Ultrafast Dynamics of Gallium Vacancy Charge States in $\beta$-Ga$_2$O$_3$

TL;DR

This study uses ultrafast supercontinuum spectroscopy to investigate the non-equilibrium charge state dynamics of gallium vacancies in $eta$-Ga$_2$O$_3$, revealing how defect charge states evolve on picosecond timescales.

Contribution

It introduces a novel ultrafast optical method to probe defect charge state dynamics in semiconductors, supported by first-principles calculations.

Findings

Defect absorption peaks at ~2.2 eV and ~1.63 eV are observed.

Absorption spectral weight shifts from lower to higher energy peaks over time.

Maximum defect absorption occurs with probe polarization along the crystal c-axis.

Abstract

Point defects in crystalline materials often occur in multiple charge states. Although many experimental methods to study and explore point defects are available, techniques to explore the non-equilibrium dynamics of the charge states of these defects at ultrafast (sub-nanosecond) time scales have not been discussed before. We present results from ultrafast optical-pump supercontinuum-probe spectroscopy measurements on $\beta$-Ga$_2$O$_3$. The study of point defects in $\beta$-Ga$_2$O$_3$ is essential for its establishment as a material platform for high-power electronics and deep-UV optoelectronics. Use of a supercontinuum probe allows us to obtain the time-resolved absorption spectra of material defects under non-equilibrium conditions with picosecond time resolution. The probe absorption spectra shows defect absorption peaks at two energies, $\sim$2.2 eV and $\sim$1.63 eV, within the 1.3-2.5 eV probe energy bandwidth. The strength of the absorption associated with each peak is time-dependent and the spectral weight shifts from the lower energy peak to the higher energy peak with pump-probe delay. Further, maximum defect absorption is seen for probe polarized along the crystal c-axis. The time-dependent probe absorption spectra and the observed dynamics for all probe wavelengths at all pump-probe delays can be fit with a set of rate equations for a single multi-level defect. Based on first-principles calculations within hybrid density functional theory we attribute the observed absorption features to optical transitions from the valence band to different charge states of Gallium vacancies. Our results demonstrate that broadband ultrafast supercontinuum spectroscopy can be a useful tool to explore charge states of defects and defect dynamics in semiconductors.