Bilayer h-BN Barriers for Tunneling Contacts in Fully-Encapsulated Monolayer MoSe₂ Field-Effect Transistors

The performance of electronic and spintronic devices based on two-dimensional semiconductors (2D SC) is largely dependent on the quality and resistance of the metal/SC electrical contacts, as well as preservation of the intrinsic properties of the SC channel. Direct Metal/SC interaction results in highly resistive contacts due to formation of large Schottky barriers and considerably affects the properties of the 2D SC. In this work, we address these two important issues in monolayer $\mathrm{MoSe_2}$ Field-Effect transistors (FETs). We encapsulate the $\mathrm{MoSe_2}$ channel with hexagonal Boron Nitride (h-BN), using bilayer h-BN at the metal/SC interface. The bilayer h-BN eliminates the metal/$\mathrm{MoSe_2}$ chemical interactions, preserves the electrical properties of $\mathrm{MoSe_2}$ and reduces the contact resistances by prevention of Fermi-level pinning. We investigate electrical transport in the monolayer $\mathrm{MoSe_2}$ FETs that yields close to intrinsic electron mobilities ($\approx 26\ \mathrm{cm^2 V^{-1} s^{-1}}$) even at room temperature. Moreover, we experimentally study the charge transport through Metal/h-BN/$\mathrm{MoSe_2}$ tunnel contacts and we explicitly show that the dielectric bilayer of h-BN provides highly efficient gating (tuning the Fermi energy) of the $\mathrm{MoSe_2}$ channel at the contact regions even with small biases. Also we provide a theoretical model that allows to understand and reproduce the experimental $I-V$ characteristics of the contacts. These observations give an insight into the electrical behavior of the metal/h-BN/2D SC heterostructure and introduce bilayer h-BN as a suitable choice for high quality tunneling contacts that allows for low energy charge and spin transport.