Guangjian Wu

Two-dimensional negative capacitance transistor with polyvinylidene fluoride-based ferroelectric polymer gating

Conventional field-effect transistors (FETs) are not expected to satisfy the requirements of future large integrated nanoelectronic circuits because of these circuits’ ultra-high power dissipation and because the conventional FETs cannot overcome the subthreshold swing (SS) limit of 60 mV/decade. In this work, the ordinary oxide of the FET is replaced only by a ferroelectric (Fe) polymer, poly(vinylidene difluoride-trifluoroethylene) (P(VDF-TrFE)). Additionally, we employ a two-dimensional (2D) semiconductor, such as MoS2 and MoSe2, as the channel. This 2D Fe-FET achieves an ultralow SS of 24.2 mV/dec over four orders of magnitude in drain current at room temperature; this sub-60 mV/dec switching is derived from the Fe negative capacitance (NC) effect during the polarization of ferroelectric domain switching. Such 2D NC-FETs, realized by integrating of 2D semiconductors and organic ferroelectrics, provide a new approach to satisfy the requirements of next-generation low-energy-consumption integrated nanoelectronic circuits as well as the requirements of future flexible electronics.Nanoelectronics: ferroelectric polymers enable ultra-low subthreshold slope in MoS 2 transistorsReplacing the conventional oxide with a ferroelectric polymer in 2D MoS2 field-effect transistors allows sub-60 mV/dec operation. A team led by Jianlu Wang at the Chinese Academy of Sciences fabricated a negative capacitance field-effect transistor based on a metal-ferroelectric-semiconductor structure, with a 2D semiconductor (MoS2 or MoSe2) as the channel. Notably, when the oxide commonly used in field-effect transistors was replaced by a ferroelectric poly(vinylidene difluoride-trifluoroethylene) polymer, the resulting device achieved a subthreshold slope of 24.2 mV/dec at a drain voltage of 0.1 V, at room temperature. Further reduction of the polymer thickness to 50 nm resulted in a 51.2 mV/dec subthreshold slope. These results show promise for overcoming the 60 mV/decade subthreshold slope limit which plagues conventional transistors.

High-Performance Photovoltaic Detector Based on MoTe2/MoS2 Van der Waals Heterostructure

Van der Waals heterostructures based on 2D layered materials have received wide attention for their multiple applications in optoelectronic devices, such as solar cells, light-emitting devices, and photodiodes. In this work, high-performance photovoltaic photodetectors based on MoTe2 /MoS2 vertical heterojunctions are demonstrated by exfoliating-restacking approach. The fundamental electric properties and band structures of the junction are revealed and analyzed. It is shown that this kind of photodetectors can operate under zero bias with high on/off ratio (>105 ) and ultralow dark current (≈3 pA). Moreover, a fast response time of 60 µs and high photoresponsivity of 46 mA W-1 are also attained at room temperature. The junctions based on 2D materials are expected to constitute the ultimate functional elements of nanoscale electronic and optoelectronic applications.

Ferroelectric polymer tuned two dimensional layered MoTe2 photodetector

Two dimensional material based photodetectors have attracted wide attention in recent years. In this work, a few-layer MoTe2 based phototransistor with a ferroelectric polymer P(VDF-TrFE) topgate is fabricated. The remanent polarization of the ferroelectrics could deplete the channel effectively to decrease the dark current of the device by more than one magnitude. As a result, the MoTe2 phototransistor has an appreciable photoresponse for visible light and near infrared. The device has a broad photoresponse range (0.6–1.5 μm), the responsivity and detectivity reach 16.4 mA W−1 and 1.94 × 108 Jones for 1060 nm light. The device works without an external gate voltage, which makes for higher reliability and lower power dissipation for practical application.

The ambipolar evolution of a high-performance WSe2 transistor assisted by a ferroelectric polymer

In recent years, the electrical characteristics of WSe2 field effect transistors (FETs) have been widely investigated with various dielectrics. Among them, being able to perfectly tune the polarity of WSe2 is a meaningful and promising work. In this work, we systematically study the electrical properties of bilayer WSe2 FETs modulated by ferroelectric polymer poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)). Compared to traditional gate dielectric SiO2, the P(VDF-TrFE) not only can tune both electron and hole concentrations to the same high level, but also improve the hole mobility of bilayer WSe2 to 265.96 cm2V-1s-1 under SiO2 gating. Its drain current on/off ratio also has been improved to 2 × 105 for p-type and 4 × 105 for n-type driven by P(VDF-TrFE). More importantly, the ambipolar behaviors of bilayer WSe2 can be effectively achieved and maintained because of the remnant polarization field of P(VDF-TrFE). This work indicates that WSe2 FETs with P(VDF-TrFE) gating have huge potential for complementary logic transistor applications, and paves an effective way to achieve in-plane p-n junction.

Graphene Dirac point tuned by ferroelectric polarization field

Graphene has received numerous attention for future nanoelectronics and optoelectronics. The Dirac point is a key parameter of graphene that provides information about its carrier properties. There are lots of methods to tune the Dirac point of graphene, such as chemical doping, impurities, defects, and disorder. In this study, we report a different approach to tune the Dirac point of graphene using a ferroelectric polarization field. The Dirac point can be adjusted to near the ferroelectric coercive voltage regardless its original position. We have ensured this phenomenon by temperature-dependent experiments, and analyzed its mechanism with the theory of impurity correlation in graphene. Additionally, with the modulation of ferroelectric polymer, the current on/off ratio and mobility of graphene transistor both have been improved. This work provides an effective method to tune the Dirac point of graphene, which can be readily used to configure functional devices such as p-n junctions and inverters.

Electrical characterization of MoS2 field-effect transistors with different dielectric polymer gate

The characteristics of MoS2-nanoflake field-effect transistors (FETs) were studied by analyzing the transfer curves in MoS2-FETs with ferroelectric and general polymers as the gate dielectric. A clear hysteresis, opposite to the electron trapping–detrapping effect in traditional MoS2-FETs, was observed in the MoS2-FETs with ferroelectric poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] films. The effect carrier mobility of MoS2 nanoflakes reached approximately 95.6 cm2/Vs under the control of the polarization field of P(VDF-TrFE), whereas the effect carrier mobility was only approximately 15.3 cm2/Vs in MoS2-FETs with traditional dielectric poly(methyl methacrylate) (PMMA) films. Furthermore, the ferroelectric MoS2-FETs possess a higher ON/OFF resistance ratio (approximately 107) than do the PMMA MoS2-FETs (approximately 105).