First-principles Calculations Of Positron Lifetimes And Band Structure And Characterization Some Thin Films By Slow Positron Beam

The positron was the first kind of antimatter discovered by Carl Anderson in 1932. If an electron and positron collide, they will annihilate with the production of two (or more) gamma photons. During the last seven decades, a lot of outstanding results have been achieved by positron annihilation method. Positron annihilation spectroscopy techniques, which are well established nuclear techniques with applications, are the versatile and non-destructive tools for the study of the microstructure of material. Those techniques are based on the detection of gamma rays, which convey the experimental information, to extract basic structural features of the studied samples. The modern theoretical physics plays an important role in discovered positron process and actually predated their experimental discovery. A fair amount of theoretical and experimental work has been finished, and the results show that positron annihilation parameters obtained from experiments and theoretical calculations are enriched or certified each other. A solid is assumed to contain many bands in which the electrons in it are packed. Band Theory can successfully explains the distinction between conductors and insulators. To semiconductor, the band gap is wide comparing with the conductor and is narrow comparing with the insulator. Electrons and holes are known as the charge carriers in a semiconductor. Positron is a particle with very similar properties to those of the hole, possessing the same charge and a mass close to that of the hole. It is interesting to calculation its properties and compares it with electrons and holes. The conventional techniques are being used for bulk sample analysis. In order to obtain a defined small penetration depth, slow positron beam is developed and many problems related to thin layers and to defects near the surface and at interfaces are investigated. The Doppler-broadening technique can be applied for slow positrons similar to the conventional procedure with foil sources.The positron bulk lifetime was calculated using first-principles pseudopotential plane-wave method, and the positron band structure of several semiconductors was investigated using first-principles pseudopotential plane-wave method and empirical pseudopotential method. Besides, several thin films were studied by slow positron beam.Chapter 1: it briefly introduces the positron history, the positron depth profiling, the experimentally observable positron parameters, the experimental techniques and positron annihilation-theory.Chapter 2: it summarizes the current knowledge of density functional theory and basic positron lifetime calculation method. And try to offer a detailed analysis on the theoretical framework of the positron lifetime calculation. The positron bulk lifetime of several structure types'crystals are calculated. The calculated results are in agreement with the measured values within a reasonable error range.Chapter 3: The positron band structures of several semiconductors are calculated using first-principles pseudopotential plane-wave method and empirical pseudopotential method. The calculated results are in good agreement with the literature results.Chapter 4: the Ni₃Al thin films, TaN thin films, La_0.3Sr_0.7MnO₃ thin films and CeO₂ thin films were analyzed by slow positron beam. The results show that slow positron beam is sensitive to variation in material microstructure at the atomic scale. It is also effective to characteristic the quantify changes in nanostructure material induced by operational conditions and surface treatments.