The Study On The Electronic Properties Of Nb-doped TiO₂Based Transparent Conductive Oxides

In this dissertation, we mainly discussed the crystal structure, electronic properties and stability of anatase and rutile TiO2, Nb-doped TiO2(NbTi cell) with different doping concentrations, and NbTi+Vo/Oi co-doped models by density functional theory based first-principle calculation. The main research work and contents are listed as follows:The calculated electronic structure and cystal structure of anatase and rutile TiO2suggest that our methods are reasonable.Based on the aforementioned discussions, anatase and rutile NbTi cell with different doping concentrations are also studied. Anatase NbTi, cell is shown to be an intrinsic metal. In contrast, the rutile NbTi cell shows insulating behaviors. Its metallic property arises from Nb substitution into the Ti site which provides electrons into the conduction band. The ionization efficiency of Nb in anatase NbTi cell is higher than that in rutile. We expect that anatase NbTi cell is a potential materials for TCOs while rutile NbTi cell is not. The ionization efficiency of dopants decreases with the increase of dopant concentration (nNb). The Nb doped TiO2is thermodynamically stable under the O-rich growth condition,which is favored to the doping of Nb atoms. On the contrary, it is not conducive to the doping of Nb atoms under the Ti-rich growth condition.With the increases of nNb, the formation energy increases.On basis of the results of anatase NbTi cell, the codoped modes are also investigated. It is found that the impurity levels mainly consist of t2g (Ti3dxy) of Ti around Nb in anatase NbTi cell, while in NbTi+Vo cell the impurity levels are contributed by t2g (Ti3dxy) of Ti near to Nb and eg (dz2, dx2-y2) of Ti around Vo. Their mn*of carriers along a(b) directions remains nearly invariant, while the mn*along c directions in NbTi+Vo is much smaller than NbTi cell. In NbTi+Vo cell, the electrons around Vo distribute homogeneously to the three Ti atoms around Vo which can contribute to the conductivity of materials. In NbTi+Oi cell, the interstitial O atom is settled next to another lattice oxygen. The two oxygen atoms are structurally identical, and they form an O2-like structure. The midgap levels of NbTi+Oi cell are mainly comprised by the2p of O atoms in O2-like structure. The midgap levels divided into an occupied level and an unoccupied level, the latter which can capture the free electrons. In NbTi+Oi cell, the extra electrons emitted by Nb are captured by O2-like structure. The electrons around Vo are free electron-gas while those distributed around Oi are localized. All these indicate that the introduction of Vo can inhance the conductivity of NbTi cell. Contrarily the Oi deteriorates to the electronic conductivity. The results of calculated defect-formation energy show that Vo can easily exist in NbTi cell under Ti-rich growth conditions. Under Ti-rich growth conditions, the defects in NbTi cell mainly exist as NbTi and Vo. Despite the O-rich growth conditions favor to the introduction of Nb, it can be also condusive to the formation of Oi, which deteriorates to the electronic conductivity of material.