Electronic band structure of wurtzite GaN under biaxial strain in theMplane investigated with photoreflectance spectroscopy

We investigate the modification of the electronic band structure in wurtzite GaN due to biaxial strain within the M plane using photoreflectance (PR) spectroscopy. The compressively strained M-plane GaN film is grown on $\gamma -{\mathrm{LiAlO}}_2$ (100). In the PR measurements, the electric-field vector (E) of the probe light is polarized parallel $\left(\Vert \right)$ and perpendicular $\left(\perp \right)$ to the c axis of GaN which lies in the growth plane. For E $\perp$ c, the spectrum exhibits only a single resonant feature at lower energies, while for E $\Vert$ c a different single resonant feature appears at higher energies. To identify these features, we calculate the strain dependence of the interband transition energies and the components of the oscillator strength using the $\mathbf{k}\cdot \mathbf{p}$ perturbation approach. Comparison with the calculations shows that the origin of the PR features and their significant in-plane polarization anisotropy is related to the influence of M-plane, biaxial compressive strain on the valence-band structure of GaN. We estimate the value of the deformation potential $D_5$ to be $-4.7\hspace{0.5em}\mathrm{eV}.$