Investigation of the p-GaN ohmic contact property by using a synchrotron radiation analysis

1We report the optimized process and Ohmic mechanism for an indium tin oxide (ITO)/p-GaN Ohmic contact. The current transfer enhanced layer (CTEL) on the top of p-type GaN was introduced to improve the Ohmic properties. The samples were annealed at temperatures in the range of 400 - 800°C under gas ambients of 0 (i.e., pure N₂), 0.1, 0.2, and 0.5 % O ₂/N₂ ratio. A small amount of O₂ addition shows much lower contact resistance than the pure N₂ only ambient while a high O₂/N₂ ratio exhibits an abruptly increasing the contact resistance and operating voltage. The contact resistance of the ITO/(CTEL)/p-GaN layer shows the best value of 1.43 × 10^-3 ohm·cm² at the annealing temperature of 650°C and a 0.1 % O₂/N₂ gas ambient. High resolution near edge X-ray absorption spectroscopy (NEXAFS) shows that more interstitial N₂ molecules were formed from the ITO/CTEL interfaces with increasing O ₂/N₂ ratio. From the NEXAFS results, the high contact resistance could be explained with a possible reaction, 2GaN + 3/2O ₂→Ga₂O₃+ N₂, during the annealing process. It could be suggested that the newly formed oxide layer makes the depletion width more narrowing for the Ohmic contact with a tunneling junction. However, the thick oxide layer formed at the high O₂/N ₂ ratio might keep the hole carriers from transporting to the electrode.