Ultrafast electron transport in layered semiconductors studied with femtosecond-laser photoemission

Femtosecond-laser photoemission was used to investigate the electron dynamics in the layered semiconductors MoSe2 and WSe2. Photoexcitation with 200-fs pulses of 2.03 eV light creates an electron gas with significant excess energy. Our measurements reveal a strong transient enhancement in the diffusive transport of the most energetic electrons relative to the conduction-band minimum. Additionally, we demonstrate that the surfaces of these layered chalcogenides are electronically passivated and we give an upper bound for the density of defect states within the band gap. @S0163-1829~97!02443-0# A detailed understanding of transient hot-electron effects on carrier transport in semiconductors is essential for various problems in modern physics and technology. It has been shown that carrier drift velocities and diffusivities can ‘‘overshoot’’ or exceed their equilibrium values whenever injected electrons are highly correlated in space or time. 1,2 The fundamental time scale for many of these transient transport processes is in the picosecond time regime. Even in the absence of external electric fields, optical techniques can be used to create a hot nonequilibrium carrier distribution. While the mechanism of generating hot carriers by photoexcitation is quite different from heating by an electric field, the same relaxation and scattering processes apply in both cases. Thus the advent of ultrafast lasers has made it possible to probe these dynamic processes directly in the time domain on a femtosecond scale. Among time-resolved techniques, time-resolved photoemission spectroscopy is unique in that it directly measures the temporal evolution of photoexcited