Ex situ observed growth stages of LPE-SiGe/Si͑001͒ Stranski-Krastanow islands with a germanium content of 10% give clear evidence of a rapid shape transition at one third of the final island height. The island shape changes from a lenslike type without a top facet to truncated pyramids with ͕111͖ side facets and an ͑001͒ top facet. High-resolution x-ray diffraction has been applied to islands with higher germanium content of about 30%. Experimental results are compared with respective kinematical scattering simulations based on finite element calculations for the strain field. From these simulations the three-dimensional germanium composition profile inside the islands can be extracted and it substantiates a similar growth scenario with a distinct shape transition at one third of the final island height also for this germanium concentration range. We attribute the observed finite island size to a distinct nucleation problem at the island bottom caused by exceptional high strain energy around the island corners in combination with a strain driven wetting layer depression.
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.
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.
To realize the ITS-90 according to its definition, among others, the melting and freezing temperatures of ideally pure metals are needed. Therefore, many national metrology institutes (NMIs) utilize a group of cells instead of one single cell as the national reference for each temperature. With direct fixed-point cell comparisons on a regular basis, it is feasible to account for the small differences between the individual fixed-point temperatures and to detect possible temperature drifts of the cells. At PTB (the German NMI), in recent years, these groups of national standard cells and the so-called transfer cells for calibrations have been complemented by newly developed slim fixed points. These cells typically contain 75 % to 80 % less fixed-point material compared with standard cells. Slim cells are used for homogeneity investigations of large batches of fixed-point material, for doping experiments to determine the influence of very small amounts of impurities on the fixed-point temperature with very small uncertainties, and for the investigation of contamination or purification effects after the manufacture of a fixed-point cell. These investigations have shown that the main limitation of slim cells is the quality of the phase boundary. The small dimensions of the cell do not allow the formation of a closed phase boundary (or even two of them). However, this can be compensated using a quasi-adiabatic realization procedure, and in this way, uncertainties comparable to those of standard fixed-point cells can be achieved. In this article, the design of the cells as well as typical measurement results and uncertainties for the direct comparison of fixed-point cells of both types, the standard size and slim design, are presented.
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