Blue Luminescence From SiO x Films Containing Ge Nanocrystals
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117
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
investigation of SixGeyOz alloy films of different chemical composition prepared by dc magnetron sputtering. The as-prepared films show a precipitation of the amorphous Ge nanosize clusters. The cluster formation is determined by the preparation parameters such as sputtering power and number of Ge chips on the Si target. Bright blue room temperature photoluminescence is observed from the films with a peak position at 410 nm for laser excitation at 325 nm. This seems to be in contrast to the peak position at 520 nm reported previously 7. However, excitation of our films with the 441nm line of the HeCd laser results in the reported peak above 500 nm. Our results do not agree with a quantum confinement origin of the room temperature PL. We will discuss possible luminescence mechanisms including defects. Experimental
Amorphous SixGe Oz alloy films were deposited on (100) silicon substrates using dc sputtering of a silicon target covered with up to 6 Ge chips in an Ar/water vapor atmosphere. The following parameters were varied in this study: the number of Ge pieces (1-6), the substrate temperature (100-200'C), the partial water pressure (0.05-0.2Pa), and the sputtering power (50-175W). This systematic variation of the sputtering parameters result in a wide range of as-prepared Ge cluster sizes (50nm down to 2nm) as well as in a variation of the chemical composition. The latter is investigated using Rutherford backscattering (RBS) providing an absolute error < 4at.% per constituent. The cluster size distribution was obtained by analyzing the small angle X-ray scattering spectra (SAXS) of the thin amorphous films 19 . In a second step the transformation of the Ge-clusters into Ge nanocrystals is induced by thermal annealing at 800°C in an argon atmosphere. The formation of the Ge nanocrystals was studied with X-ray diffraction under grazing incidence. An X-ray goniometer with Seeman-Bohlin geometry was used for the thin film analysis. The grazing incidence angle was 1° and the divergence of the Soller collimator for the analysis of the interference field was 0.38'. The X-ray intensity was measured using a secondary monochromator and a scintillations counter tube. The resulting wide angle scattering intensities were analyzed with the Scherrer multiple peak method. Photoluminescence spectra were measured with the 325nm and the 441nm line of a HeCd laser, a 0.5m monochromator, and a N2 cooled CCD matrix detector head with UV coating. Results and discussion An increase of the sputtering power from 50 to 175 W results in an increase of the deposition rate. As shown in fig. 1 the deposition rate increases from 0.2nm/s up to 1.2nm/s with the sputtering power. The higher substrate temperature results in a slight increase of the density of the films but shows no influence on the sputtering rate. The increase of the partial water pressure strongly changes the chemical composition of the ternary system.
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