Amithraj Valsaraj

Coherent Interlayer Tunneling and Negative Differential Resistance with High Current Density in Double Bilayer Graphene–WSe2 Heterostructures

We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe2. We observe large interlayer current densities of 2 and 2.5 μA/μm2 and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D-2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling. We demonstrate narrow tunneling resonances with intrinsic half-widths of 4 and 6 meV at 1.5 and 300 K, respectively.

Theoretical and experimental investigation of vacancy-based doping of monolayer MoS2 on oxide

Monolayer (ML) transition metal dichalcogenides are novel, gapped two-dimensional materials with unique electrical and optical properties. Toward device applications, we consider MoS2 layers on dielectrics, in particular in this work, the effect of vacancies on the electronic structure. In density-functional based simulations, we consider the effects of near-interface O vacancies in the oxide slab, and Mo or S vacancies in the MoS2 layer. Band structures and atom-projected densities of states for each system and with differing oxide terminations were calculated, as well as those for the defect-free MoS2-dielectrics system and for isolated dielectric layers for reference. Among our results, we find that with O vacancies, both the Hf-terminated HfO2–MoS2 system, and the O-terminated and H-passivated Al2O3–MoS2 systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoS2, in manner analogous to modulation doping. The n-type doping of ML MoS2 by high-k oxides with oxygen vacancies then is experimentally demonstrated by electrically and spectroscopically characterizing back-gated ML MoS2 field effect transistors encapsulated by oxygen deficient alumina and hafnia.

DFT simulations of inter-graphene-layer coupling with rotationally misaligned hBN tunnel barriers in graphene/hBN/graphene tunnel FETs

Van der Waals heterostructures allow for novel devices such as two-dimensional-to-two-dimensional tunnel devices, exemplified by interlayer tunnel FETs. These devices employ channel/tunnel-barrier/channel geometries. However, during layer-by-layer exfoliation of these multi-layer materials, rotational misalignment is the norm and may substantially affect device characteristics. In this work, by using density functional theory methods, we consider a reduction in tunneling due to weakened coupling across the rotationally misaligned interface between the channel layers and the tunnel barrier. As a prototypical system, we simulate the effects of rotational misalignment of the tunnel barrier layer between aligned channel layers in a graphene/hBN/graphene system. We find that the rotational misalignment between the channel layers and the tunnel barrier in this van der Waals heterostructure can significantly reduce coupling between the channels by reducing, specifically, coupling across the interface between t...

Interfacial-oxygen-vacancy mediated doping of MoS 2 by high-κ dielectrics

Dielectric engineering using high-κ oxides, such as atomic layer deposited (ALD) Al₂O_x and HfO_x, has been in widespread use to enhance the mobility of molybdenum disulfide (MoS₂) based field effect transistors (FETs) [1,2]. This performance enhancement of MoS₂ FETs in a high-κ environment is mainly attributed to the screening of Coulomb scattering from charged impurities, as well as the quenching of homopolar phonon modes of MoS₂ [3]. However, the exact mechanism is still unclear. In this work, we demonstrate, using both experiment and theory, the n-doping of MoS₂ mediated by interfacial-oxygen-vacancies at the high-κ-MoS₂ interface, and propose a mechanism for the mobility enhancement effect in MoS₂ devices upon high-κ encapsulation.

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides (TMDs). Here, the phenomenon of oxygen impurities trapping photoexcited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect-capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First-principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in TMDs, and facilitate defect engineering in relevant materials and device applications.

Semi-classical ensemble Monte Carlo simulator using innovative quantum corrections for nano-scale n-channel FinFETs

We present a three-dimensional semi-classical ensemble Monte Carlo device simulator with novel quantum corrections. The simulator includes a beyond-Fermi treatment of Pauli-Exclusion-blocked scattering, and a valley-dependent treatment of various quantum confinement effects. Quantum corrections to the potential are used not only to model redistribution of carriers in real space, but also to model altered energy valley offsets and associated redistribution of carriers in k-space, and quantum-confined scattering rates, including a new approach to model surface roughness scattering. We illustrate the capabilities of the simulator using different levels of modeling, with an emphasis on modeling nano-scale FinFETs with degenerate carrier populations, including III-V devices.

Effect of rotational misalignment on interlayer coupling in a graphene/hBN/graphene van der Waal's heterostructure

We simulate the effects of rotational misalignment of the tunnel barrier layer between aligned channel layers in a monolayer-graphene/hBN/monolayer-graphene system. Through use of density functional theory (DFT) methods, we demonstrate a reduction in tunneling current due to weakened coupling across the rotationally misaligned interfaces between the channel layers and the tunnel barrier.

Methods for modeling non-equilibrium degenerate statistics and quantum-confined scattering in 3D ensemble Monte Carlo transport simulations

Particle-based ensemble semi-classical Monte Carlo (MC) methods employ quantum corrections (QCs) to address quantum confinement and degenerate carrier populations to model tomorrows ultra-scaled metal-oxide-semiconductor-field-effect-transistors. Here, we present the most complete treatment of quantum confinement and carrier degeneracy effects in a three-dimensional (3D) MC device simulator to date, and illustrate their significance through simulation of n-channel Si and III-V FinFETs. Original contributions include our treatment of far-from-equilibrium degenerate statistics and QC-based modeling of surface-roughness scattering, as well as considering quantum-confined phonon and ionized-impurity scattering in 3D. Typical MC simulations approximate degenerate carrier populations as Fermi distributions to model the Pauli-blocking (PB) of scattering to occupied final states. To allow for increasingly far-from-equilibrium non-Fermi carrier distributions in ultra-scaled and III-V devices, we instead generate ...

Impact of gate oxide complex band structure on n-channel III–V FinFETs

FinFET geometries have been developed for the sub-22 nm regime to extend Si-CMOS scaling via improved electrostatics compared to planar technology. Moreover, engineers have incorporated high-k oxide gate stacks. Beyond leakage current, less discussed is the impact of the gate oxides complex band structure on the device performance. However, it defines the boundary condition for the channel wavefunction at the interface, which, in turn, affects the quantum confinement energy for channel electrons. Here we show that the ON-state performance of n-channel FinFETs may be sensitive to the oxides complex band structure, especially with light-mass III-V channel materials, such as In0.53Ga0.47As. We study this effect using an ensemble semi-classical Monte Carlo device simulator with advanced quantum corrections for degeneracy and confinement effects. Our simulations suggest that using a surface oxide with a heavy effective mass may lower the channel carrier confinement energies, mitigating unwanted quantum side-effects that hinder device performance. Ultimately, future high-k stacks may benefit from oxide gate stack heterostructures balancing effective mass and dielectric permittivity considerations.

ReS2-based interlayer tunnel field effect transistor

In this study, we report the fabrication and characterization of a vertical resonant interlayer tunneling field-effect transistor created using exfoliated, few-layer rhenium disulfide (ReS2) flakes as the electrodes and hexagonal boron nitride as the tunnel barrier. Due to the Γ-point conduction band minimum, the ReS2 based system offers the possibility of resonant interlayer tunneling and associated low-voltage negative differential resistance (NDR) without rotational alignment of the electrode crystal orientations. Substantial NDR is observed, which appears consistent with in-plane crystal momentum conserving tunneling, although considerably broadened by scattering consistent within low mobility ReS2 flakes.