Molecular Beam Epitaxy of β-(In_<i>x</i>Ga_1–<i>x</i>)₂O₃on β-Ga₂O₃(010): Compositional Control, Layer Quality, Anisotropic Strain Relaxation, and Prospects for Two-Dimensional Electron Gas Confinement

Abstract

In this work, we investigate the growth of monoclinic β-(In_<i>x</i>Ga_1-<i>x</i>)₂O₃ alloys on top of (010) β-Ga₂O₃ substrates via plasma-assisted molecular beam epitaxy. In particular, using different <i>in situ</i> (reflection high-energy electron diffraction) and <i>ex situ</i> (atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy) characterization techniques, we discuss (i) the growth parameters that allow for In incorporation and (ii) the obtainable structural quality of the deposited layers as a function of the alloy composition. In particular, we give experimental evidence of the possibility of coherently growing (010) β-(In_<i>x</i>Ga_1-<i>x</i>)₂O₃ layers on β-Ga₂O₃ with good structural quality for <i>x</i> up to ≈ 0.1. Moreover, we show that the monoclinic structure of the underlying (010) β-Ga₂O₃ substrate can be preserved in the β-(In_<i>x</i>Ga_1-<i>x</i>)₂O₃ layers for wider concentrations of In (<i>x</i> ≤ 0.19). Nonetheless, the formation of a large amount of structural defects, like unexpected (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>10</mml:mn><mml:mover><mml:mn>2</mml:mn><mml:mi>̅</mml:mi></mml:mover></mml:math>) oriented twin domains and partial segregation of In is suggested for <i>x</i> > 0.1. Strain relaxes anisotropically, maintaining an elastically strained unit cell along the <i>a</i>* direction vs plastic relaxation along the <i>c</i>* direction. This study provides important guidelines for the low-end side tunability of the energy bandgap of β-Ga₂O₃-based alloys and provides an estimate of its potential in increasing the confined carrier concentration of two-dimensional electron gases in β-(In_<i>x</i>Ga_1-<i>x</i>)₂O₃/(Al_<i>y</i>Ga_1-<i>y</i>)₂O₃ heterostructures.