A photoluminescence, Raman scattering, and high-resolution X-ray diffraction study was made of Si-doped n-type GaN layers, grown by metal-organic vapour phase epitaxy onto sapphire (00▪1), with a carrier concentration of 2.3 x 1017 to 9 x 1018/cm3. It was found that the band-gap reduction, deduced from photoluminescence spectra analyses, was due to both a band-gap narrowing effect and a change in the nature of the stress in the GaN:Si layers. The high-resolution X-ray diffraction spectra showed that, at high Si-doping levels (>1.6 x 1018/cm3), the GaN films came under tensile stress. Progressive decreases of the E2 Raman mode frequency with Si concentration confirmed this observation. The stress-induced E2 mode frequency shift was estimated to be 1.6/cmGPa. In addition, dislocation densities were determined via high-resolution X-ray diffraction by employing a model that exploited the linewidth of X-ray rocking curves. Atomic force microscopy was also used to investigate the defect surface structures of GaN:Si layers. These were dominated by pinned steps and surface depressions that were related to threading dislocations. The dislocation densities given by atomic force microscopic measurements were in agreement with those obtained from rocking-curve linewidths.

Stress and Density of Defects in Si-Doped GaN. Z.Chine, A.Rebey, H.Touati, E.Goovaerts, M.Oueslati, B.El Jani, S.Laugt: Physica Status Solidi A, 2006, 203[8], 1954-61