In-situ x-ray studies of Germanium nanocrystals, formation out of (GeOx/SiO2) superlattices

Semiconductor Nanocrystals (NC), consisting only of a few hundred of atoms, came recently even more into focus, because they might help to increase the efficiency of solar cells for the 3rd generation photovoltaics [1]. This could be mainly achieved by an engineered bandgap size of the material, which allows the light absorption of the complete sunlight spectrum. However, it remains a remarkable challenge to achieve a high density (>10¹² cm^-2) of equal-sized, small (< 5 nm) NC’s of Ge or Si embedded in dielectric films.
In this study we present the fabrication of Ge-NC's by decomposition of GeOx (1 < x < 2) out of a (GeOx -SiO2) superlattice structure (SL). The SL was grown by dual reactive DC magnetron sputtering from elemental targets. Different Ge/O ratios in the SL structures were realized by the variation of oxygen flow and deposition temperature. Using in-situ x-ray reflectivity (XRR) and grazing incidence diffraction (GIXRD) at the CRG Beamline ROBL at ESRF we studied the deposition of the SL and the Ge NC's evolution during subsequent annealing. The formation of Ge NC’s by the GeOx phase separation has been proofed with GIXRD. At 600°C a Ge (111) signal confirmed the existence of Ge NC’s in the size of 2-8 nm. Within the SL stability range, the NC size corresponds approximately to the GeOx sublayer thickness.
[1] Martin A. Green, Third generation photovoltaics, Springer, 2006, ISBN 1437-0379