  The semiconductor-metal junction is one of the most critical factors for high
performance electronic devices. In two-dimensional (2D) semiconductor devices,
minimizing the voltage drop at this junction is particularly challenging and
important. Despite numerous studies concerning contact resistance in 2D
semiconductors, the exact nature of the buried interface under a
three-dimensional (3D) metal remains unclear. Herein, we report the direct
measurement of electrical and optical responses of 2D semiconductor-metal
buried interfaces using a recently developed metal-assisted transfer technique
to expose the buried interface which is then directly investigated using
scanning probe techniques. We characterize the spatially varying electronic and
optical properties of this buried interface with < 20 nm resolution. To be
specific, potential, conductance and photoluminescence at the buried metal/MoS2
interface are correlated as a function of a variety of metal deposition
conditions as well as the type of metal contacts. We observe that direct
evaporation of Au on MoS2 induces a large strain of ~5% in the MoS2 which,
coupled with charge transfer, leads to degenerate doping of the MoS2 underneath
the contact. These factors lead to improvement of contact resistance to record
values of 138 kohm-um, as measured using local conductance probes. This
approach was adopted to characterize MoS2-In/Au alloy interfaces, demonstrating
contact resistance as low as 63 kohm-um. Our results highlight that the
MoS2/Metal interface is sensitive to device fabrication methods, and provides a
universal strategy to characterize buried contact interfaces involving 2D
semiconductors.
