Bottom-gate Field-effect Transistors Research Articles

A Delamination Strategy for Thinly Layered Defect-Free High-Mobility Black Phosphorus Flakes.

Extraordinary electronic and photonic features render black phosphorus (BP) an important material for the development of novel electronics and optoelectronics. Despite recent progress in the preparation of thinly layered BP flakes, scalable synthesis of large-size, pristine BP flakes remains a major challenge. An electrochemical delamination strategy is demonstrated that involves intercalation of diverse cations in non-aqueous electrolytes, thereby peeling off bulk BP crystals into defect-free flakes comprising only a few layers. The interplay between tetra-n-butylammonium cations and bisulfate anions promotes a high exfoliation yield up to 78 % and large BP flakes up to 20.6 μm. Bottom-gate and bottom-contact field-effect transistors, comprising single BP flakes only a few layers thick, exhibit a high hole mobility of 252±18 cm2 V-1 s-1 and a remarkable on/off ratio of (1.2±0.15)×105 at 143 K under vacuum. This efficient and scalable delamination method holds great promise for development of BP-based composites and optoelectronic devices.

Graphene exfoliation in the presence of semiconducting polymers for improved film homogeneity and electrical performances

We report on the production of hybrid graphene/semiconducting polymer films in one step procedure by making use of ultrasound-assisted liquid-phase exfoliation of graphite powder in the presence of π-conjugated polymers, i.e. poly(3-hexylthiophene) (P3HT) or poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine] (PCDTPT). The polymers were chosen in view of their different propensity to form crystalline structures, their decoration with alkyl chains that are known to possess high affinity for the basal plane of graphene, the energy levels of their frontier orbitals which are extremely similar to the work function of graphene, and their high electrical performance when integrated in field-effect transistors (FETs). The polymers act as a dispersion-stabilizing agent and prevent the re-aggregation of the exfoliated graphene flakes, ultimately enabling the production of homogeneous bi-component dispersions. The electrical characterization of few-layer graphene/PCDTPT hybrids, when integrated as active layer in bottom-contact bottom-gate FETs, revealed an increase of the field-effect mobility compared to the π-conjugated-based pristine devices, a result which can be attributed to the joint effect of the few-layer graphene sheets and semiconducting polymers improving the charge-transport in the channel of the field-effect transistor. In particular, few-layer graphene/PCDTPT films displayed a 30-fold increase of PCDTPT's mobility if compared to pristine polymer samples. Such findings represent a step forward towards the optimization of graphene exfoliation and processing into electronic devices, as well as towards improved electrical performance in organic-based field-effect transistors.

Dual-Gate Black Phosphorus Field-Effect Transistors with Hexagonal Boron Nitride as Dielectric and Passivation Layers.

Two-dimensional black phosphorus (BP) has attracted much attention recently because of its applicability in high-performance electronic and optoelectronic devices. BP field-effect transistors (FETs) with a tunable band gap (0.3-1.5 eV) have demonstrated a high on-off current ratio and a high hole mobility with an ambipolar behavior in global-gated devices. However, local-gated BP FETs for integrated circuits have been reported with only p-type behaviors and a low on-current compared with global-gated BP FETs. Furthermore, BP, which is not stable in air, forms sharp spikes on its surface when exposed to humid air. This phenomenon plays a role in accelerating the degradation of the electrical properties of BP devices, which can occur even within a day. In this paper, we first demonstrate the origin of transport limitations of local-gated BP FETs by comparing the transport properties of hexagonal boron nitride (h-BN)-based device architectures with those of a bottom-gated BP FET on a Si/SiO2 substrate. By using h-BN as passivation and dielectric layers, BP FETs with a low gate operating voltage were fabricated with two different transistor geometries: top-gated and bottom-gated FETs. The highest mobility extracted from the global-gated BP FETs was 249 cm2 V-1 s-1 with a subthreshold swing of 848 mV dec-1.

Laser induced forward transfer of graphene

Transfer of graphene and other two-dimensional materials is still a technical challenge. The 2D-materials are typically patterned after transfer, which leads to a major loss of material. Here, we present laser induced forward transfer of chemical vapor deposition grown graphene layers with well-defined shapes and geometries. The transfer is based on photo-decomposition of a triazene-based transfer layer that produces N2 gas, which propels a graphene layer from the donor to the acceptor substrate. The functionality of the graphene-metal junction was verified by realizing functional bottom contact bottom gate field-effect transistors.

