Osamu Kogure

Leakage current characteristics of offset-gate-structure polycrystalline-Silicon MOSFET's

Leakage current characteristics of offset-gate-structure polycrystalline-silicon (poly-Si) MOSFETs are studied as a function of dopant concentration N<inf>off</inf>in offset-gate regions. Leakage current markedly decreases from 1 × 10^-9to 2 × 10^-11A at V<inf>D</inf>= 10 V as N<inf>off</inf>is varied from 1 × 10¹⁸to 1 × 10¹⁷cm^-3. A maximum ON/OFF current ratio of 10⁸is obtained at 1 × 10¹⁷cm^-3. Calculations based on a quasi-two-dimensional model indicate that the reduction of leakage current is attributable to a decrease of the maximum lateral electric field strength in the drain depletion region. An analysis of the leakage current characteristics in terms of carrier emission from grain-boundary traps implies that thermonic emission accompanied by thermally assisted tunneling could be the dominant mechanism in determining leakage current.

Hysteresis Behavior of Texture in the Field-Induced Nematic-Cholesteric Relaxation

The effects of bias voltages on the field-induced nematic-cholesteric relaxation are investigated both for samples with perpendicular boundaries and for ones with parallel boundaries, and are discussed in relation to the hysteresis behavior of texture. When the applied voltage is reduced from a value above the threshold to a bias voltage below the threshold a relaxation from the field-induced nematic to a Grandjean-like cholesteric state occurs, the relaxation time of which increases as the bias voltage approaches a certain value Vc↓; the ratio of Vc↓ to the threshold voltage Vc for cholesteric-to-nematic transition is about 0.5 irrespective of the boundary conditions. When the bias exceeds Vc↓, the transformation shows a hysteresis behavior such that the nematic state is kept for a long time until gradually replaced by focal conic domains nucleating from some local defect points; the boundary condition is found to play no essential role in the hysteresis behavior. An interpretation of the results is presented.

Laser-recrystallized polycrystalline-silicon thin-film transistors with low leakage current and high switching ratio

Laser-recrystallized polycrystalline-silicon thin-film transistors (poly-Si TFTs) with offset-gate structures have been fabricated on quartz substrates. Offset-gate structures make it possible to reduce leakage currents to as low as 5 × 10-14A/µm at VD= 10 V, more than two orders of magnitude lower than that in conventional-structure poly-Si TFTs. Optimization of the dopant concentration in offset-gate regions minimizes degradation of drive current, enabling high switching ratios exceeding 108. Calculations based on the quasi-two-dimensional model indicate that the reduction in leakage current is due to a decrease in lateral electric field strength in the drain depletion region.

Direct pattern fabrication on silicone resin by vapor phase electron beam polymerization

A new technique for high resolution lithography with a dry‐developed multilayer resist system is demonstrated. The technique involves a direct pattern fabrication by electron beam‐induced vapor phase graft polymerization. The selective pattern formation in irradiated areas was performed using PMMA or silicone resin as a base film and styrene as a grafting monomer. The base film patterns can be fabricated by dry etching with the mask of grafted patterns. However, because of isotropic behavior in the growth of grafted polymer, the base film thickness to be directly etched with the mask of graft‐polymerized film is severely limited by the resolution to be achieved. The combination of thin silicone resin as a directly etched layer using graft‐polymerized film and thick organic polymer as the bottom layer can solve the conflict between resolution and overall resist thickness, because the silicone resin has an excellent durability against oxygen RIE of organic polymers. Submicron pattern formation with a high a...

