As a wide gap semiconductor, Ga₂O₃ is rapidly emerging as a promising candidate for power electronics applications. While most of the studies have focused on its stable β-phase, there are a handful of reports on its metastable polar ε-phase having a spontaneous polarization. Numerous experimental groups have recently attempted to stabilize ε-Ga₂O₃ using epitaxial strain using substrates such as Al₂O₃(0001)¹, GaN(0001)¹, AlN(0001)¹, MgO(111)² and SiC(0001)³. However, due to the lack of an understanding of the stability of various Ga₂O₃ phases under epitaxial strain, these trial-and-error based attempts have been of limited success. All the films are observed to be of inherently poor quality. There are also diverging reports on the structure and properties of the deposited thin films. A recent experimental report of ε-Ga₂O₃ grown on Al₂O₃ substrate has even suggested the film to be ferroelectric, where the direction of the spontaneous polarization could be switched with an external electric field. It implies that the stabilization of ε-Ga₂O₃ will open new avenue of polarization engineering in Ga₂O₃ power electronics⁴.

We have used first-principles density-functional theory (DFT) calculations in combination with coincidence site lattice theory to develop a phase-diagram of Ga₂O₃ under epitaxial strain. We show that all the previously used substrates impose an epitaxial strain over 3% on ε-Ga₂O₃, which explains the poor structural quality of the deposited thin films. In this presentation, we will discuss promising commercially available substrates that can stabilize ε-Ga₂O₃ with epitaxial strain < 1 %. We will discuss the electronic structure of ε-Ga₂O₃ under epitaxial strain, including properties such as the band gap, polarization constants and its ferroelectric nature. We will theoretically demonstrate a way to achieve two-dimensional electron gas (2DEG) in ε-Ga₂O₃ heterostructure simply by using polarization engineering. The formation of 2DEG is due to the polar catastrophe, and the formation and sheet charge density 2DEG can be controlled by thickness of ε-Ga₂O₃ and its polarization direction. Our study highlights the potential of the polar behavior of ε-Ga₂O₃ and suggest pathway to realize its stabilize epitaxial growth.

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