Isotope analysis of diamond-surface passivation effect of high-temperature H₂O-grown atomic layer deposition-Al₂O₃ films

The Al₂O₃ film formed using an atomic layer deposition (ALD) method with trimethylaluminum as Al precursor and H₂O as oxidant at a high temperature (450-°C) effectively passivates the p-type surface conduction (SC) layer specific to a hydrogen-terminated diamond surface, leading to a successful operation of diamond SC field-effect transistors at 400-°C. In order to investigate this excellent passivation effect, we carried out an isotope analysis using D₂O instead of H₂O in the ALD and found that the Al₂O₃ film formed at a conventional temperature (100-°C) incorporates 50 times more CH₃ groups than the high-temperature film. This CH₃ is supposed to dissociate from the film when heated afterwards at a higher temperature (550-°C) and causes peeling patterns on the H-terminated surface. The high-temperature film is free from this problem and has the largest mass density and dielectric constant among those investigated in this study. The isotope analysis also unveiled a relatively active H-exchange reaction between the diamond H-termination and H₂O oxidant during the high-temperature ALD, the SC still being kept intact. This dynamic and yet steady H termination is realized by the suppressed oxidation due to the endothermic reaction with H₂O. Additionally, we not only observed the kinetic isotope effect in the form of reduced growth rate of D₂O-oxidant ALD but found that the mass density and dielectric constant of D₂O-grown Al₂O₃ films are smaller than those of H₂O-grown films. This is a new type of isotope effect, which is not caused by the presence of isotopes in the films unlike the traditional isotope effects that originate from the presence of isotopes itself. Hence, the high-temperature ALD is very effective in forming Al₂O₃ films as a passivation and/or gate-insulation layer of high-temperature-operation diamond SC devices, and the knowledge of the aforementioned new isotope effect will be a basis for further enhancing ALD technologies in general.