MgCaO films grown by rf plasma-assisted molecular beam epitaxy and capped with Sc2O3 are promising candidates as surface passivation layers and gate dielectrics on GaN-based high electron mobility transistors (HEMTs) and metal-oxide semiconductor HEMTs (MOS-HEMTs), respectively. Two different plasma chemistries were examined for etching these thin films on GaN. Inductively coupled plasmas Of CH4/H-2/Ar produced etch rates only in the range 20-70 angstrom/min, comparable to the Ar sputter rates under the same conditions. Similarly slow MgCaO etch rates (similar to 100 angstrom/min) were obtained with Cl-2/Ar discharges under the same conditions, but GaN showed rates almost an order of magnitude higher. The MgCaO removal rates are limited by the low volatilities of the respective etch products. The CH4/H-2/Ar plasma chemistry produced a selectivity of around 2 for etching the MgCaO with respect to GaN. (c) 2005 Elsevier B.V. All rights reserved.