Velocity-field measurements in a GaN/AlN two-dimensional hole gas

We report measurements and an improved analysis methodology to characterize the velocity-field characteristics of a polarization-induced two-dimensional hole gas in a GaN/AlN heterostructure at both room and cryogenic temperatures, using pulsed voltage and current through a micrometer-scale constriction. These high-bias measurements are made possible by Ohmic contacts that remain sufficiently transparent (<50 Ω mm above 10 mA/mm current) at cryogenic temperatures. We observe a room temperature saturation velocity of (2.1 ± 0.2)×106 cm/s and associated mobility of ∼7–12 cm2/V s for a hole density of 4×1013 cm−2, which increases to (4.4 ± 0.4)×106 cm/s at 4.2 K, with an associated mobility of ∼25–50 cm2/V s. The measured ensemble hole saturation velocity in this geometry, which is suitable for field-effect transistor channels, is lower than that of holes measured in lightly-doped n-type 3D bulk GaN (6.63 ×106 cm/s), owing to the 2D geometry and high carrier density of the two-dimensional hole gas, and degraded hole mobility from recess etching. Measured velocity-field contours are corroborated against bulk density functional theory and two-dimensional full-band real-space cellular Monte Carlo simulations under different surface boundary conditions.