Field-effect Carrier Mobility Research Articles

Interfacial Ring Orientation in Polythiophene Field-Effect Transistors on Functionalized Dielectrics

The impact of surface chemistry on interfacial molecular orientation is studied for polymeric organic field-effect transistor (oFET) assemblies using vibrational sum frequency generation (VSFG) spectroscopy. The carbon–carbon vibrational modes on the backbone of side-chain deuterated poly(3-hexylthiophene) (DP3HT) are demonstrated to be an excellent handle for molecular orientation. DP3HT is utilized to avoid overlap in this spectral region with alkyl CH bending vibrations. Raman and FTIR spectroscopies are used to characterize the vibrational spectra of the thin films, and band assignments are confirmed by DFT calculations. Organosilane self-assembled monolayers are used to prepare oFET dielectrics with a range of surface energies, as measured by their water contact angles. The surface chemistry is found to have a profound influence on the field-effect carrier mobilities with lower surface energies producing higher mobilities. Polarization selective VSFG spectroscopy is then used to determine the relative orientation of the C═C symmetric stretching mode with respect to the surface normal. High field-effect mobilities for DP3HT on low surface energy functionalized dielectrics are directly correlated with the relative orientations of this vibrational transition dipole moment as measured by VSFG. The connection between this nonlinear spectroscopic observable and molecular structure enables this approach to confirm that low surface energy dielectrics lead to edge-on orientation of DP3HT conjugated chains with consequently higher mobilities.

Characteristics of sputtered Al-doped ZnO films for transparent electrodes of organic thin-film transistor

Aluminum-doped ZnO (AZO) thin-films were deposited with various RF powers at room temperature by radio frequency (RF) magnetron sputtering method. The electrical properties of the AZO film were improved with the increasing RF power. These results can be explained by the improvement of the crystallinity in the AZO film. We fabricated the organic thin-film transistor (OTFT) of the bottom gate structure using pentacene active and poly-4-vinyl phenol gate dielectric layers on the indium tin oxide gate electrode, and estimated the device properties of the OTFTs including drain current–drain voltage (I D–V D), drain current–gate voltage (I D–V G), threshold voltage (V T), on/off ratio and field effect mobility. The AZO film that grown at 160 W RF power exhibited low resistivity (1.54 × 10 − 3 Ω·cm), high crystallinity and uniform surface morphology. The pentacene thin-film transistor using the AZO film that's fabricated at 160 W RF power exhibited good device performance such as the mobility of 0.94 cm 2/V s and the on/off ratio of ~ 10 5. Consequently, the performance of the OTFT such as larger field-effect carrier mobility was determined the conductivity of the AZO source/drain (S/D) electrode. AZO films prepared at room temperature by the sputtering method are suitable for the S/D electrodes in the OTFTs.

Synthesis and characterization of thiazolothiazole-based polymers and their applications in polymer solar cells

A novel series of thiazolothiazole (Tz)-based copolymers, poly[9,9-didecylfluorene-2,7-diyl-alt-2,5-bis-(3-hexylthiophene-2-yl)thiazolo[5,4-d]thiazole] (P1), poly[9,9-dioctyldibenzosilole-2,7-diyl-alt-2,5-bis-(3-hexylthiophene-2-yl)thiazolo[5,4-d]thiazole] (P2), and poly[4,4′-bis(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole-alt-2,5-bis-(3-hexylthiophene-2-yl)thiazolo[5,4-d]thiazole] (P3), were synthesized for the use as donor materials in polymer solar cells (PSCs). The field-effect carrier mobilities and the optical, electrochemical, and photovoltaic properties of the copolymers were investigated. The results suggest that the donor units in the copolymers significantly influenced the band gap, electronic energy levels, carrier mobilities, and photovoltaic properties of the copolymers. The band gaps of the copolymers were in the range of 1.80–2.14 eV. Under optimized conditions, the Tz-based polymers showed power conversion efficiencies (PCEs) for the PSCs in the range of 2.23–2.75% under AM 1.5 illumination (100 mW/cm2). Among the three copolymers, P1, which contained a fluorene donor unit, showed a PCE of 2.75% with a short-circuit current of 8.12 mA/cm2, open circuit voltage of 0.86 V, and a fill factor (FF) of 0.39, under AM 1.5 illumination (100 mW/cm2). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Solution-processed polyfluorene–ZnO nanoparticles ambipolar light-emitting field-effect transistor

