Optical properties of the gap states in amorphous silicon nitride film deposited on GaAs (a-SiNx/GaAs) are characterized using piezoreflectance (PzR) and photoreflectance (PR) measurements at 15 and 300 K. The gap-state transitions of the a-SiNx film were observed clearly in the experimental PzR spectra from 1.3 to 6 eV. PR measurements of the a-SiNx/GaAs were performed to identify the transitions below 1.9 eV. The defect-state transitions of a-SiNx in both PR and PzR are temperature insensitive with respect to the direct bandgaps of GaAs and silicon nitride. The temperature-insensitive behavior is an intrinsic character of an imperfection state existing in the middle gap of a semiconductor. Transition energies of all the gap-state transitions of a-SiNx/GaAs are analyzed by detailed line-shape fits to the PzR spectra. The origins of the gap-state transitions are properly assigned. Based on the experimental results together with previous density-of-states calculations, an experimental band scheme including the transition assignments of the experimental gap states for a-SiNx/GaAs is constructed. (c) 2006 The Electrochemical Society.