178 research outputs found
Mobility engineering and metal-insulator transition in monolayer MoS2
Two-dimensional (2D) materials are a new class of materials with interesting
physical properties and ranging from nanoelectronics to sensing and photonics.
In addition to graphene, the most studied 2D material, monolayers of other
layered materials such as semiconducting dichalcogenides MoS2 or WSe2 are
gaining in importance as promising insulators and channel materials for
field-effect transistors (FETs). The presence of a direct band gap in monolayer
MoS2 due to quantum mechanical confinement, allows room-temperature
field-effect transistors with an on/off ratio exceeding 108. The presence of
high-k dielectrics in these devices enhanced their mobility, but the mechanisms
are not well understood. Here, we report on electrical transport measurements
on MoS2 FETs in different dielectric configurations. Mobility dependence on
temperature shows clear evidence of the strong suppression of charge impurity
scattering in dual-gate devices with a top-gate dielectric together with phonon
scattering that shows a weaker than expected temperature dependence. High
levels of doping achieved in dual-gate devices also allow the observation of a
metal-insulator transition in monolayer MoS2. Our work opens up the way to
further improvements in 2D semiconductor performance and introduces MoS2 as an
interesting system for studying correlation effects in mesoscopic systems.Comment: Submitted January 11, 201
Small-signal amplifier based on single-layer MoS2
In this Letter we demonstrate the operation of an analog small-signal
amplifier based on single-layer MoS2, a semiconducting analogue of graphene.
Our device consists of two transistors integrated on the same piece of
single-layer MoS2. The high intrinsic band gap of 1.8 eV allows MoS2-based
amplifiers to operate with a room temperature gain of 4. The amplifier
operation is demonstrated for the frequencies of input signal up to 2 kHz
preserving the gain higher than 1. Our work shows that MoS2 can effectively
amplify signals and that it could be used for advanced analog circuits based on
two-dimensional materials.Comment: Submitted version of the manuscrip
MoS2 Transistors Operating at Gigahertz Frequencies
The presence of a direct band gap and an ultrathin form factor has caused a
considerable interest in two-dimensional (2D) semiconductors from the
transition metal dichalcogenides (TMD) family with molybdenum disulphide (MoS2)
being the most studied representative of this family of materials. While
diverse electronic elements, logic circuits and optoelectronic devices have
been demonstrated using ultrathin MoS2, very little is known about their
performance at high frequencies where commercial devices are expected to
function. Here, we report on top-gated MoS2 transistors operating in the
gigahertz range of frequencies. Our devices show cutoff frequencies reaching 6
GHz. The presence of a band gap also gives rise to current saturation, allowing
power and voltage gain, all in the gigahertz range. This shows that MoS2 could
be an interesting material for realizing high-speed amplifiers and logic
circuits with device scaling expected to result in further improvement of
performance. Our work represents the first step in the realization of
high-frequency analog and digital circuits based on two-dimensional
semiconductors.Comment: Nano Letters (2014), Supplementary information available at
http://dx.doi.org/10.1021/nl502863
Electrical Transport Properties of Single-Layer WS2
We report on the fabrication of field-effect transistors based on single and
bilayers of the semiconductor WS2 and the investigation of their electronic
transport properties. We find that the doping level strongly depends on the
device environment and that long in-situ annealing drastically improves the
contact transparency allowing four-terminal measurements to be performed and
the pristine properties of the material to be recovered. Our devices show
n-type behavior with high room-temperature on/off current ratio of ~106. They
show clear metallic behavior at high charge carrier densities and mobilities as
high as ~140 cm2/Vs at low temperatures (above 300 cm2/Vs in the case of
bi-layers). In the insulating regime, the devices exhibit variable-range
hopping, with a localization length of about 2 nm that starts to increase as
the Fermi level enters the conduction band. The promising electronic properties
of WS2, comparable to those of single-layer MoS2 and WSe2, together with its
strong spin-orbit coupling, make it interesting for future applications in
electronic, optical and valleytronic devices
Nonvolatile Memory Cells Based on MoS2/Graphene Heterostructures
Memory cells are an important building block of digital electronics. We
combine here the unique electronic properties of semiconducting monolayer MoS2
with the high conductivity of graphene to build a 2D heterostructure capable of
information storage. MoS2 acts as a channel in an intimate contact with
graphene electrodes in a field-effect transistor geometry. Our prototypical
all-2D transistor is further integrated with a multilayer graphene charge
trapping layer into a device that can be operated as a nonvolatile memory cell.
Because of its band gap and 2D nature, monolayer MoS2 is highly sensitive to
the presence of charges in the charge trapping layer, resulting in a factor of
10000 difference between memory program and erase states. The two-dimensional
nature of both the contact and the channel can be harnessed for the fabrication
of flexible nanoelectronic devices with large-scale integration.Comment: Submitted versio
Piezoresistivity and Strain-induced Band Gap Tuning in Atomically Thin MoS2
The bandgap of MoS2 is highly strain-tunable which results in the modulation
of its electrical conductivity and manifests itself as the piezoresistive
effect while a piezoelectric effect was also observed in odd-layered MoS2 with
broken inversion symmetry. This coupling between electrical and mechanical
properties makes MoS2 a very promising material for nanoelectromechanical
systems (NEMS). Here we incorporate monolayer, bilayer and trilayer MoS2 in a
nanoelectromechanical membrane configuration. We detect strain-induced band gap
tuning via electrical conductivity measurements and demonstrate the emergence
of the piezoresistive effect in MoS2. Finite element method (FEM) simulations
are used to quantify the band gap change and to obtain a comprehensive picture
of the spatially varying bandgap profile on the membrane. The piezoresistive
gauge factor is calculated to be -148 +/- 19, -224 +/- 19 and -43.5 +/- 11 for
monolayer, bilayer and trilayer MoS2 respectively which is comparable to
state-of-the-art silicon strain sensors and two orders of magnitude higher than
in strain sensors based on suspended graphene. Controllable modulation of
resistivity in 2D nanomaterials using strain-induced bandgap tuning offers a
novel approach for implementing an important class of NEMS transducers,
flexible and wearable electronics, tuneable photovoltaics and photodetection.Comment: 12 pages, 4 figures in Nano Letters (2015
Breakdown of High-Performance Monolayer MoS2
Two-dimensional (2D) materials such as monolayer molybdenum disulfide (MoS2) are extremely interesting for integration in nanoelectronic devices where they represent the ultimate limit of miniaturization in the vertical direction. Thanks to the presence of a band gap and subnanometer thickness, monolayer MoS2 can be used for the fabrication of transistors exhibiting extremely high on/off ratios and very low power dissipation. Here, we report on the development of 2D MoS2 transistors with improved performance due to enhanced electrostatic control. Our devices show currents in the 100 mu A/mu m range and transconductance exceeding 20 mu S/mu m as well as current saturation. We also record electrical breakdown of our devices and find that MoS2 can support very high current densities, exceeding the current-carrying capacity of copper by a factor of 50. Our results push the performance limit of MoS2 and open the way to their use in low-power and low-cost analog and radio frequency circuits
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