# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1812.07942