Abstract
We report a study by photoluminescence (PL), Raman scattering, and highly resolved Xβray diffraction (HRXRD) of a series of Siβdoped nβtype GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) on sapphire (0001) with the carrier concentration of 2.3 Γ 10^17^ β 9 Γ 10^18^ cm^β3^. We found that the band gap reduction deduced from the PL spectra analysis is due to both band gap narrowing (BGN) effect and change of the nature of stress in the GaN:Si layers. The HRXRD spectra show that at high Siβdoping levels (>1.6 Γ 10^18^ cm^β3^) the GaN films become under tensile stress. Progressive decreases of the E~2~ Raman mode frequency with Si concentration confirm this observation. The stress induced E~2~ mode frequency shift is estimated to 1.6 cm^β1^/GPa. Additionally, dislocation densities are determined by HRXRD by employing a model that uses the line width of Xβray rocking curves. Atomic force microscopy (AFM) is also used to investigate the defect surface structures of GaN:Si layers which are dominated by pinned steps and surface depressions related to threading dislocations. We find that dislocation densities given by AFM measurements are in agreement with those obtained from rocking curve line widths. (Β© 2006 WILEYβVCH Verlag GmbH & Co. KGaA, Weinheim)