Enhancement Of Field-effect Mobility Research Articles

β-Disubstituted Pentacene Derivatives: Thin Film Structural Properties and Four-Probe Field Effect Mobility.

2,9- and 2,10-diphenylpentacene were synthesized by direct C-H borylation of ketal-protected pentacene, followed by halodeboronation, resolution of the dihalo isomers, Suzuki arylation, cleavage of the ketals and decarbonylation in the solid state. They were studied as main active components in organic field effect transistors (OFETs). Diphenyl substitution of pentacene affects the unit cell dimensions only slightly, preserving a face to edge molecular packing in the first layers of thin films evaporated on SiO2 substrates. Both isomers self-assemble into nanoribbons during the thin film growth upon vapor deposition. The similarity between the surface induced phases of the 2,9-isomer and unsubstituted pentacene leads to similar 4-probe hole mobilities, i. e. 0.13 cm2 V-1 s-1 for the former. Whereas 2,9-disubstitution thus does essentially preserve the thin film characteristics of unsubstituted pentacene, 2,10-disubstitution is detrimental to the molecular ordering in the thin films and therefore to the field effect mobility which is only 0.07 cm2 V-1 s-1. The known strong enhancement of field effect mobility observed upon diphenyl substitution of anthracene can thus not be emulated analogously with pentacene.

Enhancement in Performance and Reliability of Transparent IGZO Thin-Film Transistors by ITO/Ti Stacked Source/Drain Contacts

Enhancement in performance and reliability of transparent IGZO thin-film transistor (TFT) was achieved by adopting a laminated structure with a thin ITO layer and an ultrathin Ti layer as the source/drain (S/D) contacts. Compared with the transparent TFT with pure ITO S/D contacts, the transparent TFT with the ITO/Ti S/D contacts showed three times of enhancement in field-effect mobility from 4.75 to 12.10 cm2/Vs, two times of enhancement in on/off current ratio from 7.0 × 107 to 1.54 × 108, three times of reduction in contact resistance from 15.74 to 4.64 kΩ, and a decrease in threshold voltage from 3.11 V to 2.80 V. The TFT with the ITO/Ti S/D contacts also maintained an extremely low leakage current at zero gate bias (for the device with channel width/channel length of 40 μm/5 μm, the leakage current was ∼1 × 10−13 A). In addition, the TFT with the ITO/Ti S/D contacts showed a hump-free transfer curve and a smaller shift in threshold voltage under negative bias illumination stress. The enhancement in performance and reliability makes the transparent TFT with the ITO/Ti S/D contacts very promising in transparent display applications.

The influence of contact metals on epitaxially grown molybdenum disulfide for electrical and optical device applications

Bottom-gate transistors with mono-layer MoS2 channels and polycrystalline antimonene source/drain contact electrodes deposited at 75 °C are fabricated. Significant performance enhancement of field-effect mobility 11.80 cm2 V−1·s−1 and >106 ON/OFF ratio are observed for the device. Increasing photocurrents are also observed for the MoS2 transistor under light irradiation, which is attributed to the reduced carrier recombination at the metal/2D material interfaces. The results have demonstrated that besides the matching of work function values with the 2D material channel, the crystallinity of the contact electrodes is the other important parameter for the Ohmic contact formation of 2D material devices.

Self-aligned stitching growth of centimeter-scale quasi-single-crystalline hexagonal boron nitride monolayers on liquid copper.

Two-dimensional hexagonal boron nitride (hBN) atomic crystals are excellent charge scattering screening interlayers for advanced electronic devices. Although wafer-scale single crystalline hBN monolayer films have been demonstrated on liquid Au and solid Cu (110) and (111) vicinal surfaces, their reproducible growth still remains challenging. Here, we report the facile self-aligned stitching growth of centimeter-scale quasi-single-crystalline hBN monolayer films through synergistic chemical vapor deposition growth kinetics and liquid Cu rheological kinetics control. The sublimation temperature of the ammonia borane precursor, H2 content and melting temperature of the Cu substrate are revealed to be the dominant factors that regulate hBN nucleation, growth and alignment. The flowing liquid Cu catalytic surface promotes efficient rotation of floating triangular hBN domains and provokes uniform self-alignment upon merging at a critical high temperature of 1105 °C. Identical aligned grains are constantly observed at multiple regions, which corroborate the homogeneous in-plane orientation and uniform stitching over the whole growth area. Continuous quasi-single-crystalline hBN monolayer films are produced by seamless stitching of aligned domains with the same polarity. The quasi-single-crystalline hBN monolayers are successfully included as charge scattering and trap site screening interlayers in the hBN/SiO2 gate insulator stack to build high performance InGaZnO field-effect transistors (FETs). Full suppression of hysteresis and twofold enhancement of field-effect mobility are realized for InGaZnO FETs built with hBN as the interface dielectric. The facile growth of large quasi-single-crystalline hBN monolayers on liquid Cu paves the way for future high-performance electronics.

