Spin splitting in 2D monochalcogenide semiconductors

TL;DR

This study uses ab initio calculations to analyze spin splitting in layered monochalcogenide semiconductors, revealing how symmetry and stacking influence spin properties relevant for spin relaxation times.

Contribution

It provides detailed insights into the spin splitting behavior in various polytypes of GaS, GaSe, GaTe, and InSe, highlighting the effects of symmetry and layer number.

Findings

Spin splitting is finite near the Γ-point in noncentrosymmetric structures.

Centrosymmetric bulk and even-layer structures exhibit zero spin splitting.

Spin relaxation times may be longer in these materials compared to GaAs.

Abstract

We report ab initio calculations of the spin splitting of the uppermost valence band (UVB) and the lowermost conduction band (LCB) in bulk and atomically thin GaS, GaSe, GaTe, and InSe. These layered monochalcogenides appear in four major polytypes depending on the stacking order, except for the monoclinic GaTe. Bulk and few-layer $\epsilon$- and $\gamma$-type, and odd-number $\beta$-type GaS, GaSe, and InSe crystals are noncentrosymmetric. The spin splittings of the UVB and the LCB near the $\Gamma$-point in the Brillouin zone are finite, but still smaller than those in a zinc-blende semiconductor such as GaAs. On the other hand, the spin splitting is zero in centrosymmetric bulk and even-number few-layer $\beta$-type GaS, GaSe, and InSe, owing to the constraint of spatial inversion symmetry. By contrast, GaTe exhibits zero spin splitting because it is centrosymmetric down to a single layer. In these monochalcogenide semiconductors, the separation of the non-degenerate conduction and valence bands from adjacent bands results in the suppression of Elliot-Yafet spin relaxation mechanism. Therefore, the electron- and hole-spin relaxation times in these systems with zero or minimal spin splittings are expected to exceed those in GaAs when the D'yakonov-Perel' spin relaxation mechanism is also suppressed.