Design And Calculation Of High P-type Conductivity In ZnO

Semiconductor materials are not only the cornerstone of the modern information society support, but also are the important foundation of new energy development and utilization. Znic Oxide is a new type of semiconductor material, which has a wide band gap and high exciton binding energy, and it is widely used in solar cells, surface acoustic wave devices, liquid crystal display, gas sensors, and pressure-sensitive devices. However, it is difficult to fabricate ZnO-based optoelectronic devices which are performance, stability and reproducible of p-type ZnO. The reason can be listed as following: The first one is that doping asymmetry makes its difficulty in achieving p-type ZnO. The second is the doping on the basis of lattice mismatch. Therefore, we have committed in theory to calculated and design p-type ZnO systematically. In this paper, based on density functional theory (DFT), using the first-principles calculation, we have studied the influence mechanism of doping for bulk materials and surface defects on the conductivity of ZnO.1. Study on the influence of monodoping on electrical properties of ZnO. 1) From the electronic structure of pure-ZnO: The top valence band is mainly composed of Zn-d states, and the upper valence band is mainly contributed by the O-s states. The conduction band is provided by the Zn-s states. 2) For the N-single doping ZnO, it is found that the Fermi level goes into the valence band. There are certain peaks on the top of the valence band, demonstrating as the characteristic of p-type. But, because of the interaction of holes made the carriers localization strong, it reduces the solubility of nitrogen, and form deep acceptor energy near the Fermi level, which limited carrier concentration. So, it is not available for forming p-type ZnO. 3) For the electronic structure of Zn1-xMxO (M=Sn, Y), it is found that the dopant system both demonstrates as the characteristic of n-type. But it generates red-shifted for Sn doping ZnO, while it demonstrates blue-shifted for Y doping ZnO. Simultaneously, when the heavily doping with low concentration, it can enhance the conductivity of ZnO. 2. Study on the influence of codoping on electrical properties of p-type ZnO. 1) For (nN, Mg) codoping ZnO, we have calculated the defect formation energy of the systems, ionization energy, bind energy, conducting factor and electronic structure. The results show that: we can gain lower ionization energy and higher conductivity p-type ZnO through 4N-Mg codoping. 2) For (N, Ag) codoping ZnO, the results show that with the increase of the concentration of N, the p-type conductivity is strengthened. When the ration concentration of N and Ag is reach to 3:1, the conductivity is the highest. 3) For (nN, B) codoping ZnO, it is found that: when the ration concentration of N and B is 1:1, the system demonstrates as intrinsic states. But, if there is one Zn vacancy, the system demonstrates as a weak p-type. As the increase of the concentration of N, the system is indicating as strong p-type. 4) For F-Li codoping ZnO, it is found that with the increase of concentration of Li, it can improve p-type conductivity.3. Study on the influence of ZnO((10(1|-)0)) surface defects on the electrical properties. 1)We calculate the surface energy, electronic structure and work function of ZnO((10(1|-)0)) surface. The results show that: the surface energy of ZnO((10(1|-)0)) surface is the smallest, and is a cleavage plane. From the PDOS, we found that there are surface states of forming-bonding on the surface. 2) We calculated the formation energy, surface energy and electronic structure of ZnO((10(1|-)0)) surface with vacancy. It is found that: the surface with Zn vacancy and the surfaces with O vacancy have a great influence on the lattice distortion. For the surface with Zn vacancy, it demonstrates as weak p-type. For the surface with O vacancy, it demonstrates as n-type. The surface of Zn vacancies and O vacancies on the lattice distortion effects are both large. Because Zn-atoms and O-atoms form dimmers in the surface of ZnO((10(1|-)0)). 3) Based on NEB method, we have investigated the diffusion of ZnO((10(1|-)0)) surface with the intrinsic defects. The results show that: In the indirect diffusion with Zn interstitial, the diffusion barrier is the smallest for the two nearest neighbors'position, which indicate that it is the most possiblely diffusion mechanism. In the direct diffusion with Zn vacancy, the diffusion barrier is relatively high and the Zn vacancy is stable to exist in the surface. In the O interstitial sites in the direct diffusion, the diffusion barrier is also higher, which indicate that it is difficult to diffuse, and it is stable. In direct diffusion with O vacancy mechnism, diffusing from the first layer to the second layer, the diffusion barrier is relatively high, which shows that ZnO((10(1|-)0)) surface with O vacancy is stable, which is the main reason of demonstrating as n-type for fabricating ZnO.