Voids and nanocavities in silicon

In silicon, defects that are normally observed following ion implantation and annealing are interstitial based, that is they arise from the agglomeration of interstitials that are produced during ion irradiation. Vacancies that are produced in equal numbers to interstitials during irradiation only agglomerate into larger open-volume defects (almost exclusively voids) under special implantation and annealing conditions as a result of the instability of many vacancy-based defects. Hence, the observation of open-volume defects (voids and nanocavities) requires careful control of implantation and annealing conditions. Nevertheless, they have significant scientific and technological consequences and have been under active study recently. This chapter reviews open-volume defects, or nanocavities, in silicon beginning with the two main methods for producing them by ion bombardment: namely, by high-dose hydrogen or helium irradiation to first produce gas bubbles and then annealing to expel the gas and leave cavities, and during sufficiently high-dose irradiation under implantation conditions that do not amorphize the silicon to give rise to small vacancy clusters and voids at depths within the first half of the projected ion range. Such voids and nanocavities once produced have a number of interesting properties. They are very attractive trapping sites for a number of interstitial diffusers in silicon, particularly metal atoms and silicon interstitials themselves. Some intriguing nonequilibrium precipitation phenomena can be observed to occur at cavities and there are a number of ways in which cavities can be induced to shrink and disappear under subsequent irradiation and/or annealing. These aspects are especially reviewed. From the technological point of view, open-volume defects can be detrimental in terms of electronic or optoelectronic device performance but there are also beneficial applications such as the so-called "smart-cut" process, whereby a thin silicon layer can exfoliate from the host wafer under specific hydrogen implantation and annealing conditions, and also metal impurities can be removed from active device regions by strategically placing a band of voids to strongly trap them during thermal processing.