Youji Inoue

Organic Thin-Film Transistors with High Electron Mobility Based on Perfluoropentacene

We report on n-channel organic thin-film transistors (OTFTs) based on the novel n-type organic semiconductor, perfluoropentacene. The transistor exhibits excellent electrical characteristics, with a high electron mobility of 0.22 cm2/(V s) and a good current on/off ratio of 105. The electron mobility is comparable to the hole mobility of a pentacene OTFT. By combining the n-type perfluoropentacene and the p-type pentacene, we have fabricated ambipolar OTFTs and complementary inverter circuits. The OTFTs with heterostructures of the p- and n-type organic semiconductors can operate as an ambipolar device with high electron and hole mobilities of 0.042 and 0.041 cm2/(V s). The complementary inverter using an n-channel perfluoropentacene OTFT and a p-channel pentacene OTFT exhibits excellent transfer characteristics with a voltage gain of 45. A complementary inverter using the ambipolar OTFTs is also demonstrated.

Raman spectra of amorphous SiC

Abstract It has been found that the condition of the sample preparation exerts large influence on the structural disorder of the films. For the sample prepared by sputtering method, the dependence of CC band on substrate temperature T s and partial pressure of SiH 4 during the sputtering has been measured.

Influence of growth temperature on crystalline structure in Ga0.5In0.5P grown by organometallic vapor phase epitaxy

The relation between growth temperature and ordered structures in Ga0.5In0.5P grown using organometallic vapor phase expitaxy is investigated using transmission electron diffraction, electroreflectance, and Raman scattering measurements. It is found that generation of the ordered structure is not related to the immiscibility of this alloy and that the ordered structures do not simply represent ‘‘sublattice ordering.’’ The anomalous band gap may be a consequence of the variation in the atomic arrangement of neighboring atoms, but not of the long‐range ordered structure itself.

Organic Thin-Film Transistors on a Plastic Substrate with Anodically Oxidized High-Dielectric-Constant Insulators

Organic thin-film transistors (OTFTs) which use Ta2O5 as the gate insulators were fabricated on plastic substrates. The gate insulators were synthesized by anodizing the gate electrodes fabricated as stacked structures of aluminum (Al) and tantalum (Ta) at room temperature. The stacked structure suppressed the stress at the interface between the substrate and metal electrodes so that cracks on Ta could be avoided. The organic semiconductor pentacene was used as the active layer on the gate insulator. The OTFTs showed the best performance at an operating voltage of 5 V and a good field-effect mobility of 0.36 cm2/Vs on plastic substrates. The structural order of the pentacene in the film and the temperature dependence of the leakage current in the insulator were also studied to improve the device characteristics.

Anomalous temperature dependence of the ordered Ga0.5In0.5P photoluminescence spectrum

The temperature dependence of the photoluminescence spectrum of Ga0.5 In0.5 P/GaAs grown by organometallic vapor phase epitaxy is measured. Samples with a highly long‐range ordered structure show anomalous behavior, where peak energy changes with temperature exhibiting Z‐shape dependence. In the range between 100 and 30 K, peak energy decreases monotonously with decreasing temperature, and below 30 K, begins to increase, splitting into two peaks. The most probable cause of this behavior is crystal defects related to the long‐range ordered structure.

Air stable, high performance pentacene thin-film transistor fabricated on SiO2 gate insulator treated with β-phenethyltrichlorosilane

A pentacene thin-film transistor (TFT) was fabricated on a SiO2 gate insulator treated with β-phenethyltrichlorosilane (β-PTS). Employing β-PTS for the surface treatment of SiO2, large grains were present in the initial stage of pentacene crystal growth. The field effect mobility was as high as 1.5cm2∕Vs and the on/off ratio was over 106. The surface treatment dramatically improved the stability in air of the pentacene-TFT’s electrical characteristics. A field effect mobility of over 1cm2∕Vs and on/off ratio of over 105 were maintained after scanning the gate voltage 2000 times in air. This result indicates that the surface treatment with β-PTS not only improved TFT performance but also significantly suppressed the device’s degradation.

