Layer- and Strain-Dependent Electronic Properties of GaSe: A First-Principles Study

Gallium selenide (GaSe) has recently attracted renewed interest as a two-dimensional semiconductor with a highly tunable band structure and strong layer-dependent optical response, making it a promising material for next-generation optoelectronic and photonic devices. Unlike more widely studied transition-metal dichalcogenides, GaSe exhibits multiple stable stacking sequences whose electronic properties remain insufficiently explored, particularly under external perturbations such as strain. In this study, we perform systematic first-principles calculations on monolayer to multilayer (up to seven layers) and bulk GaSe for the β, and ε polytypes. The evolution of the bandgap with increasing thickness and stacking order is analyzed to uncover indirect–direct transitions and interlayer coupling effects. Furthermore, we investigate the influence of in-plane strain on each stacking configuration, revealing how tensile and compressive strain can be used to precisely tailor the band alignment and optical gap. These results provide microscopic insight into how structural and mechanical control can be leveraged to optimize GaSe for tunable, high efficiency 2D optoelectronic applications.