Christian Lang

Dependence of Surface Facet Period on the Diameter of Nanowires

Axial heterostructured silicon nanowires with varying n- and p-doping were synthesized using a vapor-liquid-solid approach. The nanowire sidewalls exhibit periodic nanofaceting in the silicon deposited directly on the sidewalls when diborane dopant gas is introduced during growth. For such nanofaceting, a model predicting the distance between facets (the facet period) is developed. For a nanowire structure, an extra energy cost term arising from the formation of apexes between facets is considered, and the facet size is predicted to decrease as the wire diameter increases. It is found that the model fits the experimental data well, and the fitted parameters in the model lie within the ranges of their expected values.

Nonuniform alloying in Ge(Si)/Si(001) quantum dots

The composition profile of pyramid shaped Ge(Si)/Si(001) quantum dots has been modeled using a combination of atomistic total energy calculations and a Metropolis Monte Carlo process. The analysis of the non-uniform composition profile has revealed the important, separate roles of the strain energy, the surface energy, and the mixing energy as driving forces of the alloying and segregation process. The surface energy was found to drive a redistribution of Ge into the surface layer of the quantum dot, which was followed by two Si-rich layers. In the vertical direction Si is found to redistribute to the bottom resulting in a Ge-rich apex of the quantum dot. This result is compared to a phenomenological description of the composition profile by Tersoff [Phys. Rev. Lett. 81, 3183 (1998)]. The possibility of a misinterpretation of experimental measurements of the composition profile as a result of the Ge-rich surface layer is discussed.

Imaging and analysis of axial heterostructured silicon nanowires

Silicon nanowires with axially varying n- and p-doping were grown by the vapor-liquid-solid approach using gold as the catalyst. The microstructures of the axial heterostructured silicon nanowires were studied using electron microscopy techniques. The nanowire sidewalls exhibit periodic nanofaceting, which is dopant-dependent. The nanofaceting occurs during the enhanced sidewall growth that arises when the diborane dopant gas is introduced. It is also found that the silicon nanowire can crystallize in a hexagonal phase, which is thermodynamically unstable in bulk silicon at ambient and may be due to the large surface stress present during the nucleation of SiNWs.

Modelling Ge/Si quantum dots using finite element analysis and atomistic simulation

Finite Element Analysis (FEA) and atomistic simulations are used to model Ge/Si quantum dots. The three dimensional non-uniform composition profile in Ge(Si)/Si(001) quantum dots is calculated using atomistic modelling. The results are compared to experimental data from the literature. FEA is used to model the contact angle dependence of the strain energy of the QD. An equation is fitted to the modelled dataset describing the strain energy of a uniformly alloyed, pyramid shaped Ge/Si quantum dot as a function of the contact angle.

The interdependency of morphology, strain and composition in buried GeSi/Si(001) quantum dots

The deposition of Ge onto Si leads to the epitaxial growth of islands with varying morphologies. The size, shape and composition of the islands are all interdependent and change continuously during growth and overgrowth of the islands. A model of the morphological changes is presented that explains various shape changes observed during overgrowth by taking into account a dynamic equilibrium between the islands, the substrate and the wetting layers.

Building blocks of amorphous Ge{sub 2}Sb{sub 2}Te{sub 5}

The structural changes during the phase change between the amorphous and crystalline phase in Ge{sub 2}Sb{sub 2}Te{sub 5} have been the subject of intense study due to the importance of Ge{sub 2}Sb{sub 2}Te{sub 5} in phase change memory devices. In our study, the energetics of the transition between the crystalline and the amorphous phase of Ge{sub 2}Sb{sub 2}Te{sub 5} is explored using density functional theory (DFT) calculations, electron diffraction, and reverse Monte Carlo model refinement. No energy barrier was found between the crystalline and the previously suggested amorphous structure of Ge{sub 2}Sb{sub 2}Te{sub 5}. Further DFT calculations have led to a different building block of the amorphous structure, which is shown to agree with the experimental reduced density function determined from electron diffraction experiments and previously reported extended x-ray absorption fine structure measurements.