  In monolayer transition metal dichalcogenides helicity-dependent charge and
spin photocurrents can emerge, even without applying any electrical bias, due
to circular photogalvanic and photon drag effects. Exploiting such circular
photocurrents (CPC) in devices, however, requires better understanding of their
behavior and physical origin. Here, we present symmetry, spectral, and
electrical characteristics of CPC from excitonic interband transitions in a
MoSe2 monolayer. The dependence on bias and gate voltages reveals two different
CPC contributions, dominant at different voltages and with different dependence
on illumination wavelength and incidence angles. We theoretically analyze
symmetry requirements for effects that can yield CPC and compare these with the
observed angular dependence and symmetries that occur for our device geometry.
This reveals that the observed CPC effects require a reduced device symmetry,
and that effects due to Berry curvature of the electronic states do not give a
significant contribution.
