Elemental redistribution and Ge loss during ion-beam synthesis of Ge nanocrystals inSiO2films

Abstract

The elemental redistribution and Ge loss in low-energy ${\mathrm{Ge}}^{+}$ implanted ${\mathrm{SiO}}_{2}$ films during wet-chemical cleaning and annealing procedures are investigated. Two effects of major importance for Ge nanocrystal formation have been found. Moisture components (${\mathrm{H}}_{2}\mathrm{O}$ vapor, ${\mathrm{H}}^{+}$, ${\mathrm{OH}}^{\ensuremath{-}}$) penetrate into the damaged oxide during storage, wet chemical cleaning, or annealing procedures and lead to a hydrogen and oxygen enrichment in the near-surface oxide. Furthermore, atomic collisions during Ge implantation result in an oxygen excess (with respect to ${\mathrm{SiO}}_{2}$ stoichiometry) underneath the Ge profile. The local net ratio of Ge and excess oxygen determines, whether the implanted Ge is incorporated into the ${\mathrm{SiO}}_{2}$ network as spatially fixed ${\mathrm{GeO}}_{2}$, oxidizes to mobile $\mathrm{GeO}$, or remains as elemental Ge forming nanocrystals. Apart from very shallow profiles, where a drastic Ge loss is observed simply by cleaning in chemical solutions containing ${\mathrm{H}}_{2}{\mathrm{O}}_{2}$, the main Ge loss occurs during annealing. The highly mobile $\mathrm{GeO}$ is identified to be responsible for both, Ge redistribution or even loss, if diffusing $\mathrm{GeO}$ meets the ${\mathrm{SiO}}_{2}$ surface and emanates into the annealing ambient. Annealing in $\mathrm{Ar}∕{\mathrm{H}}_{2}$ mixtures at $\ensuremath{\le}900\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ reduces the Ge loss due to the reduction of Ge oxides. The enhanced Ge mobility (as $\mathrm{GeO}$) is described as an oxygen vacancy assisted mechanism which also explains the influence of the $\mathrm{Si}∕{\mathrm{SiO}}_{2}$ interface on the Ge diffusivity. Finally, the consequences of Ge redistribution and loss for Ge nanocrystal memory device fabrication are discussed.