arXiv subjects

Amritesh Rai

Publications and source records attributed to Amritesh Rai.

15 recordsLinked to original sources

arXiv2020

Structural and Magnetic Properties of Molecular Beam Epitaxy Grown Chromium Selenide Thin Films

Chromium selenide thin films were grown epitaxially on Al${_2}$O${_3}$(0001) and Si(111)-(7${\times}$7) substrates using molecular beam epitaxy (MBE). Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate a flat smooth film growth along the c-axis, and is very similar to that from a hexagonal surface. X-ray diffraction pattern confirms the growth along the c-axis with c-axis lattice constant of 17.39 Å. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature dependent resistivity. Antiferromagnetic nature of the film with a Néel temperature of about 40 K is estimated from the magnetic exchange bias measurements. A larger out-of-plane exchange bias, along with a smaller in-plane exchange bias is observed below 40 K. Exchange bias training effects are analyzed based on different models and are observed to be following a modified power-law decay behavior.

cond-mat.mtrl-sci↗

arXiv2020

Stacking Order Driven Optical Properties and Carrier Dynamics in ReS2

Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principle calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blue-shifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. Our findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

cond-mat.mtrl-sci↗

arXiv2019

Visualization of Local Conductance in MoS2/WSe2 Heterostructure Transistors

The vertical stacking of van der Waals (vdW) materials introduces a new degree of freedom to the research of two-dimensional (2D) systems. The interlayer coupling strongly influences the band structure of the heterostructures, resulting in novel properties that can be utilized for electronic and optoelectronic applications. Based on microwave microscopy studies, we report quantitative electrical imaging on gated molybdenum disulfide (MoS2)/tungsten diselenide (WSe2) heterostructure devices, which exhibit an intriguing anti-ambipolar effect in the transfer characteristics. Interestingly, in the region with significant source-drain current, electrons in the n-type MoS2 and holes in the p-type WSe2 segments are nearly balanced, whereas the heterostructure area is depleted of mobile charges. The configuration is analogous to the p-i-n diode, where the injected carriers dominate in the recombination current. The spatial evolution of local conductance can be ascribed to the lateral band bending and formation of depletion regions along the line of MoS2-heterostructure-WSe2. Our work vividly demonstrates the microscopic origin of novel transport behaviors, which is important for the vibrant field of vdW heterojunction research.

physics.app-ph↗

arXiv2018

Moiré Excitons in Van der Waals Heterostructures

In van der Waals (vdW) heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moiré superlattice. While it is widely recognized that a moiré superlattice can modulate the electronic band structure and lead to novel transport properties including unconventional superconductivity and insulating behavior driven by correlations, its influence on optical properties has not been investigated experimentally. We present spectroscopic evidence that interlayer excitons are confined by the moiré potential in a high-quality MoSe2/WSe2 heterobilayer with small rotational twist. A series of interlayer exciton resonances with either positive or negative circularly polarized emission is observed in photoluminescence, consistent with multiple exciton states confined within the moiré potential. The recombination dynamics and temperature dependence of these interlayer exciton resonances are consistent with this interpretation. These results demonstrate the feasibility of engineering artificial excitonic crystals using vdW heterostructures for nanophotonics and quantum information applications.

cond-mat.mes-hall↗

arXiv2018

Accelerated Carrier Recombination by Grain Boundary/Edge Defects in MBE Grown Transition Metal Dichalcogenides

Defect-carrier interaction in transition metal dichalcogenides (TMDs) play important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in MBE grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, carrier recombination rate in MBE grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with previously reported result of theoretical calculation. Our findings provide useful reference for the fundamental parameters: carrier lifetime and sound velocity, reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics.

cond-mat.mtrl-sci↗

arXiv2017

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. Here, the phenomenon of oxygen impurities trapping photo-excited 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 the carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in transition metal dichalcogenides, and facilitates defect engineering in relevant material and device applications.

cond-mat.mtrl-sci↗

arXiv2017

Impact of Junction Depth and Abruptness on the Activation and the Leakage Current in Germanium n$^{+}$/p Junctions

