Optical properties of tensile-strained wurtzite GaN epitaxial layers

Photo-modulated electroreflectance and photoluminescence spectra of wurtzite GaN epilayers grown on a 6H-SiC (0001)_Si or a Si (111) substrate were measured as a function of temperature. The valence band structure of a biaxially tensile-strained (-0.15% in c axis) epilayer was drastically changed compared to that in unstrained and compressively strained ones; the optical transition between the conduction and the uppermost valence band exhibiting the minimum transition energy was polarized parallel to the c axis, which is characteristic of the spin split-off valence band in unstrained crystal.