C60 Field-effect Transistors Research Articles

P-Channel Field-Effect Transistors Based on C60 Doped with Molybdenum Trioxide

Fullerene (C60) is a well-known n-channel organic semiconductor. We demonstrate that p-channel C60 field-effect transistors are possible by doping with molybdenum trioxide (MoO3). The device performance of the p-channel C60 field-effect transistors, such as mobility, threshold voltage, and on/off ratio is varied in a controlled manner by changing doping concentration. This work demonstrates the utility of charge transfer doping to obtain both n- and p-channel field-effect transistors with a single organic semiconductor.

A high mobility C60 field-effect transistor with an ultrathin pentacene passivation layer and bathophenanthroline/metal bilayer electrodes

C60 field-effect transistor (OFET) with a mobility as high as 5.17 cm2/V·s is fabricated. In our experiment, an ultrathin pentacene passivation layer on poly-(methyl methacrylate) (PMMA) insulator and a bathophenanthroline (Bphen)/Ag bilayer electrode are prepared. The OFET shows a significant enhancement of electron mobility compared with the corresponding device with a single PMMA insultor and an Ag electrode. By analysing the C60 film with atomic force microscopy and X-ray diffraction techniques, it is shown that the pentacene passivation layer can contribute to C60 film growth with the large grain size and significantly improve crystallinity. Moreover, the Bphen buffer layer can reduce the electron contact barrier from Ag electrodes to C60 film efficiently.

Low-Voltage Organic Field-Effect Transistors Fabricated on Self-Assembled-Monolayer-Free SrTiO3 Insulator

Strontium titanate (SrTiO3) thin film with a dielectric constant of εr = 12.1 prepared on a heavily doped n-type silicon wafer by sputtering. Both pentacene and C60 field-effect transistors fabricated using the self-assembled monolayer (SAM)-free SrTiO3 as an insulator showed well-saturated output characteristics at a driving voltage as low as -3 or 3 V. Hole mobility of the pentacene-FET was 0.28 cm2 V-1 s-1, while electron mobility of the C60-FET was 0.09 cm2 V-1 s-1.

Low-Voltage Organic Field-Effect Transistors Fabricated on Self-Assembled-Monolayer-Free SrTiO3Insulator

Strontium titanate (SrTiO3) thin film with a dielectric constant of εr = 12.1 prepared on a heavily doped n-type silicon wafer by sputtering. Both pentacene and C60 field-effect transistors fabricated using the self-assembled monolayer (SAM)-free SrTiO3 as an insulator showed well-saturated output characteristics at a driving voltage as low as -3 or 3 V. Hole mobility of the pentacene-FET was 0.28 cm2 V-1 s-1, while electron mobility of the C60-FET was 0.09 cm2 V-1 s-1.

Spontaneous electron transfer from C60 to Au ions: oxidation of C60 and hole doping

Spontaneous oxidation of C60 by Au ions was observed. When C60 was guided to make contacts with Au3+ ions in aqueous HAuCl4 solution, electrons were spontaneously transferred from C60 to Au3+ ions, resulting in hole (h+) doped C60 cations and Au nanoparticles on a C60 layer. This spontaneous electron transfer occurs due to the galvanic displacement from C60 to Au3+ ions owing to the energy difference between Fermi energy level of C60 (−4.7 eV) and standard reduction potential of Au3+ ion (+1.002 V). The oxidation of C60 as well as the consequent formation of reduced Au nanoparticles were confirmed by atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemistry. The switch of majority charge carrier type from electron to hole and its stability in air were also confirmed by monitoring I–Vg characteristic curves of a C60 field effect transistor (FET) device before and after the reaction with Au3+ ions.

Electronic transport properties of photo-irradiated C60

We have investigated field-effect transistor (FET) application on the photo-irradiated C60 thin film. Carrier mobility can be estimated to be 3.5 X 10-4 cm2/Vs after photo-irradiation and it shows a lower mobility than that before irradiation. Probably, it would be due to introduction of many cracks in the C60 thin film induced by polymerization due to ultraviolet light irradiation. However, considering the decrease in the threshold voltage from 12 to 2 V in the ISD-VG characteristics, carrier injection barrier clearly decreases by photo-irradiated polymerization. Furthermore, the photo-irradiated C60 FET works even in atmosphere.

