Ferroelectric Rashba semiconductors, AgBiP2X6 (X = S, Se and Te), with valley polarization: an avenue towards electric and nonvolatile control of spintronic devices

Abstract

The electric and nonvolatile control of spin in semiconductors represents a fundamental step towards novel electronic devices. In this work, using first-principles calculations we investigate the electronic properties of AgBiP₂X₆ (X = S, Se, and Te) monolayers, which may be a new member of ferroelectric Rashba semiconductors due to the inversion symmetry breaking arising from the ferroelectric polarization, thus allowing for the electric control of spin. The AgBiP₂X₆ monolayers are dynamically and thermodynamically stable up to room temperature. In the AgBiP₂Te₆ monolayer, the calculated band structure reveals the direct band-gap semiconducting nature in the presence of highly mobile two-dimensional electron gas near the Fermi level. The inclusion of spin–orbit coupling yields the giant Rashba-type spin splitting with a Rashba parameter of 6.5 eV Å, which is even comparable to that of some known bulk Rashba semiconductors. Except for the Rashba-type spin splitting, spin–orbit coupling together with inversion symmetry breaking also gives rise to valley polarization located at the edge of the conduction bands. The strength of the Rashba-type spin splitting and location of the conduction band minimum can be significantly tuned by applying the in-plane biaxial strain. Also, we demonstrate that these remarkable features can be retained in the presence of the BN substrate. The coexistence of the Rashba-type spin splitting (in-plane spin direction) and band splitting at the K/K′ valleys (out-of-plane spin direction) makes the AgBiP₂Te₆ monolayer interesting for spintronics and valleytronics.