Elemental redistribution and Ge loss during ion-beam synthesis of Ge nanocrystals in SiO₂ films

The elemental redistribution and Ge loss in low-energy Ge⁺ implanted SiO₂ films during wet-chemical cleaning and annealing procedures are investigated. Two effects of major importance for Ge nanocrystal formation have been found. Moisture components (H₂O vapor, H⁺, OH^-) 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 SiO₂ stoichiometry) underneath the Ge profile. The local net ratio of Ge and excess oxygen determines, whether the implanted Ge is incorporated into the SiO₂ network as spatially fixed GeO₂, oxidizes to mobile 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 H₂O₂, the main Ge loss occurs during annealing. The highly mobile GeO is identified to be responsible for both, Ge redistribution or even loss, if diffusing GeO meets the SiO₂ surface and emanates into the annealing ambient. Annealing in Ar/H₂ mixtures at < 900°C reduces the Ge loss due to the reduction of Ge oxides. The enhanced Ge mobility (as GeO) is described as an oxygen vacancy assisted mechanism which also explains the influence of the Si/SiO₂ interface on the Ge diffusivity. Finally, the consequences of Ge redistribution and loss for Ge nanocrystal memory device fabrication are discussed.