Urbach-Martienssen tail as the origin of the two-peak structure in the photoluminescence spectra for the near-band-edge emission of a freestanding GaN crystal observed by omnidirectional photoluminescence spectroscopy

Omnidirectional photoluminescence (ODPL) and standard photoluminescence (SPL) spectroscopy measurements were carried out on a freestanding GaN crystal at various temperatures (T) between 12 K and 300 K. The intensity ratio (ρ) of the ODPL to SPL spectra for the near-band edge (NBE) emission of the crystal showed a linearly decreasing slope with respect to photon energy (E) below the fundamental absorption edge energy (Eabs), and the slope became steeper at lower T. The linear extrapolations of the slopes at various T converged at a common point (E, ρ) = (3.539 eV, -6.306). This fact implies that the slope obtained for ρ corresponds to the Urbach-Martienssen (U-M) tail of the optical absorption spectra. Numerical simulations under the assumption, where a part of photoluminescence traveled inside the crystal and the scattering occurred at a backside of the crystal, substantially reproduced the ODPL spectra. Accordingly, the origin of the two-peak structure in the ODPL spectra around the NBE emission of the GaN crystal is attributed to the existence of the U-M tail. The Eabs value gives the photon energy at which the absorption coefficient becomes approximately 16.7 cm-1, which corresponds to an inverse of a doubled value of the crystal thickness (0.03 cm).