A pulsed slow-positron beam, enabling positron lifetime studies in thin semiconductor layers and near-surface regions, is presented. The positron beam is modulated with a radio-frequency beam bunching system into pulses below 200 ps full-width at half-maximum at a repetition rate of 33 MHz. A Na-22 isotope source and W moderator are used as the source for slow positrons. The beam energy is variable within the range of 2-25 keV, corresponding to typical implantation depths in the range 0.01-3 µm. The lifetime is measured digitally with a BaF2 scintillation detector coupled to a high-speed digitiser. A unique feature is that the specimen is maintained at earth ground to facilitate sample manipulation, i.e., temperature control, biasing, etc., while the source and pulsing system are floated at high voltage. Also in this work, vacancy defects in molecular beam epitaxy (MBE) -grown indium nitride (InN) are studied with positron annihilation techniques. In addition to effects of growth conditions, those of particle irradiation and thermal annealing are investigated. In In-polar InN grown by plasma-assisted MBE, stoichiometric conditions during growth are concluded to have little effect on indium-vacancy formation. On the other hand, buffer layer optimisation is observed to lead to a lower indium-vacancy concentration, supporting the view that structural factors dictate In-vacancy formation rather than growth thermodynamics and kinetics. In He-irradiated InN films, subsequent rapid thermal annealing is observed to have notable effects on the irradiation-induced defects. The results of slow-positron Doppler-broadening and lifetime experiments indicate that in the heat-treated films, the irradiation-induced In vacancies are restructured near the film-substrate interface, where the crystal quality is poorer due to the lattice mismatch. In the rest of the InN layer, the indium vacancies and negative-ion defects produced in the irradiation are partially removed in the annealing. The observations are possibly connected to the improved electron mobility in the annealed material.
展开▼
