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 (E_abs), 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 E_abs 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).