dc.description.abstract | Strain-driven self-assembly of three-dimensional Ge or SiGe islands on Si substrates is an active topic of both fundamental and practical interests. The Ge/Si (100) material system has been regarded as prototypical for investigating the basic phenomena, for example, the formation of ‘‘self-assembled quantum dots’’. Technologically, Ge/Si (100) is of interest for possible applications in electronic, thermoelectric and optoelectronic devices.
In the first part of this thesis, the annealing effects on the composition distribution of Ge islands on Si (001) were investigated by atomic force microscopy combined with selective wet chemical etching. Experimental results demonstrate that there is a strong composition redistribution occurring during island growth and postgrowth annealing. We observe that, once Ge superdomes appear, the asymmetric composition profile of the Ge domes transforms into an almost symmetric structure. Moreover, the Ge superdomes exhibit a double-ring composition profile of Si after long-time annealing. These phenomena could be explained within a simple thermodynamic model that involves only surface diffusion process.
After the discussion on the growth of SiGe QDs and the annealing effect, the composite Ge/Si/Ge islands (CQDs) have also been investigated. After upper Ge-rich parts were removed by selective etching, the etched Ge/Si/Ge composite islands exhibit pyramid structures, which differed from the ring-like isocompositional profiles observed in conventional Ge islands. The shape and size distribution of Ge/Si/Ge composite islands can be further controlled by tuning the island parameters. Experimental results also demonstrate that the inserted Si layer less than 2 nm is disabled from blocking the Ge atoms out-diffusion from sub-dots.
In the third part of the thesis, we present an effective approach to grow high-quality thin film of composite quantum dots (CQDs) as a building block for thermoelectric materials, in which 3 times the usual Ge deposition can be incorporated within a 3-fold CQD. Selective chemical etching experiments reveal that a thin Si inserted layer in the CQDs modifies the growth mechanism through surface-mediated diffusion and SiGe alloying. Such thin-film-like CQD materials are demonstrated to exhibit reduced thermal conductivity κ with respect to the conventional QDs, perhaps as a consequence of enhanced diffusive phonon scattering from the high Si/Ge interface density and enhanced local alloying effect.
Finally, the growth of self-assembled Ge(Si) islands on a strained Si1-xGex layer is studied. The size and the surface density of islands are found to increase in the strain SiGe layer. The increased surface density and uniformity are related to augmentation of the surface roughness after deposition of the SiGe layer. The increased island size is attributed to enhanced Si intermixing and to a wetting layer thickness reduction. The influence of Si cap on SiGe strain layer is also discussed. In order to control the size and uniformity of Ge QDs, the Si spacer can tune the strain energy from SiGe layer. These results indicate that Ge QDs on SiGe strain layer would be potentially useful as an optoelectronic and thermoelectric material. | en_US |