Study Of Optical Properties Of ZnO Doped With Earth Metal La And Codoped With Transition Metal And Carbon

The research on ZnO emerged in the1930s. ZnO has lots of advantages such as a wide direct bandgap of3.37eV, large exciton bonding energy (60meV) at room temperature, low dielectric constant, lower stimulated emission threshold, low growing temperature, large optical coupling coefficient, high chemical stability, and having a high Tc and a large magnetization by doping and so on. Therefore, zinc oxide plays a role in various technological domains such as gas sensor, transparent conducting oxide (TCO), liquid crystal displays, and transparent thin-film transistors (TTFT) etc.Currently, the investigation on doping of ZnO has attracted much attention, and it is also a good method for the realization of bandgap engineering. As is known, a crucial step in designing modern optoelectronic devices is the realization of bandgap engineering to create barrier layers and quantum wells. Once the modulation of the bandgap is achievable, electro-optical devices based on ZnO films will be promising. As such, extensive researches based on impurity doping of ZnO have been carried out in order to tune the bandgap of ZnO. On the other hand, ZnO-based diluted magnetic semiconductors (DMSs), which involve charge and spin degrees of freedom in a single substance, are attracting much more attention because of the prospect of their application in the emerging field of spintronics, and there has been a major effort to produce DMSs with Curie temperatures (Tc) at or above room temperature. In this thesis, the electronic structure and optical properties of Zn1-xLaxO (x=0.0625,0.125) and Zn0.935(TM)0.062500.935C.0.0625(TM=Mn, Fe, Co, Ni, Cu) system have been investigated by using density functional theory based on first-principles ultrasoft pseudopotential method, the results are summarized as follow:1. The electronic structure and optical properties of ZnO doped with La have been investigated by using density functional theory based on first-principles ultrasoft pseudopotential method. The calculated results show that the La doping increases the bandgap of ZnO, in agreement with the experimental results; while the Fermi level shifts into the conduction band, revealing so-called Burstein-Moss effect. In comparison to pure ZnO, a new peak appears in the imaginary part of dielectric function in the system doped with La and the optical absorption edge has been obviously changed. Moreover, the covalent property of Zn1-xLaxO is found to weaken with the increase of the La concentration.2. Electronic structure and optical properties of ZnO co-doped with transition metal and carbon have been investigated by using density functional theory based on first-principles ultrasoft pseudopotential method. The calculated results show that the Fermi level has shifted codoping configuration with transition metal and carbon, in which the covalent property of ZnO has been changed remarkably. Such case seems in more favor of the FM semiconductor with high Curie temperature. Also, the optical properties in the high-energy region (>5.0eV) are almost not influenced; while in the low-energy region (<5.0eV), the optical properties are changed after codoping. The changes of optical properties are qualitatively explained in connection with the calculated electronic structure.