Reciprocal Space Analysis of the Microstructure of Luminescent and Nonluminescent Porous Silicon Films
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ABSTRACT The microstructure of anodically prepared porous silicon films was determined using a novel x-ray diffraction technique. This technique uses double-crystal difffractometry combined with position-sensitive x-ray detection to efficiently and quantitatively image the reciprocal space structure of crystalline materials. Reciprocal space analysis of newly prepared, as well as aged, p- porous silicon films showed that these films exhibit a very broad range of crystallinity. This material appears to range in structure from a strained, single-crystal, sponge-like material exhibiting long-range coherency to isolated, dilated nanocrystals embedded in an amorphous matrix. Reciprocal space analysis of n+ and p+ porous silicon showed these materials are strained single-crystals with a spatially-correlated array of vertical pores. The vertical pores in these crystals may be surrounded by nanoporous or nanocrystalline domains as small as a few nm in size which produce diffuse diffraction indicating their presence. The photoluminescence of these films was examined using 488 nm Ar laser excitation in order to search for possible correlations between photoluminescent intensity and crystalline microstructure. INTRODUCTION The goals of this study were two-fold. The first was to assess the ability of a relatively new technique for reciprocal space analysis, based on position-sensitive x-ray detection, to yield new insights into the crystalline microstructure of various types of porous silicon. In order to do this, reciprocal space analysis was used to survey the microstructure of both newly prepared and aged porous silicon films anodized under a variety of conditions. Perhaps not surprisingly -- given the wide range of conclusions reached in transmission electron microscopy (TEM), transmission electron diffraction and x-ray diffraction studies that are already published -- the porous silicon studied here was found to exhibit a wide range of crystallinity and a diverse microstructure. Using reciprocal space analysis, we found that we could observe and quantify the following microstructural phenomena: nanocrystal and nanovoid size and shape, nanocrystal orientation, lattice dilation in films containing isolated nanocrystals, lattice strain distributions in films which remain single crystalline, and mean pore spacing in single-crystalline samples containing vertical pore arrays. Results for three prototypical porous silicon structures are presented in detail below in order to demonstrate the unique capabilities offered by reciprocal space analysis. Quantitative determinations of the size, shape and volume-fraction of nanocrystalline domains that exist in various forms of porous silicon are of particular interest because of their relevance to quantum confinement as a mechanism for photoluminescence (PL) in porous silicon. Our second goal was to attempt to test the quantum confinement hypothesis by looking for the presence or absence of a quantitative correlation between relative PL efficiency and relative nanocrystal density. Thus
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