Back-gated Field-effect Transistors Research Articles

A Smart Measurement System for the Combined Nanoscale and Device Level Characterization of Electron Devices: Implementation Using Ink-Jet Printing Technologies

In this article, the integration into a single measurement system of device level and nanoscale measurement equipment is presented and applied to the electrical characterization of emerging electron devices. This system is a smart solution to simplify the test procedure, since it allows a fast switching between measurement modes (device or nanoscale level), also featuring an enlarged testing capability. Key in the system is a custom-made inkjet-printed circuit board (I-PCB) that connects the device terminals to the proper instrumentation. The flexibility offered by inkjet-printing technologies is a clear advantage, since many kinds of devices can be tested, without the need of expensive hardware modifications. As a particular case of study, the proposed strategy is demonstrated by implementing a system that alternates between standard electrical measurements with a Semiconductor Parameter Analyzer (SPA) and Conductive Atomic Force Microscopy (CAFM) nanoscale measurements on back-gate graphene field-effect transistors.

Synthesis of violet phosphorus with large lateral sizes to facilitate nano-device fabrications.

Violet phosphorus has been proven to be the most stable phosphorus allotrope and has attracted much attention recently. The growth of violet phosphorus with large lateral sizes is crucial to obtain good quality violet phosphorene for nanodevice fabrication. Herein, a large number of violet phosphorus plates have been produced from molten lead using an optimized method to achieve red bronze luster. The crystal structure of the as-produced violet phosphorus was determined by single-crystal X-ray diffraction to be monoclinic with the space group P2/n (13) (CSD-2160375), identical to the one from the chemical vapor transport method (CSD-1935087). The as-produced violet phosphorus plates were found to have lateral sizes of 1.30 ± 0.41 mm2. The violet phosphorus plates were easily exfoliated and directly transferred to silicon substrates to facilitate building of a back-gate field-effect transistor. A hole mobility of 2.308 cm2 V-1 s-1 was obtained from a violet phosphorus nanosheet with a thickness of 52 nm under ambient conditions. The absolute responsivity of 130 mA W-1 with a fast response time of 27 ms was also obtained under the irradiation of a 530 nm laser.

An individual ZnO microwire homojunction LED with ultraviolet electroluminescence spectrally purified using Pt nanoparticles cladding

Low-dimensional ultraviolet light sources are enormous significance due to their wide range of potential applications, ranging from bioscience and high-resolution biological imaging, environmental conservation to public hygiene. Nevertheless, developing energy-efficient and cost-efficient light sources still remains serious challenges. In this study, p-type ZnO microwires with Sb-incorporation (ZnO:Sb MWs) were synthesized individually. The p-type conductivity was proofed using an individual MW based back-gated field-effect transistor; while the ZnO:Sb MWs with quadrilateral cross-section exhibited stimulated emission resonant in well-defined Fabry-Perot mode on the benefit of single-crystalline quality and atomically smooth side facets. The newly synthesized ZnO:Sb MW was further used to fabricate homojunction light-emitting device by employing n-type ZnO film as electron-transporting layer. A strong ultraviolet emissions peaking at around 378.5 nm was achieved upon forward biased electrically, but illustrating much broader electroluminescence (EL) linewidth. Introducing Pt nanoparticles (PtNPs) with desired plasmons, the light-out intensity is significantly enhanced. Particularly, the broad EL profiles narrowed to about 16.6 nm at high injection regime, achieving electron-holes radiative recombination in the ZnO:Sb MWs active region. The results exhibit a prominent progress achieving high-quality ZnO nano/microstructures with genuine p-type conductivity, which has great hopes for its future applications and development in ZnO homojunction optoelectronic devices.

Salt-Assisted Low-Temperature Growth of 2D Bi2 O2 Se with Controlled Thickness for Electronics.

Bi2 O2 Se is the most promising 2D material due to its semiconducting feature and high mobility, making it propitious channel material for high-performance electronics that demands highly crystalline Bi2 O2 Se at low-growth temperature. Here, a low-temperature salt-assisted chemical vapor deposition approach for growing single-domain Bi2 O2 Se on a millimeter scale with thicknesses of multilayer to monolayer is presented. Because of the advantage of thickness-dependent growth, systematical scrutiny of layer-dependent Raman spectroscopy of Bi2 O2 Se from monolayer to bulk is investigated, revealing a redshift of the A1g mode at 162.4cm-1 . Moreover, the long-term environmental stability of ≈2.4nm thick Bi2 O2 Se is confirmed after exposing the sample for 1.5 years to air. The backgated field effect transistor (FET) based on a few-layered Bi2 O2 Se flake represents decent carrier mobility (≈287cm2 V-1 s-1 ) and an ON/OFF ratio of up to 107 . This report indicates a technique to grow large-domain thickness controlled Bi2 O2 Se single crystals for electronics.

