Light absorption in Ge nanoclusters embedded in SiO2: comparison between magnetron sputtering and sol-gel synthesis

SiGeO films have been produced by a sol-gel derived approach and by magnetron sputtering deposition. Post-thermal annealing of SiGeO films in forming gas or nitrogen atmosphere between 600 and 900 A degrees C ensured the phase separation of the SiGeO films and synthesis and growth of Ge nanoclusters (NCs) embedded in SiO2. Rutherford backscattering spectrometry analysis evidenced a similar Ge concentration (similar to 12 %), but a different Ge out-diffusion after annealing between the two types of techniques with the formation of a pure SiO2 surface layer (similar to 30 nm thick) in sol-gel samples. The thermal evolution of Ge NCs has been followed by transmission electron microscopy and Raman analysis. In both samples, Ge NCs form with similar size increase (from similar to 3 up to similar to 7 nm) and with a concomitant amorphous to crystalline transition in the 600-800 A degrees C temperature range. Despite a similar Ge concentration, a significant lower NCs density is observed in sol-gel samples attributed to an incomplete precipitation of Ge, which probably remains still dispersed in the matrix. The optical absorption of Ge NCs has been measured by spectrophotometry analyses. Ge NCs produced by the sol-gel method evidence an optical band gap of around 2 eV, larger than that of NCs produced by sputtering (similar to 1.5 eV). These data are presented and discussed also considering the promising implications of a low-cost sol-gel based technique towards the fabrication of light harvesting devices based on Ge nanostructures.