Takuo Sugano

Deep Level Transient Spectroscopy of Bulk Traps and Interface States in Si MOS Diodes

Deep-leve transient spectroscopy (DLTS) of bulk traps and interface states in Si MOS diodes are theoretically studied and energy levels, capture cross-sections and spatial and energy density distributions of majority-carrier traps are measured. In P+-implanted unannealed MOS diodes, four bulk traps are measured at Ec-0.18 eV, Ec-0.20 eV, Ec-0.31 eV and Ec-0.45 eV. Their spatial distributions are found to be the same among them within experimental error and thought to be corresponding to the distribution of implanted ions qualitatively. Bulk traps are distinguished from interface states experimentally. The capture cross-section of interface states in non-implanted MOS diodes are measured to be of the order of 10-16 cm2 in the energy range of Ec-0.15 eV to Ec-0.30 eV. The interface state density measured with DLTS is found to be in a reasonable agreement with those detetmined by other methods.

DETERMINATION OF PHOTOLUMINESCENCE MECHANISM IN INGAN QUANTUM WELLS

We report on the unambiguous experimental determination of the photoluminescence mechanism in a set of In0.25Ga0.75N quantum wells. Instead of studying the photoluminescence for different In contents, we have investigated it as a function of the quantum well width in combination with a similar study performed on GaN quantum wells. In this way, we show that the photoluminescence is not coming from quantum dots or very localized states in the quantum well, but from the quantum well itself under the influence of a piezoelectric field induced by strain. The previously reported abnormal photoluminescence shifts and temperature dependencies can thus be explained.

Fabrication and stimulated emission of Er-doped nanocrystalline Si waveguides formed on Si substrates by laser ablation

Er-doped nanocrystalline Si (nc-Si) waveguides were fabricated on Si substrates and investigated by optical pumping. A stimulated emission at 1540 nm was demonstrated at room temperature. The sizes of the fabricated Er-doped nc-Si waveguides were 5000 nm×200 nm×L, where L is the cavity length and is changed from 1 to 10 mm. Superlinear optical outputs at 1540 nm were observed for the waveguides longer than 3 mm. The threshold of the optical output where the stimulated emission occurs is in the order of 10 MW/cm2, and is demonstrated to depend on the cavity length of the waveguides. A large reduction of decay lifetimes of the light output from a cleavage facet of the Er-doped nc-Si waveguides was observed when the pumping power density exceeded the thresholds indicating an increase of transition probabilities in intra-4f electrons in Er3+ ions caused by the stimulated emission. Better 1540 nm laser performance and lower pumping power density should be obtained by optimizing the device structure and increas...

Alignment Control of a Liquid Crystal on a Photosensitive Polyvinylalcohol Film

The photoinduced optical anisotropy of an azo dye-doped polyvinylalcohol film has been utilized for controlling the azimuthal alignment of a liquid crystal. The dynamic behaviors of the optical transmission of the film and of a liquid crystal layer aligned on the film are studied using linearly polarized pumping (Ar+ laser) and probing (He-Ne laser) beams. The results show that the azo dye-doped polyvinylalcohol film memorizes the information on the polarization direction of the exciting laser beam and the resulting anisotropy induced in the film causes adjacent liquid crystal molecules on the film to rotate azimuthally.

Dielectric characteristics of fluorinated ultradry SiO2

Improvement of dielectric breakdown characteristics and hot‐electron‐induced interface degradation of metal‐oxide‐semiconductor capacitors having fluorinated ultradry oxides has been demonstrated. The fluorine is introduced through HF surface treatment of Si prior to oxidation. Secondary‐ion mass spectrometry data indicate that SiF distribution is peaked both at the surface of the oxide and at the SiO2/Si interface in the fluorinated ultradry oxide. The possible role of fluorine on the improvement of the dielectric characteristics will be discussed.

