Epitaxial engineering of polar ε-Ga2O3 for tunable two-dimensional electron gas at the heterointerface

We predict the formation of a polarization-induced two-dimensional electron gas (2DEG) at the interface of ε-Ga2O3 and CaCO3, wherein the density of the 2DEG can be tuned by reversing the spontaneous polarization in ε-Ga2O3, for example, with an applied electric field. ε-Ga2O3 is a polar and metastable ultra-wide band-gap semiconductor. We use density-functional theory (DFT) calculations and coincidence-site lattice model to predict the region of epitaxial strain under which ε-Ga2O3 can be stabilized over its other competing polymorphs and suggest promising substrates. Using group-theoretical methods and DFT calculations, we show that ε-Ga2O3 is a ferroelectric material where the spontaneous polarization can be reversed through a non-polar phase by using an electric field. Based on the calculated band alignment of ε-Ga2O3 with various substrates, we show the formation of a 2DEG with a high sheet charge density of 1014 cm−2 at the interface with CaCO3 due to the spontaneous and piezoelectric polarization in ε-Ga2O3, which makes the system attractive for high-power and high-frequency applications.