Structural Properties and Size-control of Visible Photoluminescent Porous Silicon

本研究的目的是寻找具有恒定尺寸分布的残余Si部分的多孔硅（PS）层的合适制备条件，并参考光致发光性质讨论p+PS和n+ PS层的共同结构特征，包括发射部分及其形貌和结晶度。在低电阻率小于0.02OMEGAcm的重掺杂p型和n型硅片上形成的PS层主要采用x射线多晶衍射法，电子衍射、扫描电镜和透射电镜等辅助手段进行了研究。主要实验结果如下：p+ PS层从上表面到下表面呈直径大于10nm的线状或支状结构。通过控制制备条件，可以得到尺寸分布恒定的p+ PS层。在与表面垂直的方向上，PS层的晶格间距比si基态略大。x射线双晶衍射仪测得的p+ PS层在x射线衍射谱上有一个尖峰，表明p+ PS层的结晶度与Si衬底相同。实验还表明，室温下，在He-Cd激光照射下表现出强烈光致发光的PS层不是无定形的，而是结晶的，其可见光致发光与PS层近表面的微观结构密切相关，该结构是由大量小nm大小的硅晶体组成的海绵状结构。相反，n+ PS层在大角度范围内，在n+ PS层的几个相干峰下始终表现出广泛的漫射散射，这表明n+ PS层的结晶度不如p+ PS层，这可能是由于PS层的纳米结构发生了一些修饰。研究还发现，n+ PS层是由若干层状孔隙结构组成的，这些孔隙结构与阳极氧化过程有关，卤素灯照明非常有效地形成了这种海绵状结构，可以在PS层的顶部表面附近发出可见光。综上所述，我们想说可见光的发射只来自于具有海绵状结构的PS层，这种海绵状结构由大量尺寸为几nm的硅部件组成，光致发光可能是由于这种三维结构中的载流子限制。微晶尺寸与光致发光峰位置或蓝移关系的详细研究正在进行中。少

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