| First-principles calculations are based on density functional theory. This methodcan make the system of many electrons transform into the single electron research. Theelectronic structure of materials can be obtained by solving the Schrodinger equation, sothe materials are calculated from the mesoscopic and atomic level changing to theelectronic levels. Therefore, this thesis is to employ first principles as the main researchmethod, ZnO and ZnSb as the objects of studies, the electronic structures as the basicpoints, explaining the natures of the objects and theory mechanisms of theirs propertieschanges at electronic levels.ZnO is one of widely used semiconductor materials, however, many of itsperformances have more to do with surface properties. But in the current studies, thereare no comprehensive studies on its surfaces. So the third chapter of the thesis is toinvestigate the electronic structures on four low-index ZnO surfaces, that is,(001),(100),(101) and (110), respectively. Doping is a way of improving materials properties.But the studies on adsorption after doping of ZnO surfaces by using first-principlescalculations are not too many. In addition, there is no research on Cr doping ZnO andthen adsorbing O to find its electronic performances. The fourth chapter is to researchon ZnO (001) surface with Cr doped and O adsorption; ZnSb is a promisingsemiconductor thermoelectric material, experimental studies found its semiconductortype changes after doping Te. But it didn’t instruct the further mechanism. So the fifthchapter of this thesis mainly research on the mechanism why its semiconductor typechanges at electronic level.The main conclusions are as follows:①The electronic structures of the four low-index ZnO surfaces are studied,including structural parameters, band structures, density of states and the chargedensities. Our results show that the surface energy of (110) is the lowest, and itsrelaxation is also the smallest. So the prediction that (110) is rather more stable,(001)only the second. All of the four surfaces are direct band gap, in which the minimumband width is (101) and the maximum is (110). As we all know, the bigger of the bandwidth, the bigger of the corresponding resistance value, and the weaker of the electricconductivity. The calculations of band structures and DOS suggest that valence bands ofthe four surfaces mainly consist of Zn3d and O2p orbits, and the conduction bands are mainly composed by O2p states. The differences are the size, location and number ofpeaks of the formation of the DOS and PDOS. As to the charge density,(001) and (100)surfaces are stronger than the other two surfaces. This conclusion shows the bondingcapacity and charge transfer potential of the two surfaces are also stronger. Due to theoptimal values do not match, so improveing the surface performance can be realizedthrough doping other element.②The undoped Cr on O adsorption performance are calculated. The results showthat its density of states has no obvious change after O adsorption, and the chargedensity is relatively weak. The electron transferring between the adsorption of O and thesurface electrons is not so much. So we can come to the conclusion that the Oadsorption properties of (001) surface isn’t good.③(001) surface with doped Cr is still belongs to the semiconductor type, however,the band gap change to narrow. This change facilitates the valence electron transition tothe conduction band, the conductivity enhancement and resistance reducing. Besides,the density of state position also changes. In addition, the charge strength is strongerthan undoped surface. The electronic structures of the surface on O adsorption after Crdoped have changed significantly. The position of Fermi level initially closing to theconduction band becomes closer to the valence band, which indicates that more valenceelectron transition to the conduction band. The charge density and electron transferdiagram also prove this point. Then the electron transferring increases between thesurface and O atoms, which causes the interaction enhancement, increasing theproperties of ZnO on O adsorption and improving its sensitivity. Therefore, it is a goodway to improve the gas sensitive properties of ZnO on O adsorption by doping Cr.④ZnSb belongs to P-type semiconductor. While the dopant concentrations are1.56at%and2.34at%, ZnSb changes to N-type. It’s because that Te element in ZnSbintroduces some bands originating from Te s and p orbits and a donor energy level inthe bottom of the conduction band, which induces the N-type conductivity of ZnSb.With the increasing concentration of Te up to3.12at%, ZnSb changes to P-type onceagain. That’s because there is a new compound of ZnTe appeared, which belongs toP-type semiconductor. |
