In a next future, optical interconnects will offer a solution to the limitation by metallic interconnects of working frequency of microprocessors. Micro-waveguides processed on SOI with integrated Ge photodetectors appear as an attractive option. However, the growth of a 0.4μm thick fully relaxed Ge film on Si with a low density of threading dislocations remains a challenge. The usually observed Stranski-Krastanov transition can be avoided by using a very low temperature deposition [i] which minimizes the Ge diffusion and favors the formation of vacancies. In situ RHEED was used to investigate the relaxation kinetics during the low temperature deposition. The Ge has been grown on Si at 330C by UHV-CVD at a total pressure of 1.8 10-3 Torr of hydrogen-diluted germane.
During the first 15 min. the surface was found to turn from flat to wavy, the RHEED pattern showing modulated streaks and weak 1/2 lines. For further deposition times, the surface became flat. After deposition of 45 nm of Ge (1 hour growth time), the surface exhibited a perfect 2x1 reconstruction. The variation of lattice parameter was measured as the growth proceeded and was found to increase by 2.5% after 1 hour, what is significant of a partial relaxation. In order to confirm the RHEED findings, the film roughness was measured by AFM for different deposition times. From transmission electron microscopy measurements, misfit dislocations were found to be mostly located at the interface, and emerging extended defects were identified as stacking faults. From that, it appears that the low deposition temperature process is well suited for the deposition of a thin relaxed Ge film.
[i] L.Colace et al, J.Vac.Sci.Technol.B. 17, 465 (1999).
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