The Effect of Morphology on Electron Field-Effect Mobility in Disordered C60 Thin Films

We present a model of polycrystalline C60 field-effect transistors (FETs) that incorporates the microscopic structural and electronic details of the C60 films. We generate disordered polycrystalline thin films by simulating the physical-vapor deposition process. We simulate electron hopping transport using a Monte Carlo method and electronic structure calculations. Our model reproduces experimentally observed FET characteristics, including electrical characteristics, electrochemical potentials, and charge mobilities. Our results suggest that even relatively disordered films have charge mobilities that are only a factor of 2 smaller than mobilities in single crystals.

High-performance C60 thin-film field-effect transistors with parylene gate insulator

C 60 field-effect transistors (FETs) have been fabricated with parylene gate dielectric on Si∕SiO2, on polyethylene terephthalate, and commercially available transparent sheet substrates. The best performance of the C60 FET device is achieved with parylene as gate dielectric: field-effect mobility of 0.41cm2V−1s−1 and on-off ratio of ∼107. The excellent FET characteristics are recorded without any annealing, and the devices were kept in He atmosphere after an exposure to air. This result suggests the parylene gate dielectric to be highly H2O repellent. The mechanical flexibility and air-exposure effect were studied for the C60 FET with parylene gate dielectric.

Transport properties in C60 field-effect transistor with a single Schottky barrier

C 60 field-effect transistor (FET) has been fabricated with a single Schottky barrier formed by an insertion of 1-dodecanethiol at the interface between the active layer and the gate dielectric. The suppression of drain current is observed at low drain-source voltage, showing a formation of the carrier injection barrier. Furthermore, a clear difference between forward and reverse drain currents is observed in the FET in a high temperature region, showing that this FET device is close to an ideal single Schottky diode. The quantitative analysis for carrier injection barrier has been achieved with thermionic emission model for a single Schottky barrier.

Potential barriers to electron carriers in C60 field-effect transistors

Transport properties of C60 field-effect transistors (FETs) have been investigated in the temperature range between 160 and 300K. Activation energy was estimated from temperature dependence of resistance at the linear region and of current at the saturation region for various channel lengths. Variation of activation energy values is attributed to carrier injection barrier at contact between source electrode and C60 channel, and barriers to carrier hopping between trap states in the channel of C60.

Electrical Properties of Heat-Treated C60 Field Effect Transistor Prepared on Polyimide Gate Insulator

Fullurence (C60)-based n-channel field effect transistors (FETs) have been fabricated on polyimide gate insulators as an alternative to conventional oxide gate insulators. The field effect mobility µe of C60 FET was improved to higher than 0.1 cm2 V-1 s-1 by heat treatment at 160 °C. This improvement was attributed to the de-doping of oxygen molecules, which were adsorbed by short-term exposure to air during sample transfer. The C60 films deposited on polyimide become amorphous even after the heat treatment or heating the sample during deposition. On the other hand, the crystalline phase of C60 appears when the films were deposited on pentacene-coated polyimide. Both the field effect mobility and the conductance of bulk C60 were increased markedly by the insertion of an ultrathin pentance layer between C60 and the polyimide gate insulator.

Output Properties of C60 Field-Effect Transistors with Au Electrodes Modified by 1-Alkanethiols

Field-effect transistors (FETs) with thin films of C60 have been fabricated with Au electrodes modified by a series of 1-alkanethiols. All C60 FETs show n-channels of normal FET properties. It has been found that the output properties for the FETs with the Au electrodes modified by 1-alkanethiols with long alkyl chains are largely affected by the carrier−injection barrier (i.e., the current vs drain/source voltage plots exhibited concave-up nonlinearity at low voltage regions). The output properties are substantially dominated by an additional tunneling barrier of 1-alkanethiols inserted into the junction of the Au electrode and C60 thin films, and the parameters associated with the junction barrier height and tunneling efficiency were determined from the output properties based on the thermionic emission model for double Schottky barriers.

Analysis of transient phenomena of C60 field effect transistors

Transient current excited by application of step gate voltages was investigated for C60 field effect transistors (FETs). The measurements were continued during film growth and the decay curves were obtained as a function of C60 film thickness. The decay curve contains two different relaxation times. The fast decay could be explained in terms of channel resistance and gate insulator capacitance, while the slower decay relates with trapping processes of carriers. The slower decay observed for the larger mobility film implies that mobility is not an absolute figure for evaluating the performance of organic FETs.

Intrinsic transport and contact resistance effect in C60 field-effect transistors

The autors report size dependence of characteristics of C60 field-effect transistors (FETs). The transport properties of the channel and the contact resistance between the channel and electrodes are extracted from size dependence. Contact resistances are comparable to those of channel resistances, and the gate voltage dependence of contact resistance is greater than that of channel resistance even at linear region. Results show that the Schottky barriers between the channel and the electrodes still affect device characteristics in the on state of C60 FETs.

