  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/MoS$_2$ 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 MoS$_2$ induces a large strain of ~5% in the
MoS$_2$ which, coupled with charge transfer, leads to degenerate doping of the
MoS$_2$ 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 MoS$_2$-In/Au alloy
interfaces, demonstrating contact resistance as low as 63 kohm-um. Our results
highlight that the MoS$_2$/Metal interface is sensitive to device fabrication
methods, and provides a universal strategy to characterize buried contact
interfaces involving 2D semiconductors.
