Positron annihilation spectroscopy, experimental and theoretical aspects

In summary, PAS gives microscopic information about vacancy defects in semiconductors in the concentration range 10 15 -10 19 cm -3 . The positron lifetime is the fingerprint of the open volume associated with a defect, and it can be used to identify mono- and divacancies and larger vacancy clusters. Doppler broadening of the annihilation radiation, on the other hand, can be used to identify the nature of the atoms surrounding the vacancy. Consequently, vacancies on different sub lattices of a compound semiconductor can be distinguished, and impurities associated with the vacancies can be identified. The charge state of a vacancy defect can be determined by the temperature dependence of the positron -trapping coefficient, and positron localization into Rydberg states around negative centers yields information about ionic acceptors that have no open volume. Importantly, as shown in this chapter, the methods based on positron annihilation are not restricted by the nature or physical dimensions of the semiconductor. Defects can be studied in narrow- and wide-bandgap semiconductor materials in samples of any conductivity. Bulk crystals as well as thin films can be subjected to the experiments and defects identified.