Highly Efficient Deposition of Centimeter-Scale MoS2 Monolayer Film on Dragontrail Glass with Large Single-Crystalline Domains.

Highly efficient growth of a centimeter-scale MoS2 monolayer film by oxide scale sublimation chemical vapor deposition (OSSCVD) in a time as short as 60 s is reported. Benefiting from the superior catalytic ability of Dragontrail glass (DT-glass) substrate and the controlled large vapor supersaturation of the molybdenum source, the ultrafast deposition of MoS2 is realized with maintaining large-sized single-crystalline domains over 20µm at maximum in the film. It is comparable to those reported for MoS2 grown in tens of minutes and even hours. Similar to the face-to-face precursor feed route, the gas-controlled OSSCVD with a showerhead configuration facilitates a homogeneous and controllable source supply. It enables high-quality monolayer MoS2 film deposition on 2 × 2 cm2 DT-glass with centimeter-scale uniformity confirmed by microscopic, spectroscopic, and electrical characterizations. Back-gate MoS2 field-effect transistors fabricated on polycrystalline continuous film exhibit the maximum field-effect mobility of 5.1 cm2 V-1 s-1 and a peak Ion /Ioff ratio of 5 × 108 . They reach 40 cm2 V-1 s-1 and 1.2 × 109 , respectively, on single-crystalline domains. These results are even greater than those for MoS2 grown using 1-2 orders of magnitude longer deposition time and higher temperatures. This study highlights the opportunities for low-cost high-throughput production of large-area high-quality monolayer MoS2 .

Van der Waals epitaxial growth and optoelectronics of a vertical MoS2/WSe2 p–n junction

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted extensive attention due to their unique electronic and optical properties. In particular, TMDs can be flexibly combined to form diverse vertical van der Waals (vdWs) heterostructures without the limitation of lattice matching, which creates vast opportunities for fundamental investigation of novel optoelectronic applications. Here, we report an atomically thin vertical p–n junction WSe2/MoS2 produced by a chemical vapor deposition method. Transmission electron microscopy and steady-state photoluminescence experiments reveal its high quality and excellent optical properties. Back gate field effect transistor (FET) constructed using this p–n junction exhibits bipolar behaviors and a mobility of 9 cm2/(V·s). In addition, the photodetector based on MoS2/WSe2 heterostructures displays outstanding optoelectronic properties (R = 8 A/W, D* = 2.93 × 1011 Jones, on/off ratio of 104), which benefited from the built-in electric field across the interface. The direct growth of TMDs p–n vertical heterostructures may offer a novel platform for future optoelectronic applications.Graphical

Characterization of the electric transport properties of black phosphorous back-gated field-effect transistors

We use thin layers of exfoliated black phosphorus to realize back-gated field-effect transistors in which the Si/SiO2 substrate is exploited as gate electrode. To prevent the detrimental effect of the air exposure the devices are protected by Poly(methyl methacrylate). We report the observation of an improved contact resistance at the interface between the layered material and the metal contact by electrical conditioning. We also demonstrate the existence of a hysteresis in the transfer characteristics that improves by increasing the gate voltage sweep range. Finally, we prove the suitability of such transistors as memory devices.

Visible light photodetectors based on multilayered O-doped BiI3 nanoflakes with ultrahigh sensitivity

Recently, BiI3 was discovered as a promising 2D semiconductor materials for photovoltaic and photodetector device, due to its suitable bandgap, high optical absorption coefficient, excellent electron mobility, and low carrier concentration. However, as so far, only the CVD/PVD grown BiI3 nanoflakes/films have been reported. Here, we firstly synthesized the O-doped BiI3 (OBI) bulk crystals via chemical vapor transport (CVT) method, and then systematically studied the electrical and optoelectronic properties of exfoliated multi-layer OBI nanoflakes. Back-gated field-effect transistor fabricated on the multi-layer OBI nanoflakes with van der Waals (vdW) bottom electrode contact exhibited n-type semiconducting characteristics. In addition, exfoliated multi-layer OBI based photodetector demonstrates an excellent photoresponse with good reproducibility at room-temperature for visible wavelengths, including a high photo-to-dark current ratio (2.16 ×104), a remarkable photoresponsivity (850 mA W−1), a fast response speed (rise/decay time of 3.2/2.9 ms), and a high specific detectivity (9.85 ×1012 Jones), which is favored by the ultralow dark current (∼30 fA). These results indicated that the OBI crystals have great potential for future applications in highly sensitive and ultrafast visible-light photodetector.

