Defect Profiling of Oxygen-Related Defects Using a Slow Positron Beam

Variable-energy positron annihilation spectroscopy (PAS) is a relatively new technique for probing subsurface defects, and has provided novel insights into defects associated with silicon-based systems such as SiO2/Si, silicon nitrides, SIMOX and ion-irradiated Si. The technique entails measurement of Doppler broadening of the annihilation radiation from positrons implanted monoenergetically and subsequently thermalised in the sample, which is in turn dependent on the electronic environment. Positrons trapped by defects can thus be distinguished from those in undefected material. By controlling the incident positron energy, depth profiling of defects to several microns can be achieved. The technique is described, together with a study of defects induced by oxygen implantation for fluences of 5xl011, lxl013 and lxl014cm-2. The positrons are trapped at open-volume type defects (probably divacancies) in the as-implanted samples. Changes in defect structure are observed and the sensitivity of the technique to the chemical environment of the defects is demonstrated.