Facile synthesis of a narrow-bandgap strong-donor-alt-strong-acceptor copolymer of poly(5,6-difluorobenzo-[c][1,2,5]-thiadiazole-alt-5H-dithieno[3,2-b:2′,3'-d]pyran) via direct C-H arylation polymerization

A narrow bandgap D-A conjugated polymer based on 5H-dithieno [3,2-b:2′,3'-d]pyran alternating with 5,6-difluoro-2,1,3-benzothiadiazole (denoted as PDFBT-alt-DTP) was synthesized via direct C-H arylation polymerization (DAP) as an atomically efficient protocol. The optimal reaction condition for the DAP gave the target polymers with moderate number-average molecular weights (Mn) using optimized catalytic condition of Pd2 (dba)3/(o-MeOPh)3P/PivOH/K2CO3 in o-xylene. UV-vis-NIR absorption spectra of the obtained polymers show the presence of aggregation and interchain interaction in thin films. 1H-NMR spectroscopic analysis indicates good C-H selectivity corresponding to an alternating strong-donor-alt-strong-acceptor conjugated backbone. The hole mobility of the resulting polymers in a magnitude of 10−4 cm2V−1s−1 was reached in bottom-gate top-contact field-effect transistors fabricated and tested under ambient conditions.

Poly (ethylene imine)-modulated transport behaviors of graphene field effect transistors with double Dirac points

The bottom-gated graphene field-effect transistors (GFETs) with HfO2 as the dielectric layer are fabricated and the transport behaviors with double Dirac points defined as the right conductance minima (VDirac+) and the left one (VDirac-) are obtained, which exhibit the unique W-shaped transform characteristics rather than the typical V-shape. This observation indicates that the graphene in the channel region shows the different doping behaviors from that underlying the metal contact region. The W-shaped characteristics of GFETs are affected by the ambient environment and the time-dependence of double Dirac points has been also found. After being treated by Poly (ethylene imine) (PEI), the transport behaviors of GFETs could be modulated, especially for VDirac+ showing more obvious negative shift, indicating that PEI has a remarkable n-doping effect on the graphene in the channel region. PEI also screens the environmental influence and could protect the graphene from being p-doped to some extent. At the same time, the annealing effect has been also studied and it is found that the proper annealing process could contribute to the recovery of the graphene properties.

Bio-fabrication of nanomesh channels of single-walled carbon nanotubes for locally gated field-effect transistors

Single-walled carbon nanotubes (SWNTs) are one of the promising electronic components for nanoscale electronic devices such as field-effect transistors (FETs) owing to their excellent device characteristics such as high conductivity, high carrier mobility and mechanical flexibility. Localized gating gemometry of FETs enables individual addressing of active channels and allows for better electrostatics via thinner dielectric layer of high k-value. For localized gating of SWNTs, it becomes critical to define SWNTs of controlled nanostructures and functionality onto desired locations in high precision. Here, we demonstrate that a biologically templated approach in combination of microfabrication processes can successfully produce a nanostructured channels of SWNTs for localized active devices such as local bottom-gated FETs. A large-scale nanostructured network, nanomesh, of SWNTs were assembled in solution using an M13 phage with strong binding affinity toward SWNTs and micrometer-scale nanomesh channels were defined using negative photolithography and plasma-etching processes. The bio-fabrication approach produced local bottom-gated FETs with remarkably controllable nanostructures and successfully enabled semiconducting behavior out of unsorted SWNTs. In addition, the localized gating scheme enhanced the device performances such as operation voltage and Ion/Ioff ratio. We believe that our approach provides a useful and integrative method for fabricating electronic devices out of nanoscale electronic materials for applications in which tunable electrical properties, mechanical flexibility, ambient stability, and chemical stability are of crucial importance.

OLED Operating Active Matrix Pixel Device Using Two Dimensional Transition Metal Dichalcogenides Channel FET

Two-dimensional transition metal dichalcogenides (TMDs) are fundamentally and technologically have been studied for their electronic and physical properties. The simplest Active Matrix pixel has two transistors and one capacitor, one is switch transistor and another is driver transistor. Here we fabricated p-WSe2 channel bottom gate field effect transistors (FETs) on the same glass to organize Active Matrix pixel. Linear mobilities of Switch transistor and Driver transistor were estimated to be ~30 and ~320 cm2/V s and saturation mobilities were estimated to be ~11.7 and 102.2 cm2/V s respectively. Maximum ON-current of driver transistor was measured to be ~387 uA and maximum ON/OFF ratio of switch transistor appeared to be 8x107 V. As a result, our Active Matrix pixel demonstrate organic light emitting diode (OLED) switching for green OLED and showing OLED gray scale properties. Figure 1

Highly Contorted 1,2,5-Thiadiazole-Fused Aromatics for Solution-Processed Field-Effect Transistors: Synthesis and Properties.

