arXiv subjects

Richa Mitra

Publications and source records attributed to Richa Mitra.

4 recordsLinked to original sources

arXiv2024

Gate-tunable negative differential resistance in multifunctional van der Waals heterostructure

Two-dimensional (2D) semiconductors have emerged as exciting candidates for the development of low-power and multifunctional computing applications, thanks to their qualities such as layer-dependent band gap tunability, high carrier mobility, and excellent electrostatic control. Here, we explore a pair of 2D semiconductors with nearly broken-gap (Type-III-like) band alignment and demonstrate a highly gate-tunable p-MoTe$_2$/n-SnS$_2$ heterojunction with multifunctional behavior. Employing a dual-gated asymmetric device geometry, we unveil its functionality as both a forward and backward rectifying device. Moreover, we observe a highly gate-tunable negative differential resistance (NDR), with a gate-coupling efficiency of $\eta \simeq 0.5$ and a peak-to-valley ratio of $\sim$ 3 down to 150K. By employing density functional theory, we determine that the observed NDR is dominated by valence band-to-valence band tunneling, while additional interband tunneling contributions arise at higher bias. The combination of tunneling driven transport and gate controllability of NDR opens the pathway for realizing gate-tunable 2D material-based neuromorphic and energy-efficient electronics.

cond-mat.mes-hall↗

arXiv2023

Scalable fabrication of edge contacts to 2D materials

We present a fabrication method for reliably and reproducibly forming electrical contacts to 2D materials, based on the tri-layer resist system. We demonstrate the applicability of this method for epitaxial graphene on silicon carbide (epigraphene) and the transition metal dichalcogenides (TMDCs) molybdenum disulfide ($MoS_2$). For epigraphene, the specific contact resistances are of the order of $ρ_c$ ~ $50$ $Ωμm$, and follow the Landauer quantum limit, $ρ_c \propto n^{-1/2}$, with $n$ being the carrier density of graphene. For $MoS_2$ flakes, our edge contacts enable field effect transistors (FET) with ON/OFF ratio of $> 10^6$ at room temperature ( $> 10^9$ at cryogenic temperatures). The fabrication route here demonstrated allows for contact metallization using thermal evaporation and also by sputtering, giving an additional flexibility when designing electrical interfaces, which is key in practical devices and when exploring the electrical properties of emerging materials.

cond-mat.mes-hall↗

arXiv2020

Anomalous thermopower oscillations in graphene-InAs nanowire vertical heterostructures

Thermoelectric measurements have the potential to uncover the density of states of low-dimensional materials. Here, we present the anomalous thermoelectric behaviour of mono-layer graphene-nanowire (NW) heterostructures, showing large oscillations as a function of doping concentration. Our devices consist of InAs NW and graphene vertical heterostructures, which are electrically isolated by thin ($\sim$ 10nm) hexagonal boron nitride (hBN) layers. In contrast to conventional thermoelectric measurements, where a heater is placed on one side of a sample, we use the InAs NW (diameter $\sim 50$ nm) as a local heater placed in the middle of the graphene channel. We measure the thermoelectric voltage induced in graphene due to Joule heating in the NW as a function of temperature (1.5K - 50K) and carrier concentration. The thermoelectric voltage in bilayer graphene (BLG)- NW heterostructures shows sign change around the Dirac point, as predicted by Mott's formula. In contrast, the thermoelectric voltage measured across monolayer graphene (MLG)-NW heterostructures shows anomalous large-amplitude oscillations around the Dirac point, not seen in the Mott response derived from the electrical conductivity measured on the same device. The anomalous oscillations are a signature of the modified density of states in MLG by the electrostatic potential of the NW, which is much weaker in the NW-BLG devices. Thermal calculations of the heterostructure stack show that the temperature gradient is dominant in the graphene region underneath the NW, and thus sensitive to the modified density of states resulting in anomalous oscillations in the thermoelectric voltage. Furthermore, with the application of a magnetic field, we detect modifications in the density of states due to the formation of Landau levels in both MLG and BLG.

cond-mat.mes-hall↗

arXiv2020

Anomalous Coulomb Drag between InAs Nanowire and Graphene Heterostructures

Correlated charge inhomogeneity breaks the electron-hole symmetry in two-dimensional (2D) bilayer heterostructures which is responsible for non-zero drag appearing at the charge neutrality point. Here we report Coulomb drag in novel drag systems consisting of a two-dimensional graphene and a one dimensional (1D) InAs nanowire (NW) heterostructure exhibiting distinct results from 2D-2D heterostructures. For monolayer graphene (MLG)-NW heterostructures, we observe an unconventional drag resistance peak near the Dirac point due to the correlated inter-layer charge puddles. The drag signal decreases monotonically with temperature ($\sim T^{-2}$) and with the carrier density of NW ($\sim n_{N}^{-4}$), but increases rapidly with magnetic field ($\sim B^{2}$). These anomalous responses, together with the mismatched thermal conductivities of graphene and NWs, establish the energy drag as the responsible mechanism of Coulomb drag in MLG-NW devices. In contrast, for bilayer graphene (BLG)-NW devices the drag resistance reverses sign across the Dirac point and the magnitude of the drag signal decreases with the carrier density of the NW ($\sim n_{N}^{-1.5}$), consistent with the momentum drag but remains almost constant with magnetic field and temperature. This deviation from the expected $T^2$ arises due to the shift of the drag maximum on graphene carrier density. We also show that the Onsager reciprocity relation is observed for the BLG-NW devices but not for the MLG-NW devices. These Coulomb drag measurements in dimensionally mismatched (2D-1D) systems, hitherto not reported, will pave the future realization of correlated condensate states in novel systems.

cond-mat.mes-hall↗