Absorption, Reflectance, and Luminescence of GaN Single Crystals

Abstract

Analysis of the low-temperature absorption, reflectance, and emission spectra of oriented single crystals of hexagonal GaN demonstrates that the features found at 3.62 eV ($E\perp C$) and 3.72 eV ($E\parallel C$) are due to the formation of free excitons associated with a direct energy gap. A close analogy exists between these results and similar data from ZnO, consistent with the relative position of Zn and Ga, and O and N atoms in the Periodic Table. Luminescence spectra present good, although not conclusive, evidence for the hypothesis that this direct gap is also the fundamental energy gap in GaN.