2, The University of Utah, Salt Lake City, Utah, United States
3, Washington University in St. Louis, St. Louis, Missouri, United States
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. Finally , the sheet-charge density and the electrical properties of the heterojunction of ε-Ga2O3 and the substrate will be discussed.
1. 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).
3. X. Xia, Y. Chen, Q. Feng, H. Liang, P. Tao, M. Xu and G. Du: Applied Physics Letters 108, 202103 (2016).
4. F. Mezzadri, G. Calestani, F. Boschi, D. Delmonte, M. Bosi and R. Fornari: Inorganic Chemistry 55, 12079 (2016).