The phosphorous activation in Ge n$^{+}$/p junctions is compared in terms of junction depth, by using laser spike annealing at 860°C for 400$μ$s. The reverse junction leakage is found to strongly depend on the abruptness of dopant profiles. A shallow and abrupt junction is shown to have lower phosphorous activation level, due to surface dose loss, and higher band-to-band tunneling (BTBT) leakage, when compared to the deep junction. Simulations were carried out to evaluate the lowest achievable OFF-state currents (I$_{OFF}$) for Ge double-gate FETs when using such an abrupt junction. Our results indicate that a Ge body thickness smaller than 5 nm is required to suppress the BTBT leakage and meet the requirement for the high performance devices defined by the International Technology Roadmap for Semiconductors (I$_{OFF}$ = 10$^{-7}$ A/$μ$m).

physics.app-ph↗

arXiv2017

Angular dependence of magnetization reversal in epitaxial chromium telluride thin films with perpendicular magnetic anisotropy

We investigate magnetic anisotropy and magnetization reversal mechanism in chromium telluride thin films grown by molecular beam epitaxy. We report existence of strong perpendicular anisotropy in these thin films, along with a relatively strong second order anisotropy contribution. The angular variation of the switching field observed from the magnetoresistance measurement is explained quantitatively using a one-dimensional defect model. The model reveals the relative roles of nucleation and pinning in the magnetization reversal, depending on the applied field orientation. Micromagnetic simulations are performed to visualize the domain structure and switching process.

cond-mat.mes-hall↗

arXiv2017

Intra-Domain Periodic Defects in Monolayer MoS$_2$

We present an ultra-high vacuum scanning tunneling microscopy (STM) study of structural defects in molybdenum disulfide thin films grown on silicon substrates by chemical vapor deposition. A distinctive type of grain boundary periodically arranged inside an isolated triangular domain, along with other inter-domain grain boundaries of various types, is observed. These periodic defects, about 50 nm apart and a few nanometers in width, remain hidden in optical or low-resolution microscopy studies. We report a complex growth mechanism that produces 2D nucleation and spiral growth features that can explain the topography in our films.

cond-mat.mtrl-sci↗

arXiv2017

Detection of current induced spin polarization in epitaxial Bi$_2$Te$_3$ thin film

We electrically detect charge current induced spin polarization on the surface of molecular beam epitaxy grown Bi$_2$Te$_3$ thin film in a two-terminal device with a ferromagnetic MgO/Fe and a nonmagnetic Ti/Au contact. The two-point resistance, measured in an applied magnetic field, shows a hysteresis tracking the magnetization of the Fe. A theoretical estimate is obtained for the change in resistance on reversing the magnetization direction of Fe from coupled spin-charge transport equations based on quantum kinetic theory. The order of magnitude and the sign of the hysteresis is consistent with spin-polarized surface state of Bi$_2$Te$_3$.

cond-mat.mes-hall↗

arXiv2016

Theoretical and experimental investigation of vacancy-based doping of monolayer MoS$_2$ on oxide

Monolayer transition metal dichalcogenides are novel, gapped two-dimensional materials. Toward device applications, we consider MoS$_2$ 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 MoS$_2$ 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 MoS$_2$-dielectrics system and for isolated dielectric layers for reference. Among our results, we find that with O vacancies, both the Hf-terminated HfO$_2$-MoS$_2$ system, and the O-terminated and H-passivated Al$_2$O$_3$-MoS$_2$ systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoS$_2$, in manner analogous to modulation doping. The n-type doping of monolayer MoS$_2$ by high-k oxides with oxygen vacancies then is experimentally demonstrated by electrically and spectroscopically characterizing back-gated monolayer MoS$_2$ field effect transistors encapsulated by oxygen deficient alumina and hafnia.

cond-mat.mes-hall↗

arXiv2016

Air Stable Doping and Intrinsic Mobility Enhancement in Monolayer $MoS_{2}$ by Amorphous $TiO_{x}$ Encapsulation

