Luminescence properties of strained GaN epilayers and spatial configurations of silicon impurity and related defects

Near-band-edge photoluminescence and reflection spectra have been investigated in GaN grown on Si and Al₂O₃ substrates. It has been found that the doping of GaN both with Si and Mg reduces the value of compressive strain in GaN/Al₂O₃. In GaN/Si, no evidences of the crossover of A and B excitonic resonance positions with increasing tensile strain was observed. The calculations using the non-empirical SCF MO LCAO method showed that the Si_Ga impurity may lead to an increase of the effective c/a lattice constant ratio, while SiN having greater formation energy produces a lattice relaxation which does not change the c/a ratio significantly. These results are in agreement with experimental observations of decreasing compressive strain in GaN: Si/Al₂O₃ and increasing tensile strain up to the cracking in GaN: Si/Si.