Persistent Room-Temperature Photodarkening in Cu-Doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>β</mml:mi><mml:mtext>−</mml:mtext><mml:msub><mml:mrow><mml:mi>Ga</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>
J. Jesenovec, C. Pansegrau, M. D. McCluskey, J. S. McCloy, T. D. Gustafson, L. E. Halliburton, J. B. Varley
Abstract
$\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ is an ultrawide band gap semiconductor with emerging applications in power electronics. The introduction of acceptor dopants yields semi-insulating substrates necessary for thin-film devices. In the present work, exposure of Cu-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ to UV light $>4\text{ }\text{ }\mathrm{eV}$ is shown to cause large, persistent photo-induced darkening at room temperature. Electron paramagnetic resonance spectroscopy indicates that light exposure converts ${\mathrm{Cu}}^{2+}$ to ${\mathrm{Cu}}^{3+}$, a rare oxidation state that is responsible for the optical absorption. The photodarkening is accompanied by the appearance of $\mathrm{O}─\mathrm{H}$ vibrational modes in the infrared spectrum. Hybrid function calculations show that Cu acceptors can favorably complex with hydrogen donors incorporated as interstitial (${\mathrm{H}}_{i}$) or substitutional (${\mathrm{H}}_{\mathrm{O}}$) defects. When ${\mathrm{Cu}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{H}}_{\mathrm{O}}$ complexes absorb light, hydrogen is released, contributing to the observed ${\mathrm{Cu}}^{3+}$ species and $\mathrm{O}─\mathrm{H}$ modes.