The inherently absent 2-dimensional electron gas in ultra-pure GaN/AlGaN heterostructures

TL;DR

This study demonstrates that ultra-pure GaN/AlGaN heterostructures grown by molecular beam epitaxy lack an inherent 2D electron gas at room temperature, but UV illumination can induce conductivity, enabling novel device applications.

Contribution

We show that ultra-pure GaN/AlGaN layers do not naturally host a 2DEG at room temperature, contrary to prior assumptions, and that UV light can induce a 2DEG in these structures.

Findings

Absence of 2DEG in ultra-pure GaN/AlGaN at 300 K in dark conditions.

UV illumination induces a persistent 2DEG in these heterostructures.

Residual impurity levels below 10^17 cm^-3 are critical for this behavior.

Abstract

Gallium nitride (GaN) has emerged as an essential semiconductor material for energy-efficient lighting and electronic applications owing to its large direct bandgap of 3.4 eV. Present GaN/AlGaN heterostructures seemingly feature an inherently existing, highly-mobile 2-dimensional electron gas (2DEG), which results in normally-on transistor characteristics. Here we report on an ultra-pure GaN/AlGaN layer stack grown by molecular beam epitaxy, in which such a 2DEG is absent at 300 K in the dark, a property previously not demonstrated. Illumination with ultra-violet light however, generates a 2DEG at the GaN/AlGaN interface and the heterostructure becomes electrically conductive. At temperatures below 150 K this photo-conductivity is persistent with an insignificant dependence of the 2D channel density on the optical excitation power. Residual donor impurity concentrations below 10$^{17}$ cm$^{-3}$ in the GaN/AlGaN layer stack are one necessity for our observations. Fabricated transistors manifest that these characteristics enable a future generation of normally-off as well as light-sensitive GaN-based device concepts.