Identification of subsurface damage in freestanding HVPE GaN substrates and its influence on epitaxial growth of GaN epilayers

Freestanding hydride vapor phase epitaxy grown GaN substrates subjected to various polishing methods were characterized for their surface and subsurface conditions. Although different polishing processes led to very smooth surfaces as seen in atomic force microscopy and optical microscopy, they resulted in drastically different optical properties measured by photoluminescence (PL) and cathodoluminescence (CL). PL measurements showed a large increase in defect luminescence. There was also subsurface damage evidenced in CL imaging for one polishing process. Morphological and optical characteristics of an overgrown GaN thin film indicate great differences in the initial nucleation on the substrates subjected to different polishing processes even though the starting surface roughness was similar. This difference continued for the growth of a 405 nm light emitting diode (LED) structure. A PL lifetime reduction by factors of two and four, respectively, was seen for the band edge peak of the GaN layer and the quantum well peak of the LED structure as measured by time-resolved PL. This variability in device performance emphasizes the need for a greater understanding of the effects of starting surfaces, and subsurface characteristics, on the epitaxial growth.