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[Abstract] Carrier-injection and the succeeding pre-channel-formation dynamics in organic thin-film transistor was observed using time-domain reflectometry. Having previously analyzed the depth-wise variation in the initial carrier-injection from the contact electrode to the channel region, we focus here on the succeeding pre-channel-formation dynamics. We demonstrate that a hole concentration in the semiconductor/gate insulator interface increases until its electrical capacitance is filled through contact and access resistances. Thereafter, the injected-hole dis-tribution gradually spreads. A reduction in both contact and access resistances is crucial not only for the static characteristics of the field effect transistor but also the dynamical response.

A high-performance vertical-type organic transistor has been fabricated using bis(l,2,5-thiadiazolo)-p-quinobis(l,3-dithiole) (BTQBT) for the channel layer. The BTQBT molecules are oriented horizontally, with the molecular plane of each monolayer parallel to the substrate. The π–π stacking direction of the BTQBT molecules is aligned with the carrier transport direction in this vertical transistor. The modulated drain current density exceeded 1 A cm−2 upon the application of a gate voltage of less than 5 V. In addition, the device exhibits a high on/off current ratio of over 105.

In-situ observations of vapor-phase growth of tetrathiafulvalene (TTF)-tetracyanoquinodimethane (TCNQ) crystals under an electric field were conducted without influencing the actual crystal growth process. The shortest incubation time of TTF-TCNQ nuclei and the highest initial growth rate of the crystals are obtained on the anode side and in high electric field regions. It is demonstrated that the distribution of molecules thermally diffusing on the substrate surface is controlled by an external electric field. These results indicate the potential for selective growth of highly conductive organic wires for micro- and nanoscale wiring in organic nanodevices.

To elucidate the effects of a static electric field on the crystal growth of a molecule with both a molecular dipole and a quadrupole moment, we performed physical vapor growth of monoclinic lead phthalocyanine (PbPc) crystals under quasithermal equilibrium conditions and observed distinct effects of the applied electric field on the drift motion of the molecules that diffused over the substrate surface. The density of crystals grown on the Au electrode with a high electric potential exceeds the crystal density on both an electrically grounded electrode and a glass substrate surface. In addition, this difference in crystal density increases with applied voltage. This biased distribution of crystal growth is explained by the drift motion of diffusing molecules, which is induced by the interaction of the electric field with molecular dipole and quadrupole moments. However, the long-range oriented growth that is clearly seen in the physical vapor growth with an electric field for copper phthalocyanine is not...

Conductivity anisotropy in a crystal grain of thin-film-phase pentacene has been estimated by a combination of atomic-force-microscope potentiometry (AFMP) and electrostatic simulation. The surface potential distribution and topography of a grain in a working pentacene thin-film transistor are simultaneously measured by AFMP. Then, the nonlinear potential profile due to the thickness variation is simulated by changing the anisotropic ratio of conductivity. The anisotropic ratio (corresponding to the anisotropy of carrier drift mobility) is estimated to be σ x : σ z = 45 : 1 , where x is the horizontal (harmonic mean of those in a - and b -axes) direction and z is the vertical ( c -axis) direction.

A ferroelectriclike dielectric response was observed in a field-effect transistor using oriented bis(ethylenedithio)tetrathiafulvalene-tetracyanoquinodimethane crystals. Phase transitions at 285 and 320 K were clearly observed in the temperature dependence of field-effect electron mobility. The phase transition at 320 K corresponds to the metal-insulator transition previously reported in a bulk crystal. On the other hand, the field-effect electron and hole mobilities exhibited an abrupt increase at 285 K, which had not been discovered by other physical measurements in the bulk crystal and is nonetheless sufficiently stable and reproducible. In addition, the abrupt increase in carrier mobilities was clearly correlated with the decrease in the dielectric response. The temperature variation in difference hysteresis curves demonstrated the feature of ferroelectric transition.

Crystalline domain size and temperature dependences of the carrier mobility of commonly used pentacene polycrystalline films on SiO2 have been studied by four-point-probe field-effect transistor measurements. The mobility is found to be proportional to the crystalline domain size and thermally activated. This behavior is well explained by a polycrystalline model with the diffusion theory, and thereby the barrier height at boundary and the mobility in domain are calculated to be 150meV and 1.0cm2∕Vs, respectively. The in-domain mobility is lower than those expected in single crystals, which suggests that there exist some other limiting factors of carrier transport than the domain boundaries.

Surface topography and high-resolution potential images in a thin-film transistor with a polycrystalline pentacene active layer have been measured by atomic-force-microscope potentiometry. A potential fluctuation independent of topographic features was found in large flat molecular terraces. The origin of the potential fluctuation was concluded to be the fluctuation of the top level of the highest-occupied-molecular-orbital band, which results in the variation of local carrier concentration. The full width at half maximum of the band fluctuation was estimated to be 12meV, which might reduce the mean carrier velocity in crystalline domains.

The authors report on the solution-processed planar bottom-contact (pBC) organic thin-film transistors and contact effect on gate threshold voltage incorporating regioregular poly(3-hexylthiophene) active layer. By employing pBC configuration, the transistors on SiO2∕Si without surface modification show much higher mobility, lower threshold voltage, and narrower dispersion of threshold voltage when compared to the conventional bottom-contact counterparts. The high mobility and lower threshold voltage are attributed to an improved contact at the interface between the source/drain electrodes and the poly(3-hexylthiophene) active layer.

The authors fabricated an organic nanochannel field-effect transistor (FET) that is self-wired with highly conductive organic conductors. The advantages of the transistor are a short channel (approximately 400nm in length) and spontaneous formation of an active layer of the FET. Further, in principle, the carrier-injection barrier is absent at the interface of the organic metal and organic semiconductor. Thus, the transistor is highly conductive despite the narrow cross section of the channel. The FET characteristics of the nanochannel transistor exhibit the n-channel enhancement mode behavior.

Organic static induction transistors (OSITs) with vertical channel structures that operate at low voltages have potential as components of portable or ubiquitous devices. In this study, the static characteristics of pentacene OSITs on flexible substrates were examined under bending conditions. Bending tests of the flexible substrate were carried out under various compressive and tensile strains and at various bending radii. The results demonstrate that OSITs exhibit stable electrical characteristics at compressive and tensile strains up to a bending radius of 5mm, suggesting a possible application as transistors in the field of flexible electronics.

The authors report the characteristics of novel metal-insulator-semiconductor-type organic light-emitting transistors (MIS-OLETs). The drain current and luminescent intensity of the MIS-OLET can be controlled by changing hole injection carriers by applying a gate bias voltage. In addition, the high performance (400cd∕m2 at VD=−8V) of MIS-OLETs fabricated on a plastic substrate as well as on a glass substrate is demonstrated and described.

Flexible organic static induction transistors (OSITs) based on pentacene thin films are fabricated on plastic substrates and their static characteristics are measured. The basic transistor characteristics of the flexible pentacene OSITs were comparable to those of nonflexible pentacene OSITs fabricated on glass substrates. In addition, variation in the static characteristics was negligible up to a bending radius of 20 mm. These characteristics suggest the potential for the OSITs as drivers in flexible sheet displays, such as organic light-emitting transistors in which the OSITs are combined with organic light-emitting transistors.

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−.

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.

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.