Light polarization characteristics of m-plane AlxGa1−xN films suffering from in-plane anisotropic tensile stresses

Abstract

Polarization characteristics of the near-band-edge optical transitions in m-plane AlxGa1−xN 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 E∥c 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 AlxGa1−xN. The polarization direction of observable surface emission was predicted to alter from c-axis normal (X1) 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.