Perovskite-type Oxide Semiconductor Doped With Optical And Electrical Properties Of The Surface Adsorption Theory

Strontium titanate(SrTiO3), a typical perovskite material, is a wide-gap oxide with a band gap of about 3.2eV at room temperature. It is transparent in the visible light region. Properly doped with n-type or p-type impurities, SrTiO3 becomes a good semiconductor or conductor. These characteristics become great advantages when SrTiO3 is used in the field of transparent electronics, such as transparent thin film transistor, solar cell, organic light emitting devices, and so on. In particular, the electronic performance of the all-SrTiO3 transparent thin film transistor, in which n-type doped SrTiO3 acts as a conducting channel and stoichiometric SrTiO3 as a dielectric barrier, is superior to that of the conventional amorphous silicon thin film transistor. While the electronic structure of the layered perovskite oxide Sr2TiO4 is similar to that of SrTiO3. It is a candidate for transparent conductive materials. Up to now, although large numbers of research work in n-type and p-type doped SrTiO3 has been carried out and some progress has been made, the electronic structure of n-type and p-type doped SrTiO3 is rarely reported. Research work in n-type and p-type doped Sr2TiO4 is at the initial stage and much theoretical guidance is needed. Meanwhile, with the size scale downward of devices and the increase of specific surface area of devices, the surface properties of semiconductor materials become more and more dominative in the performance of electronic devices. CO molecule that adsorbs on the surface will greatly affect the electrical properties of SrTiO3 surface. However, until now experiments in this respect is lack, the relevant theoretical work is also lack.In this thesis, the first-principles calculation based on the density functional theory has been performed to investigate the effect of acceptor and donor doping on the electrical properties, optical properties and structural stability of SrTiO3 and Sr2TiO4. The selected donor element is Nb, Sb and La, and the acceptor element is In and Sc. The theoretical study of n-type and p-type doped SrTiO3 and Sr2TiO4 will be important for the preparation of the perovskite-type transparent conductive films and manufacture of transparent electronic devices (e.g. the all-perovskite transparent thin film transistor).Moreover, the adsorption mechanism of CO on SrTiO3(100)surface is investigated, and the effect of CO adsorption on the geometrical structure and conductivity of SrTiO3(100)surface is discussed. The main research contents and results are as follows:1,The effect of Nb, Sb, and La n-type doping on the structural stability, electronic structure, and optical properties of SrTiO3 was investigated by first principles calculations. The calculated results revealed that the Nb, Sb, and La n-type doped SrTiO3 systems are stable. The dopant formation energy of La and Nb in SrTiO3 is smaller than the dopant formation energy of Sb.The ionization energy of La and Nb is also smaller than that of Sb. Therefore, La and Nb is better than Sb for n-type doping of SrTiO3. The La3+ and Nb5+ ions fully act as electron donor in doped SrTiO3, the Fermi level shifts into the bottom of conduction bands(CBs) and the SrTiO3:Nb and SrTiO3:La systems exhibit n-type degenerate semiconductor feature. As for the Sb-doped SrTiO3:Sb, there is a distortion near the bottom of CBs for SrTiO3:Sb after doping. This is an indication that some of the doped electrons have been trapped by the Ti atoms around the impurity atom. The optical transmittance of SrTiO3:Nb and SrTiO3:La improves significantly after n-type doping, and the transmittance is higher than 90% in the visible range.2,The effect of In and Sc p-type doping on the structural stability, electronic structure, and optical properties of SrTiO3 was investigated by first principles calculations. The calculated results revealed that the p-type doped SrTiO3:In and SrTiO3:Sc systems are stable and that the partial substitution of In for Ti (or Sc for Ti) merely result in local structural changes around the impurity atoms. The ionization energy of In in SrTiO3 is 0.1336eV, smaller than the ionization energy of Sc(0.2088eV). However, the dopant formation energy of In in SrTiO3 is much larger than the dopant formation energy of Sc. Moreover, a noticeable blue-shift of the absorption spectral edge is observed after doping. The optical transmittance of SrTiO3:In and SrTiO3:Sc improves significantly after