Stress induced semiconductor to semimetal transition and tunable optical anisotropy in puckered bilayer GeTe: a first-principles study

In this study, Density functional theory (DFT) calculations were employed to investigate a comparative analysis of structural, electronic, chemical bonding, phonon-dynamical, and optical properties of puckered monolayer and bilayer GeTe under applied tensile and compression stress ranging from − 1 to + 3 GPa. The indirect band gap is reduced by 76% compared to the monolayer (0.693 eV) in case of bilayer formation (0.166 eV). Under increasing biaxial compressive stress, the bilayer band gap progressively decreases and reaches a calculated zero-gap semimetallic state at approximately 0.55 GPa, characterized by band touching near the Γ point and the system is dynamically stable up to at least 3GPa, which was confirmed by phonon calculations. Based on Mulliken population analysis, the microscopic origin of gap closure is found to be progressive orbital rehybridization of Ge-Te bond. Compression is systematically red-shifted and amplified broadband optical absorption in direction dependent hierarchy inversion, leading to a mechanically tunable three-dimensional optical anisotropy. The results unveil puckered bilayer GeTe as a stress tunable platform for narrow-gap optoelectronics and pressure engineered quantum device architectures.