Formation of Uniformly Spaced Pore Array

In further investigations Lehmann ° found that the pores propagate at similar rates at different applied current densities. It was then postulated that all pore tips are limited by mass transfer in the electrolyte defined by J (see Fig. 5.1) in the steady-state condition. It was further proposed that the relative rates of carrier transport in the silicon semiconductor and mass transport in the electrolyte determine the PS morphology of n-Si. At low current densities the reaction rate is limited by the transport of carrier to the pore tips and there is no accumulation of holes so that dissolution occurs only at pore tips while the pore walls do not dissolve because of the depletion of holes. At high current densities the reaction at pore tips is mass transport limited and holes accumulate at the pore tips and some of them move to the walls resulting in the dissolution of walls and larger pore diameters. When the concentration of holes in the walls is close to that at the pore tips, the condition for the preferential dissolution at pore tips disappears and PS ceases to form. 

The rate of growth of macropores observed on -Si is independent of current density when the current density at the tip equals J. The pore diameter was related by Lehmann to the ratio of actual current density to peak current density 

Formation of Two-Layer PS on niuminated -Si. Two-layer PS with a micro PS on top of a macro PS and on the walls of the individual macropores formed on illn-minated n-Si had been reported in the late 1970s but was little investigated nntil the The micro PS may have a fractal-like geometry and can vary in the same layer in strnctnre from amorphons to single crystalline and in diameter from a few to hnndreds of nanometers. 

Whereas the formation of a macro PS layer on n-Si nnder front illumination follows the same mechanism as the macro PS formed in the dark, the formation of micro PS is believe to be mainly dne to the effect of the photogenerated carriers. Essentially, the dissolntion reaction nnder illnmination proceeds with a supply of photo holes which are generated near the snrface and distributed uniformly in the porous structure. This results in a dissolntion process different from that in the dark or back illumination and the formation of PS of extremely fine and randomly oriented structures. According to Arita, the anodic current under front illumination consists of three parts  

Alternatively, Clement et al. proposed that the micro PS found under illumination could result from shattering of the macro PS into fine filaments due to residual stress. Dissolution-precipitation was also considered to be a possible mechanism for the formation of micro PS althongh it is inconsistent with the single-crystalline nature of the miCTO PS in many two-layer PS. 

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