Silicon Nanoporous Pillar Array Formation Mechanism Of Research

The etching process of the silicon-nanoporous pillar array(Si-NPA) was investigated in this thesis. Based on common porous silicon formation mechanisms in HF solution and the anisotropic etching mechanism of silicon in alkaline etchants, the existence of silicon anisotropic etching in the acidic solution was proved from the phenomenon of formation process.A new formation mechanism which is Hole-demanded model was proposed. This model is based on the Diffusion-Limited model by introducing the anisotropic etching mechanism. The etching process of silicon in HF/HNO₃ solution will be simplified as the holes diffusion from the bulk to the surface zone with random walk. When a hole reaches the surface atom site, the atom will be annihilated with a probability determined by the number of dangling bonds. In this model there are two important factors: the holes random walk and the anisotropic etch. A detailed analysis of the two mechanisms in the etching process and the explanation of needing high-doped p-type silicon has been proposed.The Monte Carlo method has been used in the programming on the three-dimensional computer model simulation. Simulation of the process introduces holes concentration and different annihilated probability. The impact of the two parameters on morphology, porosity and fractal dimension has been analysised. Several simulations with different parameters have been tried and all etching results showed porous structures, which was different with the morphology of the Si-NPA formation process.Simulation of Hole-demanded model results in a large deviation. Based on the concept of hole and the analysis of the etching process, hole is supposed only to accelerate etching speed. Then a second etching model, Hole-assisted model has been proposed. The model is on the base of anisotropic etching and holes accelerate etching speed by the diffusion to the surface atom site randomly. There are several differences between the two models: holes' diffusion to the surface atom sites is the necessary condition in the Hole-demanded model and the assistant condition in the Hole-assisted model. This thesis provides a detailed analysis of the two etching mechanisms. We explain the porous structure on the surface and buffer layer between pillars and single silicon as well as the effect of electrolyte concentration on the pillar density.The computer simulation based on the Hole-assisted model has been operated, different types of silicon pillars has been obtained by changing the parameters, which confirmed the validity of the Hole-assisted model.