A straightforward strategy has been used to construct 1,2,5-thiadiazole-fused 12-ring π systems through twofold Stille coupling and subsequent cyclodehydrogenation by utilizing the building blocks of naphthodithiophene and 5,6-substituted benzo[b]-2,1,3-thiadidazole. Molecules 1 a and 1 b, which exhibit highly contorted π surfaces, show a butterfly-shaped conformation according to DFT calculations. Within the molecules, a plane-to-plane angle of 44.8° was found. UV/Vis absorption, thermogravimetric analysis, differential scanning calorimetry, and cyclic voltammetry (CV) were used to study their physical properties. Strong intermolecular interactions of the nonplanar molecules were also observed by concentration-dependent (1) H NMR spectroscopy measurements and thin-film XRD characterization. The low-lying LUMO and high-lying HOMO levels of the molecules are -3.73 and -5.48 eV, respectively, as estimated from CV measurements; this indicates their potential as semiconducting materials for solution-processed organic field-effect transistors (OFETS). A field-effect hole mobility of up to 0.035 cm(2) V(-1) s(-1) , a threshold voltage of 6.98 V, and a current on/off ratio of 8.65×10(5) in air for 1 a have been demonstrated with the top-contact bottom-gate field-effect transistor device structures; this represents an important step toward the solution-processed OFET application of contorted aromatics.

Graphite mediated reduction of graphene oxide monolayer sheets

Large area graphene oxide (GO) monolayer sheets were transferred on Si and SiO2/Si substrates by Langmuir–Blodgett technique and reduced by heat treatment in the presence of graphite powder, inside an evacuated and sealed enclosure. Reduction at 1000 °C results in a decrease of sheet thickness to ∼0.5 nm without any morphological changes, and de-oxygenation (O/C ratio 5%) accompanied by enhancement in sp2-C to 84%. Red-shift of G-peak to ∼1585 cm−1, decrease in I(D)/I(G) ratio and appearance of an intense G′-peak as a single Lorentzian, are indicative of substantial reduction in defects and restoration of graphitic network. Ultraviolet photoelectron spectroscopy (UPS) studies show large increase in density of states (DOS) in the immediate vicinity of Fermi level and decrease in work function after reduction. Bottom gated field effect transistors fabricated with isolated RGO sheets display ambipolar behavior, with charge neutrality point at a positive gate voltage, indicating p-type nature, consistent with UPS results. RGO sheets exhibit conductivity of (2–3) × 103 S/cm and field effect mobility of (20–45) cm2/Vs, which are substantially higher than the values usually reported for RGO sheets, particularly those obtained by chemical reduction followed by heat treatment.

N-type In2S3 films deposited by pulsed laser deposition: effect of laser power on the properties of the films

Pulsed laser deposition (PLD) with different levels of laser power was first used to deposit In2S3 films from homemade, high-purity In2S3 targets. This process was followed by post-annealing in an N2 atmosphere to improve the films’ crystallinity and conductivity. The annealed films were verified to be stoichiometric, body-centered, tetragonal In2S3 with the preferred orientation (103). The bandgap of the films decreased from 2.8 to 2.2 eV with an increase in the laser power, which was believed to be the result of the grain growth caused by the higher laser power. The electrical transport property of the bottom-gate field-effect transistor revealed the n-type conduction of the annealed In2S3 films, and the heterojunction p+-Si/annealed In2S3 film showed remarkable photovoltaic behavior upon light illumination, indicating that PLD-deposited In2S3 films may have great potential as a buffer layer in thin-film solar cells. What’s more, doped In2S3 films can be easily realized due to the fairly stoichiometric transfer of the PLD method.

