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

以氯化铵为传输剂，采用化学输运反应生长了锐钛矿型二氧化钛单晶。用相同的方法生长了Nb掺杂锐钛矿型样品(平均Nb含量为0.08wt%)，原料中加入NbO2(Nb含量为0.05wt%)。(1)Nb掺杂样品的自由电子密度在4.2K到室温之间几乎不变，这表明施主能级位于很浅的位置或在导带内。(2)掺Nb样品的电子自旋共振(ESR)谱表现出与纯晶体相同的性质。(3)晶体的颜色随氧压或氢气气氛下热处理的不同而变化。热处理后的晶体可分为无色、淡蓝色、深蓝色、深绿色和黄色晶体5种类型。(4)淡蓝色、深绿色和深蓝色晶体显示一个ESR信号，该信号与角度有关，表明存在d_(Xy)轨道上的电子。(5)用金刚石顶压室测量了高压下的X射线衍射、光吸收和光致发光。测定了晶格常数的压力依赖关系。吸收光谱表明，锐钛矿的吸收边以10 meV/Gpa的速率移动。(6)2.3 eV处的宽发光带以33 meV/Gpa的速率蓝移。这些速率的差异表明，压力对激子斯托克斯移动能的影响大于对带隙的影响。(7)吸收边在3.9~4.2 Gpa之间跃迁到较低的能级，2.3 eV处的发光消失，1.5 eV处出现一个微弱而尖锐的谱带。锐钛矿相和高压相之间的发光差异表明，2.3 eV处的发射带是由于自陷激子的复合所致。

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.