18 research outputs found
Trap-assisted space charge limited transport in short channel MoS2 transistor
We present temperature dependent measurements of short channel MoS
field effect devices at high source-drain bias. We find that although the
characteristics are Ohmic at low bias, the conduction becomes space charge
limited at high and existence of an exponential distribution of trap
states was observed. The temperature independent critical drain-source voltage
() was also determined. The density of trap states was quantitatively
calculated from . The possible origin of exponential trap distribution in
these devices is also discussed.Comment: 5 pages, 3 figure
Percolative switching in transition metal dichalcogenide field-effect transistors at room temperature
We have addressed the microscopic transport mechanism at the switching or
on-off transition in transition metal dichalcogenide (TMDC) field-effect
transistors (FET), which has been a controversial topic in TMDC electronics,
especially at room temperature. With simultaneous measurement of channel
conductivity and its slow time-dependent fluctuation (or noise) in ultra-thin
WSe2 and MoS2 FETs on insulating SiO2 substrates, where noise arises from
McWhorter-type carrier number fluctuations, we establish that the switching in
conventional backgated TMDC FETs is a classical percolation transition in a
medium of inhomogeneous carrier density distribution. From the experimentally
observed exponents in the scaling of noise magnitude with conductivity, we
observe unambiguous signatures of percolation in random resistor network,
particularly in WSe2 FETs close to switching, which crosses over to continuum
percolation at a higher doping level. We demonstrate a powerful experimental
probe to the microscopic nature of near-threshold electrical transport in TMDC
FETs, irrespective of the material detail, device geometry or carrier mobility,
which can be extended to other classes of 2D material-based devices as well
Microscopic origin of low frequency noise in MoS<sub>2</sub> field-effect transistors
We report measurement of low frequency 1/f noise in molybdenum di-sulphide (MoS2) field-effect transistors in multiple device configurations including MoS2 on silicon dioxide as well as MoS2-hexagonal Boron Nitride (hBN) heterostructures. All as-fabricated devices show similar magnitude of noise with number fluctuation as the dominant mechanism at high temperatures and density, although the calculated density of traps is two orders of magnitude higher than that at the SiO2 interface. Measurements on the heterostructure devices with vacuum annealing and dual gated configuration reveals that along with the channel, metal-MoS2 contacts also play a significant role in determining noisemagnitude in these devices
Switching of Charge-Current-Induced Spin Polarization in the Topological Insulator BiSbTeSe2
The charge-current-induced spin polarization is a key property of topological
insulators for their applications in spintronics. However, topological surface
states are expected to give rise to only one type of spin polarization for a
given current direction, which has been a limiting factor for spin
manipulations. Here we report that in devices based on the bulk-insulating
topological insulator BiSbTeSe2, an unexpected switching of spin polarization
was observed upon changing the chemical potential. The spin polarization
expected from the topological surface states was detected in a heavily
electron-doped device, whereas the opposite polarization was reproducibly
observed in devices with low carrier densities. We propose that the latter type
of spin polarization stems from topologically-trivial two-dimensional states
with a large Rashba spin splitting, which are caused by a strong band bending
at the surface of BiSbTeSe2 beneath the ferromagnetic electrode used as a spin
detector. This finding paves the way for realizing the "spin transistor"
operation in future topological spintronic devices.Comment: 11 pages, 8 figures (paper proper) + 3 pages, 4 figures (Supplemental
Material); rebuttal against recent criticisms towards topological-insulator
spin-detection experiments has been substantiated; accepted for publication
in PR
Quantum noise limited microwave amplification using a graphene Josephson junction
Josephson junctions (JJ) and their tunable properties, including their
nonlinearities, form the core of superconducting circuit quantum
electrodynamics (cQED). In quantum circuits, low-noise amplification of feeble
microwave signals is essential and the Josephson parametric amplifiers (JPA)
are the widely used devices. The existing JPAs are based on Al-AlOx-Al tunnel
junctions realized in a superconducting quantum interference device geometry,
where magnetic flux is the knob for tuning the frequency. Recent experimental
