化學傳感器：傳感材料基礎．多孔納米材料的特性及應用(第4冊)（簡體書）

When the volume filled with pores is not too large (5%), the properties of such material are closeto the properties of bulk silicon. At high indexes of porosity the situation changes completely, and such.porous silicon shows new properties, many of which are unique. For example, samples of porous silicon with porosity greater than 50% have intensive luminescence in the visible region of the spectrum(Canham 1991), although bulk Si does not have luminescence peaks in this region.
Simply removing atoms by anodic etching should leave the structural integrity of the semiconductor crystal intact-that is, porous semiconductors' should still be single crystals. Structural studies ofmicroporous silicon, where this expectation might be doubted, have shown that this material indeed isusually monocrystalline (Bomchil et al. ,1989). Moreover, it was established by numerous experimentsthat porous silicon, especially the layers With porosity of less than ~35-50%, and silicon in the substratehave the same crystallographic structure (Barla et al. 1984). However, some evidence of the presence ofan amorphous region has also been found using x-ray scattering, x-ray absorption fine structure, andRaman spectroscopy. These techniques.show thart the size of the amorphous layer varies with oxidation,aging, or postanodization treatment, yet the gross microscopic structure remains that of silicon nano,crystats embedded in an amorphous weblike matrix.
Surface characterization of porous silicon has shown that the surface is initially'covered by adsorbedhydrogen in various forms. For example, ,according to Parkhutik and Ibarra (1999), porous siliconformed in HF solution is covered b,ya hydride e passivation layer contaminated by fluoride. However,after aging in atmosphere, espececially when it is accompafiied by illumination, this surface is transformedinto an oxide-contaminated one.