Birefringence in GaN epitaxial layers and GaN microrods

Light propagating through a birefringent material undergoes a change of the state of polarization. In this article, we demonstrate the application of the non-invasive method of conoscopy to analyze the birefringence of c-plane GaN. For thick GaN bulk layers, the difference Δn between the ordinary and extraordinary refractive index results in a modification of the state of polarization from linear to elliptical to circular and back to orthogonally linear polarization within a few degree variation of the internal angle propagation. Measurements of thin epitaxial GaN layers on c-plane sapphire show the strong influence of the sapphire substrate to the observed changes of the state of polarization.