Thermoelectric transport properties in Janus Rashba semiconductors of monolayer Si2AsSb and Si2SbBi

Abstract

2D Janus Rashba semiconductors, which break both the mirror symmetry in the crystal structure and the spin degeneracy in the energy band, provide a promising platform to optimize thermoelectric performance. Herein, we use first-principles and Boltzmann transport theory to investigate the electron and phonon transport properties for Janus semiconductors of monolayer Si2AsSb and Si2SbBi. The strong Rashba spin-splitting is found in both Janus monolayers especially for Si2SbBi, which decreases the bandgaps and makes the valence bands more dispersive, resulting in decreased p-type Seebeck coefficient and increased p-type electrical conductivity. The lattice thermal conductivities of both monolayers are not low due to the weak phonon anharmonicity, strong chemical bonding, and long phonon relaxation time. The low lattice thermal conductivity of Si2SbBi than Si2AsSb mainly originates from the low phonon group velocity. Both monolayers exhibit better thermoelectric performance in n-type than in p-type. The competition among Seebeck coefficient, electrical conductivity, and electronic thermal conductivity makes the difference of optimal thermoelectric figure of merits in n-type without and with Rashba spin–orbit coupling slight for Si2AsSb, but it is significant for Si2SbBi. Within Rashba spin–orbit coupling, the optimal figure of merits at 700 K reach 0.65 and 0.59 for Si2AsSb and Si2SbBi, respectively, which indicate the potential thermoelectric applications, and will stimulate the broad study on thermoelectric properties of 2D Janus Rashba semiconductors.