Output properties of C60 field-effect transistor device with Eu source/drain electrodes

Field-effect transistor (FET) device with thin films of C60 has been fabricated with Eu electrodes exhibiting small work function. The C60 FET device shows n-channel FET properties with high field-effect mobility, 0.50cm2V−1s−1. Furthermore, nonvanishing drain current, i.e., normally on, is observed in this FET device. This originates from small energy barrier for electron from Eu source electrode to lowest unoccupied molecular orbital of C60.

Investigation of complex channel capacitance in C60 field effect transistor and evaluation of the effect of grain boundaries

Abstract Frequency and gate voltage dependences of capacitance in a C60 field effect transistor (FET) showed an intriguing power law (C ∝ f−p, p ∼ 0.3–0.35) irrespective of the gate voltage. In order to interpret this phenomenon, we formulated a complex impedance of the bottom contact FET based on a distributed constant circuit model in cases of both a single grain channel and a multi-grain channel. The power law could be well explained in terms of the complex impedance formula using only a small number of fitting parameters, the results of which indicate the validity of the model. This kind of analysis could usefully characterize the organic FETs consisting of grain boundaries, providing information on the resistance ratio of the grain interior to the grain boundary.

Molecular-Wetting Control by Ultrasmooth Pentacene Buffer for High-crystallinity Organic Field-Effect Transistors

ABSTRACTOrganic thin film devices are of interest for a variety of forthcoming ubiquitous electronics applications. In order to build ubiquitous high-performance devices, it is necessary to fabricate crystalline thin films of various organic materials onto “ubiquitous substrates” that are dictated by applications. However, many organic thin films crystallize only on a limited selection of substrates. Unfortunately, promising organic molecules, which have a large overlap of pi-orbitals between molecules, cannot migrate freely on a substrate because of stronger cohesion between molecules than interaction between the molecule and the substrate. Therefore, enhancement of the molecule-substrate interaction, i.e. ‘molecular wettability’ should promote crystallization. We found that an ultrasmooth monolayer of pentacene (C22H14), which can be grown on many general dielectric substrates, changes the molecular wettability of a substrate for other poorly wettable organic materials. We also demonstrate that a field effect transistor (FET) using a crystalline C60 thin film on a pentacene-buffered substrate can have a mobility of 4.9 cm2/Vs, which is 5-fold higher than that of C60 FETs without the buffer. Molecular wetting-controlled substrates can thus offer a general solution to the fabrication of high-performance crystalline plastic and molecular electronics.

Fabrication of C60 field-effect transistors with polyimide and Ba0.4Sr0.6Ti0.96O3 gate insulators

A flexible C60 field-effect transistor (FET) device has been fabricated with a polyimide gate insulator on the poly(ethylene terephthalate) substrate, and n-channel normally off FET properties are observed in this FET device. The field-effect mobility, μ, is estimated to be ∼10−2cm2V−1s−1 at 300K. Furthermore, the C60 FET has been fabricated with a high-dielectric Ba0.4Sr0.6Ti0.96O3 (BST) gate insulator, showing n-channel properties; the μ value is estimated to be ∼10−4cm2V−1s−1 at 300K. The FET device operates at very low gate voltage, VG, and low drain-source voltage, VDS. Thus these C60 FET devices possess flexibility and low-voltage operation characteristic of polyimide and BST gate insulators, respectively.

An in-situ Fabrication and Characterization System Developed for High Performance Organic Semiconductor Devices

We have designed and set up a fabrication and characterization system for organic devices which enables us to assemble all components of devices and to characterize the device properties without breaking the vacuum. Using this system, top and bottom contact C60 field effect transistors (FETs) were fabricated and their performance was characterized. The top contact FET exhibited a mobility as high as 1.4 cm2/(V·s), which was higher than the bottom contact FET.

Fabrication of nanoscale C60 field-effect transistors with carbon nanotubes

A nanoscale C60 field-effect transistor has been fabricated with carbon nanotubes (C60CNT-FET). A wire of multiwalled carbon nanotube has been anchored by metal pads on a Si wafer, and cut by bombardment with focused Ga2+ ion beam. The cut ends of the wire have been integrated as source-drain electrodes into the C60CNT-FET, with a vacuum evaporated C60 thin film. The C60CNT-FET has exhibited an excellent performance of a low-voltage drive operation, without any short-channel effect even at as small as 100 nm of channel length.