OPTICAL PROPERTIES AND DEFECT STATES OF TITANIUM DIOXIDE STUDIED BY TRASITIQN METAL DOPPING AND OXIGEN VACANCY CONTROL

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Single-crystals of anatse form TiO_2 were grown by means of chemical transport reaction with a transport agent of NH_4Cl. The Nb-doped anatase samples(average 0.08wt% of Nb)) were grown by same method with row material added NbO_2(0.05wt% of Nb). (1)The free electron density of Nb-doped sample is found to be almost constant between 4.2K and room temperature, which shows that donor level is located very shallowly or in the conduction band. (2) ESR (electron spin resonance) spectra of Nb-doped samples showed same property with the as-grown pure crystal. (3) The color of the crystal changes depending on the heat-treatment under oxygen pressure or hydrogen atmosphere. The heat-treated crystals can be classified into 5 types, colorless, pale-blue, dark-blue, dark-green and yellow crystals. (4) The pale-blue, dark-green and dark-blue crystals show one ESR signal, which angular-dependence indicates the presence of an electron with the d_(xy) orbital. (5) X-ray diffraction, optical absorption and photoluminescence were measured under high pressures using a diamond anvil cell. Pressure lependence of the lattice constants was determined. Absorption spectra revealed ththe absorption edge of anatase shifts at a rate of l0meV/Gpa. (6) A broad luminescence band at 2.3eV shows a blue shift at a rate of 33meV/Gpa. The difference in these rates indicates that pressure effect on the Stokes shift energy of exciton is larger than that on the band gap. (7) The absorption edge jumps to lower energy side between 3.9 Gpa and 4.2 Gpa and luminescence at 2.3 eV disappears and a weak, but sharp band appears at 1.5 eV. The difference in luminescence between anatase and high pressure phase suggest the the emission band at 2.3 eV is due to recombination of the self-trapped exciton.