Parijat Sengupta

Efficient and realistic device modeling from atomic detail to the nanoscale

As semiconductor devices scale to new dimensions, the materials and designs become more dependent on atomic details. NEMO5 is a nanoelectronics modeling package designed for comprehending the critical multi-scale, multi-physics phenomena through efficient computational approaches and quantitatively modeling new generations of nanoelectronic devices as well as predicting novel device architectures and phenomena. This article seeks to provide updates on the current status of the tool and new functionality, including advances in quantum transport simulations and with materials such as metals, topological insulators, and piezoelectrics.

Gate-Voltage Tunability of Plasmons in Single-Layer Graphene Structures—Analytical Description, Impact of Interface States, and Concepts for Terahertz Devices

The strong light-matter interaction in graphene over a broad frequency range has opened up a plethora of photonics applications of graphene. The goal of this paper is to present the voltage tunability of plasma waves in gated single-layer graphene structures and to quantify their distinction from the plasma waves in ungated graphene structures. Device concepts for plasmonic interconnects and antennas and their performance for THz communication are presented. For the first time, the role of gate voltage and the thickness of the gate dielectric on the characteristics of plasmon propagation in graphene are quantified by accounting for both the interface trap capacitance (substrate dangling bonds and gate-insulator traps) and the quantum capacitance. The gate voltage serves as a powerful knob to tweak the carrier concentration and allows building electrically reconfigurable terahertz devices. By optimizing the gate voltage to maximize the plasmon propagation length in gated graphene structures, we derive simple scaling trends that give intuitive insight into device modeling and design.

Design principles for HgTe based topological insulator devices

The topological insulator properties of CdTe/HgTe/CdTe quantum wells are theoretically studied. The CdTe/HgTe/CdTe quantum well behaves as a topological insulator beyond a critical well width dimension. It is shown that if the barrier (CdTe) and well-region (HgTe) are altered by replacing them with the alloy CdxHg1−xTe of various stoichiometries, the critical width can be changed. The critical quantum well width is shown to depend on temperature, applied stress, growth directions, and external electric fields. Based on these results, a novel device concept is proposed that allows to switch between a normal semiconducting and topological insulator state through application of moderate external electric fields.

The tuning of light-matter coupling and dichroism in graphene for enhanced absorption: Implications for graphene-based optical absorption devices

The inter-band optical absorption in graphene characterized by its fine-structure constant has a universal value of 2.3\% independent of the material parameters. However, for several graphene-based photonic applications, enhanced optical absorption in graphene is highly desired. In this work, we quantify the tunability of optical absorption in graphene via the Fermi level in graphene, angle of incidence of the incident polarized light, and the dielectric constant of the surrounding dielectric media in which graphene is embedded. The influence of impurities adsorbed on the surface of graphene on the Lorentzian broadening of the spectral function of the density of states is analytically evaluated within the equilibrium Greens function formalism. Finally, we compute the differential absorption of right and left circularly-polarized light in graphene that is uniaxially and optically strained. The preferential absorption or circular dichroism is investigated for armchair and zigzag strain.

Tunable chirality and circular dichroism of a topological insulator with C2v symmetry as a function of Rashba and Dresselhaus parameters

Polarization-sensitive devices rely on meta-materials to exhibit varying degrees of absorption of light of a given handedness. The chiral surface states of a topological insulator selectively absorb right- and left-circularly polarized light in the vicinity of the Dirac cone reaching its maximum of unity at the Γ point. In this letter, we show that a band gap open topological insulator with C2v symmetry, which is represented through a combination of Rashba and Dresselhaus Hamiltonians, alters the preferential absorption of left- and right-circularly polarized light allowing a smooth variation of the circular dichroism. This variation in circular dichroism, in a range of positive and negative values, is shown to be a function of the Rashba and Dresselhaus coupling parameters.

The influence of proximity induced ferromagnetism, superconductivity and Fermi-velocity on evolution of Berry phase in Bi2Se3 topological insulator

Bi2Se3 is a well known 3D-topological insulator (TI) with a non-trivial Berry phase of attributed to the topology of the band structure. The Berry phase shows non-topological deviations from in the presence of a perturbation that destroys time reversal symmetry and gives rise to a quantum system with massive Dirac fermions and finite band gap. Such a band gap opening is achieved on account of the exchange field of a ferromagnet or the intrinsic energy gap of a superconductor that influences the topological insulator surface states by virtue of the proximity effect. In this work the Berry phase of such gapped systems with massive Dirac fermions is considered. Additionally, it is shown that the Berry phase for such a system also depends on the Fermi-velocity of the surface states which can be tuned as a function of the TI film thickness.

