We use Auger electron spectroscopy (AES) measurements to obtain elemental distributions for Cu/Au Schottky contacts to an InGaP layer in as-deposited and thermally annealed samples. The AES depth profile shows an obvious Cu layer with no apparent interdiffusion between the Cu and the InGaP layers in either the as-deposited samples or in the samples annealed at 450 °C. However, when the annealing temperature reached 500 °C, the metallic Cu layer was released, and a distinct interdiffusion between the Cu and the InGaP layers is observed. Metallic Cu and Au intermixed to form a stable intermetallic region. Below this intermetallic region, an interaction region, composed mainly of Cu and P, is observed. The formation of this region is attributable to the diffusion of Cu into the InGaP layer, and is responsible for the thermal degradation of Cu/Au Schottky contacts to the InGaP layer. In the deep-level transient spectroscopy spectra, we find an obvious trap at 150 K. This was determined to be a native trap that exists in the InGaP materials with activation energies of 0.35–0.42eV. After annealing at 500 °C, another trap appeared. According to a thermal degradation investigation, this trap at Ec-0.70 eV can be attributed to the diffusion of Cu into the InGaP layer. © 2003 American Institute of Physics.