Infrared dielectric functions and Brillouin zone center phonons of alpha-Ga2O3 compared to alpha-Al2O3

We determine the anisotropic dielectric functions of rhombohedral alpha-Ga2O3 by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high-frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for alpha-Al2O3, the binary parent compound for the emerging alloy of alpha-(AlxGa1-x)(2)O-3, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations, we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single-mode or more complex mode behaviors.
Funding Agencies|National Science Foundation (NSF)National Science Foundation (NSF) [NSF DMR 1808715, OIA-2044049]; Air Force Office of Scientific ResearchUnited States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0360, FA9550-19-S0003, FA9550-21-1-0259]; ACCESS, an AFOSR Center of Excellence [FA9550-18-1-0529]; Knut and Alice Wallenbergs FoundationKnut & Alice Wallenberg Foundation; University of Nebraska Foundation; J. A. Woollam Foundation; JSPS Overseas Challenge Program for Young Researchers [1080033]; Swedish Research Council VRSwedish Research Council [2016-00889]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Governmental Agency for Innovation Systems VINNOVA under the Competence Center Program [201605190]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University, Faculty Grant SFO Mat LiU [2009-00971]