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

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 perpendicular to c to E<parallel to>c at the AlN molar fraction, x, between 0.25 and 0.32, where E is the electric field component of the light and perpendicular to and <parallel to> 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 (E-1) is polarized to the m-axis normal to the surface (X-3) for 0 < x < 1, meaning that E-1 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 (X-1) to c-axis parallel (X-2) for the middle energy transition (E-2) and X-2 to X-1 for the highest energy transition (E-3) between x=0.25 and 0.32. The experimental results were consistently reproduced by the calculation.