Impact of Defects on Electronic and Optical Properties of 2D Germanium Selenide*

Germanium Selenide (GeSe) and other Group IV monochalcogenides are van der Waals-bonded layered materials with potential applications in sensing, solar energy and spintronics. Importantly, their band gaps can be tuned via strain, doping, and chemical modification. Due to the reduced dimensionality and reduced screening environment in the monolayer, these modifications have a significant impact on the optoelectronic properties. We apply density functional theory (DFT) and many-body perturbation theory to understand the electronic and optical properties of point vacancies in monolayer GeSe. We find that a Selenium vacancy in the -2 charge state induces mid-gap “trap states,” which strongly localize the electron and hole density. These trap states result in a sharp optical absorption peak below the predicted pristine optical gap and a localized exciton wavefunction around the defect. Overall, these results suggest that the vacancy is a strong perturbation to the system, demonstrating the importance of considering defects in the context of materials discovery and device design.