Infrared dielectric functions and Brillouin zone center phonons of $α$-Ga$_2$O$_3$ compared to $α$-Al$_2$O$_3$

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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