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.