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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.

For the better performance of organic thin-film transistors (TFTs), gate-insulator surface treatments are often applied. However, the origin of mobility increase has not been well understood because mobility-limiting factors have not been compared quantitatively. In this work, we clarify the influence of gate-insulator surface treatments in pentacene thin-film transistors on the limiting factors of mobility, i.e., size of crystal-growth domain, crystallite size, HOMO-band-edge fluctuation, and carrier transport barrier at domain boundary. We quantitatively investigated these factors for pentacene TFTs with bare, hexamethyldisilazane-treated, and polyimide-coated SiO2 layers as gate dielectrics. By applying these surface treatments, size of crystal-growth domain increases but both crystallite size and HOMO-band-edge fluctuation remain unchanged. Analyzing the experimental results, we also show that the barrier height at the boundary between crystal-growth domains is not sensitive to the treatments. The res...

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...

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.

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.

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.

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.