Kazuyuki Uno

Structural changes on annealing of MBE grown (Ga, In)(N, As) as measured by X-ray absorption fine structure

Abstract GaInNAs grown on GaAs has recently been found to optically emit at wavelengths longer than previously possible with material grown epitaxially on GaAs substrates. To improve radiative efficiency, material is annealed after growth, after which the band gap is found to significantly blueshift. Structural changes that occur during this annealing process were studied using fluorescent X-ray absorption fine structure. By comparing the absorption data with lattice parameter and band structure simulations, it was found that the number of In atoms surrounding N atoms increased after the high-temperature annealing. According to ab initio simulations this ordering will increase the band gap of the GaInNAs alloy. We believe the reduction of free energy drives the reordering process towards increasing In–N coordination.

Crystallinity improvement of benzodithiophene-dimer films for organic field-effect transistors

We have significantly improved the crystallinity of vacuum-evaporated benzodithiophene (BDT)-dimer films using pentacene crystallinity control layers. The pentacene molecules evaporated on hexamethyldisilazane-treated SiO2 surfaces first form many small islands. Then, the islands grow two dimensionally. We used thin pentacene films with surface coverage of approximately 90% as crystallinity control layers, and deposited 30-nm-thick BDT-dimer films on them. Micron-scale planar grains are observed in atomic force microscopy images of the BDT-dimer films. The highly anisotropic shape of the grains indicates that the BDT-dimer films of good crystallinity have been obtained. The field-effect transistors based on the BDT-dimer films showed excellent hole mobility of 1.2cm2V−1s−1 at maximum.

Investigation of polysilsesquioxane as a gate dielectric material for organic field-effect transistors

We fabricated pentacene field-effect transistors using polysilsesquioxane (PSQ) as a gate dielectric material that can be cured at sufficiently low temperatures for low cost plastic substrates. The surface roughness of the PSQ films were reduced by the introduction of 3-methacryloxypropyl groups, and their surface energy was controlled to match with that of the pentacene layers by the introduction of phenyl groups. Consequently, the carrier mobility was improved by two orders of magnitude and reached 0.38 cm2 V−1 s−1.

Contact and channel resistances of organic field-effect transistors based on benzodithiophene-dimer films deposited on pentacene crystallinity control layers

We have investigated contact and channel resistances of organic field-effect transistors (FETs) based on benzodithiophene (BDT)-dimer films deposited on thin pentacene layers used as crystallinity control layers (CCLs). The contact resistance of source/drain electrodes made of conductive organic films instead of Au films has been reduced for pentacene-CCL/BDT-dimer FETs; the carrier mobility has been improved to 1.2 cm2 V−1 s−1 at maximum. Because the channel resistance of the pentacene-CCL/BDT-dimer FETs is found to be lower than that of reference pentacene FETs, the carrier transport in the BDT-dimer layers is more important than that in the pentacene CCLs for the high mobility.

Thermal Annealing Effect in GaInNAs Thin Films Estimated by Fluorescence X-Ray Absorption Fine Structure Spectroscopy

The radiative efficiency of dilute nitride GaInNAs alloys is improved by thermal annealing. However, the band gap of these alloys is found to significantly blueshift. The structural changes of these alloys occur during thermal annealing. In this study, we studied the local atomic configuration around In atoms using fluorescence X-ray absorption fine structure spectroscopy. By comparing the radial distribution functions of as-grown and annealed GaInNAs thin films, it was found that the number of In atoms surrounding N atoms increases after thermal annealing.

Carrier transport in benzodithiophene-dimer field-effect transistors with pentacene crystallinity control layers

The carrier transport in the benzodithiophene (BDT)-dimer films plays significant role for high carrier mobility of BDT-dimer field-effect transistors (FETs) using very thin pentacene layers as crystallinity control layers (CCLs). When the grains of the BDT-dimer films are small, the carrier mobility is not only low but also depends on the average thickness of the pentacene-CCLs. However, it is independent from the thickness of the CCLs when the grain size of the BDT-dimer films is large enough, and becomes as high as 0.76 cm2 V−1 s−1. This result shows that the high carrier mobility of pentacene-CCL/BDT-dimer FETs is due to the carrier transport in the BDT-dimer films.

Organic thin-film transistors based on solution-processable benzodithiophene dimers modified with hexyl groups

Benzodithiophene dimers modified with hexyl groups (2C6-BDT-dimer) were investigated as solution-processable organic semiconductors for organic thin-film transistors (OTFTs). Since 2C6-BDT-dimer crystals have an anisotropic shape, flow coating was adopted to grow polycrystalline films. The flow-coated films were inferior to the vacuum-evaporated ones in terms of their crystallinity estimated from X-ray diffraction data. However, the hole mobility of the OTFTs with the flow-coated films, which was 1.7 cm2 V−1 s−1 at maximum, was higher than that of the OTFTs with vacuum-evaporated films because the one-dimensional thin crystals of the flow-coated films were aligned in the flow-coating direction.

Temperature dependence of photoluminescence properties of zinc sulfide grown from aqueous solutions by mist chemical vapor deposition

Temperature dependence of photoluminescence properties of zinc sulfide (ZnS) films, which were successfully grown from aqueous solutions by using mist chemical vapor deposition method, was investigated. The samples were grown at 500-700°C. Although the excitation condition using 325nm HeCd laser was below-gap excitations under 80 K, strong photoluminescence (PL) emissions were observed around 450nm. Because the PL peak strength increases with increasing the growth temperature, the origin of the 450nm peak would be sulfur vacancies. The peaks have small thermal quenching properties. The ZnS film grown at 500°C has a weaker luminescence from the sulfur vacancies than the other films. Additionally, the sample shows peaks around 360nm which would be originated from zinc oxide.

Pentacene memory transistors using monolayer of ligand-removed semiconductor colloidal nano-dots as a floating gate

Summary form only given. We fabricated pentacene memory transistors, in which monolayer of semiconductor colloidal nano-dots was embedded as a floating gate. The writing time of the memory transistors was reduced to be ~1/3 when the ligand molecules of the colloidal nano-dots were removed by (NH4)2S or UV/O3 treatment. The retention time, however, became considerably short by the ligand removal. These results suggest that the ligand molecules worked as a major par of the tunneling barrier for electrons trapped in the nano-dot floating gate.

Current-voltage Characteristics of Single CdSe Colloidal Nanodots Measured by Conductive-tip Atomic Force Microscopy

We have investigated current-voltage characteristics of individual CdSe colloidal nanodots by conductive-tip atomic force microscopy (AFM). The colloidal nanodots were spun-coat and scattered on a self-assembled monolayer of thiophene molecules formed on Au (111) surfaces for single dot measurements. A thin SiO 2 layer was deposited on the sample surface in order to prevent the dots being moved by the tip during measurement. We imaged the topography of isolated single dots by AFM operated in contact mode, and measured current-voltage characteristics with the conductive tip positioned on single dots; large conductivity changes which suggest resonant tunneling through a quantized energy level in the dot was observed even at room temperature.