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Maurizio Roczen, Martin Schade, Enno Malguth, Gordon Callsen, Thomas Barthel, Orman Gref, Jan A. Töfflinger, Andreas Schöpke, Manfred Schmidt, Hartmut S. Leipner, Florian Ruske, Matthew R. Phillips, Axel Hoffmann, Lars Korte, Bernd Rech
Structural investigations of silicon nanostructures grown by self-organized island formation for photovoltaic applications.
Appl. Phys. A 108 (2012), 719-726

The self-organized growth of crystalline silicon nanodots and their structural characteristics are investigated. For the nanodot synthesis, thin amorphous silicon (a-Si) layers with different thicknesses have been deposited onto the ultrathin (2 nm) oxidized (111) surface of Si wafers by electron beam evaporation under ultrahigh vacuum conditions. The solid phase crystallization of the initial layer is induced by a subsequent in situ annealing step at 700 °C, which leads to the dewetting of the initial a-Si layer. This process results in the self-organized formation of highly crystalline Si nanodot islands. Scanning electron microscopy confirms that size, shape, and planar distribution of the nanodots depend on the thickness of the initial a-Si layer. Cross-sectional investigations reveal a single-crystalline structure of the nanodots. This characteristic is observed as long as the thickness of the initial a-Si layer remains under a certain threshold triggering coalescence. The underlying ultra-thin oxide is not structurally affected by the dewetting process. Furthermore, a method for the fabrication of close-packed stacks of nanodots is presented, in which each nanodot is covered by a 2 nm thick SiO2 shell. The chemical composition of these ensembles exhibits an abrupt Si/SiO2 interface with a low amount of suboxides. A minority charge carrier lifetime of 18 µs inside of the nanodots is determined.

Keywords: amorphous; annealing; cross section; growth; nano; preparation; quantum dots; silicon; solar cells; transmission electron microscopy
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