Vacancy-type defects in Si, Ge and SiGe studied with positron annihilation spectroscopy

Electrical (and sometimes also mechanical) properties of a semiconductor are greatly influenced by various kinds of defects present in the material. Point defects such as vacancies and vacancy-impurity complexes are very common and can be introduced into the material both during growth and device fabrication. Positron annihilation spectroscopy is a useful tool for detecting and studying these types of defects. In this work, vacancy-type defects are studied and their properties examined in silicon, silicon-germanium and germanium. Vacancy-donor complexes are one of the fundamental defects behind the electrical deactivation of n-type dopants in Si and SiGe. In this work, the vacancy-donor pair also known as the E center is studied in phosphorus- and antimony-doped SiGe. The annealing of the V-P (vacancy-phosphorus) pair in strained SiGe with different Ge contents is investigated with the help of an Arrhenius-like kinetic model and dissociation energies around 1 eV are obtained for the defect. Accumulation of germanium atoms around the defect during annealing is observed. The evolution of the V-Sb pair during annealing is studied in Si0.8Ge0.2 with different dopant concentrations. Ge accumulation around the defects is observed also in these samples. Small Ge clusters are detected in the samples after the annealing. Deliberate introduction of defects by irradiation and ion implantation are common procedures for defect studies. In this work, proton-irradiated Ge is studied with an in situ annealing experiment. Two annealing stages are observed at 100 K and 200 K. The first stage is attributed to the Frenkel pair and the second to the monovacancy. Divacancy formation is observed following the second annealing stage, and these divacancies are found to be stable at room temperature. Helium co-implantation as a means to control boron diffusion profiles in B-implanted Si is studied. Nanovoids able to trap interstitials formed during B implantation are observed after the He implantation. Novel solar cell materials consisting of Si nanocrystals embedded within an SiO2 matrix are studied in this work. The nanocrystals are formed by annealing Si/SiO2 multilayers at 1100°C in N2. Interfaces between the nanocrystals and the SiO2 matrix are probed with positrons and photoluminescence. Nanocrystal formation is found to be optimized in samples with 2 nm thick Si layers