Stress relaxation in thick-film GaN grown by hydride vapor phase epitaxy on sapphire and spinel substrates as studied by photoluminescence and raman spectroscopy

The residual strains in thick-film GaN grown on both sapphire and spinel substrates has been evaluated by photoluminescence (PL) and raman spectroscopy . The strain-free shallow donor bound exciton recombination energy (I2) is 3.468 eV at 10 K. The raman mode frequency shift with residual strain with estimated as Δw = 3.93 cm -1 per one GPa for GaN layers on both substrates . The linear relationship between the PL I2 line and the raman E2 mode frequency is ΔE/Δw = 5.12 meV/cm -1 , which leads to a stress-induced PL line shift of ΔE = 20 meV/GPa