Probing defect states in few-layer MoS2 by conductance fluctuation spectroscopy

Despite the concerted effort of several research groups, a detailed experimental account of defect dynamics in high-quality single- and few-layer transition-metal dichalcogenides remains elusive. In this paper we report an experimental study of the temperature dependence of conductance and conductance fluctuations on few-layer MoS2 exfoliated on hexagonal boron nitride and covered by a capping layer of high-κ dielectric HfO2. The presence of the high-κ dielectric made the device extremely stable against environmental degradation as well as resistant to changes in device characteristics upon repeated thermal cycling, enabling us to obtain reproducible data on the same device over a timescale of more than 1 year. Our device architecture helped bring down the conductance fluctuations of the MoS2 channel by orders of magnitude compared to previous reports. The extremely low noise levels in our devices made it possible to detect the generation-recombination noise arising from charge fluctuation between the sulfur-vacancy levels in the band gap and energy levels at the conductance band edge. Our work establishes conduction fluctuation spectroscopy as a viable route to quantitatively probe in-gap defect levels in low-dimensional semiconductors.