Band gap and band offset of Ga$_2$O$_3$ and (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys

TL;DR

This study uses density functional theory to analyze the band gaps and band offsets of Ga$_2$O$_3$ and (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys, providing key parameters for device design in UV photodetectors and transistors.

Contribution

The paper provides detailed computational analysis of the band structure and formation energies of (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys, including phase-dependent properties and implications for electronic devices.

Findings

Alloy formation enthalpies are lower than for similar In-based alloys.

(Al$_x$Ga$_{1-x}$)$_2$O$_3$ with x=0.5 can be considered as an ordered compound.

Band gaps range from 4.69 to 7.03 eV for monoclinic and 5.26 to 8.56 eV for corundum phases.

Abstract

Ga$_2$O$_3$ and (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys are promising materials for solar-blind UV photodetectors and high-power transistors. Basic key parameters in the device design, such as band gap variation with alloy composition and band offset between Ga$_2$O$_3$ and (Al$_x$Ga$_{1-x}$)$_2$O$_3$, are yet to be established. Using density functional theory with the HSE hybrid functional, we compute formation enthalpies, band gaps, and band edge positions of (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys in the monoclinic ($\beta$) and corundum ($\alpha$) phases. We find the formation enthlapies of (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys are significantly lower than of (In$_x$Ga$_{1-x}$)$_2$O$_3$, and that (Al$_x$Ga$_{1-x}$)$_2$O$_3$ with $x$=0.5 can be considered as an ordered compound AlGaO$_3$ in the monoclinic phase, with Al occupying the octahedral sites and Ga occupying the tetrahedral sites. The direct band gaps of the alloys range from 4.69 to 7.03 eV for $\beta$-(Al$_x$Ga$_{1-x}$)$_2$O$_3$ and from 5.26 to 8.56 eV for $\alpha$-(Al$_x$Ga$_{1-x}$)$_2$O$_3$. Most of the band offset of the (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloy arises from the discontinuity in the conduction band. Our results are used to explain the available experimental data, and consequences for designing modulation-doped field effect transistors (MODFETs) based on (Al$_x$Ga$_{1-x}$)$_2$O$_3$/Ga$_2$O$_3$ are discussed.