Violet and Blue Light Emissions from Nanocrystalline Silicon Thin Films

Nanocrystalline silicon thin films with a grain diameter from three to seven nanometers were fabricated on silicon substrates. It is demonstrated for the first time that the thin films show intense violet and blue luminescence at room temperature. The luminescence spectra include three peaks at wavelengths of 415 nm, 437 nm and 465 nm. Anodizations of these thin films introduce additional green and red luminescence in the spectra. Fourier transform infrared spectroscopy indicates no hydrogen- or oxygen-related absorptions in the nanocrystalline silicon thin films. Only the anodized thin films show Si–Hx absorptions. The violet luminescence should be evidence of light emission from zero-dimensionally confined silicon structures.

Effects of minute impurities (H, OH, F) on SiO/sub 2//Si interface as investigated by nuclear resonant reaction and electron spin resonance

The effects of minute amounts of impurities (H, OH, and F) in SiO/sub 2/ are investigated to obtain a guideline for improving the reliability of MOS devices. To examine the behavior of hydrogen, deuterium (D) is adopted as a tracer. The quantity of deuterium dissolved in SiO/sub 2/ is measured by the D(/sup 3/He,p)/sup 4/He nuclear resonant reaction (NRR) technique. The Influence of the impurities on the SiO/sub 2/-Si interface structure is studied by electron spin resonance (ESR) measurement. Hot-carrier injection with MOS capacitors and transistors are examined to determine the effects of minute impurities on the electrical characteristics of gate SiO/sub 2/ and the correlation of this effect with the NRR and ESR experimental results. It was found that significant amounts of D/sub 2/O are diffused into SiO/sub 2/, even at 200 degrees C, and these dissolved D/sub 2/O molecules are eliminated at temperatures above 700 degrees C. The number of unpaired bonds at the interface increases with decrease of dissolved water in SiO/sub 2/. The disappearance of the interface traps after high-temperature annealing above 800 degrees C is thought to be due to the viscous flow of SiO/sub 2/ and to the interface reoxidation. Reducing the hydrogen and relaxing the interface strain are essential for improving the MOS device endurance against hot carriers. >

Violet luminescence from anodized microcrystalline silicon

Microcrystalline silicon (μ‐Si) thin films were anodized in dilute HF solutions in the same manner as forming porous materials. It is demonstrated for the first time that the anodized μ‐Si thin films show strong violet luminescence (415 nm) at room temperature. Visible green and red emissions were also observed accompanying the violet luminescence. Structural investigations with scanning electron microscopy indicate that any formation of micrometer‐sized pores which is typical for porous silicon does not exist in the anodized μ‐Si thin films as reported here. This fact is useful for device applications of silicon‐based materials.

Selective anodic oxidation of silicon in oxygen plasma

Plasma-enhanced oxidation process at low temperature was studied for the selective oxidation of silicon. Silicon dioxide films as thick as 1 µm were routinely obtained at the substrate temperature of 600 °C. The oxidation was found to proceed through the motion of both oxygen and silicon ions and/or their vacancies under the electric field applied across the oxide. Aluminum oxide film was used as a mask for selective oxidation and very small lateral oxidation under the mask was found. Furthermore, no generation of dislocations was found at mask edges. The properties of the oxide were found comparable to those of thermally grown oxide. Electron-spin resonance measurements revealed one kind of defects in the oxide, which was attributed to the interface states by comparing with results obtained by measuring the capacitance-voltage characteristics. The morphology of the Si-SiO 2 interface, studied by transmission electron microscopy was not significantly different from that of thermally grown SiO 2 -Si systems. Impurity redistribution in the substrate was not observed.

Determination of the interface states in GaAs MOS diodes by deep‐level transient spectroscopy

The capture cross sections and density‐energy distribution of the trap states at the interface between GaAs epitaxial layers and oxide films grown by anodization in oxygen plasma have been determined by deep‐level transient spectroscopy (DLTS). The capture cross sections are of the order of 10−12–10−13 cm2. The state density in the energy space range from 1×1013 to 3×1013 cm−2 eV−1. It has a peak 0.43 eV below the conduction band edge, but increases again near the valence band edge. The existence of traps in the oxide is also suggested. The activation energies of the emission rates determined by constant‐capacitance DLTS with a small pulse voltage are in good agreement with the surface potentials, and no particular band structure at the interface, such as an interface band, has been found.