Ferroelectric Rashba semiconductors AgBiP2X6 (X = S, Se and Te) with valley polarization: An avenue towards electric and nonvolatile control of spintronic devices
The electric and nonvolatile control of spin in semiconductor represent a fundamental step toward 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 semiconductor 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 AgBiP2Te6 monolayer, the calculated band structure reveals the direct band-gap semiconductor nature with the presence of highly mobile two-dimensional electron gas near the Fermi level. Inclusion of spin-orbit coupling yields the giant Rashba-type spin splitting with a Rashba parameter of 6.5 eVÅ, 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 give rise to the valley polarization located at the edge of conduction bands. The strength of the Rashba-type spin splitting and location of conduction band minimum can be significantly tuned by applying in-plane biaxial strain. Also, we demonstrate that these remarkable features can be survived in the presence of BN substrate. The coexistence of Rashba-type spin splitting (in-plane spin direction) and band splitting at the K/K valleys (out-of-plane spin direction) makes AgBiP2Te6 monolayer interesting for spintonics and valleytronics.