Two-dimensional tetragonal AlP monolayer: strain-tunable direct–indirect band-gap and semiconductor–metal transitions

Abstract

Recently, pristine group 13–15 monolayers have been attracting increasing interest. However, most of them have wide band-gaps, which have greatly limited their application in optoelectronic devices with photo response in the visible region. Based on first-principles calculations, we report a two-dimensional tetragonal (t-) AlP monolayer with a direct band-gap (1.56 eV). Furthermore, the intrinsic electron mobility of the t-AlP sheet can reach ∼10³ cm² V⁻¹ s⁻¹, higher than that of black phosphorus. Interestingly, the band-gap feature of t-AlP can be modulated by the strain and layer stacking order. Specifically, t-AlP exhibits direct-indirect gap and semiconductor–metal transitions at rather small biaxial strains. If synthesized, the t-AlP monolayer may have applications in designing tunable optoelectronic devices.