Search

Potential mapping of organic thin-film transistors (TFTs) has been carried out using originally developed atomic-force-microscope potentiometry (AFMP). The technique is suitable for the accurate measurement at metal–semiconductor boundaries of working TFTs. Potential drops near metal–organic boundaries are observed for both source and drain Au top contacts of a pentacene TFT. The approximate width of the steeper potential slope is 400 nm, which is larger than the spatial resolution of AFMP. The potential drop is considered to be due to a damaged area with low carrier mobility caused by the Au evaporation, which is also reproduced by device simulation.

Wire-like crystals of tetrathiafulvalene-tetracyanoquinodimethane (TTF–TCNQ) charge-transfer complexes were grown under a static electric field by employing electric-field assisted evaporation. TTF–TCNQ molecular wires grew from the edges of two gold electrodes opposite to each other along the electric lines of force, and finally make a connection at their front end to form a single wire. Self-organization of the wire bridge is derived from a higher local electric field between the tips of the opposing molecular wires. Oriented molecular wires, which have diffuse branches, exhibit the effects of a local electric field. Preferential growth of TCNQ at the tip of the molecular wire during coevaporation of TTF and TCNQ is clearly revealed by microscopic Raman spectroscopy. Asymmetrical growth of coevaporated TTF–TCNQ wire under a static electric field is dominated by the drift motion of TCNQ−.

Poly(3-hexylthiophene) field-effect transistors (P3HT-FETs) have been prepared by an electric-field casting method. P3HT was cast onto a substrate with source and drain electrodes in nitrogen atmosphere saturated with CHCl3 vapor. An electric-field was applied between the electrodes during film formation. Thermal treatment and electric-field application were performed after film formation to control the device parameters. The threshold voltage (V th) of P3HT-FETs without electric-field application during film formation was almost invariable after the thermal treatment. On the other hand, the V th of P3HT-FETs formed with electric-field application can be controlled using thermal treatment. The device parameters can also be controlled by a post-electric-field application. These variations are a result of the doping or dedoping of O2. Two types of inverter showing different inversion points were fabricated using such controlled P3HT-FETs.

Crystalline copper phthalocyanine (CuPc) films have been fabricated by physical vapor deposition using a cylindrical furnace. Field effect transistors (FETs) of the crystalline CuPc film are fabricated by optimizing the deposition conditions. Crystal orientation along the electric field is enhanced by an application of a DC electric field during crystal growth. The electric-field-assisted orientation is effective in a submillimeter area. The degree of crystal orientation improves with increasing electric field strength. High current and a remarkable threshold for the drain voltage are observed in the FET characteristics of the oriented crystalline film.

We have studied optical and electrical properties in donor molecule adsorbed TCNQ Langmuir-Blodgett (LB) films. In fabricating the adsorption LB films, C 18 TCNQ was used as a spreading material for LB films and TMPD or PPD was used as an adsorbate. The results show that the concentration of donor molecule in aqueous subphase affects the quantity of charge-transfer (CT) complex of the adsorption LB films and the electric conductivity strongly depends on the degree of CT complex formation in the adsorption LB films.

We have investigated the formation process of charge-transfer (CT) complex and the electric conductivity of TTF-TCNQ co-evaporated films by varying the growth temperature (Tg). The molar ratio of TTF to TCNQ in the co-evaporated film is constant at a value of 1.0 in the Tg range between 31°C and 42°C. The formation of the single crystal of the CT complex is enhanced and the electric conductivity increases with Tg. The temperature dependence of the surface conductivity is explained by the combination of the single-crystalline conduction of the grains and the activation-type conduction of the amorphous phase between the grains.

We have fabricated organic static induction transistors (SITs) using copper phthalocyanine (CuPc) films. The organic SITs have a layered structure of Au (drain)/CuPc/Al (gate)/CuPc/Au (source)/glass. The electrical characteristics of SITs show that the source-drain current is controlled by the bias voltage applied to the Al gate electrode and a typical SIT operation with unsaturated current characteristics is examined. Furthermore, excellent characteristics such as low voltage and high speed operation as organic transistors are obtained by choosing an appropriate thickness for each layer.

