Light Helicity Dependent Photocurrent in Layered Transition Metal Dichalcogenides

In this chapter, we will first explain how a spin-valley dependent photocurrent can be generated in monolayers of transition metal dichalcogenides (TMDs), particularly in terms of photogalvanic photocurrents and contrast it with other effects such as the photon drag and the Berry curvature. The nonlinear-in-electric field-dependent photocurrent data of 1L-TMD phototransistors will be introduced together with detailed data analysis, under various light and measurement geometries. The phenomenology has been already covered in Chap.  4 ; here, we will highlight the cases that deviate from the expectation. The microscopic origin of helicity dependent photocurrent in 1L-TMD is quite distinct from graphene and other similar 2D materials. More specifically, features like being a direct band gap semiconductor with strong spin-orbit coupling and broken inversion symmetry can be decisive in the second-order response dependent photocurrent contributions. This can be better understood by comparing the results for thicker TMDs to those with monolayers for excitation above and below the gap.