Space charge layer doping

Before constructing an electrode for microwave electrochemical studies, the question of microwave penetration in relation to the geometry of the sample has to be evaluated carefully. Typically only moderately doped semiconductors can be well investigated by microwave electrochemical techniques. On the other hand, if the microwaves are interacting with thin layers of materials or liquids also highly doped or even metallic films can be used, provided an appropriate geometry is selected to allow interaction of the microwaves with a thin oxide-, Helmholtz-, or space-charge layer of the materials. 

Non-heavily doped n-Si Space charge layer Space charge layer Oxide film... 

The applied anodic potential may mostly or partially drop in the space charge layer or in the Helmholtz layer depending on doping type and concentration as well as on the potential range. 

For moderately doped substrates, when the surface is free of oxide the change of potential is mostly dropped in the space charge layer and in the Helmholtz double layer. The reactions are very sensitive to geometric factors. The reaction that is kinetically limited by the processes in the space charge layer is sensitive to radius of curvature, while that limited by the processes in the Helmholtz layer is sensitive to the orientation of the surface. Depending on the relative effect of each layer the curvature effect versus anisotropic effect can vary. 

Two-layer PS with a micro PS on top of a macro PS layer is formed on lowly doped p-Si or illuminated n-Si. For lowly doped p-Si two-layer PS can form when the conditions are such that the space charge layer and the resistive layer differ in dimension by several orders of magnitude. 

The formation of two-layer PS on p-Si involves two different physical layers in which the potential-current relations are sensitive to the radius of curvature. The space charge layer of p-Si under an anodic potential is thin, which is responsible for the formation of the micro PS. The non-linear resistive effect of the highly resistive substrate is responsible for that of macro PS. The effect of high substrate resistivity should also occur for lowly doped n-Si. However, under normal conditions, the thickness of the space charge layer under an anodic potential, at which macro PS is formed, is on the same order of magnitude as the dimension... 

This is a good point to bring up briefly a property of very small crystals (often obtained in CD), which is dealt with in more detail in Chapter 9. The crystal size is, in most cases, much smaller than the size of any space charge layer that would be formed. This means that in an isolated nanocrystal, unless the doping level is very high (usually it is not, as attested to by the high resistivities more of-... 

Inorganic rectifiers are, of course, still superior to organic systems. However the experiments show that /—n junctions and space-charge layers at contacts can be obtained with the help of organic solids. Further experiments should be made, especially with doped systems. 

---