Conduction Mechanisms at Interface of AlN/SiN Dielectric Stacks with AlGaN/GaN Heterostructures for Normally-off High Electron Mobility Transistors: Correlating Device Behavior with Nanoscale Interfaces Properties.

In this work, the conduction mechanisms at the interface of AlN/SiN dielectric stacks with AlGaN/GaN heterostructures have been studied combining different macroscopic and nanoscale characterizations on bare materials and devices. The AlN/SiN stacks grown on the recessed region of AlGaN/GaN heterostructures have been used as gate dielectric of hybrid metal-insulator-semiconductor high electron mobility transistors (MISHEMTs), showing a normally-off behavior (Vth = +1.2 V), high channel mobility (204 cm2 V-1 s-1), and very good switching behavior (ION/IOFF current ratio of (5-6) × 108 and subthreshold swing of 90 mV/dec). However, the transistors were found to suffer from a positive shift of the threshold voltage during subsequent bias sweeps, which indicates electron trapping in the dielectric stack. To get a complete understanding of the conduction mechanisms and of the charge trapping phenomena in AlN/SiN films, nanoscale current and capacitance measurements by conductive atomic force microscopy (C-AFM) and scanning capacitance microscopy (SCM) have been compared with a macroscopic temperature-dependent characterization of gate current in MIS capacitors. The nanoscale electrical analyses showed the presence of a spatially uniform distribution of electrons trapping states in the insulator and the occurrence of a density of 7 × 108 cm-2 of local and isolated current spots at high bias values. These nanoscale conductive paths can be associated with electrically active defects responsible for the trap-assisted current transport mechanism through the dielectric, observed by the temperature-dependent characterization of the gate current. The results of this study can be relevant for future applications of AlN/SiN bilayers in GaN hybrid MISHEMT technology.