We report on a solution-processed polyfluorene (PFO)–ZnO nanoparticles composite light-emitting organic field-effect transistor (LE-OFET). The behavior of absorption, photoluminescence spectra and electroluminescence intensity of the PFO:ZnO hybrid films with different concentration of ZnO nanoparticles is analyzed. By changing the PFO/ZnO nanoparticles concentration ratio the PFO:ZnO OFET shows either unipolar or ambipolar behavior of current–voltage characteristics and operates in the hole/electron accumulation mode with current saturation behavior. The field effect mobility of charge carriers depends on the ZnO concentration. For the ambipolar PFO:ZnO OFET with modest PFO/ZnO nanoparticles ratio (1:0.2), well balanced electron and hole mobility values at 300 K are ∼0021 and ∼0029 cm 2/Vs, respectively, whereas for films with high ZnO concentration (1:1) the mobility (∼2 cm 2/Vs) is close to that of polycrystalline ZnO. The ambipolar PFO:ZnO LE-OFET emits light at both positive and negative gate bias. The working mechanism of the PFO:ZnO LE-OFET is investigated.

Electrodeposited Single Crystalline PbTe Nanowires and Their Transport Properties

Single crystalline PbTe nanowires were potentiostatically electrodeposited by a template-directed method using track-etched polycarbonate membranes as scaffolds in acidic nitrate baths. They exhibited a face-centered cubic (FCC) structure with a preferred growth direction about 31° against the [200] direction. By galvanic displacing the ends of PbTe nanowire with gold prior to electrode microfabrication, the Schottky barrier (i.e., native PbTe oxide) at the interfaces between nanowire and electrodes was eliminated/reduced to form an ohmic contact between nanowire and electrodes. Field effect transistor (FET) transfer characteristics indicated that the electrodeposited single-crystalline PbTe nanowires are p-type semiconductors with the estimated field effect carrier mobility and concentration of 3.32 ± 0.15 cm2/(V s) and 1.85 ± 1.06 × 1018 cm−3, respectively.

Synthesis and characterization of a thiazolo[5,4-d]thiazole-based copolymer for high performance polymer solar cells

A highly processable, new semiconducting polymer, PCDTTz, based on alternating thiazolothiazole and carbazole units was synthesized. The new polymer exhibited a field-effect carrier mobility of up to 3.8 × 10(-3) cm(2) V(-1) s(-1) and bulk heterojunction solar cells made from PCDTTz produced a power conversion efficiency of 4.88% under AM 1.5 G (100 mW cm(-2)) conditions.

Lyotropic chromonic liquid crystal semiconductors for water-solution processable organic electronics

We propose lyotropic chromonic liquid crystals (LCLCs) as a distinct class of materials for organic electronics. In water, the chromonic molecules stack on top of each other into elongated aggregates that form orientationally ordered phases. The aligned aggregated structure is preserved when the material is deposited onto a substrate and dried. The dried LCLC films show a strongly anisotropic electric conductivity of semiconductor type. The field-effect carrier mobility measured along the molecular aggregates in unoptimized films of LCLC V20 is 0.03 cm2 V−1 s−1. Easy processibility, low cost, and high mobility demonstrate the potential of LCLCs for microelectronic applications.

Investigation and Fabrication of Bottom Gate ZnO:Al TTFTs with Various Thicknesses of ZnO Buffer Layers

An Al doped ZnO (ZnO:Al) transparent thin film transistors (TTFTs) with various thickness of ZnO buffer layer sandwiched between gate insulator and channel ayer were deposited by a magnetron radio frequency co-sputter system. When the thickness of the buffer layer was 80 nm, the field-effect carrier mobility of the TTFTs was as high as 122.0 cm^2/V-s. Furthermore, the associated gate voltage swing was 0.24 V/decade, and the maximum state density was 2.69×10^11 eV^−1cm^−2. The on-to-off current ratio of the TTFTs with 80 nm-thick ZnO buffer layer was up to 5×10^7.

