Graphene and hexagonal boron nitride heterostructures for beyond CMOS applications

Kang, Sangwoo
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Scaling limits of conventional complementary metal oxide semiconductor (CMOS) technology has motivated the research of numerous beyond CMOS device concepts. One such device is the interlayer tunnel FET (ITFET). This device is demonstrated using the two-dimensional (2D) materials bilayer graphene and hexagonal boron nitride (hBN). Stacking these materials together, we fabricate a double bilayer graphene and hBN heterostructure where the two graphene layers function as the top and bottom electrodes and the hBN as the tunnel barrier of the ITFET. Significant negative differential resistance (NDR) in the interlayer current-voltage characteristic is demonstrated at room temperature. Electrostatic analysis reveals that the multiple NDR peaks are due to energetic band alignment between the two sub-bands of the top and bottom bilayer graphene at the K-point of the Brillouin zone. Temperature dependent and parallel magnetic field measurements are conducted to further confirm that the conduction mechanism is momentum and energy conserving resonant tunneling. In addition, we demonstrate how the NDR can be used for implementing a one-transistor static random access memory element. Improvements in the transfer method which allowed rotationally aligned top and bottom electrode layers, made possible extensive experimental studies of 2D heterostructure based ITFETs. Utilizing such technique, we conducted experiments involving thicker multilayer graphene. We analyze how the graphene thickness and stacking order can influence the resonance condition and how this affects the overall device characteristic. The effects of interlayer hBN scaling such as increased tunneling current and peak position shifting is also briefly dealt with. Current-voltage simulations based on Bardeen transfer Hamiltonian approach were conducted for these devices, and it is shown that the peak positions predicted by theory match well with those obtained through measurements.