We studied experimentally the crystallization of a low-temperature (LT)-GaN buffer layer and its effect on the subsequent crystalline quality of the GaN epitaxial layer. The degree of crystallization was sensitive to the thickness and the temperature ramp rate of the GaN buffer layer. When the LT-GaN buffer layer was more than 100 nm thick, there was relatively little crystallization of the buffer layer and subsequently poor crystalline quality of the GaN epitaxial layer. The optimum crystallization condition for the GaN buffer layer just prior to high-temperature growth was found when the single-crystal regions in the GaN buffer layer occupied a volume 40% as large as the randomly aligned matrix. We could reduce the threading dislocation density in the GaN epitaxial layer by inserting the LT-GaN buffer layers in periodically. To explain this result, we propose a mechanism by which the periodic layer reduces the density of threading dislocations.
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