A polarization-induced 2D hole gas in undoped gallium nitride quantum wells

The long-missing polarization-induced two-dimensional hole gas is finally observed in undoped gallium nitride quantum wells. Experimental results provide unambiguous proof that a 2D hole gas in GaN grown on AlN does not need acceptor doping, and can be formed entirely by the difference in the internal polarization fields across the semiconductor heterojunction. The measured 2D hole gas densities, about $4 \times 10^{13}$ cm$^{-2}$, are among the highest among all known semiconductors and remain unchanged down to cryogenic temperatures. Some of the lowest sheet resistances of all wide-bandgap semiconductors are seen. The observed results provide a new probe for studying the valence band structure and transport properties of wide-bandgap nitride interfaces, and simultaneously enable the missing component for gallium nitride-based p-channel transistors for energy-efficient electronics.