Self-assembled Au nanoparticles in SiO₂ by ion implantation and wet oxidation

Implantation, annealing, and oxidation processes have been used to form Au nanoparticles with a narrow size and depth distribution in a SiO₂ layer. Different approaches have been attempted: in particular, the gettering of Au to fill preformed nanocavities (obtained by H-implantation and annealing) and thus overcome the broad particle size distribution that is normally associated with nanoparticles formed by implantation and annealing. The results suggest that nanocavities cannot be directly formed in SiO₂ by H-implantation and a subsequent annealing due partly to the high mobility of H atoms in SiO₂. However, cavities formed in Si are useful in obtaining a narrow size and depth distribution of Au precipitates: the Si substrate can then be oxidized to form Au nanoparticles in SiO₂. Sequential wet oxidations of Si samples containing Au nanoparticles have revealed several interesting phenomena, namely, segregation of Au particles at a growing oxide interface, Au-enhanced oxidation, dissolution and reprecipitation of Au precipitates during oxidation, and preferential wetting of Au on the oxide layer. In particular, the Au dissolution and reprecipitation processes are Si interstitial mediated. By completely oxidizing the top Si layer, an array of Au precipitates can be confined at a precise depth within a SiO₂ layer corresponding to the front interface of a buried oxide layer. The size distribution of the resulting Au precipitates in SiO₂ is smallest when Au is first gettered to cavities and vacancies are subsequently introduced into the Si layer prior to oxidation.