Postdeposition annealing effect on the reliability of atomic-layer-deposited Al₂O₃films on GaN

Atomic-layer-deposited (ALD) Al2O3 is a promising gate insulation material for wide-bandgap semiconductor devices of increasing importance for high-speed and high-power switching operation. This study comprehensively reports on postdeposition annealing (PDA) effects on the reliability of ALD-Al2O3/GaN metal-insulator-semiconductor capacitors. High-temperature (450 °C) ALD for the Al2O3 growth was effective for reducing the bias instability (BI) of the capacitors and for suppressing the blisters caused by PDA in the Al2O3 films. The BI of the high-temperature Al2O3 capacitors was reduced more remarkably by PDA at high temperatures. The conduction current in the capacitors was also reduced by PDA at 800 °C and higher by 2 orders of magnitude. The high-temperature PDA, however, caused a positive flat-band voltage shift and increased the distribution of times to breakdown of the capacitors and the interface-state density from 1 × 1011 to 3 × 1012 cm-2 eV-1, causing the large frequency dispersion of C-V characteristics. This increase in the interface-state density was found to be the major cause of the aforementioned flat-band voltage shift by PDA, whereas the stress-induced flat-band voltage shift was mostly due to the negative Al2O3 charging. In agreement with the literature, transmission electron microscope observations demonstrated the crystallization of Al2O3 films by annealing at 800 °C and higher, ascribing the increased distribution of times to breakdown to some crystalline defects in the Al2O3 films. Hence, the best PDA effect was achieved at 700 °C, reducing BI by half in 200 °C operation. For this PDA, the breakdown lifetimes of capacitors were confirmed to be the same, if not longer, as those of unannealed samples even at an elevated temperature of 200 °C, achieving 300 years at the rating of 3 MV/cm, well over the reliability target of 20 years. To put the ALD-Al2O3/GaN capacitors into practical use, PDA possibly needs to be performed before contact formation and their BI should preferably be further improved, simultaneously carrying out its long-term projections.