SiGe-on-insulator fabricated via germanium condensation following high-fluence Ge+ ion implantation

摘要

Germanium condensation is demonstrated using a two-step wet oxidation of germanium implanted Silicon-On-Insulator (SOI). Samples of 220nm thick SOI are implanted with a nominal fluence of 5x1016cm-2 Ge+ at an energy of 33keV. Primary post-implantation wet oxidation is performed initially at 870°C for 70 minutes, with the aim of capping the sample without causing significant dose loss via Ge evaporation through the sample surface. This is followed by a secondary higher temperature wet oxidation at either 900°C, 1000°C or 1080°C. The germanium retained dose and concentration profile, and the oxide thickness is examined after primary oxidation, and various secondary oxidation times, using Rutherford backscattering analysis. A mixed SiGe oxide is observed to form during the primary oxidation followed by a pure silicon oxide after higher temperature secondary oxidation. The peak germanium concentration, which varies with secondary oxidation condition, is found to range from 43 at- % to 95 at- %, while the FWHM of the Ge profile varies from 13 to 5nm, respectively. It is also observed that both the diffusion of germanium and the rate of oxidation are enhanced at 870°C and 900°C compared to equilibrium expectations. Transmission electron microscopy of a representative sample with secondary oxidation at 1080oC for 20 minutes shows that the SiGe layer is crystalline in nature and seeded from the underlying silicon. Raman spectroscopy is used to determine residual strain in the SiGe region following secondary oxidation. The strain is compressive in nature and increases with Ge concentration to a maximum of approximately 1% in the samples probed. In order to elucidate the physical mechanisms, which govern the implantation-condensation process, we fit the experimental profiles of the samples with a model that uses a modified segregation boundary condition; a modified linear rate constant for the oxidation; and an enhanced diffusion coefficient of germanium where the enhancement is inversely proportional to the temperature and decays with increasing time. Comparison of the modelled and experimental results shows reasonable agreement and allows conclusions to be made regarding the dominant physical mechanisms, despite the semi-empirical nature of the model used.