Indium Gallium Oxide Alloys: Electronic Structure, Optical Gap, Surface Space Charge, and Chemical Trends within Common-Cation Semiconductors.

Tim D Veal,Joel B Varley,Marius Grundmann,Anna Hassa,Jack E N Swallow,Michael Lorenz,Philip A E Murgatroyd,Robert G Palgrave,Holger Von Wenckstern,Anna Regoutz

doi:10.1021/acsami.0c16021

Abstract

The electronic and optical properties of (InxGa1-x)2O3 alloys are highly tunable, giving rise to a myriad of applications including transparent conductors, transparent electronics, and solar-blind ultraviolet photodetectors. Here, we investigate these properties for a high quality pulsed laser deposited film which possesses a lateral cation composition gradient (0.01 ≤ x ≤ 0.82) and three crystallographic phases (monoclinic, hexagonal, and bixbyite). The optical gaps over this composition range are determined, and only a weak optical gap bowing is found (b = 0.36 eV). The valence band edge evolution along with the change in the fundamental band gap over the composition gradient enables the surface space-charge properties to be probed. This is an important property when considering metal contact formation and heterojunctions for devices. A transition from surface electron accumulation to depletion occurs at x ∼ 0.35 as the film goes from the bixbyite In2O3 phase to the monoclinic β-Ga2O3 phase. The electronic structure of the different phases is investigated by using density functional theory calculations and compared to the valence band X-ray photoemission spectra. Finally, the properties of these alloys, such as the n-type dopability of In2O3 and use of Ga2O3 as a solar-blind UV detector, are understood with respect to other common-cation compound semiconductors in terms of simple chemical trends of the band edge positions and the hydrostatic volume deformation potential.

Highlights

Indium oxide (In2O3) is the material of choice for applications requiring transparent conductive electrodes because of its propensity for n-type dopability, providing great electrical performance, and its large energy gap (∼2.9 eV fundamental band gap and ∼3.7 eV for the first dipole-allowed transition1), giving high optical transparency.[2]
The evolution of the electronic structure of a variable composition (InxGa1−x)2O3 alloy has been determined by using a combination of combinatorial X-ray photoemission spectroscopy (XPS), measurements of the optical gap, and state-of-the-art hybrid functional density functional theory (DFT) calculations
We show the simple trends regarding the evolution of the valence band edges in (InxGa1−x)2O3

Summary

Introduction

Indium oxide (In2O3) is the material of choice (most often doped with Sn) for applications requiring transparent conductive electrodes because of its propensity for n-type dopability, providing great electrical performance, and its large energy gap (∼2.9 eV fundamental band gap and ∼3.7 eV for the first dipole-allowed transition1), giving high optical transparency.[2]. An intermediate hexagonal InGaO3 phase has been reported, which has a symmetric In site and a trigonal-bipyramidal Ga site (Figure 1b).[8−10] It is not yet well explored how these different structures affect the surface electronic properties of these materials. This fundamental understanding is of great importance for the development of new electronic devices and Received: September 8, 2020 Accepted: December 23, 2020 Published: January 11, 2021

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Results

Conclusion