Drain induced barrier increasing in multilayer ReS2

The interlayer tunneling resistivity (Rint) and Thomas-Fermi charge screening effects play critical roles in the carrier transport of two-dimensional (2D) multilayer devices. For example, the vertical electric field modifies the Rint, resulting in a channel migration along the c-axis. However, because Rint varies considerably with the drain electric field in addition to the vertical field, the effective contribution of each layer to the total current varies with the drain bias (VD). Here, we demonstrate a drain induced barrier increasing (DIBI) in 2D multilayer rhenium disulfide (ReS2) as a possible reverse short channel effect (rSCE). The reported decoupled layer interaction and much higher interlayer resistivity of ReS2 compared to other 2D materials allow us to observe the DIBI clearly. As VD increases, the effective amplitude of Rint decreases dramatically, leading to (i) the increase of off-current, (ii) the enhancement of field-effect mobility, and (iii) the blue shift of the flat band voltage (VFB), which is in sharp contrast to a previous report on a short-channel bulk-Si device with drain induced barrier lowering (DIBL). We attribute this difference to the weakened Fermi level (EF) tunability when the channel migrates from the bottom to the top surface of ReS2, and to an opposite electrostatic force resulting from VD, implying the increased importance of VD-dependent Rint in the carrier transport mechanism of 2D multilayer systems. The VD-dependent Coulomb scattering parameter probed via a low frequency (LF) noise analysis provides deeper insights for the channel shift. Our findings pave the way for understanding and exploiting the fundamental charge transport mechanism in 2D multilayer systems.

Fluoroelastomer encapsulation for enhanced reliability of solution-processed carbon nanotube thin-film transistors

Single-walled carbon nanotube (SWCNT) has attracted considerable attention as a promising semiconducting material due to its high field-effect mobility, high current-carrying capability, and superior mechanical durability. However, SWCNT-based thin-film transistors (SWCNT-TFTs) generally show insufficient electrical reliability in terms of the drain-current hysteresis and threshold voltage shift under the bias stress. To facilitate the great advantages of SWCNT in commercial electronic devices, strategies are required for enhancing the electrical reliability of SWCNT-TFTs without compromising their mechanical flexibility. In this work, we demonstrate solution-processed, hysteresis-free, and bias-stable SWCNT-TFTs encapsulated with elastomeric poly(vinylidene fluoride-co-hexafluoropropylene) (e-PVDF-HFP) using a lamination method. The dipolar nature of e-PVDF-HFP suppresses the charge trapping, resulting in elimination of hysteresis as well as enhancement of field-effect mobility and subthreshold slope. The bias instability under the negative gate bias stress is also alleviated because the hydrophobic encapsulation layer can exclude physisorbed water/oxygen molecules on the hydrophilic surface. Besides, our solvent-free lamination method effectively prevents the potential damages of the active and electrode layers from the use of the solvents. The proposed encapsulation method with laminating the e-PVDF-HFP film can provide a possibility for realization of wearable applications based on highly reliable solution-processed SWCNT-TFT.

Enhanced piezoelectric tactile sensing behaviors of high-density and low-damage CF4-plasma-treated IGZO thin-film transistors coated by P(VDF-TrFE) copolymers

Piezoelectric pressure sensing behaviors of high-density and low-damage CF4-plasma-treated indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) coated by poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) films have been investigated. The CF4 plasma treatment was performed on the IGZO channel by using an inductively coupled plasma (ICP) system with a quartz filter. Prior to the fabrication of devices, X-ray diffractometer (XRD) and atomic force microscopy (AFM) were applied to identify the enhancement in the crystallinity of P(VDF-TrFE) films on the plasma-fluorinated IGZO films. With the analyses of X-ray photoelectron spectroscopy (XPS), it is proved that the fluorine radicals reacted with the metal-oxygen bonds will increase the oxygen vacancies in IGZO films, contributing to an enhancement in field effect mobility (μFE) and drain current (IDS) of IGZO TFTs. Furthermore, the fluorine atoms diffused from the IGZO channel into the bottom SiO2 layer were confirmed by secondary ion-mass spectroscopy (SIMS), reducing the interfacial and oxide charges of the devices for a negative shift in threshold voltage (Vt). Under a 0.5-kg applied force press/release cyclic test, a 4.8-fold increase in drain current response of 1-min CF4-plasma-treated piezoelectric pressure sensors was optimized, because of the enhanced electrical behaviors of IGZO TFTs and an increase in aligned dipole moments of P(VDF-TrFE) copolymers. The promising results make the piezoelectric pressure sensors with high-density and low-damage CF4-plasma-treated IGZO TFTs coated by P(VDF-TrFE) copolymer suitable for future high-performance tactile sensing applications.

