Properties and microstructure of ion-implanted silicon

Abstract

The implantation of helium or deuterium ions into single-crystal silicon to high dose levels products amorphous silicon and nanocavities. Raman spectroscopy and transmission electron microscopy (TEM) are used to study the microstructure of the implanted silicon and the modifications to the microstructure induced by heat treatment or by a combination of sputtering gold onto the silicon and subsequent heat treatment. Raman spectroscopy is used to study the relative proportions of crystalline to amorphous silicon as a function of depth, for implantation energies from 20 to 160keV and implantation temperatures in the range 20-300°C. This is done by monitoring the phonon peaks from crystalline and amorphous silicon, following progressive removal of the sample surface by argon ion-beam milling. For room temperature implantations, fully amorphous Si is formed for implantation energies of 45keV and above, with the amorphous layer moving from the surface region to depth as the energy increases. The ratio of amorphous to crystalline Si decreases with increasing implantation temperature. The crystal peak in the Raman spectrum is broad near the surface and becomes narrower with increasing depth. The broadening is attributed to stress in the implanted layer. For implantation with deuterium, additional broadening is attributable to Si-D bonds. Helium is insoluble in silicon and precipitates out to form cavities. For implantation temperatures up to 300°C, the cavities in the implanted layer are at low concentration. Subsequent thermal annealing at 650-850°C in vacuum causes the cavities to grow and their visibility under TEM to increase. These cavities are at high concentration and may be faceted. The diffusion of gold to cavities in as-implanted specimens is also investigated. A gold layer is sputtered into the front surface of the silicon and samples annealed for different temperatures and times. Rutherford backscattering spectrometry and TEM studies show that gold is gettered by the cavities. Bubble coalescence and cavity faceting are enhanced by the presence of the gold.