Electron energy barriers between the valence band of (100)Si and the conduction bands of ultrathin Al2O3 and ZrO2 insulators grown by atomic-layer deposition were determined using internal photoemission of electrons. The barriers for Al2O3 and ZrO2 are found to be 3.25 +/-0.08 and 3.1 +/-0.1 eV, respectively, i.e. significantly lower than for SiO2 (4.25 +/-0.05 eV). Thermal oxidation at 650-800 degreesC strongly suppresses tunneling current through enhancement of the barriers at the Si/Al2O3 and Si/ZrO2 interfaces. However, it does not reduce the high density of band tail states in the insulators suggesting that silicates are formed. (C) 2001 Elsevier Science B.V. All rights reserved.