Time-dependent dielectric breakdown of atomic-layer-deposited Al₂O₃ films on GaN

Atomic-layer-deposited (ALD) Al₂O₃ films are the most promising surface passivation and gate insulation layers in non-Si semiconductor devices. Here, we carried out an extensive study on the time-dependent dielectric breakdown characteristics of ALD-Al₂O₃ films formed on homo-epitaxial GaN substrates using two different oxidants at two different ALD temperatures. The breakdown times were approximated by Weibull distributions with average shape parameters of 8 or larger. These values are reasonably consistent with percolation theory predictions and are sufficiently large to neglect the wear-out lifetime distribution in assessing the long-term reliability of the Al₂O₃ films. The 63% lifetime of the Al₂O₃ films increases exponentially with a decreasing field, as observed in thermally grown SiO₂ films at low fields. This exponential relationship disproves the correlation between the lifetime and the leakage current. Additionally, the lifetime decreases with measurement temperature with the most remarkable reduction observed in high-temperature (450 °C) O₃-grown films. This result agrees with that from a previous study, thereby ruling out high-temperature O₃ ALD as a gate insulation process. When compared at 200 °C under an equivalent SiO₂ field of 4 MV/cm, which is a design guideline for thermal SiO₂ on Si, high-temperature H₂O-grown Al₂O₃ films have the longest lifetimes, uniquely achieving the reliability target of 20 years. However, this target is accomplished by a relatively narrow margin and, therefore, improvements in the lifetime are expected to be made, along with efforts to decrease the density of extrinsic Al₂O₃ defects, if any, to promote the practical use of ALD Al₂O₃ films.