Proving Optical Anisotropy and Polarization Effects in <i>β</i>‐Ga2O3$\left(\text{Ga}\right)_{2} \left(\text{O}\right)_{3}$ Nanomembranes via X‐Ray Excited Optical Luminescence

Monoclinic β ‐ is a key representative material of the ultrawide‐bandgap semiconductor family. The distinct atomic arrangement in β ‐ 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 β ‐ nanomembranes with a thickness of nm, produced through mechanical exfoliation, is employed. Polarization‐resolved X‐ray excited optical luminescence (XEOL) measurements reveal a strong ultraviolet (UV) emission band at eV, which is strongly polarized along the c ‐axis. Additionally, XEOL data show blue ( eV) and deep‐UV ( 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 ‐ nanomembranes as a robust material platform for developing polarization‐sensitive devices. The pronounced anisotropy of β ‐ causes orientation‐dependent optoelectronic properties, making it a highly promising candidate for a wide range of advanced applications.