Ultra-long bismuth telluride nanoribbons synthesis by lithographically patterned galvanic displacement

We demonstrated the wafer level batch synthesis and fabrication of single semiconducting thermoelectric nanoribbon based devices by Lithographically Patterned Galvanic Displacement (LPGD). The shape, composition, and dimension of nanoribbons were tailored by adjusting deposition conditions. High resolution TEM images with fast Fourier transform (FFT)-converted selected area electron diffraction (SAED) patterns confirmed the formation of polycrystalline Bi2Te3 intermetallic compound with a rhombohedral structure without elemental Te and Bi. The thickness dependent electrical resistivity of BixTey nanoribbons shows a classic size effect due to the increase in surface boundary scattering. The as-synthesized nanoribbons were n-type semiconductors with no clear trend between field effect carrier mobility and composition, which might be attributed to the trapped charges at the interface between the channel and dielectric layer. The preliminary results on thermoelectric properties (i.e. Seebeck coefficient and power factor) show that the Seebeck coefficient of as-synthesized 0.1 µm thick Bi30Te70 nanoribbon is comparable with bulk counterparts, however, the power factor was lower because of poor crystallinity which leads to higher resistivity.

Electrical Characterization of 5 MeV Proton-Irradiated Few Layer Graphene

Few layer graphene (FLG) samples contacted were irradiated with protons at an energy of 5 MeV and doses up to 2 × 10 15 /cm 2 . The electrical properties of ungated FLG sheets contacted by Pd/Au in a source (S)-drain (D) configuration, including V DS -I DS , V G -I DS , and the hole mobility, were compared before and after proton irradiation. After irradiation, it is observed that the ambipolar conduction of the FLG sheets was changed to a p-type conduction. The field-effect mobility of the hole carriers and the resistance in the graphene sheets greatly decreased because the proton irradiation increased the number of the surface states.

Periodic arrays of organic crystals on polymer gate dielectric for low-voltage field-effect transistors and complementary inverter

Periodic arrays of highly oriented 7,7,8,8-tetracyanoquinodimethane (TCNQ) crystals, directly grown on a polymeric gate dielectric through a solution process, are used for the fabrication of a low-voltage organic field-effect transistor (OFET). Consequently, an organic complementary inverter using the TCNQ periodic array (n-channel) and pentacene (p-channel) is also reported. The TCNQ-based n-channel OFET exhibited very stable field-effect characteristics with low operational (2 V) and threshold voltages (<0.5 V). The highest field-effect carrier mobility in the saturation region was found to be 0.03 cm2 V−1 s−1. Furthermore, the organic complementary inverter showed good response characteristics in the low-voltage regime. The swing range of VOUT is same as VDD, ensuring “zero” static power consumption in the digital logic circuit. For the inverter with VDD = 2 V, the noise margin for low and high voltages are 1.0 V and 0.3 V, respectively. The logic threshold (VIN = VOUT) is 1.3 V and the maximum gain (−dVOUT/dVIN) of 4 is obtained at VIN = 1.3 V.

Synthesis of tellurium nanotubes by galvanic displacement

Tellurium nanotubes with controlled diameter and wall thickness were synthesized by galvanic displacement of cobalt nanowires and their temperature dependent field effect transistor and magnetoresistance properties were systematically investigated. The nanotube diameter was slightly larger than the sacrificial cobalt nanowire diameter with a wall thickness of range from 15 to 30 nm depending on the diameter of cobalt nanowires. Te nanotubes show p-type semiconducting property with the field effect carrier mobility of approx. 0.01 cm 2/V s which is relatively lower than other 1D nanostructure. Low mobility might be attributed to porous morphology with small grain size (<10 nm). Temperature dependent mobility also exhibiting a Conwell–Weisskopf relationship to temperatures below 250 K, indicating that the dominant scattering sites are ionized impurity centers. Unique MR behavior was observed from nanotube with a maximum magnetoresistance ratio of 37% at 260 K.

