Transient hot-carrier dynamics and intrinsic velocity saturation in monolayer MoS2

Abstract

Drift velocity saturation (at some characteristic value, ${v}_{d}^{\mathrm{sat}}$) is a critical process that limits the ultimate current-carrying capacity of semiconductors at high electric fields $(\ensuremath{\sim}{10}^{4}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{cm})$ . With the recent emergence of two-dimensional (2D) semiconductors, there is a need to understand the manner in which velocity saturation is impacted when materials are thinned to the monolayer scale. Efforts to determine ${v}_{d}^{\mathrm{sat}}$ are typically hampered, however, by self-heating effects that arise from undesirable energy loss from the active 2D layer to the dielectric substrate that supports it. In this work, we explore this problem for an important 2D semiconductor, namely monolayer molybdenum disulfide ($\mathrm{Mo}{\mathrm{S}}_{2}$). By applying a strategy of rapid (nanosecond duration), single-shot, pulsing, we are able to probe the true hot-carrier dynamics in this material, free of the influence of self-heating of its $\mathrm{Si}{\mathrm{O}}_{2}$ substrate. Our approach allows us to realize high current densities $(\ensuremath{\sim}\mathrm{mA}/\ensuremath{\mu}\mathrm{m})$ in the $\mathrm{Mo}{\mathrm{S}}_{2}$ layers, representing a significant enhancement over prior studies. We similarly infer values for the saturated drift velocity $({v}_{d}^{sat\phantom{\rule{4pt}{0ex}}}\ensuremath{\sim}5\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}{\mathrm{s}}^{\ensuremath{-}1})$ that are higher than those reported in earlier works, in which the influence of self-heating (and carrier injection into oxide traps) could not be excluded. In fact, our estimates for ${v}_{d}^{\mathrm{sat}}$ are somewhat close to the ideal velocity expected for normal (parabolic) semiconductors. Since a proper knowledge of this parameter is essential to the design of active electronic and optoelectronic devices, the insight into velocity saturation provided here should provide useful guidance for such efforts.