Bias stability enhancement of metal/Al2O3/AlN/GaN capacitors by high-temperature plasma-enhanced atomic layer deposition of thin AlN interlayer

AlN films formed on (0001) GaN by a plasma-enhanced atomic layer deposition (PEALD) method at 200 and 450 °C without remote-plasma pretreatment (RPP) were crystalline as formed and epitaxially aligned with the GaN substrate. Even a 0.7-nm AlN film formed by high-temperature (HT, 450 °C) PEALD suppressed the bias instability of Al/Al2O3/AlN/GaN capacitors to a practically negligible level. By contrast, the bias instability improvement for low-temperature (LT, 200 °C) PEALD was quite limited. Although RPP was not conducted, the interface-state density of the HT-PEALD capacitors was reduced to 1.4 × 1011 cm−2 eV−1 for AlN thicknesses of ≤1.5 nm, supposedly owing to the RPP effect inherent in HT-PEALD. By contrast, LT-PEALD AlN films generated a considerable number of interface states. The HT-PEALD AlN interlayer possessed a polarization charge of −7.7 × 1013 cm−2 in units of electronic charge, comparable to a reported value of −8.7 × 1013 cm−2. The flatband-voltage shift of an Al/Al2O3/0.7-nm AlN/GaN capacitor caused by this polarization was −1.2 V. Similar to previous results, the net insulator charge, which comprised interface-state and GaN-polarization charges, was considerably smaller in magnitude than the GaN polarization charge, indicating the presence of highly positive interface-state charge compensating for the negative GaN polarization charge. A physical model for the interface-state charge was proposed. As the thin HT-PEALD AlN interlayer effectively suppressed the bias instability of Al2O3/AlN/GaN capacitors, retaining a good interface with the GaN substrate and causing a small flatband-voltage shift, as aforementioned, it will be a key to realizing high-performance and high-reliability GaN metal-insulator-semiconductor field-effect transistors.