The interfacial and electrical properties of RF sputtered La2O3 on p-GaAs substrates with and without ultrathin Si interface passivation layer (IPL) are reported. X-ray photoelectron spectroscopy (XPS) studies revealed the formation of thermally stable interfacial native oxide (Ga-oxide and As-oxide) for La2O3/Si/p-GaAs gate stacks. The presence of (La2O3)(1-x)(SiO2)(x) at the interface prevented the formation of La-hydroxide which improved the metal-oxide-semiconductor (MOS) device characteristics, such as interface state density (similar to 1.2x10(12)eV(-1)cm(-2)), frequency dispersion (similar to 7%), and hysteresis voltage (similar to 260 mV). A gate leakage current density of 1.1x10(-5) A.cm(-2) has been achieved at V-fb-1 V for an equivalent oxide thickness of 3.4 nm and small flat-band voltage instability under constant voltage stressing was observed. The electrical properties of Al/La2O3/Si/p-GaAs MOS capacitors were found to improve after post deposition annealing at 500 degrees C. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.072202jes] All rights reserved.