Positron annihilation studies on vacancy defects in group IV semiconductors

Electrical properties of semiconductor materials are greatly influenced by point defects such as vacancies and interstitials. These defects are common and form during the growth and processing of the material. Positron annihilation spectroscopy is a method suitable for detecting and studying vacancy-type lattice defects. In this work, the formation, properties, and annealing of vacancy defects is studied in silicon, silicon-germanium, and germanium. Defects consisting of a vacancy and one or several donor atoms are one of the most common defects causing electron trapping and deactivation of ntype doping in silicon and silicon-germanium. In this work, the studies in silicon-germanium show that several germanium atoms accumulate around the vacancy-phosphorus (V -P) pair during the annealing of the samples. The increased Ge-decoration pulls the energy level (-/-) down into the band-gap and makes the V -P pair decorated by several Ge atoms an especially effective trap for conduction electrons. The positron trapping in a vacancy surrounded by three As atoms (V -As3) is studied in highly As-doped Si. The positron detrapping from the V -As3 defect at high temperatures is observed and a binding energy of 0.27(3) eV of a positron to the V -As3 complex is determined. Defects can also be introduced deliberately by neutron-irradiation and ionimplantation. These techniques offer possibilities for studying the generation and annealing of vacancy defects. In this work, neutron-irradiated germanium is studied. Irradiation induced divacancy defects that are stable at room temperature are observed. A negative charge state of a divacancy is found to stabilize the defect even at 400◦C. The divacancy is shown to form bigger clusters before the final recovery at 500◦C. Finally, B-doping related problems are studied. The results show that He-implantation produces nanovoids that trap interstitials formed during the B-implantation, reducing the implantation related damage. The positron studies on the excimer laser annealed Si support theoretical calculations, which suggest vacancy formation at the maximum melt depth.