Effect of non-chlorinated solvents on the enhancement of field-effect mobility in dioctylbenzothienobenzothiophene-based top-gate organic transistors processed by spin coating

Abstract The use of non-chlorinated solvents for film deposition in organic field-effect transistors (OFETs) is highly desirable to fabricate flexible and printable electronic circuits by environmentally friendly processes. Here, we investigate the influence of the use of non-chlorinated solvents, such as toluene, p-xylene, cyclohexanone, and mesitylene, for the deposition of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films by spin coating on the morphologies of these semiconductor films and the electrical characteristics of OFETs with a top-gate configuration. The results show that the size of polycrystalline domains in spin-coated C8-BTBT films tends to increase as the boiling point (b.p.) of the solvent increases, whereas an improvement in the field-effect mobility is restricted by the decrease in the extent to which the C8-BTBT film covers the substrate due to the long evaporation time of high b.p. solvents. The addition of the high b.p. solvent p-xylene to toluene allows a significant increase in polycrystalline domain sizes in spin-coated C8-BTBT films. Further, spin coating C8-BTBT using the high b.p. solvent mesitylene at a high spin rate is effective for the production of thinner C8-BTBT films with diminished surface roughness. Top-gate C8-BTBT FETs processed by spin coating using a mixed binary solvent and by high-speed spin coating achieved high average field-effect mobilities of 4.9 and 5.4 cm2 V−1 s−1, respectively.

Field effect enhancement in buffered quantum nanowire networks

III-V semiconductor nanowires have shown great potential in various quantum transport experiments. However, realizing a scalable high-quality nanowire-based platform that could lead to quantum information applications has been challenging. Here, we study the potential of selective area growth by molecular beam epitaxy of InAs nanowire networks grown on GaAs-based buffer layers. The buffered geometry allows for substantial elastic strain relaxation and a strong enhancement of field effect mobility. We show that the networks possess strong spin-orbit interaction and long phase coherence lengths with a temperature dependence indicating ballistic transport. With these findings, and the compatibility of the growth method with hybrid epitaxy, we conclude that the material platform fulfills the requirements for a wide range of quantum experiments and applications.

All-dry transferred single- and few-layer MoS2 field effect transistor with enhanced performance by thermal annealing

We report on the experimental demonstration of all-dry stamp transferred single- and few-layer (1L to 3L) molybdenum disulfide (MoS2) field effect transistors (FETs), with a significant enhancement of device performance by employing thermal annealing in moderate vacuum. Three orders of magnitude reduction in both contact and channel resistances have been attained via thermal annealing. We obtain a low contact resistance of 22 kΩ μm after thermal annealing of 1L MoS2 FETs stamp-transferred onto gold (Au) contact electrodes. Furthermore, nearly two orders of magnitude enhancement of field effect mobility are also observed after thermal annealing. Finally, we employ Raman and photoluminescence measurements to reveal the phenomena of alloying or hybridization between 1L MoS2 and its contacting electrodes during annealing, which is responsible for attaining the low contact resistance.

Interplay of Orientation and Blending: Synergistic Enhancement of Field Effect Mobility in Thiophene-Based Conjugated Polymers

Trade-off between mechanical flexibility due to amorphicity and highly facile charge transport emanating from molecular crystallinity demands the orientation of conjugated polymers (CPs) for their utilization as active semiconducting material for flexible organic electronics. We have already demonstrated that it is rather easy to orient nonregiocontrolled poly(3-hexylthiophenes) (NR-P3HT) as compared to their highly regioregular counterparts due to very high alkyl chain interdigitation. To provide an amicable solution, efforts have been directed to orient blends of two CPs such as NR-P3HT (amorphous and flexible) and poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) (crystalline and facile charge transport) using a solution-based procedure floating film and transfer method (FTM). FTM-processed thin films of this blend system exhibited very high field effect transistor (FET) mobility reaching up to 0.1 cm2/V s, which is much higher than the corresponding individual CPs. In spite of onl...

