The analysis of Raman spectra obtained for different germanium nanostructures grown on silicon substrates is presented. Comparison of these spectra with a Raman spectrum of a silicon wafer reveals a one-to-one correspondence of features located around 229, 300, and 435 cm−1. It is argued that the peaks observed at these frequencies and often ascribed to Ge nanostructures are, in fact, coming from the Si substrate. The erroneous ascription of the peaks makes the corresponding conclusions incorrect.
The authors demonstrate the epitaxy of Ge–Sb–Te alloys close to the Ge2Sb2Te5 composition on GaSb(001). Using molecular beam epitaxy with elemental sources, amorphous films are obtained at growth temperatures below 120 °C and films with a cubic structure and a predominant cube-on-cube epitaxial relationship above 180 °C. Using a high-power pulsed laser, the epitaxial films are switched between the crystalline and the amorphous phases. Streaks in the diffraction data help to resolve the apparent ambiguity in interatomic distances between earlier x-ray absorption and powder diffraction measurements. The structural changes are confirmed by Raman spectroscopy.
A new method is proposed for the fabrication of thin films consisting of closely packed nm-size metallic grains. The method is based on laser ablation of metallic (Cu) target and cascade fission of ejected liquid metallic drops charged in the laser torch plasma. It is suggested that cascade fission is limited by field emission current from the drop surface, which stops this process sharply when the Cu granule size diminishes down to about 10 nm. The fabricated films, free oxidized in air, are shown to consist of one or several layers of monodisperse spherical Cu grains, 8 nm in diameter, covered by about a 1 nm thick Cu2O layer.
High-resolution photoelectron spectroscopy of in situ prepared films of GeSb2Te4 reveals significant differences in electronic and chemical structure between the amorphous and the crystalline phase. Evidence for two different chemical environments of Ge and Sb in the amorphous structure is found. This observation can explain the pronounced property contrast between both phases and provides new insight into the formation of the amorphous state.
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