Organic Vapor Sensing Mechanisms by Large-Area Graphene Back-Gated Field-Effect Transistors under UV Irradiation.

The gas sensing properties of graphene back-gated field-effect transistor (GFET) sensors toward acetonitrile, tetrahydrofuran, and chloroform vapors were investigated with the focus on unfolding possible gas detection mechanisms. The FET configuration of the sensor device enabled gate voltage tuning for enhanced measurements of changes in DC electrical characteristics. Electrical measurements were combined with a fluctuation-enhanced sensing methodology and intermittent UV irradiation. Distinctly different features in 1/f noise spectra for the organic gases measured under UV irradiation and in the dark were observed. The most intense response observed for tetrahydrofuran prompted the decomposition of the DC characteristic, revealing the photoconductive and photogating effect occurring in the graphene channel with the dominance of the latter. Our observations shed light on understanding surface processes at the interface between graphene and volatile organic compounds for graphene-based sensors in ambient conditions that yield enhanced sensitivity and selectivity.

Al back-gated graphene field-effect transistors for capacitive sensing applications based on quantum capacitance effect

We report a new device design of a graphene field-effect transistor (G-FET) for capacitive sensing application. A channel area exposed to ambient conditions in G-FET is known to be a promising candidate for molecular level sensing applications because graphene can attract certain molecules with its freely hanging sigma bonds. In addition, molecules that adhere to graphene act as impurities that affect the electron transport within graphene. Two of the most common ways to evaluate such a change are measuring the changes in resistance and in quantum capacitance. Previous research studies have been largely focused on using resistive measurement due to restrictions from device design even though capacitive measurement can be cost-effective. To overcome the obstacles, we developed G-FET with high capacitance and a large exposed channel area by incorporating Al back-gate electrodes with naturally oxidized AlOx surface as an insulating layer. The measured capacitance was well-modulated in vacuum by the gate voltage due to the quantum capacitance effect. Also, the capacitance curve was shifted up to the right in the 100 ppm NO2 environment. The capacitance at zero gate bias was increased by 56.6% from the vacuum to the 100 ppm NO2 environment. These results indicate that the proposed device can be used for capacitive sensing applications.

(Digital Presentation) Novel Back gate-Field Effect Transistor (BG-FET) Based on Curcumin Functionalized Zinc Oxide Nanorods for Detection of Arsenic(III) Ions in Water

Human activities and industrial pollutants that are released regularly in the surrounding, causes toxic metallic impurities like Hg, As, Cd and Pb to accumulate and enter the water bodies in different forms. These toxic metal contaminats can cause severe biological and environmental degradation and has the capability to threat human life.These severe consequences have led many organizations to determine maximimum quantities of metal ions in potable water and food. Safe limits for lead, mercury, arsenic and cadmium in potable water are 15,2, 10 and 5 parts per billion (ppb) respectively, as determined by EPA (Environmental Protection Agency). Various disorders in functioning of nucleic acid, immune system attack and other nervous system disorders can be caused due to repletion of As3+ in human body.Detection of As3+ ions in drinking water in presence of other competing analytes is a challenging task. Therefore, developing sensitive and accurate techniques for monitoring these toxic metal ions in water is the need of the hour. Analytical techniques such as AAS/AES, ICP-MS, ASV that were conventionally used, require expensive instruments, incur complicated measuring techniques and are time consuming. Alternatively, various sensors are fabricated for As3+ ion detection, such as electrochemical, fluorescent, colorimetric and surface-enhanced Raman scattering. Inspite of these developments, sensors with ease of operation, portable analytical platform and low cost for user friendly analysis are still needed.In this work, curcumin functionalized zinc oxide nanorods (Cur-ZnO) were used as an electrical sensing platform for highly selective and sensitive As3+ detection. Zinc oxide nanorods were synthesized using solvothermal approach and attachment of curcumin to ZnO nanrods were obtained through an in-situ approach. High resolution transmission electron microscopy (HRTEM), Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Ultraviolet-visible (UV-Vis) absorbance analysis were done to confirm the attachment of curcumin sheets to ZnO nanorods. The sensing platform was fabricated in a novel staggered gate-field effect transistor (SG-FET) configuration with Cur-ZnO as the sensing layer. A high-k dielectric (hafnium oxide) was used as gate dielectric and aluminium was used as gate metal. The performance of the sensor towards As3+ detection was investigated electrically by varying the gate voltage of the sensor device. The sensor was found to be highly selective towards As3+ in presence of Pb2+, Hg2+,Cd2+, Na+, Fe3+, K+ and Cu2+. The interaction of As3+ ions with the sensing layer was also studied using UV-Vis and PL spectrophotometer. The sensitivity of the sensor was seen to improve by tuning the gate voltage. Our work suggests that Cur-ZnO based staggered gate-field effect transistor (SG-FET) are promising towards development of portable, selective and sensitive arsenic ion detector in drinking water. Figure 1