Proximity induced ferromagnetism, superconductivity, and finite-size effects on the surface states of topological insulator nanostructures

Bi2Te3 and Bi2Se3 are well known 3D-topological insulators (TI). Films made of these materials exhibit metal-like surface states with a Dirac dispersion and possess high mobility. The high mobility metal-like surface states can serve as building blocks for a variety of applications that involve tuning their dispersion relationship and opening a band gap. A band gap can be opened either by breaking time reversal symmetry, the proximity effect of a superconductor or ferromagnet or adjusting the dimensionality of the TI material. In this work, methods that can be employed to easily open a band gap for the TI surface states are assessed. Two approaches are described: (1) Coating the surface states with a ferromagnet which has a controllable magnetization axis. The magnetization strength of the ferromagnet is incorporated as an exchange interaction term in the Hamiltonian. (2) An s-wave superconductor, because of the proximity effect, when coupled to a 3D-TI opens a band gap on the surface. Finally, the hybrid...

Low-temperature thermal transport and thermopower of monolayer transition metal dichalcogenide semiconductors

We study the low temperature thermal conductivity of single-layer transition metal dichalcogenides (TMDCs). In the low temperature regime where heat is carried primarily through transport of electrons, thermal conductivity is linked to electrical conductivity through the Wiedemann-Franz law (WFL). Using a k.p Hamiltonian that describes the [Formula: see text] and [Formula: see text] valley edges, we compute the zero-frequency electric (Drude) conductivity using the Kubo formula to obtain a numerical estimate for the thermal conductivity. The impurity scattering determined transit time of electrons which enters the Drude expression is evaluated within the self-consistent Born approximation. The analytic expressions derived show that low temperature thermal conductivity (1) is determined by the band gap at the valley edges in monolayer TMDCs and (2) in presence of disorder which can give rise to the variable range hopping regime, there is a distinct reduction. Additionally, we compute the Mott thermopower and demonstrate that under a high frequency light beam, a valley-resolved thermopower can be obtained. A closing summary reviews the implications of results followed by a brief discussion on applicability of the WFL and its breakdown in context of the presented calculations.

Photo-modulation of the spin Hall conductivity of mono-layer transition metal dichalcogenides

We report on a possible optical tuning of the spin Hall conductivity in mono-layer transition metal dichalcogenides. Light beams of frequencies much higher than the energy scale of the system (the off-resonant condition) do not excite electrons but rearrange the band structure. The rearrangement is quantitatively established using the Floquet formalism. For such a system of mono-layer transition metal dichalcogenides, the spin Hall conductivity (calculated with the Kubo expression in presence of disorder) exhibits a drop at higher frequencies and lower intensities. Finally, we compare the spin Hall conductivity of the higher spin-orbit coupled WSe2 to MoS2; the spin Hall conductivity of WSe2 was found to be larger.

Graphene nanoribbon plasmonic waveguides: Fundamental limits and device implications

The 2D carbon material graphene exhibits strong light-matter interaction over a very wide wavelength range from the far infrared to the ultraviolet [1]. The tunability of the density-of-states and Fermi energy in graphene along with its excellent transport properties provide a path for graphene photonic applications such as quantum optics, photo-voltaics, photo-detectors, and biological sensing [2]. In this paper, we propose exploiting collective electron-light oscillations or plasmons in patterned graphene nano-ribbons (GNRs) for low energy, high-speed on-chip interconnects that can potentially overcome the latency and power constraints of the current copper/low-K on-chip interconnects [3-4]. The contributions of this paper are threefold. First, compact models for evaluating the plasmon-damping rate in GNRs are introduced. The models account for plasmon-damping pathways through phonons (intrinsic and substrate), substrate charged impurities, and edge-states in ribbons. The compact models introduced in this paper are also applicable to other photonic applications of graphene beyond just on-chip interconnects. Secondly, compact models for evaluating the propagation speed and energy consumption of plasmonic waveguides based on their shot-noise limits are introduced. Finally, the fundamental limits and device implications of on-chip plasmonic waveguides are quantified. In particular, propagation speed and energy consumption are compared with copper/low-K on-chip interconnects at advanced technology nodes.