We have fabricated thin-film transistors consisting of evaporated thin films of metal-free, copper and lead phthalocyanines, and estimated the electrical parameters of carrier mobility, carrier concentration and electrical conductivity by in-situ field effect measurements. We have also investigated the effect of introducing of oxygen gas into the vacuum chamber and of thermal annealing on the electrical parameters of the films. We found that the carrier concentration and carrier mobility are strongly influenced by the metals in the phthalocyanine molecules and by oxygen gas exposure.

We have investigated the effect of bias voltage applied to an electrically conductive substrate on the formation of adsorption Langmuir-Blodgett (LB) films. In fabricating the adsorption LB films, a merocyanine dye with a long alkyl chain and a triphenylmethane dye are employed as spreading and adsorbing molecules from an aqueous subphase, respectively. The results demonstrate that the bias voltage LB technique is a powerful tool for controlling the adsorption ratio of a variety of functional dyes and the formation of dye aggregates.

Electron tunneling spectroscopy is important from both characterization and potential application viewpoints because it provides valuable information about the structure, orientation, and interactions of molecules in organic thin films. In particular, inelastic electron tunneling spectroscopy (IETS) using a scanning electron tunneling microscope reveals spatial and spectral information and is attractive as a high-density storage system. We describe the basic experimental results of changes in IETS patterns of organic monolayers induced by the voltage pulse, and the potential for application to high-density recording systems.

The InAs/GaAs strained quantum well structure growths by molecular beam epitaxy on variously oriented GaAs substrates have been investigated. X-ray diffraction and photoluminescence measurements have been used to examine the properties' dependence on the substrate orientation of a few monolayers of InAs quantum wells. Experimental results that show strong influence of substrate orientation on the growth mode and growth rate are obtained.

Surface pressure variations of merocyanine or arachidic acid monolayers formed at the air-water interface are investigated. Under the constant area condition, a marked increase of surface pressure is observed during the adsorption of triphenylmethane dyes from the subphase. The surface pressure variation is closely related to the molecular interaction and arrangement between the spreading and adsorbing materials. Experimental results and data, calculated by means of the molecular orbital method, of molecular interactions and the quantity of adsorbed molecules are compared.

Absorption coefficients for the resist materials (polymethyl methacrylate (PMMA) and partially chloromethylated polystyrene (CMS-DU)) were obtained in the vacuum ultraviolet (VUV) region. Negative and positive resist characteristics were investigated using a coherent VUV radiation source (λ=110-150 nm) generated by two-photon resonant four-wave mixing in Cd vapor. A very low exposure energy of approximately 1 mJ/cm2 was realized by employing a very thin resist film and using VUV light from the high-absorption region of the resist materials.

An inelastic electron tunneling spectroscopy (IETS) technique is employed to investigate Langmuir-Blodgett monolayers deposited on silicon substrates. Thermally formed SiO2 layers are used as a tunneling barrier to obtain reliable and sensitive signals. IETS data, which are compared to IR and Raman data, show the stability and chemical reactions of organic monolayers at the metal or SiO2 interface.

The output properties of hydrogen light sources and the resist parameters, such as spectral sensitivity and absorption coefficient, in the vacuum-ultraviolet (VUV) region were investigated. A very low exposure energy of about 1 mJ/cm2 was obtained by employing a very thin polymethyl methacrylate film and the VUV light of hydrogen microwave discharge. Contact photolithography using a Si/MgF2 mask and a trilayer resist system showed that submicron patterns as small as 0.2 µm had been replicated at an exposure time of under 30 s.

Several triphenyl methane derivatives (TPMs) having different substituents can be adsorbed from an aqueous solution to Langmuir films consisting of arachidic acid (AR) or a mixture of AR and merocyanine dye (MC). The substituent dependence of the adsorption phenomenon shows that the adsorption mechanisms of TPM to AR and MC are mainly due to charge-charge and dipole-dipole interactions, respectively. Moreover, a peculiar property such as a negative photoelectric effect was obtained in the dye-adsorbed Langmuir-Blodgett films.

