Thermal oxidation of Si (001) single crystal implanted with Ge ions

The thermal oxidation of Ge-implanted Si single crystals has been investigated for different Ge doses (3x1015 cm-2 and 3x1016 cm-2) and different oxidation processes (in wet ambient at 920 °C for 30, 60 and 120 minutes, or dry ambient at 1100 °C for 30 minutes). The oxide roughness, the oxidation rate, the Ge diffusion, precipitation and clustering, have been followed by several experimental techniques: atomic force microscopy, transmission electron microscopy, Rutherford backscattering spectrometry and X-ray absorption spectroscopy. We found that the surface roughness is related to the segregation of Ge at the oxide/substrate interface, occurring when the oxidation rate is faster than the Ge diffusion, in particular at the higher implanted dose (3x1016 cm-2) processed in wet ambient. In these conditions we also observed the oxidation rate enhancement with respect to pure Si and a strong indication that pure Ge clusters have been formed. When a critical Ge concentration at the interface is reached, the oxidation mechanisms changes and a lowering of the oxidation rate is found, along with an evident Ge diffusion into the substrate and a consequent reduction of the Ge fraction at the interface. Nevertheless, the oxide roughness still increases despite of the Ge concentration reduction, keeping memory of initial nucleation of precipitates.