Ultra thin tunnel oxides(SiOx) were grown on silicon, pretreated with 5% hydrofluoric acid, using high pressure low temperature oxidation. These oxides were characterised using variable illumination current-voltage (Voc - Jsc) measurements on semi-transparent metal gate MIS diodes. The open circuit voltage (Voc), short circuit current (Jsc), ideality factor (n) and reverse saturation current (Jo) are studied as a function of oxidation time. The interface state density Dits for the HF treated sample was found to decrease from 2 × 1012 cm-2eV-1 to 9.21 × 1011 cm-2eV-1. Highly reproducible good quality ultra thin oxides were obtained by pre-treatment of the wafer in hydrofluoric acid. © 1992 SPIE. All rights reserved.