Optical Anisotropy in van der Waals materials: Impact on Direct Excitation of Plasmons and Photons by Quantum Tunneling

Inelastic quantum mechanical tunneling of electrons across plasmonic tunnel junctions can lead to surface plasmon polariton (SPP) and photon emission. So far, the optical properties of such junctions have been controlled by changing the shape, or the type of the material, of the electrodes, primarily with the aim to improve SPP or photon emission efficiencies. Here we show that by tuning the tunneling barrier itself, the efficiency of the inelastic tunneling rates can be improved by a factor of 3. We exploit the anisotropic nature of hexagonal boron nitride (hBN) as the tunneling barrier material in Au//hBN//graphene tunnel junctions where the Au electrode also serves as a plasmonic strip waveguide. As this junction constitutes an optically transparent hBN–graphene heterostructure on a glass substrate, it forms an open plasmonic system where the SPPs are directly coupled to the dedicated strip waveguide and photons outcouple to the far field. We experimentally and analytically show that the photon emission rate per tunneling electron is significantly improved (~ ×3) in Au//hBN//graphene tunnel junction due to the enhancement in the local density of optical states (LDOS) arising from the hBN anisotropy. With the dedicated strip waveguide, SPP outcoupling efficiency is quantified and is found to be ∼ 80% stronger than the radiative outcoupling in Au//hBN//graphene due to the high LDOS of the SPP decay channel associated with the inelastic tunneling. The new insights elucidated here deepen our understanding of plasmonic tunnel junctions beyond the isotropic models with enhanced LDOS.

Device Simulation of The GeSe Homojunction And vdW GeSe/GeTe Heterojunction TFETs For High-Performance Application

Compared with a 2D homogeneous channel, the introduction of a 2D/2D homojunction or heterojunction is a promising method to promote the performance of a TFET mainly by controlling the tunneling barrier. We simulate the 10-nm-Lg double-gated GeSe homojunction TFETs and vdW GeSe/GeTe heterojunction TFETs using the ab initio quantum transport calculations. Two constructions are considered for both the homojunction and heterojunction TFETs by placing the BL GeSe and vdW GeSe/GeTe heterojunction as the source or drain while the channel and the remaining drain or source use ML GeSe. The on-state current (Ion) of the optimal n-type BL-ML GeSe source homojunction TFET and the optimal p-type vdW GeSe/GeTe drain heterojunction TFET are 2320 and 2387 μA μm-1, respectively, which are 50% and 64% larger than Ion of the ML GeSe homogeneous TFET. Inspiringly, the device performances (Ion, intrinsic delay time τ, and power delay product PDP) of both the optimal n-type GeSe homojunction and p-type vdW GeSe/GeTe heterojunction TFETs meet the requirement of the International Roadmap for Device and Systems high-performance device for the year of 2034 (2020 version).

Effect of Ge Mole Fraction on Performance of Underlapped Gate-All-Around SiGe-Source Tunneling Field-Effect Transistors

In this paper, we propose the design optimization of underlapped Si1–xGex-source tunneling field-effect transistors (TFETs) with a gate-all-around structure. The band-to-band tunneling rates, tunneling barrier widths, I–V transfer characteristics, threshold voltages, on/off current ratios, and subthreshold swings (SSs) were analyzed by varying the Ge mole fraction of the Si1–xGex source using a commercial device simulator. In particular, a Si0.2Ge0.8-source TFET among our proposed TFETs exhibits an on/off current ratio of approximately 1013, and SS of 27.4 mV/dec.

Nanomechanical Effects

This chapter discusses the effect of force and deformation of the tip apex and the sample surface in the operation and imaging mechanism of STM and AFM. Because the contact area is of atomic dimension, a very small force and deformation would generate a large measurable effect. Three effects are discussed. First is the stability of the STM junction, which depends on the rigidity of the material. For soft materials, hysterisis is more likely. For rigid materials, the approaching and retraction cycles are continuous and reproducible. Second is the effect of force and deformation to the STM imaging mechanism. For soft material such as graphite, force and deformation can amplify the observed corrugation. For hard materials as most metals, force and deformation can decrease the observed corrugation. Finally, the effect of force and deformation on tunneling barrier height measurements is discussed.

Hetero - Gate Oxide Dual Metal Vertical Tunnel FET

A Silicon based Vertical Dual metal Double gate Tunnel FET (Si-VDMDGTFET) has been proposed and simulated in Sentaurus TCAD tool with improved DC and analog/RF characteristics. The vertical In-line tunneling dominates in the proposed device which results in better subthreshold slope (SS). The vertical in-line tunneling tunes the tunneling barrier and eventually controls the ON current. The dual metal gate and the heterogeneous gate stack oxide within the proposed device design gives the mouldability for controlling and improving the DC characteristics such as ON current, OFF current. The analog/RF behaviour of the proposed device has been calculated and compared with conventional lateral Silicon based dual metal double gate Tunnel FET furthermore it is seen that the proposed device outperforms the conventional lateral device.