Optical polarization properties of m-plane AlxGa1-xN epitaxial films grown on m-plane freestanding GaN substrates toward nonpolar ultraviolet LEDs

Light polarization characteristics of the near-band-edge optical transitions in m-plane AlxGa1-xN epilayers suffering from anisotropic stresses are quantitatively explained. The epilayers were grown on an m-plane freestanding GaN substrate by both ammonia-source molecular beam epitaxy and metalorganic vapor phase epitaxy methods. The light polarization direction altered from E perpendicular to c to E//c at the AlN mole fraction, x, between 0.25 and 0.32, where E is the electric field component of the light and perpendicular to 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 are calculated as functions of strains using the Bir-Pikus Hamiltonian. The calculation predicts 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 is 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 are consistently reproduced by the calculation. (C) 2011 Optical Society of America