Proving optical anisotropy and polarization effects in β-Ga2O3 nanomembranes via X-Ray excited optical luminescence

Abstract

Monoclinic beta-Ga2O3 is a key representative material of the ultrawide-bandgap semiconductor family. The distinct atomic arrangement in beta-Ga2O3 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 (100) beta-Ga2O3 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 beta-Ga2O3 nanomembranes as a robust material platform for developing polarization-sensitive devices. The pronounced anisotropy of beta-Ga2O3 causes orientation-dependent optoelectronic properties, making it a highly promising candidate for a wide range of advanced applications