Synthesis and characterization of low-bandgap cyclopentadithiophene-biselenophene copolymer and its use in field-effect transistor and polymer solar cells

Novel series of conjugated copolymers, incorporating cyclopentadithiophene (CPDT) and the biselenophene (R-CPDT-Se2), were synthesized by Pd-catalyzed Stille coupling polymerization. The optical, electrochemical, field-effect carrier mobilities, and photovoltaic properties of the R-CPDT-Se2 were investigated and compared with cyclopentadithiophene (CPDT) and the bithiophene (EHex-CPDT-T2). The highest hole mobility of thin film transistor devices fabricated with new p-type polymer semiconductors, Oct-CPDT-Se2, was 1.3 × 10−3 cm2/Vs with an on/off ratio of about 105. The maximum power conversion efficiency of polymer solar cell fabricated with the blend of EHex-CPDT-Se2/C71-PCBM reached 1.86% with an open circuit voltage (VOC) of 0.55 V, a short circuit current density (Jsc) of 7.27 mA/cm2, and a fill factor (FF) of 0.47 under AM 1.5G irradiation (100 mW/cm2). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009

Utilization of a Buffered Dielectric to Achieve High Field-Effect Carrier Mobility in Graphene Transistors

We utilize an organic polymer buffer layer between graphene and conventional gate dielectrics in top-gated graphene transistors. Unlike other insulators, this dielectric stack does not significantly degrade carrier mobility, allowing for high field-effect mobilities to be retained in top-gate operation. This is demonstrated in both two-point and four-point analysis and in the high-frequency operation of a graphene transistor. Temperature dependence of the carrier mobility suggests that phonons are the dominant scatterers in these devices.

Charge carrier mobility in sulphonated and non-sulphonated Ni phthalocyanines: experiment and quantum chemical calculations

The objective of this interdisciplinary paper was to study theoretically and experimentally the electronic part of charge carrier transport in the class of sodium salts of sulphonated Ni phthalocyanine as candidates for p-type channels in organic field-effect transistors. These materials were selected because of their enhanced solubility as compared to their non-sulphonated counterparts. The values of the field-effect charge carrier mobility determined on the OFET structures using NiPc(SO3Na)x films were much higher than the charge carrier mobility obtained on the respective device prepared from non-substituted phthalo- cyanine. In order to explain differences between charge carrier mobility of sulphonated and non-sulphonated Ni phthalocyanines, quantum chemistry studies of molecular aggregates were performed. Quantum chem- istry modeling of the semiconductive molecular systems is new and progressive - we highlighted factors at the molecular level which led to the enhancement of the charge carrier mobility in systems containing SO3Na groups.

Enhancement of the field-effect mobility of solution processed organic thin film transistors by surface modification of the dielectric

The electrical performance of organic thin film transistors (OTFTs) is profoundly affected by organic semiconductor crystal formation and molecular ordering. The molecular ordering induced by self-assembly mechanisms can improve carrier mobility by a few orders of magnitude. In this work, the dielectric surface was modified to obtain preferred chemical functional groups that promote semiconductor ordering on the dielectric surface. OTFTs were fabricated using heavily p-doped silicon wafers as the gate electrode and thermally grown SiO 2 as the dielectric. Organosilanes, (3-mercaptopropyl)triethoxysilane and p-tolyltrimethoxysilane were used to modify the dielectric surface. Organic semiconductor, bis(triisopropylsilylethynyl) pentacene, was solution-cast as the semiconductor layer. X-ray photoelectron spectroscopy was used to characterize the modified dielectric surface. The analysis from atomic force microscopy indicated improved ordering in the semiconductor layer on silane treated dielectric surfaces. Electrical characterizations showed that the field-effect carrier mobility was enhanced by two orders of magnitude with dielectric surface modifications.

