Theoretical Studies Of P-type Doping And External Field Control Of Low-dimensional ZnO Systems

Zinc oxide is a direct band gap semiconductor, with the wide band gap of3.37eV at room temperature. The exciton binding energy of ZnO is as large as60meV, which allows it to have potential applications in ultraviolet optoelectronic devices at room temperature and higher temperatures. Owing to these properties, much attention has been paid to exploring the properties and applications of ZnO by experimental and theoretical methods. In this dissertation, with using first-principles calculations we investigate the properties of ZnO surface and its low-dimensional structures, including ZnO bilayer、ZnO single sheet and ZnO nanoribbons. Our study will be helpful for understanding the surface effect on the p-type conductivity and how to modulate the properties of ZnO low-dimensional structures.This dissertation consists of four chapters. In the first chapter, we review the former experimental and theoretical studies for ZnO. Beginning with the ZnO wurtzite bulk, we address the electronic structures、native defects and p-type defects. Then we focus on the electrical properties of the polar surfaces and nonpolar surfaces. At the end of the first chapter, many different kinds of low-dimensional nanostructures are reviewed, which might allow them to have potential in nanodevices. Owing to the properties mentioned above, in our dissertation we mainly focus on the p-type doping in ZnO thin films and how to tune the properties of ZnO low-dimensional structures.In the second chapter, we brifly introduce the density functional theory (DFT) method, which is widely used in our dissertation. Within the framework of the DFT, all properties of a system is the functional of the ground state charge density. According to the Kohn-Sham equation, the many-body problem is approximated as a single-particle problem. Finally, the simulation packages used in our dissertation are also introduced.From the third chapter, we begin to focus on our research.In the third chapter, we report the investigated stability and the thermal ionization energy of p-type defect LiZn、No and LiZn-No at different atomic layers in ZnO thin films. Our calculations show that the doped Lizn、No and Lizn-No prefer to locate in the outmost atomic layer of ZnO films, and the thermal ionization energy of the impurities in the surface region is much larger than that in the bulk region, which is mainly attributed to the different distributions of electrostatic potential between the surface and the bulk. Our results strongly suggest that the surface effect arising from ZnO surfaces significantly degrades the p-type conductivity of ZnO films. Furthermore, we investigate the adsorption behavior of Li、N on ZnO surface. Our results indicate that only the proper concentration of p-type defects can make the p-type conductivity of ZnO films stable.In the fourth chapter, we mainly handle several low-dimensional ZnO nanostructures. First we investigate the relative stabilities and electronic structures of various stacking configurations for ZnO bilayer, which reveals that only the electrostatic attraction and the dipole moment interaction exist between the two single ZnO layers. Moreover, based on the different electronegativity between Zn atoms and O atoms, the external electric field can efficiently modulate the electronic properties and optical absorption spectra of a ZnO bilayer. The good response of ZnO bilayers to the external electric field might allow them to have potential applications in nanodevices. Then, we investigate the adsorption behavior of hydrogen on the planar hexagonal ZnO sheet. Our calculations find that the planar ZnO monolayer preferably adsorbs hydrogen molecules, where a hydrogen molecule attaches to one oxygen atom with binding energy of-0.13eV. This implies that the interaction between a hydrogen molecule and the ZnO sheet is stronger than that between a hydrogen molecule and graphene. We predict that the gravimetric density for hydrogen storage on ZnO sheet is evaluated to be about4.7wt%at zero temperature. Furthermore, our calculations show that the gravimetric density for hydrogen storage on ZnO sheet reaches1.5-2.1wt%at298K and5MPa. This suggests that ZnO sheets may have potential applications in hydrogen storage. At the end of this chapter, we present that under the loaded strain along the periodic axis, the ZnO zigzag nanoribbons are transformed into new ground state featuring square lattice. We find that this new ground state is semiconducting, which is completely different from the initial zigzag nanoribbons having metallic behavior. Such a metallic-semiconducting phase transition might allow ZnO nanoribbons to have potential applications in nanoswitch.