Optical investigations of germanium nanoclusters - Rich SiO2 layers produced by ion beam synthesis

In this work, refractive index and extinction coefficient spectra of germanium nanoclusters - rich SiO2 layers have been determined using variable angle spectroscopic ellipsometry (VASE) in the 2501000 nm range. The samples were produced by Ge+ ion implantation into SiO2 layers on Si substrates and subsequent annealing at temperatures from 700 to 1100 degrees C. It is known from previous investigations of similar samples that the Ge nanoclusterization process starts already at 800 degrees C and spherical Ge nanocrystallites 5-8 nm in diameter are observed in the SiO2 layers after annealing for I h at even higher temperatures of 1000-1100 degrees C. Rutherford backscattering spectrometry (RBS) was employed to measure the Ge atom concentration depth profiles in the studied samples. The RBS results helped us choose realistic models for the VASE analysis which were necessary for a proper interpretation of the VASE data. It has been found that the refraction index value for the SiO2/Si layer increases after Ge implantation. This effect can be explained by a defect-dependent compaction of ion-bombarded layers. A band's tail in the extinction coefficient spectra for all the samples is observed which originates from a strong ultraviolet absorption band at 6.8 eV due to a Germanium Oxygen-Deficiency Center (GeODC) and/or a Ge-E'center in SiO2. The annealing process results in the emergence of weaker extinction coefficient bands in the 400-600 nm region, associated with direct band-to-band transitions in Ge nanostructures. Transformation of these bands, including their blue-shift with the increasing annealing temperature could be explained via a quantum-confinement mechanism, by size and structural changes in Ge nanostructures.