Observation of vacancy defects at silicon grain boundaries formed via suppressed solid phase epitaxy

Polycrystalline silicon has been formed via the complete amorphization and subsequent thermal re-crystallization of the overlayer of silicon-on-insulator substrates. The formation of the polycrystalline phase was confirmed via cross-sectional transmission electron microscopy, with the samples also being characterized by variable energy positron annihilation spectroscopy (VEPAS). By comparing the response of the VEPAS spectra for samples in which polycrystal formation was confirmed with samples in which the silicon overlayer was regrown as a single crystal, it was possible to observe the existence of vacancy-type defects at the film grain boundaries. For a sample with an overlayer of 1500 nm thickness a saturation of the measured Doppler-broadened S-parameter of 1.025 was estimated for the polycrystalline sample. This leads us to conclude that the majority of the vacancy defects at the grain boundaries are close in structure to that of the silicon monovacancy.