All deformation potentials in GaN determined by reflectance spectroscopy under uniaxial stress: Definite breakdown of the quasicubic approximation

Reflectance spectroscopy was performed for nonpolar and semipolar bulk GaN substrates under uniaxial stress. The exciton-transition energies and oscillator strengths clearly depended on uniaxial stress. The results were interpreted by group theory on excitons and analyzed in terms of the effective Hamiltonian proposed by Bir and Pikus. In the Hamiltonian, the short-range Coulomb interaction was taken into consideration. This approach allows all exciton deformation potentials to be determined without the quasicubic approximation and we found that the exciton deformation potentials in GaN considerably deviate from the quasicubic approximation.