Raman and photoluminescence studies of biaxial strain in GaN epitaxial layers grown on 6H–SiC

The effect of biaxial strain on optical phonons in high-quality GaN epitaxial layers grown on 6H–SiC substrates by metal organic chemical vapor deposition has been studied. The deformation potential constants for the $E_2^{\text{(1)}},$ $A_1(\mathrm{TO}\mathrm{),}$ $E_1(\mathrm{TO}\mathrm{),}$ and $E_2^{\text{(2)}}$ optical phonon modes in hexagonal GaN have been obtained. A method for calculating strain in hexagonal GaN layers from Raman data alone is suggested. A comparative analysis of the strain measured by x-ray diffraction and Raman spectroscopy shows that these data agree well. It is found that the biaxial stress of 1 GPa results in a shift of the excitonic photoluminescence lines of 20±3 meV.