Toshio Osawa

Rapid growth of AlN films by particle-precipitation aided chemical vapor deposition

A colorless and transparent AlN film 0.3 mm in thickness was grown on a quartz glass substrate by the reaction between AlCl3 and NH3 at 1073 K and atmospheric pressure. The growth rate of this film was as large as 80 nm/s. The formation of fine particles in the gas followed by their precipitation onto the substrate kept colder than the gas, due to thermophoretic and diffusional movements, was presumably responsible for this rapid growth.

A Kinetic Study of the Chemical Vapor Deposition of Silicon Carbide from Dichlorodimethylsilane Precursors

Silicon carbide (SiC) films were prepared from dichlorodimethylsilane (DDS) precursors at temperatures ranging from 1173 to 1373 K by atmospheric pressure chemical vapor deposition (APCVD). A comprehensive model of the chemical vapor deposition of SiC from DDS was developed, which includes gas-to-surface mass transfer, surface sticking, and gas-phase chemistry. This model successfully reproduced our experimental results as well as those previously reported in the literature. This model assumed that the gas-phase chemistry consists of two reaction paths to form SiC: (i) gas-phase decomposition of DDS to form a growth species, most plausibly initiated by the fission reaction of the Si-C bond in DDS, followed by sticking on the surface and (ii) polymerization between this growth species and DDS, followed by deposition on the surface. The relative importance of mass transfer relative to chemical reactions in the gas phase was also determined.

Reaction of Si with HCl to form chlorosilanes time dependent nature and reaction model

We propose a chemical vapor deposition (CVD) process with closed gas recycling for making low-cost, crystalline silicon thin films for solar cells, which connects chlorosilane synthesis from Si and HCI with Si thin-film growth by CVD from chlorosilanes. In this work we studied the formation of chlorosilanes by the reaction of Si with HCl at temperatures ranging from 623 to 723 K. The reaction rate is time dependent, and many pores are formed on the surface of particles after reaction. These pores are active sites for chemical reactions, and the reaction rates increase with increasing pore area. The rate can be correlated with the conversion ratio of Si, and the temporal evolution of the reaction rate can be explained by a reaction model called the shrinking-core model with growing pores. By using this model, we estimated the reaction rates per unit area of activated surfaces and converted them into a rate equation that can be used for the reactor design. The incubation time of the reaction can be shortened by pretreating the Si particles in a fluidized bed, which probably creates defects in the native oxide layers on the particles, which in turn become reactive sites.

The effects of UV irradiation on titania deposition from titanium tetra-isopropoxide

Abstract The influence of UV irradiation on the deposition of titania from titanium tetra-isopropoxide (TTIP) under reduced pressure was studied. It was found that UV irradiation accelerates the titania growth rate by a factor of 2–3 in the temperature range of 573 to 623 K. The coverage quality of micron-size trenches in the cases with and without UV irradiation indicates the role of the photo-enhanced surface reaction. The threshold wavelength of 380 nm which is equivalent to the energy gap of anatase indicates strongly that band-to-band excitation is responsible for the photo-enhanced surface reaction.

Rapid vapour deposition and in situ melt crystallization for 1 min fabrication of 10 μm-thick crystalline silicon films with a lateral grain size of over 100 μm

We developed a film deposition method which yielded continuous polycrystalline Si films with large lateral grain sizes of over 100 μm and thicknesses of ∼10 μm in 1 min on growth substrates other than silicon wafers in a single-step process. The silicon source is heated to ∼2000 °C, much higher than the melting point of Si, which enables a high deposition rate. Controlling the temperature of the growth substrate, initially above and later below the melting point of Si, allows the seamless lateral to vertical growth of crystalline silicon grains. Thermally and chemically stable substrates of quartz glass and alumina with a 0.1 μm-thick amorphous carbon layer were effective; liquid silicon wetted well by forming a thin SiC interlayer while substrates stayed stable. Such large-grain polycrystalline silicon films synthesized rapidly in 1 min may be used for low-cost, stable and flexible thin film photovoltaic cells.

Surface protrusions of chemical vapor deposited TiN films caused by Cu contamination of silicon substrates

We found that surface protrusions of chemical vapor deposited TiN films are caused by reactions between copper contaminants and the silicon substrate. Depending on the size of the copper contaminant, two kinds of defects were formed: copper silicide (CuSi) and silicon dioxide. The silicon dioxide is formed because of the catalytic role of copper silicide. The defects grow both into and out of the silicon substrate. In the formation of copper silicide and silicon dioxide, copper, silicon, and oxygen are the major participating species.

Synchrotron‐radiation photochemical‐vapor deposition of amorphous carbon

High‐energy synchrotron‐radiation (SR) light was applied to the photochemical‐vapor deposition of amorphous carbon films. The source gases of CH4 and H2 have been directly photodissociated by the ultraviolet SR light to deposit carbon films on the heated Si substrate. The film structure was characterized by field‐emission scanning electron microscopy, Raman scattering, and infrared absorption, which indicate the film to be a rigid amorphous structure comprised of mostly sp3‐bonded carbon. The advantage as well as the limitation of the synchrotron‐radiation process to the synthesis of functional materials is discussed.

Global Sustainability and the Role of Asia

The 20th century was an era of expansion. Every human activity expanded drastically, revealing that the earth is finite. The concentration of atmospheric C02 started to increase sharply (Fig. 1), and the concentration of ozone in the ozone layer decreased. Also, a significant portion of the natural environment on the earth was lost. For example, as much as 40% of the forests have already been lost through conversion into agricultural land, urban areas, or arid land. If the expansion of the 20th century continues, the future will be bleak in the 21st century, and degradation of the environment and depletion of natural resources will prevail.