Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

TLDR

Two‑dimensional electron gases in AlGaN/GaN heterostructures arise from strong polarization effects, and their sheet carrier concentration and confinement are highly sensitive to polarity, alloy composition, strain, thickness, and barrier doping. The study also examines how nonabrupt interfaces, dislocations, and surface states can reduce sheet carrier concentrations. We investigated undoped and silicon‑doped transistor structures using high‑resolution X‑ray diffraction, atomic force microscopy, Hall effect, and capacitance–voltage profiling, calculated polarization‑induced sheet charge from literature piezoelectric constants, and self‑consistently determined sheet carrier concentrations with a coupled Schrödinger–Poisson solver for pseudomorphically and partially relaxed barriers of varying alloy composition. Comparison of theoretical and experimental results demonstrates that two‑dimensional electron gases in both undoped and doped AlGaN/GaN structures depend on both piezoelectric and spontaneous polarization effects.

Abstract

Two dimensional electron gases in AlxGa1−xN/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance–voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined self-consistently from a coupled Schrödinger and Poisson equation solver for pseudomorphically and partially relaxed barriers with different alloy compositions. By comparison of theoretical and experimental results, we demonstrate that the formation of two dimensional electron gases in undoped and doped AlGaN/GaN structures rely both on piezoelectric and spontaneous polarization induced effects. In addition, mechanisms reducing the sheet carrier concentrations like nonabrupt interfaces, dislocations, and the possible influence of surface states on the two dimensional electron gases will be discussed briefly.