Simple yet Versatile Synthesis of CuInSexS2–x Quantum Dots for Sunlight Harvesting

Common approaches to synthesizing alloyed CuInSexS2–x quantum dots (QDs) employ high-cost, air-sensitive phosphine complexes as the selenium precursor. Such methods typically offer low chemical yields and only moderate emission efficiencies, particularly for selenium-rich compositions. Here we demonstrate that such hazardous and air-sensitive selenium precursors can be completely avoided by utilizing a combination of thiols and amines that is very effective at reducing and then complexing with elemental selenium to form a highly reactive selenium precursor at room temperature. The optical properties of the CuInSexS2–x QDs synthesized by this new approach can be finely tuned for optimal sunlight harvesting through control of QD size and composition. In order to demonstrate the importance of such material tunability, we incorporate QDs into liquid-junction Grätzel solar cells and study correlations between varied QD size and composition and the resulting device performance. We also investigate charge transport in films of CuInSexS2–x QDs by incorporating them into bottom-gate field effect transistors. Such films exhibit measurable p-type conductance even without exchange of the long native surface ligands, and the film’s conductance can be improved by more than 3 orders of magnitude by replacing native ligands with shorter ethanedithiol molecules. The results of this study indicate the significant promise of CuInSexS2–x QDs synthesized by this method for applications in photovoltaics utilizing both sensitized and p–n junction architectures.

Solution-processed single-crystalline organic transistors on patterned ultrathin gate insulators

Single-crystalline organic transistors of 3,11-didecyl-dinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10-DNBDT-NW) and 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) were fabricated by solution processes on top of the patterned hybrid ultrathin gate dielectrics consisting of 3.6nm-thick aluminum oxide and self-assembled monolayers (SAMs). Due to the excellent crystallinity of the channel films, bottom-gate and top-contact field-effect transistors exhibited the average field-effect mobility of 3.7cm2/Vs and 4.3cm2/Vs for C10-DNBDT-NW and C10-DNTT, respectively. These are the first successful devices of solution-processed single-crystalline transistors on ultrathin gate dielectrics with the mobility above 1cm2/Vs, opening the way to develop low-power-consumption and high-performance printed circuits.

Blending effect of 6,13-bis(triisopropylsilylethynyl) pentacene–graphene composite layers for flexible thin film transistors with a polymer gate dielectric

Solution processible poly(4-vinylphenol) is employed as a transistor dielectric material for low cost processing on flexible substrates at low temperatures. A 6,13-bis (triisopropylsilylethynyl) (TIPS) pentacene–graphene hybrid semiconductor is drop cast to fabricate bottom-gate and bottom-contact field-effect transistor devices on flexible and glass substrates under an ambient air environment. A few layers of graphene flakes increase the area in the conduction channel, and form bridge connections between the crystalline regions of the semiconductor layer which can change the surface morphology of TIPS pentacene films. The TIPS pentacene–graphene hybrid semiconductor-based organic thin film transistors (OTFTs) cross-linked with a poly(4-vinylphenol) gate dielectric exhibit an effective field-effect mobility of 0.076 cm2 V−1 s−1 and a threshold voltage of −0.7 V at V gs = −40 V. By contrast, typical TIPS pentacene shows four times lower mobility of 0.019 cm2 V−1 s−1 and a threshold voltage of 5 V. The graphene/TIPS pentacene hybrids presented in this paper can enhance the electrical characteristics of OTFTs due to their high crystallinity, uniform large-grain distribution, and effective reduction of crystal misorientation of the organic semiconductor layer, as confirmed by x-ray diffraction spectroscopy, atomic force microscopy, and optical microscopy studies.

A Route for Modulating the Diameter of Cylindrical Silicon Nanowires by Using Thermal Self-Ordering Silver Nanoparticles

For the synthesis of uniform sub-80-nm silicon nanowires (Si NWs), we introduce a metal-assisted chemical etching (MCE)-based facile and high-yield route, employing simple thermal annealing and vacuum deposition processes. Under rapid thermal annealing, an ultrathin silver (Ag) film on a Si substrate is self-organized into Ag nanoparticles (NPs), which are used for making Si nanoholes through a short MCE process. After sputter deposition of Au (10 nm)/Ag (20 nm) on the caved Si substrate with nanoholes, a nanomesh is obtained. Finally, with the nanomesh as an etching mask, Si NWs are successfully produced through a second MCE process. The size of the Si NWs can be modulated by controlling the thickness of the initial Ag film. The minimum diameter of the synthesized Si NWs is 30 ± 5 nm, and the maximum diameter is 68 ± 10 nm. Furthermore, to determine the uniformity of our Si NWs, bottom-gate field-effect transistors were fabricated and the linearity of the on-current level of these transistors with the number of addressed Si NWs was confirmed.

