Energy Levels For3d~6Ions On The Distorted Oxygen Octahedron And Carriers Passivation Mechanism For Band Engineering Design

Semiconductor information materials and devices are foundation and precursor for the development of information science and technology. In recent years, with the rapid research progress of diluted magnetic semiconductor, the discrete component possessing semiconductor logic function and information storage function of magnetic components attracts a large of attention due to its potential application. That is, combining electron charge with spin in a material. It also encourages us to speed up search and design room temperature magnetic semiconductor materials. It will bring a new revolution in the field of computer once room temperature magnetic semiconductor materials were found. In addition, with the increasingly serious problems of energy and environment, it is demanded and imperative to focus on the new energy exploration, energy conservation and emission reduction, pollution abatement. TiO2has broad application prospects in the solar energy acquisition and pollutants abatement because of its unique properties. In the dissertation, using first-principle computer simulations, we have studied the electronic structures, doping characteristics and magnetic origins of n type wide-band-gaps transparent conductive oxide semiconductors, such as TiO2, In2O3and SnO2, which have the most important applications in optoelectronics. The main innovation achievements are shown as follows:Firstly, we discussed the energy levels for3d6ions on the distorted oxygen octahedron, taking Ni doped SnO2and TiO2, Co doped bulk and nanocrystalline In2O3as the research paradigm, because of semiconductor magnetism depends on the spin-orbital split size and coupling of magnetic ion. It is found:(ⅰ) In low doping concentration, oxygen octahedron with small distortion will not lead3d6ion energy levels to spin splitting. At this time, all the research systems are no magnetic. If making two magnetic ions in adjacent cationic positions by increasing doping concentration or enhancing the lattice distortion,3d6ion energy levels will spin splitting, and create an effective magnetic moment,(ⅱ) Oxygen-vacancy exists in transition metal oxides will induce local magnetic moment, coupling with3d6ion through super exchange interaction. If oxygen-vacancy exists in the main group oxides, it will offer itinerant electron, long-range coupling with3d6ion through changing the number of charge of3d6ion. The results show that the magnetic of semiconductor can be modulated by enhancing magnetic ion spin split degree caused by crystal structure variation.Secondly, The wave function characters which create by surface dangling bonds of nonstoichiometric In2O3quantum dots are investigated by using band structure calculations, due to the existence of nonstoichiometric ratio problems and surface dangling bonds in nano structure compounds. The results show that the surface dangling bonds are the main source of unconventional magnetism. It provides a new way to regulate and control semiconductor magnetic. It will make the system to keep the neutral by using pseudo-hydrogen atoms with fraction electrons saturate surface dangling bonds, which is in agreement with the experimetal result that the materials generally won’t be charged. Generally, oxygen ion surface electronic states are mainly distributed in the top of valence band, which is equivalent to the shallow acceptor level provide holes, leading to a large spin-polarization. While indium ion surface electronic states are mainly distributed slightly below the conduction band, that is equivalent to the shallow donor level provide electrons, leading to a little spin-polarization. When the surface dangling bonds are full passivated by pseudo-hydrogen atoms, the localized surface states and the spin-polarization states in the gap disappear.Then, taking Ni doped rutile SnO2nanowires as the research paradigm, we comprehensive investigated the effection of lattice distortion and surface dangling bonds. It is clear that we can synthetically regulate and control semiconductor magnetic. The research results show:compared with the nonstoichiometric In2O3quantum dots, the spin splitting energy of rutile SnO2nanowires with a small surface distortion surface distortion is much smaller. And no spin magnetic moment generates when dope pure rutile SnO2nanowires with3d6ion. But if there are surface dangling bonds,3d6ion energy levels appear partial occupy state, so spin polarization will generate. Moreover, the spin polarization interactions in3d6ion doped rutile SnO2nanowires with cation surface dangling bonds are stronger than that in anion surface dangling bonds. When the ratio of cation surface dangling bonds and3d ion is3:2, the rutile SnO2nanowires with3d6ion will possess the biggest magnetic moment.Last but not the least, based on the passivation doping idea, we applied the passivation principle in improving the photocatalytic efficiency in TiO2and strengthen the magnetism in semiconductors. The results show that:(ⅰ) The donor-acceptor complex can be passivated by codoping, then avoid the photo-induced electrons compound with acceptor impurities holes. In particular, the carbon(C)/tungsten(W) codoped TiO2has a substantial increase in the valence band edge, while leaving the conduction band edge almost unchanged. The band gap reduces about2.2eV. These features are all beneficial to absorb visible light,(ⅱ) It is difficult to p-type doping in TiO2because of the very low valence band edge. Doping the host with group-IVA and group-VIB impurities can not only greatly enhance the valence band edge, but also move up the conduction band edge in a certain extent. It provides a possible approach to overcome the p-type doping bottleneck.(ⅲ) Passivated group doping also can strengthen spin splitting. In TiO2, a nitrogen-dopant induces a hole and occurs about0.7μB magnetic moment. Meanwhile, fermi level is higher than the valence band maximum, and the hole level is local near the fermi energy. But the fermi energy is further away from the valence band after passivated group doping, and the hole level became more local. The increscent spin exchange energy indicates that the spin polarization improves.