To reduce Schottky-barrier-induced contact and access resistance, and the impact of charged impurity and phonon scattering on mobility in devices based on 2D transition metal dichalcogenides (TMDs), considerable effort has been put into exploring various doping techniques and dielectric engineering using $high-κ$ oxides, respectively. The goal of this work is to demonstrate a $high-κ$ dielectric that serves as an effective n-type charge transfer dopant on monolayer (ML) molybdenum disulfide ($MoS_{2}$). Utilizing amorphous titanium suboxide (ATO) as the '$high-κ$ dopant', we achieved a contact resistance of ~ $180$ $Ω.μm$ which is the lowest reported value for ML $MoS_{2}$. An ON current as high as $240$ $μA/μm$ and field effect mobility as high as $83$ $cm^2/V-s$ were realized using this doping technique. Moreover, intrinsic mobility as high as $102$ $cm^2/V-s$ at $300$ $K$ and $501$ $cm^2/V-s$ at $77$ $K$ were achieved after ATO encapsulation which are among the highest mobility values reported on ML $MoS_{2}$. We also analyzed the doping effect of ATO films on ML $MoS_{2}$, a phenomenon which is absent when stoichiometric $TiO_{2}$ is used, using ab initio density functional theory (DFT) calculations which shows excellent agreement with our experimental findings. Based on the interfacial-oxygen-vacancy mediated doping as seen in the case of $high-κ$ ATO - ML $MoS_{2}$, we propose a mechanism for the mobility enhancement effect observed in TMD-based devices after encapsulation in a $high-κ$ dielectric environment.

cond-mat.mes-hall↗

arXiv2016

Structural and Electrical Properties of MoTe$_2$ and MoSe$_2$ Grown by Molecular Beam Epitaxy

We demonstrate the growth of thin films of molybdenum ditelluride and molybdenum diselenide on sapphire substrates by molecular beam epitaxy. In-situ structural and chemical analyses reveal stoichiometric layered film growth with atomically smooth surface morphologies. Film growth along the (001) direction is confirmed by X-ray diffraction, and the crystalline nature of growth in the 2H phase is evident from Raman spectroscopy. Transmission electron microscopy is used to confirm the layered film structure and hexagonal arrangement of surface atoms. Temperature dependent electrical measurements show an insulating behavior which agrees well with a two-dimensional variable-range hopping model, suggesting that transport in these films is dominated by localized charge-carrier states.

cond-mat.mtrl-sci↗

arXiv2015

Perpendicular magnetic anisotropy and spin glass-like behavior in molecular beam epitaxy grown chromium telluride thin films

Reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM), vibrating sample magnetometry and other physical property measurements are used to investigate the structure, morphology, magnetic and magneto-transport properties of (001)-oriented Cr$_2$Te$_3$ thin films grown on Al$_2$O$_3$(0001) and Si(111)-(7$\times$7) surfaces by molecular beam epitaxy (MBE). Streaky RHEED patterns indicate flat smooth film growth on both substrates. STM studies show the hexagonal arrangements of surface atoms. Determination of the lattice parameter from atomically resolved STM image is consistent with the bulk crystal structures. Magnetic measurements show the film is ferromagnetic having the Curie temperature of about 180 K, and a spin glass-like behavior was observed below 35 K. Magneto-transport measurements show the metallic nature of the film with a perpendicular magnetic anisotropy along the $c$-axis.

cond-mat.mtrl-sci↗

arXiv2015

High-Mobility Holes in Dual-Gated WSe$_2$ Field-Effect Transistors

We demonstrate dual-gated $p$-type field-effect transistors (FETs) based on few-layer tungsten diselenide (WSe$_2$) using high work-function platinum source/drain contacts, and a hexagonal boron nitride top-gate dielectric. A device topology with contacts underneath the WSe$_2$ results in $p$-FETs with $I_{ON}$/$I_{OFF}$ ratios exceeding 10$^7$, and contacts that remain Ohmic down to cryogenic temperatures. The output characteristics show current saturation and gate tunable negative differential resistance. The devices show intrinsic hole mobilities around 140 cm$^2$/Vs at room temperature, and approaching 4,000 cm$^2$/Vs at 2 K. Temperature-dependent transport measurements show a metal-insulator transition, with an insulating phase at low densities, and a metallic phase at high densities. The mobility shows a strong temperature dependence consistent with phonon scattering, and saturates at low temperatures, possibly limited by Coulomb scattering, or defects.

cond-mat.mes-hall↗