The annealing behavior of very thin SiO 2 /Ge multilayers deposited on Si substrate by e-gun deposition in high vacuum was explored. It is shown that, after annealing at moderate temperatures (800 ∘ C) in inert atmosphere, Ge is completely outdiffused from the SiO 2 matrix leaving small (about 3 nm) spherical voids embedded in the SiO 2 matrix. These voids are very well correlated and formed at distances governed by the preexisting multilayer structure (in vertical direction) and self-organization (in horizontal direction). The formed films produce intensive photoluminescence (PL) with a peak at 500 nm. The explored dynamics of the PL decay show the existence of a very rapid process similar to the one found at Ge/SiO 2 defected interface layers.
pivac@irb.hr, Phone: 385 1 4561068, Fax: 385 1 4680114We explore the process of Si nanoparticles formation in SiO 2 matrix suitable for advanced solar cells application. To this purpose a superstructure consisting of alternating 5 nm thick SiO x and SiO 2 layers was deposited by high vacuum evaporation from solid sources. After high temperature annealing of such structures in high vacuum, the SiO x decomposed and Si nanoparticles aggregated at their former position forming therefore a superstructure of Si nanoparticles embedded in dielectric SiO 2 matrix. To explore such process of nanoobjects formation in different matrix, grazing incidence small-angle X-ray scattering (GISAXS) seems to be as relatively easy non-destructive technique; a logical choice. Despite the fact that due to the rather small difference in electron density between Si and SiO 2 . GISAXS contrast is very small, by performing GISAXS with intense synchrotron light and subtracting the dominant surface contribution to the overall signal we managed to retrieve information on the structural changes within the layers. The conclusions from the GISAXS analysis were confirmed by photoluminescence measurements.
In this work, self‐assembled Ge quantum dot (QD) formation in a dielectric matrix is explored. Of particular interest were their structural and optical properties, in order to understand the stress build‐up in such a process and its impact on the material properties during processing. To this end, thin films consisting of (Ge + SiO2)/SiO2 multilayers grown by RF magnetron sputtering were deposited at room temperature. Annealing of such films at 873 K in inert N2 atmosphere produced, at the position of the Ge‐rich SiO2 layers, a high lateral density (about 1012 cm−2) of Ge QDs with a good crystallinity. SiO2 spacer layers separated the adjacent Ge‐rich layers, where the Ge QDs were formed with a diameter of about the size of the (Ge + SiO2) as‐deposited layer thickness, and created a good vertical repeatability, confirmed by the appearance of a Bragg sheet in two‐dimensional small‐angle X‐ray scattering patterns. The structural analysis, by wide‐angle X‐ray diffraction, grazing‐incidence small‐angle X‐ray scattering and transmission electron microscopy, has shown that the described processing of the films induced large compressive stress on the formed QDs. Optical analysis by time‐resolved photoluminescence (PL) revealed that the high density of crystalline Ge QDs embedded in the amorphous SiO2 matrix produced a strong luminescence in the visible part of the spectrum at 2–2.5 eV photon energy. It is shown that the decay dynamics in this energy range are very fast, and therefore the transitions that create such PL are attributed to matrix defects present in the shell surrounding the Ge QD surface (interface region with the matrix). The measured PL peak, though wide at its half‐width, when analysed in consecutive short spectral segments showed the same decay dynamics, suggesting the same mechanism of relaxation.
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