First-ever measurement of hole velocity vs. electric field in polarization-induced two-dimensional hole gases in GaN/AlN heterojunctions

We present the first-ever measurements of the velocity vs. electric field (v-E) characteristic for a polarization-induced two-dimensional hole gas (2DHG) in undoped gallium nitride (GaN) on metal-polar aluminum nitride (AlN) heterostructures. The v-E characteristic provides an essential benchmark of carrier transport in low-field (mobility) and high-field (saturation) regimes and places an intrinsic limit on tenable RF device performance. Our sample, grown by molecular beam epitaxy, has a high-density 2DHG at the GaN/AlN interface degenerately occupying both the heavy- and light-hole valence bands with densities of 3.1×10¹³/cm² and 0.4×10¹³/cm², respectively, as determined with magneto-transport measurements fit to a 2-carrier model. We measure v-E from 4.2 to 300K with a pulsed voltage input and two-point current and voltage (IV) measurement of a test structure with an etched constriction of micron-scale dimensions. The ensemble hole velocity and average electric field in the constriction are calculated from the measured IV, hole density, and constriction dimensions. We observe ensemble saturation velocities of 3.4×10⁶ cm/s at 4.2 K and 2.5×10⁶ cm/s at 300 K, 2-3x slower than previous photo-assisted measurements in undoped bulk GaN with four orders lower equivalent hole density.