Silica Porous Glasses With Silicon Impregnations
- PDF / 1,270,264 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 15 Downloads / 225 Views
Two methods of porous oxidized silicon formation were employed in this work. In the first case (samples B) porous silicon was obtained by anodization of p-type (I Ohm*cm) crystalline Si substrates in hydrofluoric acid-ethanol solution at a constant current density. Thickness of the fabricated films was of the order of 50 micron (measured interferometrically after detaching the film from the substrate). The total volume of voids in Si porous layer was about 50% (typical size of voids 20-30 A,measured, as also in the case of A, and A2 samples, by the Robert's method 7). In the second cue (C) an original sparkdischarge technique was used to obtain the porous layer. The voltage from Ruhmkorff induction coil was applied to the silicon needle placed at a distance of 7 mm from the polished silicon wafer. At negative potentials on the needle and at currents of about 0.5 mA a porous film, 20 micron thick, was obtained on the surface of the silicon wafer. Fig. 1 shows SEM pictures of a type C porous silicon specimen. It consists of silicon clusters
a
b
Fig. I. SEM photographs of the porous silicon films on silicon substrates obtained in the spark-discharge: a - top view, b - view of the split (bar scale - 10 micron). encapsulated in thin dielectric coatings smintered between themselves. The obtained films behave as typical insulators in standard electrical measurements. By filling the volume of voids with dielectric liquids with known values of dielectric constant it was possible to calculate the volume of voids and the effective dielectric constant of the porous framework material.8 The porosity in our case was about 40% while the dielectric constant of the matrix was about 11. We argue that dielectic coatings consist of silicon oxynitride rather than of pure SiO 2. In case the spark discharge process was carried out in pure nitrogen atmosphere the properties of the obtained films were almost the same as in the case of open-air process. Porous layer oxidation was performed in water vapours in standard oxidation furnaces at temperatures as low as 7600 C to reduce coarsening of the porous texture. A convincing evidence that the porous silicon had oxidized was that it had become completely transparent. The obtained porous layer thickness remained on the preoxidation level. To fill the voids with carbon all porous samples were placed in analytic grade 40% glucose water solution. Superfluous solution was removed from the surface. Then the samples were dried and annealed at 200-220'C. As the result glucose was decomposed and the samples (A, B) gained a typical black or reddish (C) colour. Two types of subsequent
774
treatment were employed. In the first procedure samples were annealed at T=550-7500 C for 10-15 min. As the result carbon disappeared and the samples retained their transparence.
In the second process the mentioned above spark discharge system was used. Regions subjected to the action of the discharge also became transparent. The electrical conductivity in the already processed places decreased drastically, so
Data Loading...
