Coexistence of doping and strain to tune electronic and optical properties of GaN monolayer

GaN monolayer is a prospective two-dimensional (2D) graphene-like material due to its excellent physical and chemical properties. Band gap engineering plays a crucial role for extending potential applications of GaN-based optoelectronic devices. We performed first-principles calculations to study the structural, electronic and optical properties of GaN monolayers by Al and In atoms doping with different concentrations. We found that Al atom doping can increase the band gap and result in the blue-shift with the increase of doping concentration, while In atom doping presents the opposite results. However, the band gap characteristics remains indirect which is unfavourable for the optical transition and solar energy conversion. Next, biaxial strains within magnitude range of -10% to 13% are applied in Al and In-doped GaN monolayers. Coexistence of doping and strain to tune electronic and optical properties of GaN monolayer are systematically studied for the first time. The results show the band gap increases at first then decreases under compressive strain and decreases monotonically under tensile strain. Moreover, the band gap characteristics changes from indirect to direct in the certain compressive strain. For the optical property modification, compressive strain can cause blue-shift in the whole energy range, while tensile strain can result in red-shift in the visible energy range and blue-shift at first then red-shift in the ultraviolet energy range. Our results confirm coexistence of doping and strain induced in GaN monolayer is an effective method in band gap engineering for the applications in tunable nano-optoelectronic devices.