Vacuum-ultraviolet lithography of a trilayer resist system which consists of polymethyl methacrylate (PMMA), Si and polyimide has been studied using a deuterium lamp. The exposure dose needed for a 0.1-µm-thick PMMA film is less than 1 mJ/cm2, and a high etch rate ratio of 40 for polyimide to Si is obtained with the reactive ion beam etching (RIBE) technique. Using a simple contact exposure system and RIBE, photomask patterns are accurately transferred in the polyimide down to a linewidth of 0.2 µm, and 50-nm line patterns of polyimide are obtained.

Low temperature photoluminescence measurements were performed for Zn+-implanted GaAs, where the concentration of Zn, [Zn], was widely varied from 3 × 1016 to 1 × 1021 cm−3. A newly discovered Zn-associated emission denoted by [g-g] was found to move towards lower energy levels with increasing [Zn]. [g-g] was theoretically suggested to be an acceptor-acceptor pair emission. [g-g] presents a strong optical compensation at high [Zn] and in order to study further properties of [g-g] we investigated the local atomic structure of Zn+-implanted GaAs by fluorescence extended X-ray absorption fine structure (F-EXAFS). The results show that the implanted Zn atoms occupy Ga sites and/or tetrahedral interstitial sites. It was also observed that the lattice in the region longer than the second-nearest neighbor is highly disordered, which brings about the selective self-optical compensation (SSOC) effect of [g-g] at high [Zn].

Pair-type emissions like donor-acceptor pair emissions, (D0,A0) are generally not observed in GaAs. In rather impure MBE (molecular beam epitaxially) grown GaAs, however, a sharp line series of seemingly pair-type emissions (temporally denoted by CDIBE, i.e., carbon and defect-induced bound excitons) are commonly observed at the energy position just below the free and bound exciton emissions regions. In this report we demonstrate that CDIBE can be reproducibly formed by the dual implantation of C+ (carbon) and O+ (oxygen) ions and that CDIBE is the first complex-type emission centers (CAs + OINT) ever discovered.

Low-temperature photoluminescence spectra have been studied in dually Zn+ (acceptor)- and Se+ (donor)-implanted GaAs made by molecular beam epitaxy. Three new quasi-donor-acceptor pair emissions at ∼ 38, ∼ 74 meV, and ∼ 118 meV below the free exciton emissions are observed. These emissions are strongly dependent on the combinations of Zn and Se concentration and one of them shows a strong blue shift with increasing excitation density.

Surface and bulk conduction properties have been measured in low density polyethylene (LDPE), after 30 keV N+ and Ar+ ion bombardment, in the ion dose range of 1014-1016 ions/cm2. The surface resistivity of the films decreased by 9 orders of magnitude from its non-irradiated value of 1 × 1017 Ω/□ to 2 × 108 Ω/□ at a dose of 1016 ions/cm2. Temperature dependence of surface conductivity was consistent with the charge transport due to hopping between conducting particles, as proposed by Sheng et al. The bulk conductivity increases with increasing the dose, and considerable asymmetry in the current-voltage (I-V) characteristics appears at an ion dose of 1016 ions/cm2. The rectifying ratio is 4 orders of magnitude. The behavior of the bulk conduction was dominated by space charge limited current.

The effect of thermal reoxidation of N+-ion-implanted silicon on the SiO2-Si interface properties have been investigated. We propose that it is possible to obtain high quality insulating layers with a very low density of interface traps by employing the proper thermal reoxidation process after N+ ion implantation. Thermal reoxidation of N+-ion-implanted silicon is stopped at the early stage even at a relatively low dose of about 5 × 10 15 N+ ions cm−2, which is indicative of the formation of a thin oxynitride layer. The oxynitride formation at the SiO2-Si interface by the reoxidation process has positive effects on metal-oxide-semiconductor characteristics.

The incorporation mechanisms of residual impurities in GaAs layers grown by molecular‐beam epitaxy has been investigated by high‐resolution photoluminescence (PL) spectroscopy at 2 K. A systematic study of near‐band‐edge emissions of undoped GaAs layers grown at a wide range of growth temperatures (Tg), 470–750 °C, demonstrates that PL spectra related with residual impurities are significantly dependent upon Tg. It was found that maximum emission intensity of free exciton is obtained at Tg ∼550 °C, and the minimum impurity incorporation is established at Tg of 550–650 °C.

