We have studied the strain state, film and surface morphology of SiGe virtual substrates grown by reduced pressure chemical vapour deposition (RP-CVD). The macroscopic strain relaxation and the Ge composition of those virtual substrates have been estimated in high resolution x-ray diffraction, using Omega-2Theta scans around the (004) and (224) orders. Typically, linearly graded Si 0.7 Ge 0.3 virtual substrates 5 µm thick are 96% relaxed. From transmission electron microscopy, we confirm that the misfit dislocations generated to relax the lattice mismatch between Si and SiGe are mostly confined inside the graded layer. The threading dislocations density obtained for Ge concentrations of 20% and 27% is indeed typically of the order of (7.5 ± 2.5) ×10 5 cm −2 . The surface roughness of the relaxed SiGe virtual substrates increases significantly as the Ge concentration approaches 30%. We find for the technologically important Ge concentration of 30% a surface root mean square roughness of 5 nm, with an undulation wavelength for the cross-hatch of the order of 1 µm. We have also studied the electronic quality of our RP-CVD grown SiGe virtual substrates. We have grown a MODFET-like heterostructure for this purpose, with a buried tensile-strained Si channel 8 nm thick embedded inside SiGe 31%. We have obtained a well-behaved two-dimensional electron gas in the Si channel, with electron sheet densities and mobilities at 1.45 K of 5.7 × 10 11 cm −2 and 180 000 cm 2 V −1 s −1 , respectively.

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We have measured the resistivity of a dilute two-dimensional electron gas near the (111) silicon surface as a function of a temperature. Since the valley degeneracy in such structures g v is 6, the dimensionless radius r s approaches 50 at electron densities significantly larger than in previously studied (100)Si or p-AlGaAs/GaAs systems. We have observed a nonmonotonical behavior of (T), the resistivity slowly decreasing with the temperature decreasing for temperatures above TϷ1 K and increasing at lower temperatures for electron densities corresponding to ϳh/e 2 , when the metal-insulator transition is expected. Such nonmonotonic behavior can be tentatively described by corrections to the conductivity due to electron-electron interaction with negative Fermi-liquid constant F 0 ϷϪ0.25.