T. Clausen

Surfactant-mediated epitaxy of Ge on Si(111): Beyond the surface

For a characterization of interface and “bulk” properties of Ge films grown on Si(111) by Sb surfactant-mediated epitaxy, grazing incidence x-ray diffraction and transmission electron microscopy have been used. The interface roughness, defect structure, and strain state have been investigated in dependence of film thickness and growth temperature. For all growth parameters, atomically smooth interfaces are observed. For thin Ge layers, about 75% of the strain induced by the lattice mismatch is relaxed by misfit dislocations at the Ge∕Si interface. Only a slight increase of the degree of relaxation is found for thicker films. At growth temperatures below about 600°C, the formation of twins is observed, which can be avoided at higher temperatures.

Self-organized 2D nanopatterns after low-coverage Ga adsorption on Si (1 1 1)

The evolution of the Si(1 1 1) surface after submonolayer deposition of Ga has been observed in situ by low-energy electron microscopy and scanning tunnelling microscopy. A phase separation of Ga-terminated -R 30° reconstructed areas and bare Si(1 1 1)-7 × 7 regions leads to the formation of a two-dimensional nanopattern. The shape of this pattern can be controlled by the choice of the surface miscut direction, which is explained in terms of the anisotropy of the domain boundary line energy and a high kink-formation energy. A general scheme for the nanopattern formation, based on intrinsic properties of the Si(1 1 1) surface, is presented. Experiments performed with In instead of Ga support this scheme.

Correlated stacks of CdSe/ZnSSe quantum dots

The observation and quantitative investigation of the spatial correlation of CdSe quantum dots in stacked CdSe/ZnSSe quantum-dot layers is reported. Using grazing incidence x-ray small angle scattering, the influence of the ZnSSe spacer layer thickness ranging from 20 to 80 A, as well as the influence of the stacking number (3–10) has been analyzed. Satellite spots, indicative of quantum-dot ordering, have been observed for a spacer thickness of up to 45 A, and for a stacking number of at least 5. This finding can be explained by a self-organized ordering process driven by the lattice mismatch induced strain. A mean lateral quantum-dot distance ranging from 116 to 145 A, depending on the spacer thickness, has been found. In addition, an anisotropy of twofold symmetry has been observed, with the strongest correlation signal along 〈110〉.

Interfacial interactions at Au/Si3N4/Si(111) and Ni/Si3N4/Si(111) structures with ultrathin nitride films

Synchrotron photoemission spectromicroscopy has been used to study the interfacial interactions, metal diffusivity, and electronic barriers of Au and Ni contacts on ultrathin silicon nitride films. The Au/Si3N4 interface was found to be nonreactive, and only in the case of a very thin nitride film and elevated temperatures, Si can segregate from the Si(111) substrate and interact with Au. In the case of Ni/Si3N4/Si(111) structures, Ni diffusion and degradation of the Si3N4 lattice are evidenced even at room temperature and strongly enhanced at elevated temperatures, leading to formation of a Ni silicide interlayer.

Ultra-thin high-quality silicon nitride films on Si(111)

Ultra-thin silicon nitride films grown by exposure of Si(111) substrates to a flux of atomic nitrogen at temperatures between 700 °C and 1050 °C have been investigated by means of X-ray spectromicroscopy, atomic force microscopy, X-ray reflectivity, and X-ray photoemission spectroscopy. The films show a Si3N4 stoichiometry. For reactive nitride growth at temperatures below 800 °C, a smooth surface and interface morphology is found. Higher temperatures lead to the formation of rough films with holes and grooves of increasing size, approaching a lateral size of several hundred nanometers for growth temperatures above 900 °C. Nonetheless, X-ray spectromicroscopy shows that the bottom of the holes consists of Si3N4.

CoPt3 nanoparticles adsorbed on SiO2: a GISAXS and SEM study

Ultra-thin CoPt 3 nanoparticle films have been prepared on SiO 2 surfaces using a Langmuir-Blodgett (LB) deposition technique. The structural properties of the overlayers have been investigated by grazing-incidence small-angle x-ray scattering (GISAXS) and high-resolution scanning electron microscopy (SEM) for the first time. Self-assembly of the nanoparticles is found and with GISAXS an average particle-particle distance of (8.23 ± 0.06) nm is determined, in good agreement with the SEM results. A particle correlation length of (22.3 ± 1.2) nm was derived which is shown to be independent of the surface coverage. The latter quantity may be controlled by choice of a suitable retraction speed during the LB step.