Low-Temperature Fabricated TFTs on Polysilicon Stripes

This paper presents a novel approach to make high-performance CMOS at low temperatures. Fully functional devices are manufactured using back-end compatible substrate temperatures after the deposition of the amorphous-silicon starting material. The amorphous silicon is pretextured to control the location of grain boundaries. Green-laser annealing is employed for crystallization and dopant activation. A high activation level of As and B impurities is obtained. The main grain boundaries are found at predictable positions, allowing transistor definition away from these boundaries. The realized thin-film transistors (TFTs) exhibit high field-effect carrier mobilities of 405 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vmiddots (NMOS) and 128 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vmiddots (PMOS). CMOS inverters and fully functional 51-stage ring oscillators were fabricated in this process and characterized. The process can be employed for large-area TFT electronics as well as a functional stack layer in 3-D integration.

High Aspect-Ratio ZnO Nanowires Based Nanoscale Field Effect Transistors

High aspect-ratio ZnO nanowires were grown onto the copper foil, in a large-quantity, by non-catalytic thermal evaporation method. The detailed morphological observations revealed that the diameters and lengths of as-grown nanowires are in the range of 60-100 nm and 10-30 microm, respectively exhibiting a very high-aspect ratio. Detailed structural characterizations confirmed that the as-grown nanowires are well crystalline and possess a wurtzite hexagonal phase, grown along the c-axis direction in preference. The presence of a sharp and strong UV emission at 381 nm in the room temperature photoluminescence (PL) spectrum affirmed that the obtained nanowires have good optical properties. The electrical transport properties of the as-grown nanowires was explored by fabricating the field effect transistors (FETs) using a single ZnO nanowire. From the fabricated single ZnO nanowire based FET, the electron carrier density and field effect mobility were estimated to be approximately 6.7 x 10(13) cm(-3) and approximately 3.8 cm2/Vs, respectively.

Improvement of electrical properties of silicon-based thin-film transistors by modifying technological fabrication processes

This paper is a review of technological process evolution associated to electrical performance improvement of silicon-based thin-film transistors (TFTs) that were performed mainly in the GM/IETR laboratory. The main objective in agreement with the fields of applications is to fabricate TFTs at a temperature low enough to be compatible with the substrates, glass substrates in a first place and flexible substrates in a second one, which implies several approaches. In fact, the electrical properties of the TFTs, mainly field-effect mobility of carriers in the channel, Ion/Ioff drain current ratio, and subthreshold slope, are strongly dependent on the quality and the nature of the channel material, on the material quality and thus on the density of states at the interface with the gate insulator, and on the quality of the gate insulator itself. All the improvements are directly linked to all these aspects, which means an actual combination of the efforts. For the glass substrate, compatible technology processes such as deposition techniques, or solid phase, or laser crystallizations of active layers were studied and compared. The paper details all these approaches and electrical performances. In addition, some results about the use of a silicon–germanium compound as channel active layer and airgap transistors for which the insulator is released, complete the presentation of the evolution of the silicon-based TFTs during the last twenty years.

Bottom-up organic integrated circuits

Self-assembly—the autonomous organization of components into patterns and structures1—is a promising technology for the mass production of organic electronics. Making integrated circuits using a bottom-up approach involving self-assembling molecules was proposed2 in the 1970s. The basic building block of such an integrated circuit is the self-assembled-monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. In the SAMFETs fabricated so far, current modulation has only been observed in submicrometre channels3–5, the lack of efficient charge transport in longer channels being due to defects and the limited intermolecular p–p coupling between the molecules in the selfassembled monolayers. Low field-effect carrier mobility, low yield and poor reproducibility have prohibited the realization of bottom-up integrated circuits. Here we demonstrate SAMFETs with long-range intermolecular p–p coupling in the monolayer. We achieve dense packing by using liquid-crystalline molecules consisting of a p-conjugated mesogenic core separated by a long aliphatic chain from a monofunctionalized anchor group. The resulting SAMFETs exhibit a bulk-like carrier mobility, large current modulation and high reproducibility. As a first step towards functional circuits, we combine the SAMFETs into logic gates as inverters; the small parameter spread then allows us to combine the inverters into ring oscillators. We demonstrate real logic functionality by constructing a 15-bit code generator in which hundreds of SAMFETs are addressed simultaneously. Bridging the gap between discrete monolayer transistors and functional selfassembled integrated circuits puts bottom-up electronics in a new perspective.