Optical absorption studies of ion implanted and amorphous silicon

A detailed study of the implantation induced damage in Si on sapphire, carried out by optical absorption measurements extending from energies above the band gap down to energies far into the sub gap region of Si, is presented. The changes induced in the optical band gap, band edge slopes and in the sub gap features of the spectra are carefully described. The various stages of formation and quenching of divacancies were monitored as a function of implantation conditions and annealing cycles through their 1.8 μm absorption band. It is shown that the divacancies strongly affect the population of band tail states and the annealing studies revealed that the progressive quenching of the divacancy band is followed by the appearance of another absorption band, characteristic of some intrinsic secondary defect, whose annealing behaviour is similar to the one observed for the 5-vacancies EPR spectrum. The study of the structural relaxation process in implanted a-Si gave indications that the process is indeed associated with annihilation of defects as well as average strain reduction in the material, in agreement with earlier indications.