M. Kitamura

Insitu fabrication of self‐aligned InGaAs quantum dots on GaAs multiatomic steps by metalorganic chemical vapor deposition

Self‐alignment of InGaAs quantum dots was achieved by growing the quantum dots on the multiatomic steps in metalorganic chemical vapor deposition. In this technique, first GaAs epilayer with multiatomic step structures along straight lines was grown on a vicinal GaAs substrate under appropriate growth conditions. Then, the InGaAs quantum dots were grown selectively on the multiatomic step edges using strain effects. This growth technique results in spontaneously aligned InGaAs quantum dots without any preprocessing technique prior to the growth.

High field-effect mobility amorphous InGaZnO transistors with aluminum electrodes

We report on the device characteristics of amorphous indium gallium zinc oxide thin-film transistors (TFTs) with aluminum (Al) electrodes. The TFTs exhibited a high performance with a field-effect mobility of 11.39 cm2/V s, a subthreshold swing of 181 mV/ decade, and an on-off ratio of 107. Further improvement in device performance was achieved by doping the source/drain contact regions, resulting in an enhanced mobility of 16.6 cm2/V s at an operating voltage as low as 5 V.

Organic light-emitting diodes driven by pentacene-based thin-film transistors

Integrated circuits with an organic light-emitting diode (OLED) and a thin-film transistor (TFT) based on small molecules were fabricated on a glass substrate. The field-effect mobility and the current on/off ratio of the integrated TFTs with a pentacene layer deposited at room temperature were 0.12 cm2/V s and higher than 103, respectively. The OLEDs had a multilayer structure containing tris-(8-hydroxyquinoline) aluminum doped with coumarin 6 to obtain high luminance efficiency. The luminance was controlled in the range of up to approximately 1000 by applying the gate voltage of 10 to −10 V. The value of the luminance is sufficient for conventional displays.

Organic/inorganic hybrid complementary circuits based on pentacene and amorphous indium gallium zinc oxide transistors

Complementary inverters composed of pentacene for the p-channel thin-film transistors (TFTs) and amorphous indium gallium zinc oxide for the n-channel TFTs have been fabricated on glass substrates. The p- and n-channel TFTs have field-effect mobilities of 0.6 and 17.1 cm2/V s, respectively, and inverters yield a high gain of ∼56. Complementary five-stage ring oscillator exhibits a good dynamic operation with an output frequency of 200 Hz at 10 V. Since both channel layers are stable in air and can be formed by room temperature deposition process, the hybrid circuits are applicable to flexible electronic devices.

Hybrid p-n junction light-emitting diodes based on sputtered ZnO and organic semiconductors

We fabricated light-emitting hybrid p-n junction devices using low temperature deposited ZnO and organic films, in which the ZnO and the organic films served as the n- and p-type component, respectively. The devices have a rectification factor as high as ∼103 and a current density greater than 2 A/cm2. Electroluminescence of the hybrid device shows the mixture of the emission bands arising from radiative charge recombination in organic and ZnO. The substantial device properties could provide various opportunities for low cost and large area multicolor light-emitting sources.

Enhanced light emission from an organic photonic crystal with a nanocavity

An organic photonic crystal (PC) with a nanocavity has been fabricated on a SiO2 membrane with air holes. The emission peak caused by the resonant mode of the nanocavity was clearly observed from the nanocavity area of the PC. The emission peak corresponded to degenerated dipole modes from comparison between measured and calculated results. Modification of a nanocavity caused the peak splitting of dipole modes and the appearance of a peak corresponding to a hexapole mode. The emission peak with a quality factor of about 1000 was obtained from a PC with a modified nanocavity.

Conductance of single thiolated poly(GC)-poly(GC) DNA molecules

We use ultrahigh vacuum scanning tunneling microscopy∕spectroscopy (UHV-STM∕STS) to investigate the electronic properties of single thiolated 12-base-pair poly(GC)-poly(GC) DNA molecules on a Au(111) surface at room temperature. Reproducible current-voltage curves of the DNA are obtained at variable sample-tip separations. The normalized conductance, which can be interpreted as the density of states, shows a well-defined wide band gap. UHV-STM∕STS opens up a novel technique to probe the electronic properties of biomolecules on surfaces at the atomic level.

High performance flexible pentacene thin-film transistors fabricated on titanium silicon oxide gate dielectrics

Pentacene thin-film transistors (TFTs) with a high dielectric constant gate insulator, titanium silicon oxide (TiSiO2), were fabricated on flexible polyethylene naphthalate films coated with indium tin oxide layers. In order to define the device characteristics, the thickness dependence of the gate dielectric properties was studied. The pentacene TFT with a 132-nm-thick TiSiO2 film showed high performance with a threshold voltage of −0.88V, an inverse subthreshold slope of 317mV/decade. From the current-voltage characteristics, the field-effect mobility was obtained, resulting in an impressive mobility of 0.67cm2∕Vs at an operating voltage as low as −5V.

Ferromagnetic properties of epitaxial La2NiMnO6 thin films grown by pulsed laser deposition

La2NiMnO6 (LNMO) thin films were fabricated on (100) oriented SrTiO3 single crystal substrates by pulsed laser deposition method. The film growth conditions were found to significantly affect the magnetic moment and ferromagnetic transition temperature of LNMO films. Epitaxial LNMO films exhibiting the ferromagnetism equivalent to that of bulk polycrystal were grown only in such a narrow region of growth temperature and oxygen pressure that stabilized Ni2+–O–Mn4+ superexchange ferromagnetic interaction. Out of this narrow region, the ferromagnetism is significantly suppressed by the appearance of secondary phases, locally disordered Ni and Mn ions, and oxygen vacancies.

High-performance pentacene thin-film transistors with high dielectric constant gate insulators

Pentacene thin-film transistors (TFTs) with titanium silicon oxide (Ti1−xSixO2) gate insulator have been fabricated to realize high field-effect mobility at practical voltages. The Ti1−xSixO2 films deposited by rf sputtering exhibited a flat surface and high dielectric constant. The pentacene TFT with the Ti1−xSixO2 gate insulator had high performance with a threshold voltage of −1.6V, an inverse subthreshold slope of 0.13V∕decade, and a current on/off ratio of 107 at a voltage of −10V. The field-effect mobilities of higher than 1cm2∕Vs were obtained in the whole voltage range of −2to−15V.