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

Abstract

A high-conductivity two-dimensional (2D) hole gas, analogous to the ubiquitous 2D electron gas, is desirable in nitride semiconductors for wide-bandgap p-channel transistors. We report the observation of a polarization-induced high-density 2D hole gas in epitaxially grown gallium nitride on aluminium nitride and show that such hole gases can form without acceptor dopants. The measured high 2D hole gas densities of about 5 × 1013 per square centimeters remain unchanged down to cryogenic temperatures and allow some of the lowest p-type sheet resistances among all wide-bandgap semiconductors. The observed results provide a probe for studying the valence band structure and transport properties of wide-bandgap nitride interfaces.

Wurtzite phonons and the mobility of a GaN/AlN 2D hole gas

“…(The hole population is enhanced at negative kx, which, given the negative group velocity for holes, implies a rightward current.) (c) Model mobility versus Hall measurements reported by Chaudhuri et al7 Dashed durves are also shown for the various scattering mechanisms alone (polar optical, acoustic, and extrinsic). Note: the model is obtained by a full solution incorporating all mechanisms simultaneously, not by a Matthiessen approximation of the component limitations.…”

Molecular Beam Epitaxy Growth of Large‐Area GaN/AlN 2D Hole Gas Heterostructures

“…This is analogous to the 2DEG in an Al(Ga)N/GaN heterostructure. In a previous work, we reported the observation of this undoped GaN/AlN 2DHG with one of the highest hole densities seen across all material systems and sheet resistances ranging from ∼6–10 kΩ sq −1 , a 3× improvement in room‐temperature conductivity over previous doped 2DHGs . We next analyze the effect of the growth conditions on the resultant 2DHG channel, identify the optimum condition, and show that large‐area 2DHG wafers can be obtained.…”

“…The lowest 2DHG sheet resistances measured were ≈6 kΩ sq −1 at 300 K and ≈2 kΩ sq −1 at 77 K. These are some of the highest conductivity hole channels reported so far in III‐nitrides. We previously reported the temperature‐dependent transport of similar 2DHGs, showing that their conductivity increases as the temperature is lowered, down to ≈1 kΩ sq −1 at 10 K. In recent reports, Poncé et al and Bader et al derived that the 300 K transport of the GaN/AlN 2DHG is dominated by acoustic phonons (AP) scattering. At the low temperatures of 77 K, phonons freeze out and extrinsic scattering mechanisms such as due to interface roughness (IR) dominate.…”

“…We have recently reported the observation of the long‐missing polarization‐induced 2DHG at the undoped GaN/AlN interface—the p‐type dual of the ubiquitous undoped Al(Ga)N/GaN 2DEG, which has three times a higher conductivity than the previous reported 2DHGs. These high‐density holes give us experimental access to the largely unknown transport physics in the valence band of GaN.…”

“…Technologically, it is a promising platform to enable high‐performance p‐channel transistors toward realizing wide‐bandgap complementary circuits. In the previous report, we had demonstrated the growth of these GaN/AlN structures on small pieces less than 1 cm 2 in size. Moving these growths to larger‐area wafers will accelerate research and process development efforts, similar to how the wafer‐scale uniform growth of the AlGaN/GaN 2DEGs was a step toward enabling a production‐suitable process for HEMTs .…”