Off-State Degradation and Recovery in Oxide/AlGaN/GaN Heterointerfaces: Importance of Band Offset, Electron, and Hole Trapping

The properties of the oxide/AlGaN heterointerface are investigated from field-dependent off-state degradation and recovery in thermally grown NiOx-, TiO2-, and Al2O3-based metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs). AI- and Ti-oxides form type-I straddling band alignment with positive and negative band offsets, respectively, and Ni-oxide forms type-II staggered band alignment with AlGaN. These oxides show promising results for high-performance HEMTs suitable for switching applications. The oxides are grown by rapid thermal oxidation of a thin film of the respective metals. The properties are quantified from the degradation and recovery of critical device parameters. The oxides largely follow the field-dependent degradation by electron and hole trapping. The critical fields for electron-hole pair generation and dominance of hole trapping over electron trapping have great significances in applied power electronics due to the presence of inherently large voltage transients. In addition to the quality of interfaces, oxide band alignment with AlGaN is important. The degradation and recovery are faster in Al2O3, indicating the presence of shallow traps and type-I band alignment. Even though NiOx shows the highest gate leakage current due to the type-II heterostructure, it shows negligible degradation in most of the parameters. The prestressed TiO2 devices show a performance similar to that of Al2O3 samples. Meanwhile, TiO2 shows the least degradation and endures the largest off-state voltage owing to the better heterointerfacial properties and type-I band alignment with negative band offsets. Both Al2O3 and TiO2 follow the field-dependent degradation due to electron trapping followed by hole trapping, which is concluded from the crossover in the threshold voltage for the formation of two-dimensional electron gas and confirmed by repeating the measurements for various gateto-drain separated devices. The device breakdown for the applied off-state step stress occurs at 120, 140, and 100 V for NiOx, TiO2, and Al2O3, respectively. The corresponding gate-connected field plate devices show breakdown voltages in an excess of 600 V.