Enhancement of Field-Effect Mobility By Controlling the Surface Potential of PVP Insulator in Pentacene-Based Thin Film Transistors

In recent years, the surface potential at insulator/semiconductor interface has received considerable attention for improving the electrical performance of various electronic and optoelectronic device. Many studies of the effect of a surface potential at insulator/semiconductor interface have been carried out. Most of these studies have focused on achieving efficient hole or electron injection at the inorganic insulator (i.e., such as SiO2)/ semiconductor interface. But the surface potential effects at the polymer insulator/semiconductor interface of organic thin-film transistors have not been sufficiently studied. In this presentation, we focus in particular on the effects of varying the surface potential on the polymer insulator/semiconductor interface. We demonstrated a method of reducing the surface energy in a polymer insulator for use in high-performance OTFTs. The surface energy of a poly(4-vinyl phenol) insulator was reduced from 45 mJ/m2 to 32 mJ/m2 through treatment with UV/ozone and ODTS. These treatments facilitated the deposition of the pentacene film onto the ODTS-treated poly(4-vinyl phenol) insulator to prepare a single phase that showed better π-orbital overlap among the pentacene molecules. As a result, the field-effect mobility in the insulator increased by 209 %. These results demonstrated that adjusting the surface energy of a polymer insulator provided a suitable approach to enhancing the performances of OTFTs. Acknowledgments This study was supported by the R&D program of the Korea Research Council for Industrial Science and Technology of Republic of Korea (Grant 2015K000217/ 10052802/ KM3710/ NK186C/ SC1130/ MO5970/ NM8270).

Enhancement of field-effect mobility in molybdenum-disulfide transistor through the treatment of low-power oxygen plasma

We experimentally demonstrate that the treatment with low-power oxygen plasma can raise the field-effect mobility in molybdenum-sulfide (MoS2) transistors from 0.01 to 9.6 cm2 V−1 s−1, which is an increment of about three orders of magnitude. The decrease in threshold voltage and the increase in the drain current of the devices indicate that the electron density increases significantly after the mild plasma treatment. The emergence of Mo–O characteristics but the suppression of Mo–S features on the X-ray photoelectron spectrum of the plasma-treated sample suggests that a portion of the MoS2 film becomes conductive molybdenum oxide. This transformation may considerably upgrade the performance of MoS2 transistors.

Wavy Channel TFT-Based Digital Circuits

We report a wavy channel (WC) architecture thin-film transistor-based digital circuitry using ZnO as a channel material. The novel architecture allows for extending device width by integrating vertical finlike substrate corrugations giving rise to 50% larger device width, without occupying extra chip area. The enhancement in the output drive current is 100%, when compared with conventional planar architecture for devices occupying the same chip area. The current increase is attributed to both the extra device width and 50% enhancement in field-effect mobility due to electrostatic gating effects. Fabricated inverters show that WC inverters can achieve two times the peak-to-peak output voltage for the same input when compared with planar devices. In addition, WC inverters show 30% faster rise and fall times, and can operate up to around two times frequency of the planar inverters for the same peak-to-peak output voltage. WC NOR circuits have shown 70% higher peak-to-peak output voltage, over their planar counterparts, and WC pass transistor logic multiplexer circuit has shown more than five times faster high-to-low propagation delay compared with its planar counterpart at a similar peak-to-peak output voltage.

High Performance HfO2 Back Gated Multilayer MoS2 transistors

A new substrate ( $\sim 30$ -nm HfO2/Si) is developed for high-performance back-gated molybdenum disulfide (MoS2) transistors. Record drain current $I_{\mathrm{ ds}} \sim $ 180 $\mu \text{A}/\mu \text{m}$ and transconductance value $g_{m} \sim 75~\mu \text{S}/\mu \text{m}$ at $V_{\mathrm{ ds}} = 1$ V have been achieved for 1- $\mu \text{m}$ channel length multilayer MoS2 transistors on HfO2/Si substrate. The transistors on HfO2 substrate show $> 2.5\times $ enhancement in field effect mobility ( $\mu _{\mathrm{ FE}}\sim 65$ cm2/V $\cdot $ s) compared with the transistors on SiO2 ( $\mu _{\mathrm{ FE}} \sim 25$ cm2/V $\cdot $ s) substrate. The intrinsic mobility extracted from $Y$ function technique ( $\mu _{\mathrm{ FE}} \sim 154$ cm2/V $\cdot $ s) is $3\times $ more than SiO2 substrate. The drastic improvement in transistor performance is attributed to a combination of three factors: 1) efficient gate coupling with an EOT of 6.2 nm; 2) charge impurity screening due to high- $k$ dielectric; and 3) very low contact resistance through sulfur treatment.

