Inspection of Line Defects in Transition Metal Dichalcogenides Using a Microscopic Hyperspectral Imaging Technique

Direct Assessment of Defective Regions in Monolayer MoS2 Field-Effect Transistors through In Situ Scanning Probe Microscopy Measurements

Implementing two-dimensional materials in field-effect transistors (FETs) offers the opportunity to continue the scaling trend in the complementary metal-oxide-semiconductor technology roadmap. Presently, the search for electrically active defects, in terms of both their density of energy states and their spatial distribution, has turned out to be of paramount importance in synthetic transition metal dichalcogenides layers, as they are suspected of severely inhibiting these devices from achieving their highest performance. Although advanced microscopy tools have allowed the direct detection of physical defects such as grain boundaries and point defects, their implementation at the device scale to assess the active defect distribution and their impact on field-induced channel charge modulation and current transport is strictly restrained. Therefore, it becomes critical to directly probe the gate modulation effect on the carrier population at the nanoscale of an FET channel, with the objective to establish a direct correlation with the device characteristics. Here, we have investigated the active channel in a monolayer MoS2 FET through in situ scanning probe microscopy, namely, Kelvin probe force microscopy and scanning capacitance microscopy, to directly identify active defect sites and to improve our understanding of the contribution of grain boundaries, bilayer islands, and defective grain domains to channel conductance.

Resonant Coherent Raman Scattering from WSe2

Low-dimensional transition-metal dichalcogenides (TMDs) continue to comprise a subject of intense research because of their unique optical and electronic properties that may be harnessed in modern devices. Intense photoluminescence (PL) from few-/monolayer TMDs rendered PL-based micro- and nanospectroscopic characterization ideal in the quest to understand the correlation between structure and function in these materials. Nonlinear optical methods are by comparison far less utilized for this purpose. In this work, we describe an approach based on electronically resonant four-wave-mixing that allows spatio-spectral characterization of excitons in monolayer WSe₂. Due to the coherent nature of the response that we exploit to trace exciton resonances, and recent demonstrations of electronic four-wave-mixing-based nanoimaging and nanospectroscopy, our present work is an important step toward characterizing TMDs on the nano-femto scale using light.