p-type doping and the transmittance is higher than 85% in the wavelength region of 350nm to 625nm.3,The electronic structure and optical properties of intrinsic Sr2TiO4 were investigated by the first principles calculations. The calculated results revealed that the interaction between the Ti atoms is localized in the ab plane due to the SrO layers distributed in the perovskite network along the c-axis. The bond strength of the Ti-O(2) bond along the c-axis is weaker than that of the Ti-O(1) bond in the ab plane. The absorption spectral edge of is strongly dependent on the polarization direction of the electromagnetic field. The optical absorption edge ofε2//(ω) along the (001) polarization direction appears at a higher energy region. This implies that the c-axis is the most suitable crystal growth direction for obtaining the Sr2TiO4 materials with a wide transparent region.4,The effect of Nb and La n-type doping on the structural stability, electronic structure, and optical properties of Sr2TiO4 was investigated by first principles calculations. The electronic structure of La-doped Sr1.9375La0.0625TiO3.96875 was explored in detail. Emphasis is placed on the changes in structural and electrical properties by the simultaneous substitution of La for Sr and the introduction of oxygen vacancies. The calculated results revealed that the doped Sr2Nb0.125Ti0.875O4 and Sr1.9375La0.0625TiO4 systems are stable. The dopant formation energy of La substituting for Sr site is smaller than that of Nb substituting for Ti site. At the same time, the ionization energy of La in SrTiO3 is 0.1078eV, much smaller than the ionization energy of Nb(0.1345eV). Therefore, La is better than Nb for n-type doping of Sr2TiO4. The Nb5+ and La3+ ions fully act as electron donor in doped Sr2TiO4, the Fermi level shifts into the bottom of CBs and the Sr1.9375La0.0625TiO4 and Sr2Nb0.125Ti0.875O4 systems show n-type degenerate semiconductor character. After doping, a noticeable blue-shift of the absorption spectral edge is observed. The optical transmittances of Sr2Nb0.125Ti0.875O4 and Sr1.9375La0.0625TiO4 improve significantly after doping.As for the Sr1.9375La0.0625TiO3.96875 system, the oxygen vacancies have a tendency to distribute in the ab planes of the perovskite SrTiO3 layer. An oxygen vacancy related donor level appears near the bottom of CBs for Sr1.9375La0.0625TiO3.96875.The donor level is highly localized and makes little contribution to the electrical activity of Sr1.9375La0.0625TiO3.96875.5,The effect of In and Sc p-type doping on the structural stability, electronic structure, and optical properties of Sr2TiO4 was investigated by first principles calculations. The calculated results revealed that the doped Sr2Ino0.125Ti0.875O4 and Sr2Sc0.125Ti0.875O4 systems are p-type degenerate semiconductors. The structural stability of Sr2TiO4 is weakened after In and Sc doping. The dopant formation energy of Sc substituting for Ti site is much smaller than that of In substituting for Ti site. The ionization energy of Sc in Sr2TiO4 is 0.2348eV, slightly larger than the ionization energy of In(0.2147eV). Therefore, Sc is better than In for p-type doping of Sr2TiO4. At the same time, the optical absorption edge becomes smoother and a noticeable red-shift of the absorption spectral edge is observed after doping. The optical transmittance of the doped Sr2In0.125Ti0.875O4 and Sr2Sc0.125Ti0.875O4 reduces badly in the visible range.6,The adsorption mechanism of CO on SrTiO3(100)surface was investigated, and the effect of CO adsorption on the geometrical structure and conductivity of SrTiO3(100)surface was discussed. The calculated results revealed that the CO molecule adsorbs weakly on the SrTiO3 (100) surface and it is a weak physisorption. The CO molecule is preferentially adsorbed on the SrTiO3 (100) surface by C-downward and the TiO2-terminated surface is more benefit for CO adsorption than the SrO-terminated surface. The electrical properties of the TiO2-terminated SrTiO3 (100) surface is not changed after CO adsorption.After introducing the oxygen vacancy in TiO2-terminated SrTiO3 (100) surface, the interaction between the CO molecule and the SrTiO3 (100) surface is strengthened significantly. The DOS of the TiO2-terminated SrTiO3 (100) defective surface is changed significantly. The localized surface energy band appears near the Fermi level and the Fermi level is pinned in the localized surface stated. This results in that the conductivity of SrTiO3 can not be significantly improved by semiconducting doping.