  Two dimensional (2D) crystal heterostructures are shown to possess a unique
opportunity for novel THz nonlinear devices. In contrast to the oxide tunneling
barrier, the uniformity of 2D insulators in the thickness control provides an
ideal condition for tunneling barriers in the atomic scale. Numerical
calculations based on a first principles method clearly indicate the
feasibility of diode operation with barriers as thin as two monolayers of
hexagonal boron nitride or molybdenum disulfide when placed between
graphene-metal asymmetric electrodes. Further analysis predicts the cut-off
frequencies of the proposed device over 10 THz while maintaining strong
nonlinearity for zero-bias rectification. Application of the tunneling
structure to hot electron transistors is also investigated, illustrating the
THz operation with superior power performance. The proposed concept provides an
excellent opportunity for realizing active nonlinear devices in the frequency
range inaccessible thus far.
