Proving Optical Anisotropy and Polarization Effects in β- Ga 2 O 3 $\left(\text{Ga}\right)_{2} \left(\text{O}\right)_{3}$ Nanomembranes via X-Ray Excited Optical Luminescence

Monoclinic β-${\text{Ga}}_2{O}_3$ is a key representative material of the ultrawide-bandgap semiconductor family. The distinct atomic arrangement in β-${\text{Ga}}_2{O}_3$ introduces two coordination environments for Ga ions, resulting in pronounced anisotropy in its optical, electronic, and thermal properties. In this study, a synchrotron nanoprobe to investigate the anisotropic optical properties of well-oriented $\left(100\right)$ β-${\text{Ga}}_2{O}_3$ nanomembranes with a thickness of $200$ nm, produced through mechanical exfoliation, is employed. Polarization-resolved X-ray excited optical luminescence (XEOL) measurements reveal a strong ultraviolet (UV) emission band at $3.4$ eV, which is strongly polarized along the c-axis. Additionally, XEOL data show blue ($2.9$ eV) and deep-UV ($3.8$ eV) emissions. Notably, the deep-UV band, rarely reported in conventional photoluminescence studies, is attributed to the presence of Ga vacancies, as supported by first-principles calculations. Polarization-dependent X-ray absorption near-edge structure (XANES) spectroscopy allows one to probe the distinct symmetries of the b and c crystallographic planes. Furthermore, by combining XANES and XEOL, this study investigates the site-specific contributions of Ga ions to the luminescence process. These findings highlight the potential of β-${\text{Ga}}_2{O}_3$ nanomembranes as a robust material platform for developing polarization-sensitive devices. The pronounced anisotropy of β-${\text{Ga}}_2{O}_3$ causes orientation-dependent optoelectronic properties, making it a highly promising candidate for a wide range of advanced applications.