Ab initio studies of vacancy defects in bulk and nanostructured semiconductors
Student thesis: Doctoral Thesis
As intrinsic point defects, vacancies commonly exist in many semiconductors and play crucial roles in determining many of the properties and associated applications. In this thesis we perform ab initio studies to investigate vacancy defects in semiconductors. Typically we focus on the negatively charged nitrogen-vacancy (N-V-) center in bulk diamond and the oxygen vacancy in zinc oxide (ZnO) nanowires, as the nature of defects in these two systems is still not well understood and lots of issues remain unresolved. ZnO is a wide band-gap semiconductor with a large excitonic binding energy and a promising candidate for a wide variety of applications such as optoelectronics, photocatalysis and sensing. As a native defect, oxygen vacancy (VO) has been suggested to be responsible for the intrinsic n-type conductivity in ZnO bulk, acting as a compensation center for p-type doping. The surface of nanowires (NWs) also presents significant challenges as well as opportunities for tuning the electronic properties of ZnO. For example, VO has been suggested to enhance the photocatalytic activity in ZnO NWs, since it can capture photo-generated electrons and holes separately, and play a role in the decomposition of organic contaminants and small molecules. Furthermore, it is also important to investigate how this defect migrates inside the NWs, since it plays an important role in mediating impurity diffusion. Recently, it has been suggested that oxygen vacancies are likely to be located on the surface of the NWs. However, little is known on the diffusion of these defects in ZnO nanostructures. It is known that the coherent coupling between quantum objects and optical photons is of crucial importance in quantum information science. The associated quantum entangled states are widely used for applications in quantum information processing. Among the solid-state hosts which are preferred for these applications because of scalability reasons, the N-V- center in diamond is a realizable solid-state quantum logic bit (qubit) as it offers long spin coherence time, and the spin states can be optically initialized and read out. Due to high photostability the N-V- center also is a good candidate as a single-photon source used in quantum networks. However, successful realization of long-distance spin-photon entanglement is still a challenge. One of the difficulties arises from the zero-phonon line (ZPL) emission of this defect including very little part of the total emission; while the frequency shifted phonon-side band (PSB) normally gives rise to deterioration of the spin-photon entanglement. In this thesis we present an ab-initio investigation of migration behaviors for oxygen vacancy (VO) in ultra-thin ZnO nanowires. We find that VO prefers to be located at surface sites and have lower migration barriers than in bulk ZnO. The results suggest that one would expect a lower concentration of VO in inner sites of the ZnO NW compared to surface sites and sub-surface sites. These results provide a possible explanation for the enhanced photocatalytic activities in ZnO NWs observed in experiments. Furthermore, our results show that the migration of VO from inner sites to the surface is more favorable than the reverse path. This could lead to the pathway for successful p-type doping in ZnO where the impurity could diffuse to inner sites of the NW with the mediation of VO, leaving the vacancy at the surface and thus suppressing the self-compensation for p-type doping, which can have important implications for the use of ZnO in optoelectronics and photocatalysis. Moreover, we extensively present a theoretical work that probes the spin-conserving optical excitation of the N-V- center. Using density-functional-theory (DFT) calculations, we find that the relaxation energy in the optical absorption/emission of the N-V- center arises from the defect orbital relaxation together with the deformation of delocalized states, which are both correlated strongly with the local ionic arrangement. In optical excitation, the relaxation energy associated with the orbital relaxation is relatively larger than that from the deformation of delocalized states and thus dominates the SS, whereas in the deexcitation, due to different electronic occupancies the ASS is derived mostly from the latter. This provides an understanding of how ionic relaxation affects the excitation/deexcitation of the N-V- system and interprets the experimental observation well. The obtained results also demonstrate the dependence of the excitation spectrum of the N-V- center on the axial strain of the lattice, which is directly tunable via external pressure. We expect based on these results that pressure-control for the optical transitions of the N-V- center will be possible. Furthermore, the relatively small ASS obtained at 100 GPa demonstrates a suppressed PSB of the emission spectrum at low temperatures. This might prove useful as a means to improve the ZPL emission of the N-V- center for applications to quantum cryptography and quantum communication. Beyond these specific applications, the realizable control of the PSB emission also paves a pathway towards improving the efficiency of photon-spin coherent coupling, which is currently the key limitation in the realization of long-distance photon-spin entanglement.
- Semiconductors, Nanostructured materials, Defects