Malachite-green (MG) dye which does not contain a long alkyl chain can be adsorbed from aqueous solution onto Langmuir-Blodgett films consisting of arachidic acid (AR) only or a mixture of the acid and merocyanine (MC) dye. A special interaction exists between MC and MG molecules; MG molecules are more selectively adsorbed to MC molecules than to AR molecules. The adsorption Langmuir-Blodgett films show negative photoelectrical properties which are strongly related to the interaction between the two functional dyes.

The photovoltaic effects of ITO/ZnO/merocyanine/Ag cells are reported. The ZnO layer was deposited by a dc sputtering technique on the light-transparent electrode. The dye film was fabricated by a vacuum vapor deposition onto the ZnO layer. The short-circuit photocurrent and fill factor of these cells were increased compared to those of Al/merocyanine/Ag cells. Furthermore, the stability of the device operation under the high light intensity (100 mW/cm2) was considerably improved.

The selective optical compensation effect in which exclusively acceptor‐associated emissions g and [g‐g] are selectively quenched by the simultaneous presence of acceptor and donor atoms, was investigated in dually Zn+ (acceptor)‐implanted and Se+(donor)‐implanted GaAs as a function of Se to Zn concentration ratio, [Se]/[Zn], at a fixed Zn concentration of 1×1017 cm−3. It was revealed for the first time that Se atoms with one‐tenth of the concentration of Zn have the ability to significantly suppress the [g‐g] emission. However, the g emission is not significantly suppressed by the presence of even an equal concentration of Se atoms.

Low‐temperature photoluminescence (PL) spectra for Si‐doped AlAs grown by molecular‐beam epitaxy is reported for the first time. It is revealed that a sharp emission which is 63 meV below the indirect excitonic band gap Eg,ind(X) is related to the Si‐donor level. From the dependence of PL spectra upon Si dosage, it is suggested that a broad emission band which is 135 meV below Eg,ind(X) is due to donor‐acceptor (SiAl‐SiAs) pair recombinations.

Low‐temperature photoluminescence studies of undoped, N+ (nitrogen)‐implanted and C+ (carbon)‐implanted AlAs grown by molecular beam epitaxy are reported. It was experimentally demonstrated that a dominant emission temporarily denoted by A, observed at 13 meV below the indirect excitonic band gap, Eg,ind(X), is closely related with N isoelectronic impurity atoms. It was also found that the A emission accompanies many one‐phonon and two‐phonon replicas, among which the longitudinal optical phonon replica is predominant. Carbon atoms were determined not to be principal residual impurities in undoped AlAs. The two conspicuous C‐related emissions were revealed by the intentional incorporation of C atoms, which are situated at 60 and 64 meV below Eg,ind(X).

Photoluminescence studies of C+, Mg+, Zn+, and Cd+ ion‐implanted GaAs layers were carried out at 2 K. Two conspicuous emissions denoted by g and [g‐g] were observed to be situated between bound exciton and band to acceptor emissions. It was found that these two are common emissions among the above acceptor impurities, and that they can be explicitly observed only when the background impurity concentration is extremely small.

Photoluminescence spectra of Mg‐doped GaAs grown by molecular beam epitaxy (MBE) are for the first time reported. Mg was introduced during MBE growth by using a synergic reaction of Mg3As2. Two near‐band‐edge emissions, g and [g‐g], were observed below bound exciton emissions which were originally obtained in Mg+ ion‐implanted GaAs. It is explicitly demonstrated that there is no essential difference of photoluminescence spectra between Mg+ ion‐implanted GaAs and the present material. These results conclusively indicate that the above two emissions are definitively related with the acceptor levels and exclusively not associated with crystalline defects produced by ion implantation. It is additionally presented that [g‐g] is principally established also in Mg‐doped GaAs prepared by liquid phase epitaxy, where [g‐g] is significantly suppressed due to the unintentional incorporation of Si donor atoms. For moderately Mg‐doped GaAs prepared by MBE a new fine emission with a doublet structure, temporarily denote...

