Various rapid thermal annealing (RTA) conditions for the Ti/Al/Ta/Au ohmic contact process and the resulting surface analysis have been investigated to characterise AlGaN/GaN high-electron-mobility transistors (HEMTs) on Si (111) substrates. The use of Ta as an effective diffusion barrier in Ti/Al/metal/Au ohmic metallisation yields better ohmic contacts than other metal formations. To achieve low ohmic contact resistance (R-c) and good surface morphology, we tested different Ta layer thicknesses (20, 40,60, and 80 nm) in a deoxidation process with smooth O-2/H-2 plasma-etching pre-treatments comprised of five annealing temperatures (700,750,800, 850, and 900 degrees C) and two annealing times (15 and 30 s). The best ohmic resistance, 0.03 Omega mm (7.27 x 10(-7) Omega cm(2)), is obtained for a Ta barrier layer thickness of 40 nm, annealing temperature of 850 degrees C and annealing time of 30 s. In atomic force microscopy (AFM) images, nano-scale surface morphology with a root-mean-square (RMS) deviation of 6.9 nm is observed. Through comparative energy dispersive spectrometry (EDS) analysis of the surface morphologies at the bulge and sunken areas at 800 degrees C annealing temperature, we found that the surface degradation phenomenon is easily generated at 800 degrees C annealing temperature by a significant reaction of the Au-Al alloy. As a result, Au and Al are diffused into the Ta barrier layer, and a bulge and sunken are generated on the ohmic contact surface. From the EDS spectra, the Au ratio continually decreases from the optimised area (63%) to the sunken area (32%) due to the diffusion of Au through the Ta barrier layer. Conversely, the Au ratio continually increases from the optimised area (63%) to the bulged area (90%) due to the inability of Au to diffuse downward; Al diffuses upward and causes the Au bulge. (C) 2013 Elsevier Ltd. All rights reserved.