Photoluminescence and Raman Scattering Signatures of Anisotropic Optical Properties in Freestanding m‑, a- and c‑Plane GaN Substrates

Abstract

Anisotropic optical properties in m-, a-, and c-plane GaN substrates are investigated with photoluminescence and Raman scattering spectroscopic techniques. At room temperature, the Raman light scattering of polar phonon modes, i.e., A1 and E1 modes, exhibits vividly polarization property, while two nonpolar modes (e.g., E2L and E2H) and a defect-mediated mode at ∼418 cm–1 can be observed in all the substrates with three orientations. More interestingly, a silent Raman mode (e.g., B1L) is observed in the Raman spectra of the two nonpolar substrates. At cryogenic temperature, the band-edge emissions and respective LO phonon replicas of the m-plane substrate show a strong polarization dependence on the direction of electric field of excitation light no matter what the recombination mechanism is. Furthermore, the polarization is found to be strongly temperature-dependent. These experimental findings are well consistent with the predictions by Hopfield’s model (e.g., J. Phys. Chem. Solids 1959, 10, 110–119).