Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

Abstract

Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}{\mathrm{N/GaN/Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ and N-face ${\mathrm{GaN/Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations $\text{(0.15<x<0.5)}$ and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from ${\text{6×10}}^{12}$ to ${\text{2×10}}^{13} {\mathrm{cm}}^{\mathrm{-2}}$ in the GaN channel with increasing Al-concentration from $\text{x=0.15}$ to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by piezoelectric and spontaneous polarization effects.