T. Boeck

Equilibrium shape of SiGe Stranski-Krastanow islands on silicon grown by liquid phase epitaxy

SiGe Stranski–Krastanow islands coherently grown on Si(001) substrates by liquid phase epitaxy are typically made of truncated pyramids with {111} side facets, whereas the persistent presence of an (001) top facet indicates an energetical disadvantage of complete pyramids compared to truncated ones. We attribute this to a surface minimization process during the island evolution under the assumption of isotropically distributed surface energies and stable island facets. For the presence of {111} side facets we have theoretically derived a final geometrical aspect ratio of island base versus island height of 1.96, which is in excellent agreement with the experimentally derived averaged value of 2.08±0.10 within a concentration window between 9% and 30% germanium.

Coherent diffraction tomography of nanoislands from grazing-incidence small-angle x-ray scattering

The combination of grazing-incidence small-angle x-ray scattering with tomographic and phase retrieval methods is presented for the reconstruction of the three-dimensional (3D) electron density of nanometer sized objects. The measured 3D intensity distribution in reciprocal space is used for the phase retrieval and reconstruction of the shape and electron density of epitaxial SiGe islands with the shape of truncated pyramids with a 200 nm square base. A spatial resolution below 20 nm demonstrated in this work cannot be achieved by traditional tomographic methods.

Regularly arranged indium islands on glass/molybdenum substrates upon femtosecond laser and physical vapor deposition processing

A bottom-up approach is presented for the production of arrays of indium islands on a molybdenum layer on glass, which can serve as micro-sized precursors for indium compounds such as copper-indium-gallium-diselenide used in photovoltaics. Femtosecond laser ablation of glass and a subsequent deposition of a molybdenum film or direct laser processing of the molybdenum film both allow the preferential nucleation and growth of indium islands at the predefined locations in a following indium-based physical vapor deposition (PVD) process. A proper choice of laser and deposition parameters ensures the controlled growth of indium islands exclusively at the laser ablated spots. Based on a statistical analysis, these results are compared to the non-structured molybdenum surface, leading to randomly grown indium islands after PVD.

Investigation of the chemical composition profile of SiGe∕Si(001) islands by analytical transmission electron microscopy

The authors have determined the composition profile within individual Si1−xGex nanoscale islands on Si(001). Samples have been grown by means of liquid phase epitaxy in the Stranski-Krastanov mode. By applying electron energy loss spectroscopy, the intensities of Si K and Ge L edges have been measured to determine the relative atomic concentration of germanium. The quantification of the composition suggests a profile comprising of two regions with different linear concentration gradients.

Ge in-plane nanowires grown by MBE: influence of surface treatment

We present a detailed study of morphological phenomena during molecular beam epitaxy (MBE) of Ge nanowires on Ge substrates by means of the vapor–liquid–solid mechanism. Different wet chemical surface passivation methods were tested for their effect on Ge nanowire growth. Clean, smooth and well-passivated surfaces enable the preferential formation of in-plane nanowires, instead of conventional out-of-plane nanowires. Depending on the type of passivation, different growth directions of the self-aligned in-plane wires were observed: exclusively -grown wires on substrates with a very stable passivation layer, both and growth on substrates with a less stable passivation. The morphology of the wires was studied by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). For the -grown in-plane wires, nanofaceting of the top and side walls was observed. Based on the analysis, a coherent hypothesis is formulated to explain the experimental findings.

Germanium nanowire growth controlled by surface diffusion effects

Germanium nanowires (NWs) were grown onto Ge(111) substrates by the vapor-liquid-solid process using gold droplets. The growth was carried out in a molecular beam epitaxy chamber at substrate temperatures between 370 °C and 510 °C. The resulting nanowire growth rate turns out to be highly dependent on the substrate temperature exhibiting the maximum at T = 430 °C. The temperature dependence of growth rate can be attributed to surface diffusion both along the substrate and nanowire sidewalls. Analyzing the diffusive material transport yields a diffusion length of 126 nm at a substrate temperature of 430 °C.

Continuous polycrystalline silicon layers on glass grown from tin solutions

Polycrystalline silicon on glass for photovoltaic applications is grown at low temperatures in a two-step process. In the first step, nanocrystalline Si (nc-Si) films are formed by direct deposition on heated glass substrates at about 400 °C. In the second step, these seed layers serve as templates for the growth of crystalline silicon (c-Si) at temperatures around 600 °C by steady-state solution growth, a process that exhibits significant similarities with float glass production. Using Sn solutions, continuous microcrystalline Si layers with a thickness of about 10 μm are grown. Structural characterization by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) give insights into the growth process of both the seed layer and the thick microcrystalline layer. Chemical characterization by secondary ion mass spectrometry (SIMS) shows low impurity levels, which makes the material interesting for the use in thin-film silicon solar cells.

Size, shape, and ordering of SiGe/Si(001) islands grown by means of liquid phase epitaxy under far-nonequilibrium growth conditions

Applying scanning electron microscopy, we have studied the evolution of shape and lateral positional correlation of Si1−xGex/Si(001) Stranski–Krastanov islands grown by means of liquid phase epitaxy (LPE). However, in contrast to conventional near-equilibrium LPE, a distinctly higher cooling rate of 10 K/min ensures extremly nonequilibrium growth conditions. The facet inclination of subsequent island stages decreases from nearly vertical sidewalls toward {111}- and {101}-type facets. Energy dispersive x-ray microanalysis yields a size-independent germanium content of 8.9% within islands between 760 and 1700 nm base width which is—by more than a factor of 2—smaller than islands of the same concentration grown in a near-equilibrium LPE process. Square-like formations of subsequently smaller islands around a large central island indicate only next to island interactions during the lateral self-assembling.

Dedicated fabrication of silicon-based ensembles of dot molecules with a specific and unique number of dots

We have performed a two-step liquid phase epitaxy yielding ensembles of SiGe∕Si(001) dot molecules with a specific and unique number of dots. An undersaturation of the initial bismuth solution causes strain-induced pits in the epitaxial Si0.985Ge0.015 layer which are effectively preserved during subsequent Si0.68Ge0.32 dot growth at considerably lower temperatures. Since the latter process happens extremely close to thermodynamic equilibrium, we are able to interrupt it after the formation of ensembles of dimers, trimers or quadruplets, respectively. The crosslike ensemble symmetry is discussed in terms of strain energy distribution as revealed by finite element calculations.

Unidirectional self-assembling of SiGe Stranski-Krastanow islands on Si(113)

We report on a dedicated two-step liquid phase epitaxy (LPE) experiment yielding ensembles of SiGe∕Si(113) nanoscale islands with unidirectional ordering along the [1¯10] direction. Initial homoepitaxy of silicon on a polished Si(113) wafer from an indium solution at 930°C results in a highly regular pattern consisting of (111) and (116) facets. For the subsequent heteroepitaxy with SiGe the solvent has been exchanged for bismuth since it enables an LPE process at considerably lower temperatures around 590°C, and thus preserves the initial template which strictly enforces the linear self-assembling of SiGe islands. However, with respect to the initial grooves the established island–island correlation length perpendicular to them has been increased by a factor of about 1.6 indicating that not every groove acts as a nucleation place. This can be explained by finite element calculations on the three-dimensional deformation field which proves energetically favorable nucleation sites in the island vicinity wit...