomly. They are considered barriers for the device manufacturing process.

In order to improve this issue we made a new approach. A initial substrate having a layer of round shape patterns with the spacing of 400μm on the surface was used as a starting substrate. The GaN was grown on that by HVPE, then, a thick GaN epitaxial layer was obtained, on which all the pits arranged orderly. The spacing of the center of the pits was 400μm, which was determined by the patterned layer. After flattend this new type substrate was examined. The distribution of dislocations was examined by cathodoluminescence (CL). The CL image showed that in each pit area at the center a core (We named this area a core) was observed. Each core was clearly distinguished by a boundary from the other area in the CL image. These cores and pit areas were positioned regularly. The dislocation density near the core was as high as 10⁷cm^-2. It decreased as low as the order of 10⁴cm^-2 at a distance from the core. Total amount of dislocation is greatly reduced. We named this new method for the reduction of dislocations as advanced-DEEP (A-DEEP). The A-DEEP can have other type of facet structure, which will be introduced at the oral presentation.

[1] K. Motoki et al.: Journal of Crystal Growth 237-239 ( 2002 ) 912-921.

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