Evaluation of the Initial Stage of Formation of Ti/Al Ohmic Contacts Using Photoresponse Method

Ti/Al contacts formed on n-GaN and AlGaN/GaN layers upon annealing at temperatures below the melting point of Al were evaluated by photoresponse (PR), current–voltage (I–V), and secondary ion mass spectroscopy (SIMS) measurements. In the PR spectra, the photocurrent based on the internal photoemission was detected, and the Schottky barrier height (qφ_B) was determined for all the samples, even though the I–V characteristics were very leaky. The AlGaN/GaN samples had constant qφ_B values of 0.85–1.0 eV independent of the annealing temperature. In contrast, the n-GaN samples had very low qφ_B values of 0.2 eV under the as-deposited condition. Upon annealing, qφ_B significantly increases and finally reaches almost the same values as those of the AlGaN/GaN samples. This can be explained by the change from Ti- to Al-rich interfaces in conjunction with the SIMS results.