realizations of 2D van der Waals JJs provide an opportunity to implement
various cQED devices with the added advantage of tuning the junction properties
and the operating point using a gate potential. While other components of a
possible 2D van der Waals cQED architecture have been demonstrated -- quantum
noise limited amplifier, an essential component, has not been realized. Here we
implement a quantum noise limited JPA, using a graphene JJ, that has linear
resonance gate tunability of 3.5 GHz. We report 24 dB amplification with 10 MHz
bandwidth and -130 dBm saturation power; performance on par with the best
single-junction JPAs. Importantly, our gate tunable JPA works in the
quantum-limited noise regime which makes it an attractive option for highly
sensitive signal processing. Our work has implications for novel bolometers --
the low heat capacity of graphene together with JJ nonlinearity can result in
an extremely sensitive microwave bolometer embedded inside a quantum
noise-limited amplifier. In general, our work will open up exploration of
scalable device architecture of 2D van der Waals materials by integrating a
sensor with the quantum amplifier.Comment: 15 pages, 4 figures, and supplementary informatio
The Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors
We present low temperature electrical transport experiments in five field
effect transistor devices consisting of monolayer, bilayer and trilayer MoS2
films, mechanically exfoliated onto Si/SiO2 substrate. Our experiments reveal
that the electronic states in all films are localized well up to the room
temperature over the experimentally accessible range of gate voltage. This
manifests in two dimensional (2D) variable range hopping (VRH) at high
temperatures, while below \sim 30 K the conductivity displays oscillatory
structures in gate voltage arising from resonant tunneling at the localized
sites. From the correlation energy (T0) of VRH and gate voltage dependence of
conductivity, we suggest that Coulomb potential from trapped charges in the
substrate are the dominant source of disorder in MoS2 field effect devices,
which leads to carrier localization as well.Comment: 10 pages, 5 figures; ACS Nano (2011
Anomalous Fraunhofer Patterns in Gated Josephson Junctions Based on the Bulk-Insulating Topological Insulator BiSbTeSe2
One-dimensional Majorana modes are predicated to form in Josephson junctions based on three-dimensional topological insulators (TIs). While observations of supercurrents in Josephson junctions made on bulk-insulating TI samples have been reported recently, the Fraunhofer patters observed in such Tl-based Josephson junctions, which sometimes present anomalous features, are still not well-understood. Here, we report our study of highly gate-tunable Tl-based Josephson junctions made of one of the most bulk-insulating TI materials, BiSbTeSe2, and Al. The Fermi level can be tuned by gating across the Dirac point, and the high transparency of the Al-BiSbTeSe2 interface is evinced by a high characteristic voltage and multiple Andreev reflections, with peak indices reaching 12 Anomalous Fraunhofer patterns with missing lobes were observed in the entire range of gate voltage. We found that, by employing an advanced fitting procedure to use the maximum entropy method in a Monte Carlo algorithm, the anomalous Fraunhofer patterns are explained as a result of inhomogeneous supercurrent distributions on the TI surface in the junction. Besides establishing a highly promising fabrication technology, this work clarifies one of the important open issues regarding Tl-based Josephson junctions
Current crowding mediated large contact noise in graphene field-effect transistors
The impact of the intrinsic time-dependent fluctuations in the electrical resistance at the graphene-metal interface or the contact noise, on the performance of graphene field-effect transistors, can be as adverse as the contact resistance itself, but remains largely unexplored. Here we have investigated the contact noise in graphene field-effect transistors of varying device geometry and contact configuration, with carrier mobility ranging from 5,000 to 80,000 cm(2)V(-1) s(-1). Our phenomenological model for contact noise because of current crowding in purely two-dimensional conductors confirms that the contacts dominate the measured resistance noise in all graphene field-effect transistors in the two-probe or invasive four-probe configurations, and surprisingly, also in nearly noninvasive four-probe (Hall bar) configuration in the high-mobility devices. The microscopic origin of contact noise is directly linked to the fluctuating electrostatic environment of the metal-channel interface, which could be generic to two-dimensional material-based electronic devices