Tuning transport coefficients of monolayer MoSi_2N_4 with biaxial strain

Experimentally synthesized MoSi₂N₄ (Science 369 670 (2020)) is a piezoelectric semiconductor. Here, we systematically study the large biaxial (isotropic) strain effects (0.90-1.10) on electronic structures and transport coefficients of monolayer MoSi₂N₄ by density functional theory (DFT). With a/a₀ from 0.90 to 1.10, the energy band gap firstly increases, and then decreases, which is due to transformation of conduction band minimum (CBM). Calculated results show that the MoSi₂N₄ monolayer is mechanically stable in the considered strain range. It is found that the spin-orbital coupling (SOC) effects on Seebeck coefficient depend on the strain. In unstrained MoSi₂N₄, the SOC has neglected influence on Seebeck coefficient. However, the SOC can produce important influence on Seebeck coefficient, when the strain is applied, for example, 0.96 strain. The compressive strain can change relative position and numbers of conduction band extrema (CBE), and then the strength of conduction bands convergence can be enhanced, to the benefit of n-type ZT_e. Only about 0.96 strain can effectively improve n-type ZT_e. Our works imply that strain can effectively tune the electronic structures and transport coefficients of monolayer MoSi₂N₄, and can motivate farther experimental exploration.