Microstructural investigation of Ti/Au ohmic contacts on Ga doped single crystalline n-ZnO films

Characteristics of Ti/Au ohmic contacts on Ga doped n-type ZnO films were investigated in detail, before and after the thermal annealing. It was observed that a minimum specific contact resistance was obtained after annealing the samples at 200–300 °C, and the contact properties deteriorated by annealing at higher temperatures. Observation using the transmission electron microscopy and the X-ray diffractometer showed no phase reactions at the film interfaces after the annealing at 200 °C. It was observed in the case of 500 °C annealed samples that Ti-Zn oxide formed within the Ti layer and substantial out-diffusion of Zn and O occurred from ZnO to Ti. It was proposed that the Zn/O out-diffusion and the Ti-Zn oxide formation are responsible for the deterioration of contact properties after the high temperature annealing. Formation of Ti–Au compounds was not observed in any of the annealed samples.