Ultrawide Bandgap Semiconductors for Photonic Applications: Recent Advances in Epitaxial Ga₂O₃, hBN, and ScAlN

The epitaxial growth of semiconductor materials plays a pivotal role in photonic applications by enabling precise control over material composition and facilitating flexible heterogeneous integration. Sophisticated epitaxial techniques have been extensively developed for mature, narrow-bandgap semiconductor platforms such as silicon (Si) and indium phosphide (InP), which have laid the foundation for photonic integrated circuits (PICs) used in data centers and optical communication systems. In contrast, the epitaxial growth of emerging ultrawide-bandgap (UWBG) semiconductors and the exploration of their potential for photonic applications remain an active area of research. This review summarizes recent progress in the epitaxial growth, optical properties, and photonic applications of three representative UWBG semiconductors: gallium oxide (Ga₂O₃), hexagonal boron nitride (hBN), and scandium aluminum nitride (ScAlN). For each material, we review state-of-the-art epitaxial growth techniques, optical properties across linear, nonlinear, and quantum optical regimes, and unique application opportunities that arise from these properties. This review aims to provide a timely broadband resource for researchers interested in advancing the field of epitaxial UWBG semiconductors for photonics.