Reactive Magnetron Sputtering of (GeOx-SiO2) Superlattices for Nanocrystal Synthesis

Recently semiconductor nanocrystals (NC) attracted additional interest because they might have the potential for adapting solar cell devices to a broader irradiance spectra. It is believed that this could be realized by size-controlled bandgap engineering of multiple junction solar cells. The feasibility of bandgap shifts up to 2 eV has been proofed for NC's of Ge or Si in the size from 1 to 5nm [1].
However, the fabrication of dense (>10^-12cm²), small and equally sized NC’s in a suitable matrix remains still a remarkable challenge.

The main focus of this work is the manufacturing of Ge-NC superlattice structures in silica matrix for photovoltaic application. DC reactive sputtering was used to deposit sequentially GeOx and SiO2 layers on Si wafers with thermally oxidized silica surface layer (500 nm). The sputter rate from the elemental targets was quite small (< 0.2 Å/s) to achieve good layer quality. The GeOx and SiO2 layer thicknesses could be tuned independently with the deposition time. It was possible to vary the composition from elemental Ge to GeO2 by adjusting the partial pressure of oxygen (p = 0 to 0.02 Pa) in the sputter chamber (sputter gas: Ar, p = 0.5 Pa). With increasing substrate temperature (RT up to 400°C) the oxygen content had to be increased as well in order to get the same x-value. Subsequent annealing led to Ge crystallisation with intrinsic cut-off size due to the silica separation layers.

In-situ characterization revealed the temperature dependent growth of the Ge-NC by grazing incidence x-ray diffraction (GID) and the layer interface roughness by x-ray reflectometry (XRR). Ge-NC’s being 2 to 5 nm small could be detected above 500°C. Interface roughnesses of about 1 nm showed that the fabrication of very thin separation layers allowing direct tunnelling should be possible.

Ex-situ analysis via Rutherford backscattering (RBS) provided the matrix of dependencies between the temperature, the deposition rate, the partial pressure of oxygen and the stoichometry. With the help of transmission electron microscopy (TEM) it was possible to gain local information about shape, size and crystallinety of the Ge-NC’s. Raman spectroscopy allowed a more global view on the size and shape of the nanocrystals and on the ratio of amorphous and crystalline Ge parts.

[1] Martin A. Green, Third generation photovoltaics, Springer, 2006, ISBN 1437-0379