Strain Engineering and Two-Dimensional Electron Gas in Polar ε-Ga2O3

As a wide gap semiconductor, Ga 2 O 3 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 -Ga 2 O 3 using epitaxial strain using substrates such as Al 2 O 3 (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 Ga 2 O 3 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 -Ga 2 O 3 grown on Al 2 O 3 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 -Ga 2 O 3 will open new avenue of polarization engineering in Ga 2 O 3 power electronics 4 . We have used first-principles density-functional theory (DFT) calculations in combination with coincidence site lattice theory to develop a phase-diagram of Ga 2 O 3 under epitaxial strain. We show that all the previously used substrates impose an epitaxial strain over 3% on ε -Ga 2 O 3 , 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 -Ga 2 O 3 with epitaxial strain < 1 %. We will discuss the electronic structure of ε -Ga 2 O 3 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 -Ga 2 O 3 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 -Ga 2 O 3 and its polarization direction. Our study highlights the potential of the polar behavior of ε -Ga 2 O 3 and suggest pathway to realize its stabilize epitaxial growth. Y. Oshima, E.G. Víllora, Y. Matsushita, S. Yamamoto and K. Shimamura: Journal of Applied Physics 118 , 085301 (2015). 2. H. Nishinaka, D. Tahara and M. Yoshimoto: Japanese Journal of Applied Physics 55 , 1202BC (2016). X. Xia, Y. Chen, Q. Feng, H. Liang, P. Tao, M. Xu and G. Du: Applied Physics Letters 108 , 202103 (2016). F. Mezzadri, G. Calestani, F. Boschi, D. Delmonte, M. Bosi and R. Fornari: Inorganic Chemistry 55 , 12079 (2016).