Multifunctional Two-Dimensional VSi 2 N

Semiconducting monolayer $\mathrm{Mo}\mathrm{Si}$${}_{2}$$\mathrm{N}$${}_{4}$ and monolayer $\mathrm{WSi}$${}_{2}$$\mathrm{N}$${}_{4}$ have recently been fabricated and a number of two-dimensional (2D) materials with the same chemical formula and symmetry have been predicted, the mechanical, optical, and thermoelectric properties of which have been investigated. However, research regarding device applications of this 2D family have been very limited so far. Here, we propose a photodetector based on a 2D $\mathrm{VSi}$${}_{2}$$\mathrm{N}$${}_{4}$/$\mathrm{WSi}$${}_{2}$$\mathrm{N}$${}_{4}$/$\mathrm{VSi}$${}_{2}$$\mathrm{N}$${}_{4}$ lateral heterojunction, and investigate the transport properties of the photocurrent by using quantum transport simulations. The dependence of the photocurrent on the incident angle $\ensuremath{\alpha}$ and on the light-polarization angle $\ensuremath{\theta}$ is formulated. A robust photogalvanic effect is generated under the illumination of linearly polarized light due to the noncentrosymmetric ${C}_{2v}$ symmetry of the photodetector. The photocurrent has a $\mathrm{cos}(2\ensuremath{\theta})$ and $\mathrm{cos}(2\ensuremath{\alpha})$ dependence on the light polarization and on the incident angle, respectively. A pure spin current, a perfect spin-filtering effect, and an excellent spin-valve effect can be achieved at zero bias for a broad photon energy range with an appropriate polarization angle at both normal and horizontal incidence. Moreover, at horizontal incidence, the photocurrent is highly polarization sensitive with an extinction ratio of up to ${10}^{4}$. These results suggest that the 2D $\mathrm{WSi}$${}_{2}$$\mathrm{N}$${}_{4}$ family has promising multifunctional applications in low-power optoelectronics and spintronics.