Minoru Dohi

Paramagnetic defects in ultrafine silicon particles

Two defects in the surface oxide layer of ultrafine Si particles, temporarily named an EXL center and an EXH center, were investigated by electron spin resonance (ESR). The Si particles were prepared by a conventional gas evaporation method. The specimens were exposed to air at room temperature to oxidize the surfaces. After the heat treatment in vacuum at 600 and 1000 °C, the ESR signal of the EXL and EXH center appeared, respectively, and after successive annealing in O2 gas at that temperature, the signal disappeared completely. The ESR signals of the defects have different microwave power saturabilities. The EXL center and the EXH center are different defects though they have the same g value. The relationship between ESR signals and infrared spectra was studied. The EXL center was found to be a defect related to the Si–OH structure.

Gas evaporation in space

Silver (Ag) was evaporated in argon (Ar) and xenon (Xe) gases at various pressures in the low-gravity environment aboard the space shuttle. Four glass bulbs with filament tips coated with 50 mg of silver were filled with Ar gas of 6.7 Pa (A) or 40 Pa (B) or Xe gas of 0.67 Pa (C) or 1.33 Pa (D) and ignited one by one in the low gravitational field of space. The evaporation temperatures were maintained at 1150°C at which smoke plumes were barely detectable in all cases in the ground experiment. A ball of smoke particles appeared to grow around the evaporation source instead of rising as it would under earth gravity conditions. No smoke was observed in (A), but it was observed in (B) and (C) and bursts of smoke extended in various directions from the smoke ball in the case of (D). The experiment suggested that vapor could be confined locally around the source with high pressure and temperature by the surrounding gas in the low gravity. This suggestion cannot be derived from any conventional model of evaporation in the gas.

Gas Evaporation of Zinc under Low Gravity with a Drop Capsule.

Fine zinc particles were produced in an argon gas atmosphere and in a helium gas atmosphere of 1.33 ×104 Pa without thermal convection. To investigate the initial process of crystal growth, particles deposited at various places and at different moments during evaporation were collected. The shape and size of the particles were observed using a scanning electron microscope. When the evaporation time increased, the mean size of the deposited particles decreased in the argon gas atmosphere, while it increased in the helium gas atmosphere. These experimental results can be interpreted in terms of the assumption that nucleation and crystal growth are dominated only by the diffusion of vapor from the evaporation source.

Electron Spin Resonance of Ultrafine Si Particles

A previously reported EX center is a new defect observed in thermal SiO2 layer on Si. The detailed characteristics of this defect have not been clarified yet. The present work reports on the creation, the activation and the passivation of the EX center by thermal treatment in ultrafine Si particles. The electron spin resonance signal of the EX centers was emphasized by annealing in vacuum and reduced by annealing in oxygen. The intensity of the EX signal varied reversibly with the pressure of ambient oxygen.

Molecular Process of Evaporation in Gas Atmosphere without Convection

A proposed modified diffusion equation for the molecular process of evaporation of materials in the gas atmosphere without convection was solved, and the existence of a zone of maximum vapor density higher than the saturation near the evaporation source was found. Actual density distributions of Zn vapor in He gas atmosphere at various pressures were calculated to be sensitively dependent on the gas pressure and the shape of the evaporation source.

Measurement of Vapor Distribution in Gas Evaporation without Convection by Atomic Absorption Method

Measurement of the distribution of zinc vapor atoms is carried out by means of atomic light absorption spectroscopy in the top-heating gas evaporation furnace which is developed for the gas evaporation without convection on the ground. Distribution of supersaturated vapor atoms in the wide range from the evaporating surface in the gas atmosphere is measured. The sizes and number densities of collected particles at various places on the collecting rod are interpreted in terms of measured vapor densities and temperatures at respective places.

Gas Evaporation of Zn by Means of the Top-Heating Vertical Furnace

Gas evaporation of Zn without convection was carried out in the top-heating furnace. The temperature distribution and the pressure of He gas in the furnace were controlled and the particles deposited at various places of temperature in the furnace were observed by scanning electron microscope. The conditions of nucleation and growth mechanisms of the evaporated Zn vapors were discussed.