A semi-empirical model for the field-effect mobility of hydrogenated polycrystalline-silicon MOSFETs

The quantitative relationship between field-effect mobility ( mu /sub FE/) and grain-boundary trap-state density (N/sub t/) in hydrogenated polycrystalline-silicon (poly-Si) MOSFETs is investigated. The focus is on the field-effect mobility in MOSFETs with N/sub t/ 1*10/sup 2/ cm/sup -2/. It is found that reducing N/sub t/ to as low as 5*10/sup 11/ cm/sup -2/ has a great impact on mu /sub FE/. MOSFETs with the N/sub t/ of 4.2*10/sup 11/ cm/sup -2/ show an electron mobility of 185 cm/sup 2//V-s, despite a mean grain size of 0.5 mu m. The three principal factors that determine mu /sub FE/, namely, the low-field mobility, the mobility degradation factor, and the trap-state density N/sub t/ are clarified. >

AC-Thin Film ZnS:Mn Electroluminescent Device Prepared by Metal Organic Chemical Vapor Deposition

AC-thin film ZnS:Mn electroluminescent devices are fabricated by the Metal Organic Chemical Vapor Deposition, using dimetylzinc and H2S as source materials. The crystal structure of the MOCVD-prepared ZnS film on glass is (111)-oriented cubic polycrystalline. The electroluminescent center Mn is added into MOCVD-prepared ZnS film through thermal diffusion. The brightness and efficiency of the MOCVD-prepared EL devices are higher than those of the EB-evaporation prepared EL devices having the same structure. The MOCVD-prepared EL devices exhibit few dark spots, and therefore exhibit high stable operation and high breakdown voltage.

Light Scattering Characteristics in Nematic-Cholesteric Mixtures with Positive Dielectric Anisotropy

The angular dependence of light scattering in nematic-cholesteric mixtures with positive dielectric anisotropy was measured; the quiescent transparent satate S, the focal-conic state F, the storage focal-conic state F0, and the field-induced nematic state H were investigated. The F0 state showed Bragg scattering peaks caused by the lattice spacing of P0/2 where P0 was the helical pitch of the mixture used. While, the light scattering profile for the F state under an applied electric voltage was found to be characterized by scattering peaks caused by the optical lattice spacing of P0 itself, though the detailed physical reason has not yet been fully understood. The contrast ratio of the light scattering between F0 and S was also discussed in connection with application to storage-type liquid crystal displays.

Formation of high‐quality, magnetron‐sputtered Ta2O5 films by controlling the transition region at the Ta2O5/Si interface

Magnetron‐sputtered Ta2O5 films on Si are analyzed for their applications to semiconductor devices. It is clarified that the transition region formed at the Ta2O5/Si interface plays a significant role in determining electrical characteristics. This transition region formed during the initial stage of deposition consists of a mixture of Ta, Si, and O, and its thickness is in the range of 4 to 6 nm. The composition profile of this transition region and its thickness depend on two dominant factors: (1) oxygen partial pressure in the sputtering gas and (2) the surface oxide thickness on Si prior to deposition. Furthermore, it is initially indicated that 2.5‐nm‐thick surface oxide formed on Si prior to deposition produces a significant deterioration in leakage current. By reducing the surface oxide thickness to as low as 1.8 nm and adjusting the oxygen partial pressure, an optimum transition region is formed, which makes it possible to obtain high‐quality Ta2O5 films with high dielectric breakdown strength exc...

Low-Threshold Strained-Layer InGaAs Ridge Waveguide Lasers

Strained-layer InGaAs graded-index separate confinement heterostructure single-quantum-well (SQW) lasers have been fabricated. The threshold current dependence on cavity length using a 150- µm-wide broad contact laser yielded a transparency current of 49 A/cm2 and a differential gain coefficient of 10.8 cm/A, values which are both superior to those of conventional GaAs SQW lasers. A CW threshold current of fabricated 3- µm-wide ridge waveguide lasers was reduced to 2.8 mA by HR-HR coating at a lasing wavelength of 972 nm.

Study of ECR Hydrogen Plasma Treatment on Poly-Si Thin Film Transistors

In order to improve the characteristics of poly-Si thin film transistors (TFTs), hydrogen plasma treatment using electron cyclotron resonance plasma (ECR plasma treatment) has been studied. ECR plasma treatment has improved carrier mobility and the ON/OFF current ratio of poly-Si TFTs, which have been found to be greater than those using conventional hydrogen plasma treatment with radio frequency plasma. The improvement in the poly-Si TFTs characteristics is due to the reduction in the carrier trap density at the poly-Si grain boundary. Furthermore, the ECR plasma treatment improves leakage current.