Photonic effects during low-temperature ultraviolet-assisted oxidation of SiGe

The behavior of Ge atoms during dry oxidation of Si_0.8Ge_0.2 films at 300°C under 10 mbar of oxygen induced by vacuum-ultraviolet (VUV) illumination from an array of Xe₂* excimer lamps (λ = 172 nm) has been studied. During VUV oxidation, samples are exposed to both a high concentration of ozone and atomic oxygen and a large flux of energetic photons. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy investigations showed that the layers grown for shorter periods of time contain mostly SiO₂ with a few percent GeO₂. Most of the Ge atoms, initially present uniformly within the SiGe layer, were segregated and accumulated at the interface between the grown oxide and remaining SiGe. Angle-resolved XPS showed that the amount of GeO₂ within the grown oxide layer decreased for longer irradiation times and was located adjacent to the SiGe layer. When the grown SiO₂ layer reached a thickness around ∼70 Å and the amount of Ge that had accumulated in the segregated layer more than doubled, a sharp increase in the Ge oxidation rate was observed. Continuing the oxidation for longer irradiation times resulted in the formation of a mixed oxide layer. The Ge segregation was not previously observed during other low-temperature oxidation treatments, including ozone-assisted oxidation, which provides the same oxidation species as VUV-assisted oxidation and similar growth rates. It is, therefore, concluded that a VUV photon-irradiation enhancement effect on Si and Ge interdiffusion has been introduced, possibly involving either Si-Si or Si-Ge bond softening or even breaking.