Single β-Ga2O3 nanowire back-gate field-effect transistor

In this work, a normally-on single-monocrystal β-Ga2O3 nanowire (NW) back-gate field-effect transistor (FET) has been demonstrated by transferring metal-organic chemical vapor deposition-grown β-Ga2O3 NWs on sapphire onto SiO2(300 nm)/p +-Si substrate. When the gate voltage (V G) exceeds −14 V, the device is pinched off, with an on/off ratio greater than 108 and a drain leakage current density as low as ∼7.34 fA. The maximum field-effect carrier mobility for these n-doped single β-Ga2O3 NW FETs reaches ∼62.2 cm2 (V s)−1. A prompt degradation in the on/off ratio for these β-Ga2O3 NW back-gate FETs is observed as the operation temperature increased up to 400 K. With strong evidence, the temperature-dependent degradation in the performance is determined by the activation of self-trapped holes and intrinsic vacancy-related defects, both of which would lead to a rapid increase in the channel leakage current at high temperatures.

In-plane optical and electrical anisotropy in low-symmetry layered GeS microribbons

Layered group-IV monochalcogenides, including GeS, GeSe, SnS, and SnSe, garner attention because of their anisotropic structures and properties. Here, we report on the growth of GeS microribbons via chemical vapor transport (CVT), which affords each of them with a low-symmetry orthorhombic structure and anisotropic optical and electronic properties. The single-crystalline nature of the GeS microribbon, which has a typical thickness of ~30 nm, is confirmed. Polarized Raman spectra reveal angle-dependent intensities that are attributed to the anisotropic layered structure of GeS microribbons. The photoluminescence (PL) spectra reveal a peak at ~1.66 eV. The angle-dependent PL and anisotropic absorption spectroscopy results provide evidence for a distinct anisotropic optical transition near the energy band edges; this phenomenon is also predicted by our density functional theory (DFT)-based calculations. Strong in-plane direct-current transport anisotropy is observed under dark and white illumination by using back-gate cross-shaped field effect transistors (CSFETs) fabricated with the GeS microribbon; significant gate-tunable conductivity is also confirmed. The strong anisotropy is further confirmed by the DFT-calculated effective mass ratio. Our findings not only support the application of GeS microribbons in anisotropic photoelectronic transistors but also provide more possibilities for other functional device applications.

Electrical characteristics of WSe2 transistor with amorphous BN capping layer

Back-gated WSe2 field effect transistor (FET) with amorphous BN (a-BN) capping layer was fabricated by a direct chemical vapor deposition (CVD) assemble technique, and the electrical performance was investigated. The field effect mobility and the on/off current ratio of the a-BN capped WSe2 FET both improved by one order of magnitude due to the clean and conformal contact between the BN and WSe2 interface that further reduced the extrinsic scattering. This technique could be further applied for other 2D material based devices.

Highly Sensitive Photodetectors Based on Monolayer MoS2 Field-Effect Transistors.

Molybdenum disulfide (MoS2) is a promising candidate for the development of high-performance photodetectors, due to its excellent electric and optoelectronic properties. However, most of the reported MoS2 phototransistors have adopted a back-gate field-effect transistor (FET) structure, requiring applied gate bias voltages as high as 70 V, which made it impossible to modulate each detecting device in the fabricated array. In this paper, buried-gate FETs based on CVD-grown monolayer MoS2 were fabricated and their electric and photoelectric properties were also systematically investigated. A photoresponsivity of around 6.86 A/W was obtained at 395 nm, under the conditions of zero gate bias voltage and a light power intensity of 2.57 mW/cm2. By application of a buried-gate voltage of 8 V, the photoresponsivity increased by nearly 10 times. Furthermore, the response speed of the buried-gate MoS2 FET phototransistors is measured to be around 350 ms. These results pave the way for MoS2 photodetectors in practical applications.

Layer-Dependent Raman Spectroscopy and Electronic Applications of Wide-Bandgap 2D Semiconductor β-ZrNCl.

