Anisotropic strain and phonon deformation potentials in GaN

We report optical phonon frequency studies in anisotropically strained c-plane- and a-plane-oriented GaN films by generalized infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The anisotropic strain in the films is obtained from high-resolution x-ray diffraction measurements. Experimental evidence for splitting of the GaN E₁(TO), E₁(LO), and E₂ phonons under anisotropic strain in the basal plane is presented, and their phonon deformation potentials c_E₁(TO), c_E₁(LO), and c_E₂ are determined. A distinct correlation between anisotropic strain and the A₁(TO) and E₁(LO) frequencies of a-plane GaN films reveals the a_A₁(TO), b_A₁(TO), a_E₁(LO), and b_E₁(LO) phonon deformation potentials. The a_A₁(TO) and b_A₁(TO) are found to be in very good agreement with previous results from Raman experiments [V. Yu. Davydov et al., J. Appl. Phys. 82, 5097 (1997)]. Our a_A₁(TO) and a_E₁(LO) phonon deformation potentials agree well with recently reported theoretical estimations [J.-M. Wagner and F. Bechstedt, Phys. Rev. B 66, 115202 (2002)], while b_A₁(TO) and b_E₁(LO) are found to be significantly larger than the theoretical values. A discussion of the observed differences is presented.