Ultrafast photocurrent and absorption microscopy of few-layer TMD devices isolate rate-limiting dynamics driving fast and efficient photoresponse

Abstract

Despite inherently poor interlayer conductivity, photodetectors made from few-layer devices of 2D transition metal dichalcogenides (TMDs) such as WSe${}_2$ and MoS${}_2$ can still yield a desirably fast ($\le$90 ps) and efficient ($\epsilon$$>$40\%) photoresponse. By combining ultrafast photocurrent (U-PC) and transient absorption (TA) microscopy, the competing electronic escape and recombination rates are unambiguously identified in otherwise complex kinetics. Both the U-PC and TA response of WSe${}_2$ yield matching interlayer electronic escape times that accelerate from 1.6 ns to 86 ns with applied $E$-field to predict the maximum device PC-efficiency realized of $\sim$44\%. The slope of the escape rates versus $E$-field suggests out-of-plane electron and hole mobilities of 0.129 and 0.031 cm${}^2$/V$s$ respectively. Above $\sim$10${}^{11}$ photons/cm${}^2$ incident flux, defect-assisted Auger scattering greatly decreases efficiency by trapping carriers at vacancy defects. Both TA and PC spectra identify a metal-vacancy sub-gap peak with $\sim$5.6 ns lifetime as a primary trap capturing carriers as they hop between layers. Synchronous TA and U-PC microscopy show the\ net PC collected is modelled by a kinetic rate-law of electronic escape competing against the linear and nonlinear Auger recombination rates. This simple rate-model further predicts the PC-based dynamics, nonlinear amplitude and efficiency, $\epsilon$ over a 10${}^5$ range of incident photon flux in few-layer WSe${}_2$ and MoS${}_2$ devices.

10 pages, 4 figures