Local Structure of Indium-Plated Porous Silicon
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Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
conventional electric furnace oxidation at 500 'C for 30 min. RBS data of the In-plated specimens thus obtained were taken using a Pelletron accelerator (model: National Electrostatic Corp. 3S-R10). The beam size of 2.1-MeV He 2+ ions employed was 2 mm in diameter. For TEM measurements using a 200-kV electron microscope (model: JEOL JEM-2010) with an energy-dispersive X-ray (EDX) analysis system (model: Noran 5500), the specimens were cut into small pieces, some of which were thinned by mechanical lapping and subsequent ion etching. The probing beam size was approximately 5 nm in diameter for TEM EDX measurements. Scanning electron microscope (SEM) images with mapping images of EDX data were taken using a field-emission SEM. RESULTS AND DISCUSSION As-Indium-Plated Porous Silicon We have found that In-plating can be successfully done for freshly prepared PS specimens whereas it is difficult to plate naturally-oxidized PS specimens. Therefore, special care was required so as to restrain an unnecessary air-exposure of the PS specimens appearing below as shortly as possible when the solution in the anodization cell was changed from the HF solution to the In 2(SO 4)3 solution. The depth distribution of plated In was measured non-destructively by the conventional RBS technique. Two of RBS spectra thus obtained are typically shown in Figs.l(a) and 1(b). As schematically demonstrated by the insets in Figs. 1, In metal can be plated into the PS layers. However, the In depth distribution depended on the samples, and was roughly grouped into the following two cases. For the one case (type I ) as shown in Fig. 1(a), the whole PS layer was plated although the amount of plated In is larger near the PS - Si substrate interface than in the subsurface PS layer. For the other case (type 11l),there was a depth region where almost no In atoms existed while considerable amounts of the In atoms were populated near the PS - substrate interface and near the specimen surface (Fig. 1(b)). The In signal from the surface region is, however, mostly originated from an In2(SO4)3 salt in the solution since a small amount of S was also present near the surface. The total amount of the plated In atoms ranged typically from I X
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Fig. 1. Rutherford backscattering spectrometry (RBS) spectra taken for a type I specimen ((a)) and a type IIspecimen ((b)). Insets show schematic distributions ofSi, In and 0 atoms. A partial channeling effect was observed for the substrate Si especially in (b). 486
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Fig.2. Transmission electron microscope (TEM) images taken for the type I specimen ((a)) and type IH specimen ((b)) shown in Figs. 1. Plated In at
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