Characterization of Chromium Impurities in $\beta$-Ga$_2$O$_3$

TL;DR

This paper thoroughly characterizes chromium impurities in $eta$-Ga$_2$O$_3$, revealing their optical properties, formation energetics, and potential as quantum defects in a promising wide-bandgap semiconductor.

Contribution

It combines theoretical calculations and experimental measurements to understand Cr behavior in Ga$_2$O$_3$, including luminescence and phonon coupling, which is novel for this material.

Findings

Cr preferentially incorporates as a neutral impurity on octahedral sites

Cr exhibits a zero-phonon line near 1.8 eV with a small Huang-Rhys factor of 0.05

Cr in Ga$_2$O$_3$ shows promise as a quantum defect for quantum technologies

Abstract

Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al$_2$O$_3$, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behavior of Cr in Ga$_2$O$_3$ through theoretical and experimental techniques. $\beta$-Ga$_2$O$_3$ samples are grown with the floating zone method and show evidence of a sharp photoluminescence signal, reminiscent of ruby. We calculate the energetics of formation of Cr from first principles, demonstrating that Cr preferentially incorporates as a neutral impurity on the octahedral site. Cr possesses a quartet ground-state spin and has an internal transition with a zero-phonon line near 1.8 eV. By comparing the calculated and experimentally measured luminescence lineshape function, we elucidate the role of coupling to phonons and uncover features beyond the Franck-Condon approximation. The combination of strong emission with a small Huang-Rhys factor of 0.05 and a technologically relevant host material render Cr in Ga$_2$O$_3$ attractive as a quantum defect.