Structural Properties and Size-control of Visible Photoluminescent Porous Silicon

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The purposes of this study are to find suitable preparation conditions for porous silicon (PS) layrs having a constant size-distribution of residual Si parts, and to discuss common structural features including emitting parts and their morphology and crystallinity for the p+PS and n+ PS layrs with reference to photoluminescent natures. The PS layrs, formed on heavily doped p-type and n-type silicon wafers of low resistivi less than 0.02OMEGAcm, have been investigated by mainly X-ray multi-crystal diffractometry, supplemented with electron diffraction and scanning electron microscopy and transmission electron microscopy. Main experimental results are as follow : The p+ PS layr presents a wirelike or branched structure of more than 10nm in diameter from the top surface to the bottom part of the PS layr. It was easy to get p+ PS layrs with a constant size-distribution by controlling preparation conditions. The PS layr exibits a slightly larger lattice spacing than the si substate along th … More e direction normal to the surface. The p+ PS layr shows a sharp peak on X-ray diffraction profile measured by X-ray double-crystal diffractometer, indicating that the p+ PS exhibits the same crystallinity as that of the Si substrate. It was also shown that PS layrs which exhibit intensive photoluminescence under He-Cd laser illumination at room temperature are not amorphous, but crystalline, and that the visible photoluminescence is strongly related to the microstructure of PS layr near the surface that shows a spongelike structure consisting of a large number of silicon crystallites of few nm's in size. On the contrary, the n+ PS layr always shows broad diffuse scattering under severl coherent peaks for the n+ PS layr in a large angular range, indicating that the crystallinity of the n+ PS layrs is inferior to that of the p+ PS layrs, probably due to some modifications of the nanostructure of PS layr. It was also found that the n+ PS layrs are composed of several layred pore-structures which are associated with anodization processes, and that halogen-lamp illumination was very effective in forming such a spongelike structure which can emit visible photoluminescence near the top surfaces of the PS layrs.In conclusion, we would like to say that visible light emission comes from only the PS layr that has a spongelike structure consisting of a large number of silicon parts of few nm's in size, and that photoluminescence is probably due to charge carrier confinement in such three dimensional structure. Detailed studies on the correlation between microcrystallite size and photoluminescence peak position or blueshift are now in grogress. Less