Structural and micromechanical analyses by polarized Raman spectroscopy of wurtzitic GaN films grown on (0001) sapphire substrates.

Polarized Raman spectroscopy is applied to non-destructively unfold the variation of crystallographic orientation in wurtzitic GaN films that occurred during their growth on (0001)-oriented sapphire substrates as a function of growth conditions. Variations of polarized Raman spectra were clearly observed at different locations along the growth direction by systematically performing line scans on cross sections of different film/substrate systems. Such variations showed a strong dependence on the type of buffer layer, deposition temperature, and reactor pressure. The observed trends suggested the occurrence of inhomogeneous lateral growth of GaN films near the buffer layer and a continuous adjustment of the structure of the tilted-grown epilayer during the growth process until reaching the expected stable structure. The large elastic/thermal mismatch, which could not be fully relaxed by the effect of the buffer layer, resulted in a non-uniform distribution of residual strain. Such strain, mainly stored at the interface, gradually relaxed by the occurrence of tilting and twisting of the crystal cells along the film growth direction: a process that spontaneously occurred minimizing the strain energy stored in the film/substrate system. Varying reactor pressure caused a variation of residual strain in the epilayer and thus a different adjustment of the GaN microstructure during the growth process. Relatively homogenous GaN films with a low density of dislocations could be achieved on patterned sapphire substrates, despite the presence of crystallographic tilting of the film. [ABSTRACT FROM AUTHOR]