Ultrahigh electron mobility induced by strain engineering in direct semiconductor monolayer Bi₂TeSe₂

The successful commercial applications as thermoelectric devices and, due to their exotic electronic properties, as topological insulators of bismuth telluride (Bi₂Te₃) and bismuth selenide (Bi₂Se₃) have stimulated research interest on Bi₂Se₃/Bi₂Te₃-based chemical compounds. Based on the first-principles calculations, we investigate the electronic, optical, vibrational and transport properties of new monolayer Bi₂TeSe₂ obtained by transmuting one Se atom into its neighboring Te atom in the same group from Bi₂Se₃. We find that the monolayer Bi₂TeSe₂ maintains a stable hexagonal structure up to 700 K. Monolayer Bi₂TeSe₂ possesses a direct bandgap of 0.29 eV due to the strong spin-orbit coupling effects, and it remains a direct semiconductor for strains in a moderate range. The optical absorption covers a wide range from the green region to the ultraviolet region, which may lead to applications in optoelectronic devices like saturable absorbers. An extremely high electron mobility of 20 678 cm² V^-1 s^-1 along the zigzag direction can be achieved by strain engineering with -6% compressive strain, which is nearly ten times larger than the intrinsic mobility. These indicate that monolayer Bi₂TeSe₂ is a promising candidate for future high-speed (opto)electronic devices.