Kavli Affiliate: Debdeep Jena

| First 5 Authors: Megan Stokey, Rafal Korlacki, Matthew Hilfiker, Sean Knight, Steffen Richter

| Summary:

We determine the anisotropic dielectric functions of rhombohedral

$alpha$-Ga$_2$O$_3$ 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 $alpha$-Al$_2$O$_3$, the

binary parent compound for the emerging alloy of

$alpha$-(Al$_{x}$Ga$_{1-x}$)$_2$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.

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