Photoluminescence measurements were carried out at 2 K for Zn+ ion‐implanted GaAs, where the concentration of Zn was widely varied from 3×1016 to 1×1021 cm−3. Two Zn+‐associated emissions were formed. One emission is at 1.512 eV, g, and the other emission [g‐g] is just below g and this moves towards the lower energy level with increasing Zn concentration, [Zn]. The intensity of [g‐g] was enhanced with increasing [Zn], up to [Zn]=3×1017 cm−3, and was gradually suppressed for [Zn] beyond that concentration. This selective self‐optical compensation effect (SSOC) for [g‐g] was found to occur for moderately heavy ion acceptor species such as Zn and Cd, although [g‐g] is a common emission among many other acceptor impurities. In addition two new emissions were observed between g and [g‐g] at certain [Zn]. Preliminary theoretical explanations are presented for this SSOC effect.

Newly discovered Zn‐associated emissions in GaAs, ‘‘g’’ and [g‐g], were found to be completely suppressed by the simultaneous presence of Zn and Se atoms. This selective optical compensation (SOC) effect observed when acceptor (Zn) and donor (Se) atoms exist in combination was established by simultaneously implanting Zn+ and Se+ ions into almost carbon‐free ultrapure GaAs made by molecular‐beam epitaxy. It is suggested that the failure to observe ‘‘g’’ and [g‐g] emissions in acceptor‐incorporated GaAs prepared by conventional epitaxial methods may be ascribed to this SOC effect, in which donor atoms were unintentionally introduced into the epitaxial layer.

Copper phthalocyanine (CuPc) tetra sulfonic acid, tetrasodium salt (CuPcSAS) and polyaniline (PA) have been chosen as functional organic materials to adsorb each other spontaneously. The alternate structure consisting of ultrathin films with the two materials promises new properties, because CuPc has been known to a p-type organic semiconductor and PA to a conductive polymer in the doped state by acid. In this paper we describe the fabrication of alternate structures using the self-assembly and spin-coating (SASC) method.

We have investigated the conductivity variation of copper phthalocyanine nano-crystals deposited on SiO2 and mica. The density of surface hydroxyl group on SiO2 was estimated to be more than 10 times larger than that on mica by X-ray photoelectron spectroscopy. The grown crystals on both substrates were categorized to be a-axis orientation of α–form. Conductance-distance (G-d) measurements for various crystalline grains having different heights have been carried out with current imaging using atomic force microscope. The obtained conductivity vs. crystal height plot indicates that lower height crystals have higher conductivities on both surfaces. Besides, the lower height crystals on SiO2 have higher conductivity than those on mica. These results suggest that the electrons in the bottommost molecular layer are transferred to the electrophilic functional groups on the substrate surface. Such a phenomenon at organic semiconductor/insulator interface is reported for the first time.

Organic p-n diodes based on n-type and p-type dyes have been fabricated. The cells have layered structure of ITO/n-type dye/p-type dye/Ag. They show rectifying properties and photovoltaic effects. The open-circuit photovoltage as high as 0.75 V and the short-circuit photocurrent of about 0.13 mA/cm2 were obtained. The present values are significantly higher than those reported previously.

The conduction of the monolayer Langmuir-Blodgett(LB) film consisting of Cadmium-Stearate(CdSt) may be explained by an electron tunneling process. The tunneling current through the Al/CdSt/Al sample showed a great decrease with time when a constant voltage was applied in air. Such a phenomenon seems to result from the growth of an Al2O3 layer by the anodic oxidation of the aluminum electrode enhanced by the electron tunneling.

The spectral sensitivity of the organic diodes with merocyanine dye films was controlled in the visible spectral range by choosing proper dye molecules, or mixing two or three dyes. The photovoltaic cells with the layered structure of ZnO and merocyanine dye exhibited improved photovoltaic properties compared to the Al/day/Ag cell reported before. An electron diffraction study revealed that the dye films evaporated on the ZnO layer show a weak crystallization, while the dye films on an Al film are amorphous.

Photovoltaic behavior of the merocyanine dye family with a rhodanine ring has been systematically studied by employing eight different derivatives in the diode structure Al/evaporated dye/Ag. Each dye film behaves like a p-type semiconductor with a rectifying barrier at Al interface and an Ohmic contact at Ag electrode. The open circuit photovoltage as high as V oc≃1.5 V was observed for one of the derivatives, while the fill factor remained f.f.≃0.25 in all cases. The chemical stability of the dye molecules during and after evaporation is also discussed on the basis of the results obtained by the thin-layer chromatography.