High mobility organic field-effect transistor based on water-soluble deoxyribonucleic acid via spray coating

High mobility organic field-effect transistors (OFETs) by inserting water-soluble deoxyribonucleic acid (DNA) buffer layer between electrodes and pentacene film through spray coating process were fabricated. Compared with the OFETs incorporated with DNA in the conventional organic solvents of ethanol and methanol: water mixture, the water-soluble DNA based OFET exhibited an over four folds enhancement of field-effect mobility from 0.035 to 0.153 cm2/Vs. By characterizing the surface morphology and the crystalline structure of pentacene active layer through atomic force microscope and X-ray diffraction, it was found that the adoption of water solvent in DNA solution, which played a key role in enhancing the field-effect mobility, was ascribed to both the elimination of the irreversible organic solvent-induced bulk-like phase transition of pentacene film and the diminution of a majority of charge trapping at interfaces in OFETs.

Performance improvement of a pentacene organic field-effect transistor through a DNA interlayer

Organic field-effect transistors (OFETs) with a top-contact structure were fabricated by applying deoxyribonucleic acid (DNA) as a hole injection layer using a simple spray-coating process. Compared with that of the bare OFET, the OFETs incorporated with a DNA hole injection layer exhibited a remarkable enhancement of field-effect mobility from 0.02 to 0.104 cm2 V−1 s−1. By analysing the electrical characteristics of OFETs and the surface morphology of pentacene film, the results showed that the dipole formation effect of the DNA interlayer effectively reduced the contact resistance between gold electrodes and pentacene. Consequently, improved hole injection was obtained along with enhanced electrical characteristics. This effect was also observed in OFETs based on an α-hexathiophene organic semiconductor and OFETs based on silver electrodes. In addition, a guanine base-pair, a major constituent of DNA, was selected to be further analysed and confirmed the existence of the dipole formation effect.

Enhancement of field-effect mobility due to structural ordering in poly(3-hexylthiophene) films by the dip-coating technique

Organic field-effect transistors (OFETs) were fabricated by depositing a regioregular poly(3-hexylthiophene) (P3HT) active layer using a dip-coating method. The field-effect mobility in OFETs depends on chain orientation and crystallinity and is related to direction and withdrawal speed with respect to the source/drain orientation. In this paper, how to control the structural and transport properties of P3HT films by coating parallel and perpendicular to the dipping direction is demonstrated. X-ray diffraction curves taken in the perpendicular direction exhibit a higher degree of crystalline ordering and edge-on conformation compared with those in the parallel direction; this finding correlates with the directional anisotropy of the OFET mobility. Both structural anisotropy and transport properties are enhanced upon thermal treatment.

Mobility enhancement in amorphous InGaZnO thin-film transistors by Ar plasma treatment

The effects of Ar plasma treatment on the back-channel of amorphous InGaZnO (a-IGZO) thin-film transistors are investigated. A decrease in metallic ion-oxygen bonding in the Ar plasma-treated a-IGZO channel layer was observed by X-ray photoelectron spectroscopy (XPS) depth profile analysis. An increase in the channel charge carrier concentration is estimated from the increased oxygen vacancy atomic ratio using XPS curve decomposition analysis. The plasma-treated area of the a-IGZO back-channel is varied with a photoresist screening layer with a varied open window length (Lp). From the Lp-dependent channel resistance analysis, a carrier concentration-dependent field-effect mobility enhancement is observed.

Light exposure dependence of field-effect mobility of pentacene thin films deposited on very thin polyimide photo-alignment layers

We report the anisotropic field-effect hole mobility of 30 nm-thick pentacene films deposited on 1.6 nm-thick polyimide photo-alignment layers with different in-plane anisotropy. To induce different in-plane anisotropy, the light exposure was varied up to 16 J/cm2. In this light exposure range, the long molecular axis of pentacene was standing with respect to the substrate surface (c*-axis oriented “thin film phase”), and the pentacene molecular plane was oriented on average parallel to the average alignment direction of the underlying polyimide backbone structures. For light exposures of 6 and 11 J/cm2, the field-effect mobility for the current flow direction parallel (perpendicular) to the alignment direction of pentacene molecular plane was enhanced (suppressed) compared to that of the pentacene film deposited on non-aligned polyimide film. This field-effect mobility enhancement shows a positive effect of the in-plane alignment control of pentacene molecules. However, the field-effect mobility decreased at a light exposure of 16 J/cm2, even though the in-plane orientation order of the pentacene molecular plane slightly increased. The causes for the mobility enhancement at 6 and 11 J/cm2 and the mobility decrease at 16 J/cm2 were discussed.