The GaSb-on-GaAs growth was optimized for the fabrication of metal-oxide-semiconductor (MOS) capacitors (Caps) with low interface state trap density (D-it) using in-situ deposited amorphous silicon (a-Si) interface passivation layer (IPL) and high-k oxides. The best top surface with the average roughness R-a = 0.37 nm and with spiral type "step-flow" growth mode was observed in the GaSb structure with the initial 0.5 mu m grown at 410 degrees C and the top 0.5 mu m grown at 485 degrees C. N- and p-type GaSb MOSCaps with reasonable capacitance-voltage(C-V) characteristics at room temperature (RT) were demonstrated using all in-situ 0.5 nm a-Si IPL and 10 nm Al2O3+ HfO2 or Al2O3. A-Si IPL was found essential for n-MOSCaps but not in the case of p-MOSCaps where comparable C-V characteristics with a similarly low D-it = 1-2 x 10(12) cm(-2) eV(-1)were demonstrated without IPL. (c) 2013 Elsevier B.V. All rights reserved.