Spectroscopic Ellipsometry Insights into Optical Response of MBE-Grown Epitaxial Nanoscale-Thick GaSe Films Toward Optoelectronic and Photocatalytic Applications

The large-scale epitaxial growth of two-dimensional (2D) layered materials with superior quality is essential for advancing next-generation device technologies. In this work, we demonstrate the controlled growth of high-quality nanometer thick GaSe films using molecular beam epitaxy (MBE) and systematically investigate their structural, chemical, and optical characteristics. Comprehensive analyses employing AFM, SEM, EDX, XRD, Raman spectroscopy and XPS were carried out to assess the morphology, crystallinity, composition and electronic states of the films. Particular emphasis was placed on exploring the optical response of GaSe thin films using spectroscopic ellipsometry (SE), a precise and nondestructive method for determining optical constants. The refractive index ( n ), extinction coefficient ( k ), and dielectric function (ϵ 1 and ϵ 2 ) were extracted across a wide photon energy range to examine their connection with interband transitions and excitonic features. Additionally, the absorption coefficient (α) and optical bandgap (E g ) were derived from SE data through Tauc analysis, confirming the direct bandgap behavior of GaSe. By correlating these optical parameters with the underlying electronic band structure, this study provides valuable insights into light–matter interactions in GaSe and highlights its promise for optoelectronic and photonic applications.