Ambipolar/unipolar conversion in graphene transistors by surface doping

An ambipolar/unipolar conversion of conduction polarity in bottom-gate graphene field-effect transistor (FET) was realized by intended/unintended surface doping. Exposing the graphene FET in air made it fully p-type while covering graphene with Al nanofilm or poly(ethylene imine) (PEI) layer yielded a recovery of ambipolar conduction. The alteration of the conduction polarity in graphene FET was due to hole or electron-doping effect on graphene. Distinct changes in carrier mobility and current-voltage relationship were discussed between graphene with Al and PEI doping, and the dielectric screening by PEI was proposed as the possible mechanism.

Using soft lithography to fabricate gold nanoparticle patterns for bottom-gate field effect transistors

In this study we developed an efficient method for the direct patterning of Au, through the self-assembly of gold nanoparticles (AuNPs) from colloidal solutions, onto Si substrates. We used an integrated process involving soft lithography and self-assembly to fabricate patterns of sub-micrometer AuNPs on the Si surface. By exploiting soft lithography, we obtained nanoscale patterns of self-assembled monolayers (SAMs) without the need for traditional photolithography or etching processes. After using soft lithography to pattern a SAM of (3-mercaptopropyl)trimethoxysilane on a Si surface, we immobilized AuNPs, synthesized in a two-phase system, to form the bottom layer. We then used a cross-linking agent, 1,6-hexanedithiol, to construct multiple layers of AuNP aggregates, ultimately self-assembling five-layer structures that we employed as nanoscale metal electrodes in the fabrication of organic field effect transistors (OFETs), which exhibited output characteristics, including sheet resistance, drain–source voltage, and drain–source current, similar to those of corresponding OFETs prepared using traditional processing.

Electric Bistability Induced by Incorporating Self-Assembled Monolayers/aggregated Clusters of Azobenzene Derivatives in Pentacene-Based Thin-Film Transistors

Composite films of pentacene and a series of azobenzene derivatives are prepared and used as the active channel material in top-contact, bottom-gate field-effect transistors. The transistors exhibit high field-effect mobility as well as large I-V hysteresis as a function of the gate bias history. The azobenzene moieties, incorporated either in the form of self-assembled monolayer or discrete multilayer clusters at the dielectric surface, result in electric bistability of the pentacene-based transistor either by photoexcitation or gate biasing. The direction of threshold voltage shifts, size of hysteresis, response time, and retention characteristics all strongly depend on the substituent on the benzene ring. The results show that introducing a monolayer of azobenzene moieties results in formation of charge carrier traps responsible for slower switching between the bistable states and longer retention time. With clusters of azobenzene moieties as the trap sites, the switching is faster but the retention is shorter. Detailed film structure analyses and correlation with the transistor/memory properties of these devices are provided.

Annealing-induced conductivity transition in ZnO nanowires for field-effect devices

We report on the fabrication of ZnO nanowire (NW) devices in which the NWs were annealed in air ambient for their conductivity change from conducting to semiconducting states. Ambient annealing at 600 °C effectively gained a good semiconducting state of ZnO NW. Either top- or bottom-gate NW field-effect transistors (FETs) with optimally annealed ZnO NW showed a high on/off current ratio of ∼106, while the NW FETs with the initially conducting NWs appeared to keep on-state only. Schottky diode with the annealed NW displayed an ideality factor of ∼1.51 along with an on/off ratio of ∼103.

Effect of Temperature and Humidity on $\hbox{NO}_{2}$ and $\hbox{NH}_{3}$ Gas Sensitivity of Bottom-Gate Graphene FETs Prepared by ICP-CVD

Graphene was used for a sensing part of bottom-gate field-effect transistor (FET)-type gas sensors, and the sensing performance of the sensors was studied. We investigated the effect of temperature <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(T)$</tex></formula> and relative humidity on the gas sensitivity of bottom-gate graphene FETs prepared by the inductively coupled plasma chemical vapor deposition method. The conductivity change of graphene exposed to nitrogen oxide <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\hbox{NO}_{2})$</tex></formula> and ammonia <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\hbox{NH}_{3})$</tex></formula> was increased with increasing temperature and humidity. For <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{NO}_{2}$</tex></formula> gas, the graphene FET shows increasing <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$I_{D}$</tex></formula> with increasing <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$T$</tex></formula> and also increasing <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $I_{D}$</tex></formula> with increasing humidity. However, the FET shows an opposite trend with the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$T$</tex></formula> and humidity for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \hbox{NH}_{3}$</tex></formula> gas.