Research On Magnetic And Thermoelectric Properties Of Functional Semiconductor Materials

There is a growing recognition that new functional semiconductor materials which need to exploit spins charge attributes as information carriers (so-called Dilute magnetic semiconductors) could provide such advantages as increased data processing speed, decreased electric power consumption, and increased integration densities, so Dilute magnetic semiconductors has become a focused interdisciplinary field involving electronics, physics, materials science and other disciplines. Meanwhile, with the growing public interest in environmental and energy problems in recent years, thermoelectric materials has the advantages of high reliability, being environmental friendly, and being easy to control, so it can play an important role in a global sustainable energy solution. Supported by National High Technology Research and Development Program of China (Grant No2009AA03Z405), National Natural Science Foundation of China (Grant No.60971068,61102024), and BUPT Excellent Ph.D. Students Foundation (Grant No. CX201114, CX201223), the research works presented in this doctoral thesis focus on the magnetic and thermoelectric properties of the functional semiconductor materials. The main contents and innovative ideas are listed below:1. We perform a first-principles simulation to study the electronic and optical properties of wurtzite Zn1-xCuxO. The simulations are based upon the Perdew-Burke-Ernzerhof form of generalised gradient approximation within the density functional theory. Calculations are carried out in different concentrations. With increasing Cu concentration, the band gap of Zn1-xCuxO decreases due to the shift of valence band. The imaginary part of the dielectric function indicates that the optical transition between O2p states in the highest valence band and Zn45states in the lowest conduction band shifts to the low energy range as the Cu concentration increases. Besides, it is shown that the insertion of Cu atom leads to redshift of the optical absorption edge.2. The electronic structure and optical properties of Co-doped A1N are investigated based upon the Perdew-Burke-Ernzerhof form of generalized gradient approximation within the density functional theory. The band gaps narrowing of Al1-xCoxN are found with the increase of Co concentrations. The analyses of the band structures and density of states show that Al1-xCOxN alloys exhibit a half-metallic character. Moreover, it is shown that the insertion of Co atom leads to redshift of the optical absorption edge.3. The electronic and magnetic properties of (Mn,N)-codoped ZnO are studied within the framework of the density functional theory, by using the Perdew-Burke-Ernzerhof form of generalized gradient approximation. Five geometrical configurations of Mn doped ZnO are investigated and antiferromagnetic (AFM) properties of Mn doped ZnO are demonstrated. Furthermore, by investigating13geometrical configurations, for (Mn,N)-codoped ZnO, the ground state is changed from no-metallic AFM to half-metallic ferromagnetic, which is due to the strong hybridization between N2p and Mn3d states. In addition, the most stable configurations are found to be-O-Mn-N-Mn-O-.4. The electronic and magnetic properties of (Mn,C)-codoped ZnO are studied in the Perdew-Burke-Ernzerhof form of generalized gradient approximation of the density functional theory. By investigating five geometrical configurations, we find that Mn doped ZnO exhibits anti-ferromagnetic or spin-glass behaviour, and there are no carriers to mediate the long range ferromagnetic (FM) interaction without acceptor co-doping. We observe that the FM interaction for (Mn,C)-codoped ZnO is due to the hybridization between C2p and Mn3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. Meanwhile, We demonstrate that ZnO co-doped with Mn and C has a stable FM ground state and show that the (Mn,C)-codoped ZnO is FM semiconductor with high Curie temperature. These results are conducive to the design of dilute magnetic semiconductors with codopants for spintronics applications.5. A first-principles study has been performed to evaluate the electronic structure and thermoelectric properties of Bi2(Te1-xSex)3compound. Our calculations indicate that the spin-orbit effect is very important and necessary to descript the band structure for these materials. The transport coefficients are then calculated within the semiclassical Boltzmann theory, and further evaluated as a function of chemical potential assuming a constant relaxation time and an averaged thermal conductivity. Our theoretical calculations show that the ZT value is1.43for Bi2(Te1-xSex)3system, and it agrees well with previous experimental data.