Investigation Of Hydrogen-related Defects And Properties In ZnO Semiconductors

The past twenty years have seen dramatic development of wide band gap semiconductors in various applications from blue lasers to solar cells, from light-emitting diodes to liquid-crystal displays. Zinc oxide (ZnO), as the representative of group ?-? wide band gap semiconductors, has attracted more and more interest for its useful advantages in direct and wide band gap, large exciton binding energy, availability of large single crystal, amenability to wet chemical etching, and radiation hardness. With the development of the spintronics, ZnO based diluted magnetic semiconductors (DMS), which exploit spin along with charge of electrons in semiconductors, have been the most potential candidates for these spin-based electronic devices.Though ZnO has many potential applications, the lack of control over the defects and dopants is still a big difficulty to practical devices. As-grown ZnO usually exhibits a natural n-type conductivity, while reliable p-type conductivity of ZnO is difficult to achieve. At the same time, room-temperature ferromagnetism (RTFM) has been achieved in transition-metal-doped or nontransition-metal-doped ZnO based DMS, but the origin of RTFM is still controversial. Hydrogen is one of the most abundant elements on earth, and is also easily incorporated in semiconductor materials. Hydrogen in ZnO exclusively acts as a shallow donor and is responsible for the n-type behavior of ZnO. Furthermore, hydrogen can induce RTFM in ZnO. Therefore, we need to pay more attention to the hydrogen-related problems in ZnO semiconductor materials.In this dissertation, hydrogen was incorporated into the ZnO single crystals and ZnO nanocrystals via electrochemical charging with hydrogen in acid solutions and annealing in H2/Ar mixed gas, respectively. The microstructural defects, optical and magnetic properties of these samples were thoroughly investigated by using positron annihilation techniques, combined with X-ray deffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), Raman scattering, photoluminescence (PL) and physical property measurement system (PPMS). The main contents and results are as follows:1. The ZnO single crystal was electrochemically charged with hydrogen in acid solution, and the corrosion tests of samples were performed by direct immersion in the same solution. Slow positron beam measurements revealed that for hydrogen-charged samples, a near-surface damaged region with a high concentration of open-volume defects was formed. SEM and AFM images showed that the hillocks of hexagonal pyramids were observed, and the open-volume defects resulted from the plastic deformation on the surfaces. For corroded samples, though many hexagonal pits or holes were observed, the slow positron beam results showed corrosion couldn't lead to the formation of Zn-related defects. The enhancement of NBE emission intensity confirmed that hydrogen was introduced via electrochemical charging.2. The interaction of hydrogen with defects in ZnO nanoparticles annealing under H2/Ar mixed gas at different temperatures was investigated. The thermal stability of hydrogen-related defects was carried out by isochronal annealing of hydrogenated samples. XRD results and SEM images showed that the crystallite of ZnO nanoparticles grow slowly in H2/Ar ambient compared to that in pure Ar ambient. Uniform particle morphology was observed in the the samples annealed in Ar gas, while the grain size for hydrogenated samples was quite different. HRTEM images confirmed that the distortion of ZnO lattice structure occurred due to the effect of hydrogenation. Positron annihilation results showed that more vacancy clusters formed above550?in the hydrogenated samples. The reason was that hydrogen hindered the growth of small grains at the boundaries via the removal of oxygen atoms and zinc atoms, leading to the overlap of the small grains and the formation of more vacancy clusters at the interfaces. Raman, EPR and PL measurements showed that there existed two main forms of hydrogen in ZnO nanoparticles:hydrogen occupying the oxygen vacancy Ho and interstitial hydrogen Hi. With subsequent annealing of the hydrogenated sample in air, Hi diffused out at about150?, and Ho, hydrogen trapped in oxygen vacancy, annealed out completely at700?.3. We observed that room-temperature ferromagnetism in ZnO nanoparticles could be switched between "on" and "off" states through hydrogen annealing and oxygen annealing process, respectively. Combining the XPS and PAS analysis, Zn vacancy and OH bonding complex (Vzn+OH) was crucial to the observed ferromagnetism. The Raman and PL measurements excluded the possibility of O vacancy as the origin of the ferromagnetism. Based on first-principles calculations, Vzn+OH complex was favorable to be presented due to the low formation energy. At the same time, this configuration gave a magnetic moment of0.57?B.