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

Abstract

We 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 similar to 800 degrees C under gas ambients of 0 (i.e., pure N-2), 0.1, 0.2, and 0.5 % O-2/N-2 ratio. A small amount Of O-2 addition shows much lower contact resistance than the pure N-2 only ambient while a high O-2/N-2 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 x 10(-3) ohm.cm(2) at the annealing temperature of 650 degrees C and a 0.1 % O-2/N-2 gas ambient. High resolution near edge X-ray absorption spectroscopy (NEXAFS) shows that more interstitial N-2 molecules were formed from the ITO/CTEL interfaces with increasing O-2/N-2 ratio. From the NEXAFS results, the high contact resistance could be explained with a possible reaction, 2GaN + 3/2O(2)-> Ga2O3 + N-2, 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 O-2/N-2 ratio might keep the hole carriers from transporting to the electrode.