Light polarization characteristics of m -plane Al_x Ga _1-x N films suffering from in-plane anisotropic tensile stresses

Polarization characteristics of the near-band-edge optical transitions in m -plane Al_x Ga_1-x N epilayers suffering from anisotropic stresses were quantified. The epilayers were grown by both ammonia-source molecular beam epitaxy and metalorganic vapor phase epitaxy methods on an m -plane freestanding GaN substrate. The light polarization direction altered from E⊥c to Ec at the AlN molar fraction, x, between 0.25 and 0.32, where E is the electric field component of the light and ⊥ and represent perpendicular and parallel, respectively. To give a quantitative explanation for the result, energies and oscillator strengths of the exciton transitions involving three separate valence bands were calculated as functions of strains using the Bir-Pikus Hamiltonian. The calculation predicted that the lowest energy transition (E₁) is polarized to the m -axis normal to the surface (X₃) for 0<x ≤ 1, meaning that E₁ emission is principally undetectable from the surface normal for any in-plane tensile strained Al_x Ga_1-x N. The polarization direction of observable surface emission was predicted to alter from c -axis normal (X ₁) to c -axis parallel (X₂) for the middle energy transition (E₂) and X₂ to X₁ for the highest energy transition (E₃) between x=0.25 and 0.32. The experimental results were consistently reproduced by the calculation.