Memristors such as phase-change memory and resistive memory have been proposed to emulate the synaptic activities in neuromorphic systems. However, the low reliability of these types of memories is their biggest challenge for commercialization. Here, a highly reliable memristor array using floating-gate memory operated by two terminals (source and drain) using van der Waals layered materials is demonstrated. Centimeter-scale samples (1.5 cm × 1.5 cm) of MoS as a channel and graphene as a trap layer grown by chemical vapor deposition (CVD) are used for array fabrication with Al O as the tunneling barrier. With regard to the memory characteristics, 93% of the devices exhibit an on/off ratio of over 10 with an average ratio of 10 . The high on/off ratio and reliable endurance in the devices allow stable 6-level memory applications. The devices also exhibit excellent memory durability over 8000 cycles with a negligible shift in the threshold voltage and on-current, which is a significant improvement over other types of memristors. In addition, the devices can be strained up to 1% by fabricating on a flexible substrate. This demonstration opens a practical route for next-generation electronics with CVD-grown van der Waals layered materials.
Piezoelectricity of transition metal
dichalcogenides (TMDs) under mechanical strain has been theoretically
and experimentally studied. Powerful strain sensors using Schottky
barrier variation in TMD/metal junctions as a result of the strain-induced
lattice distortion and associated ion-charge polarization were demonstrated.
However, the nearly fixed work function of metal electrodes limits
the variation range of a Schottky barrier. We demonstrate a highly
sensitive strain sensor using a variable Schottky barrier in a MoS2/graphene heterostructure field effect transistor (FET). The
low density of states near the Dirac point in graphene allows large
modulation of the graphene Fermi level and corresponding Schottky
barrier in a MoS2/graphene junction by strain-induced polarized
charges of MoS2. Our theoretical simulations and temperature-dependent
electrical measurements show that the Schottky barrier change is maximized
by placing the Fermi level of the graphene at the charge neutral (Dirac)
point by applying gate voltage. As a result, the maximum Schottky
barrier change (ΔΦSB) and corresponding current
change ratio under 0.17% strain reach 118 meV and 978, respectively,
resulting in an ultrahigh gauge factor of 575 294, which is
approximately 500 times higher than that of metal/TMD junction strain
sensors (1160) and 140 times higher than the conventional strain sensors
(4036). The ultrahigh sensitivity of graphene/MoS2 heterostructure
FETs can be developed for next-generation electronic and mechanical–electronic
devices.
Two-dimensional (2D)
layered materials with properties such as
a large surface-to-volume ratio, strong light interaction, and transparency
are expected to be used in future optoelectronic applications. Many
studies have focused on ways to increase absorption of 2D-layered
materials for use in photodetectors. In this work, we demonstrate
another strategy for improving photodetector performance using a graphene/MoS2 heterojunction phototransistor with a short channel length
and a tunable Schottky barrier. The channel length of sub-30 nm, shorter
than the diffusion length, decreases carrier recombination and carrier
transit time in the channel and improves phototransistor performance.
Furthermore, our graphene/MoS2 heterojunction phototransistor
employed a tunable Schottky barrier that is only controlled by light
and gate bias. It maintains a low dark current and an increased photocurrent.
As a result, our graphene/MoS2 heterojunction phototransistor
showed ultrahigh responsivity and detectivity of 2.2 × 105 A/W and 3.5 × 1013 Jones, respectively. This
is a considerable improvement compared to previous pristine MoS2 phototransistors. We confirmed an effective method to develop
phototransistors based on 2D materials and obtained ultrahigh performance
of our phototransistor, which is promising for high-performance optoelectronic
applications.
Graphene is one of the most promising materials for photodetectors due to its ability to convert photons into hot carriers within approximately 50 fs and generate long-lived thermalized states with lifetimes longer than 1 ps. In this study, we demonstrate a wide range of vertical photodetectors having a graphene/h-BN/Au heterostructure in which an hexagonal boron nitride (h-BN) insulating layer is inserted between an Au electrode and graphene photoabsorber. The photocarriers effectively tunnel through the small hole barrier (1.93 eV) at the Au/h-BN junction while the dark carriers are highly suppressed by a large electron barrier (2.27 eV) at the graphene/h-BN junction. Thus, an extremely low dark current of ∼10 −13 A is achieved, which is 8 orders of magnitude lower than that of graphene lateral photodetector devices (∼10 −5 A). Also, our device displays an asymmetric photoresponse behavior due to photothermionic emission at the graphene/h-BN and Au/h-BN junctions. The asymmetric behavior generates additional thermal carriers (hot carriers) to enable our device to generate photocurrents that can overcome the Schottky barrier. Furthermore, our device shows the highest value of the I ph /I dark ratio of ∼225 at 7 nm thick h-BN insulating layer, which is 3 orders of magnitude larger than that of the previously reported graphene lateral photodetectors without any active materials. In addition, we achieve a fast response speed of 12 μs of rise time and 5 μs of fall time, which are about 100 times faster than those of other graphene integrated photodetectors.
A highly reliable memristor array using floating‐gate memory operated by two terminals (source and drain) using van der Waals' layered materials is demonstrated by Young Hee Lee, Woo Jong Yu, and co‐workers in article number https://doi.org/10.1002/adma.201703363. Centimeter‐scale samples with MoS2 as a channel and graphene as a trap layer grown by chemical vapor deposition are used for array fabrication, with Al2O3 as a tunneling barrier. This work opens up the possibility to realize reliable and flexible memristors.
Two-dimensional
transition metal dichalcogenides (TMDs) offer numerous
advantages over silicon-based application in terms of atomically thin
geometry, excellent opto-electrical properties, layer-number dependence,
band gap variability, and lack of dangling bonds. The production of
high-quality and large-scale TMD films is required with consideration
of practical technology. However, the performance of scalable devices
is affected by problems such as contamination and patterning arising
from device processing; this is followed by an etching step, which
normally damages the TMD film. Herein, we report the direct growth
of MoSe2 films on selective pattern areas via a surface-mediated
liquid-phase promoter using a solution-based approach. Our growth
process utilizes the promoter on the selective pattern area by enhancing
wettability, resulting in a highly uniform MoSe2 film.
Moreover, our approach can produce other TMD films such as WSe2 films as well as control various pattern shapes, sizes, and
large-scale areas, thus improving their applicability in various devices
in the future. Our patterned MoSe2 field-effect transistor
device exhibits a p-type dominant conduction behavior with a high
on/off current ratio of ∼106. Thus, our study provides
general guidance for direct selective pattern growth via a solution-based
approach and the future design of integrated devices for a large-scale
application.
As the tight contact interface of the lateral PN junction enables high responsivity, specific detectivity, and fast response speed, atomic-scale two-dimensional (2D) lateral PN heterostructures are emerging as viable alternatives to silicon-based photodiodes.
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