We have developed and tested the efficacy of a method for pre-atomic layer deposition (ALD) surface preparation that removes native oxides from the (100) In0.2Ga0.8As surface and provides a clean starting surface for ALD of ultrathin Al2O3 layers. Successive wet etching by aqueous HCl and NH4(OH) solutions and in situ pre-ALD thermal desorption of residual elemental As were performed. Photoelectron spectra obtained after ALD of Al2O3 on In0.2Ga0.8As prepared by this method revealed that the interface was free of In, Ga, and As oxides. The resultant metal-oxide-semiconductor capacitors with Pt electrodes exhibited capacitance-derived equivalent oxide thicknesses as small as 1.8nm.