Injection current in single-layer group IV-monochalcogenides

Single-layer group-IV monochalcogenides GeS, GeSe, SnS, SnSe are two-dimensional ferroelectrics predicted to exhibit giant second harmonic generation and photovoltaic shift current. Using density functional theory methods, we show that these materials also exhibit giant injection current (circular photogalvanic effect). The magnitude can reach values up to 10¹¹ A/V²S in the visible spectrum which is two orders of magnitude larger than that of prototypical nonlinear semiconductor CdS. We discuss the correlations between injection current, density of states and electric polarization using analytical and numerical methods. The injection current varies nonmonotonically with polarization reaching a maximum at an optimal polarization but is not correlated with the density of states. Our results establish GeS, GeSe, SnS, SnSe as versatile nonlinear materials suitable for optoelectronic applications.