Perfluoropentacene and Perfluorotetracene: Syntheses, Crystal Structures, and FET Characteristics

The syntheses and FET characteristics of perfluoropentacene and perfluorotetracene are described. Both acenes are planar and crystalline materials that adopt π-stack structures with the short interpalnar distances of 3.27 Å in perfluorotetracene and 3.21 Å in perfluoropentacene. The oxidation and reduction peak potentials of the perfluorinated acenes shift positively compared with those of the corresponding acenes, suggesting that the HOMO and LUMO energies are diminished by fluorine substituents. Organic field-effect transistors (OFETs) with perfluoropentacene exhibit n-type semiconducting properties with high electron mobility of 0.22 cm2/V s.

Improvement of Characteristics of Organic Thin-Film Transistor with Anodized Gate Insulator by an Electrolyte Solution and Low-Voltage Driving of Liquid Crystal by Organic Thin-Film Transistors

The characteristics of pentacene-based organic thin-film transistors (OTFTs) on plastic substrates with anodized gate insulators using different electrolyte solutions were investigated. The characteristics were significantly improved by using ammonium borate as the solution. The results show a good current-saturation characteristic, field-effect mobility of 0.51 cm2/Vs and a current on/off ratio of 105 at a low drain voltage of 3 V. To investigate the reason for the improvements, the structural and electrical characteristics of OTFTs and insulators were examined using various methods such as X-ray diffraction, atomic force microscope, capacitance-voltage measurement and secondary ion mass spectroscopy. From the results of these experiments, it was found that the orientation of pentacene was improved and the concentration of impurities in the insulator decreased in the case of ammonium borate. We also examined low-voltage driving of liquid crystal devices by the OTFT, targeting its application for flexible displays. Improvement of characteristics by the solution was also confirmed in the driving.

Crystalline and electronic energy structure of OMVPE-grown AlGaInP/GaAs

Abstract The crystalline and electronic energy structure of AlGaInP/GaAs grown by organometallic vapor phase epitaxy (OMVPE) is investigated using transmission electron microscopy (TEM) and electroreflectance (ER), as well as photoluminescence (PL) and Raman scattering measurements. In TEM observation, sharp superstructure spots at the h+ 1 2 , k− 1 2 , l± 1 2 position, corresponding to CuPt type structure, are present in the (110) diffraction pattern. Based on this observation, the relationship between the bond and the ordered structure configuration on 9001) growth surface is discussed. It is also found by photoluminescence and Raman scattering measurements that the GaInP grown on (111)B is in a disordered state. Photoluminescence measurement for AlGaInP grown under various conditions shows that an ordered structure exists not only in GaInP but throughout the entire compositional range of the AlGaInP/GaAs. The electroreflectance spectrum shows anomalous structures specific to OMVPE-grown GaInP. The structures around 2.2 and 2.4 eV suggest that there exist additional interband transition edges caused possibly by zone-folding from the L point to the Γ point.

Liquid Crystal Display Cells Fabricated on Plastic Substrate Driven by Low-Voltage Organic Thin-Film Transistor with Improved Gate Insulator and Passivation Layer

Organic thin-film transistor (OTFT)-driven 5×5 polymer-dispersed liquid crystal (PDLC) display cells on a flexible plastic substrate have been developed. It is necessary to increase the maximum usable gate voltage of OTFT to obtain a high contrast ratio. We propose a stacked gate insulator that consists of polyvinylphenol and anodized Ta2O5 for decreasing the gate leakage current and increase the maximum voltage. The OTFT with the insulator showed a field-effect mobility of 0.4 cm2/(V s), a current on/off ratio of 105, a low threshold voltage of 1.1 V, and a subthreshold slope of 0.2 V/decade. Leakage current was successively decreased up to a gate voltage of 15 V, maintaining a low-voltage operation of OTFT. Double passivation layers using polyvinylalcohol and photosensitive acrylic material are also proposed to prevent the degradation of OTFT by liquid crystal. The bending characteristics of OTFT on plastic substrates were also measured for various radiuses of curvature. The OTFT can operate at a radius of curvature exceeding 20 mm. On the fabricated display cells, we confirmed a good display operation with a contrast ratio of 10:1 with a low driving voltage of 12–13 V.