In recent years, 2D layered semiconductors have received much attention for their potential in next-generation electronics and optoelectronics. Wide-bandgap 2D semiconductors are especially important in the blue and ultraviolet wavelength region, although there are very few 2D materials in this region. Here, monolayer β-type zirconium nitride chloride (β-ZrNCl) is isolated for the first time, which is an air-stable layered material with a bandgap of ≈3.0eV in bulk. Systematical investigation of layer-dependent Raman scattering of ZrNCl from monolayer, bilayer, to bulk reveals a blueshift of its out-of-plane A1g peak at ≈189cm-1 . Importantly, this A1g peak is absent in the monolayer, suggesting that it is a fingerprint to quickly identify the monolayer and for the thickness determination of 2D ZrNCl. The back gate field-effect transistor based on few-layer ZrNCl shows a high on/off ratio of 108 . These results suggest the potential of 2D β-ZrNCl for electronic applications.

Enhancing the electrical performance of InAs nanowire field-effect transistors by improving the surface and interface properties by coating with thermally oxidized Y2O3.

Due to their excellent electrical characteristics, InAs nanowires (NWs) have great potential as conducting channels in integrated circuits. However, the surface effect and loose native oxide coverage can deteriorate the performance of InAs NW transistors. Y2O3, a high-k dielectric with low Gibbs free energy, has been proposed to modify the InAs NW surface. Here, we systematically investigate the effect of Y2O3 coating on the performance of InAs NW field-effect transistors (FETs). We first explore the influence of the thermal oxidation process of Y2O3 on the performance of back-gated FETs. We then observe that the coverage of Y2O3/HfO2 bilayers on the NW decreases the hysteresis (the smallest value reaches 0.1 V), subthreshold swing (SS, down to 169 mV dec-1) and on-state resistance Ron, and increases the field-effect mobility μFE (up to 4876.1 cm2 V-1 s-1) and the on-off ratio, mainly owing to the passivation effect on the NW surface. Finally, paired top-gated NW FETs with a Y2O3/HfO2 bilayer and a single layer of HfO2 dielectric are fabricated and compared. The Y2O3/HfO2 bilayer provides better gate control (SSmin = 113 mV dec-1) under a smaller gate oxide capacitance, with an interface trap density as low as 1.93 × 1012 eV-1 cm-2. The use of the Y2O3/HfO2 stack provides an effective strategy to enhance the performance of III-V-based transistors for future applications.

Memory effects in black phosphorus field effect transistors

We report the fabrication and the electrical characterization of back-gated field effect transistors with a black phosphorus (BP) channel. We show that the hysteresis of the transfer characteristic, due to intrinsic defects, can be exploited to realize non-volatile memories. We demonstrate that gate voltage pulses allow to trap and store charge inside the defect states, which enable memory devices with endurance over 200 cycles and retention longer than 30 min. We show that the use of a protective poly(methyl methacrylate) layer, positioned on top of the BP channel, does not affect the electrical properties of the device but avoids the degradation caused by the exposure to air.

Protein biosensor based on Schottky barrier nanowire field effect transistor

A top-down nanofabrication approach involving molecular beam epitaxy and electron beam lithography was used to obtain silicon nanowire-based back gate field-effect transistors with Schottky contacts on silicon-on-insulator (SOI) wafers. The resulting device is applied in biomolecular detection based on the changes in the drain-source current (IDS). In this context, we have explained the physical mechanisms of charge carrier transport in the nanowire using energy band diagrams and numerical 2D simulations in TCAD. The results of the experiment and numerical modeling matched well and may be used to develop novel types of nanowire-based biosensors.

Monolayer MoS2 photodetectors with a buried-gate field-effect transistor structure

Unlike zero-bandgap graphene, molybdenum disulfide (MoS2) has an adjustable bandgap and high light absorption rate, hence photodetectors based on MoS2 have attracted tremendous research attention. Most of the reported MoS2 photodetectors adopted back-gate field-effect transistor (FET) structure due to its easy fabrication and modulation features. However, the back-gate FET structure requires very high gate voltage up to 100 V, and it is impossible to modulate each device in an array with this structure independently. This work demonstrated a monolayer MoS2 photodetector based on a buried-gate FET structure whose experimental results showed that both the electrical and photoelectrical properties could be well modulated by a gate voltage as low as 3 V. A photoresponsivity above 1 A W−1 was obtained under a 395 nm light-emitting diode light illumination, which is over 2 orders of magnitude higher than that of a reported back-gate photodetector based on monolayer MoS2 (7.5 mA W−1). The photoresponsivity can be further improved by increasing the buried gate voltage and source-drain voltage. These results are of significance for the practical applications of MoS2 photodetectors, especially in the low voltage and energy-saving areas.