We report on the fabrication of modulation-doped compressively strained Ge quantum wells by low-energy plasma enhanced chemical vapor deposition. A virtual substrate consisting of a thick linearly graded SiGe buffer layer and a cap layer of constant composition is first grown at a high rate (>5 nm/s). The active layer stack, grown at a reduced rate, contains strain compensating cladding layers with modulation doping above the channel. Mobilities of up to 3000 cm2/V s and 87 000 cm2/V s have been achieved at room temperature and liquid He temperature, respectively.

A mechanism of strain relief of H+ ion implanted and annealed pseudomorphic Si1−xGex/Si(100) heterostructures grown by molecular beam epitaxy is proposed and analyzed. Complete strain relaxation was obtained at temperatures as low as 800 °C and the samples appeared free of threading dislocations within the SiGe layer to the limit of transmission electron microscopy analysis. In our model, H filled nanocracks are assumed to generate dislocation loops, which glide to the interface where they form strain relieving misfit segments. On the basis of this assumption, the conditions for efficient strain relaxation are discussed.

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The influence of He implantation and annealing on the relaxation of Si 0.7 Ge 0.3 layers on Si ͑100͒ substrates is investigated. Proper choice of the implantation energy results in a narrow defect band Ϸ100 nm underneath the substrate/epilayer interface. During annealing at 700-1000°C, He-filled bubbles are created, which act as sources for misfit dislocations. Efficient annihilation of the threading dislocations is theoretically predicted, if a certain He bubble density with respect to the buffer layer thickness is maintained. The variation of the implantation dose and the annealing conditions changes density and size of spherical He bubbles, resulting in characteristic differences of the dislocation structure. Si 1Ϫx Ge x layers with Ge fractions up to 30 at. % relax the initial strain by 70% at an implantation dose of 2ϫ10 16 cm Ϫ2 and an annealing temperature as low as 850°C. Simultaneously, a low threading dislocation density of 10 7 cm Ϫ2 is achieved. The strain relaxation mechanism in the presence of He filled bubbles is discussed.

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