Identification of open-volume defects in disordered and amorphized Si: A depth-resolved positron annihilation study

Depth-resolved positron beam studies have been carried out on Ar-irradiated Si using Doppler S parameter and lifetime measurements. Si samples have been irradiated with 140-keV Ar ions to a dose of $2\times {10}^{13}$ and $5\times {10}^{16}\hspace{0.5em}\mathrm{Ar}/{\mathrm{cm}}^2,$ respectively, so as to produce disordered and amorphous states in near-surface regions. The observed features of the defect sensitive line shape S parameter indicate the presence of small vacancylike defects in the disordered sample and higher-order vacancy clusters in an amorphous sample. Pulsed positron beam lifetime results indicate that the disordered Si sample exhibits lifetime distribution ascribable to mostly divacancies. In the case of an amorphous sample, the lifetime distribution is broad with larger lifetime values indicating the presence of a distribution of large vacancy clusters or nanovoids. By using theoretical lifetime values for Si reported in the literature, an empirical fit to the lifetime variation as a function of vacancy cluster size is obtained. By comparing the experimental lifetime distribution with this data, the vacancy cluster size distribution in disordered and amorphous Si is deduced. In disordered Si, divacancies are found to be the dominant defects species followed by small concentration of ${\mathrm{V}}_3.$ In amorphous Si, nanovoids in the size range of four to seven vacancy clusters are present with ${\mathrm{V}}_5$ and ${\mathrm{V}}_6$ clusters being the dominant defect species. The implication of these results is discussed in light of recent computer-simulation studies.