Strain effects in GaN epilayers grown on different substrates by metal organic vapor phase epitaxy

For practical applications, it is important to control strain by carefully tuning growth conditions for stress modification. Strain can have a pronounced impact on device behavior and is caused by extrinsic thermal stress and a tunable intrinsic growth stress. The impact of strain on GaN epilayers was investigated by photoluminescence and high resolution x-ray diffraction (XRD). The GaN samples were grown by metal organic vapor phase epitaxy, on sapphire, 4H–SiC, freestanding GaN and Si (111) substrates. Both free and bound exciton transitions were observed at low temperature, and their energy shift was analyzed with respect to the strain values derived from XRD. We also characterized the valence band split and the GaN bandgap as a function of the strain at 4 K.