Anodization of silicon in aqueous HF solutions generates a curious labyrinth of pores which penetrate deep within the bulk of the silicon. The morphology and size of the pores may be controlled by varying the anodization conditions such as silicon doping, HF concentration, and anodization current densities. In addition to the fascinating interfacial process that control pore formation, the pores have a wide ranging of uses from SOI formation, micromachining, and the generation of a direct bandgap material with the emission of light from silicon. Below are a few photographs of various pore morphologies.
Two types of pores formed in n-type silicon under different current densities. The upper half of the photograph shows pore morphologies under high current densities--large spike-like trenches. In the bottom half of the photograph the current density was reduced and a more filament like morphology is realized.
Close up view of spikes formed in n-type silicon under high current density conditions. Side dendrites are clearly shown.
The dissolution process of porous silicon formation is crystallographically anisotropic. Dissolution preferentially occurs in the <100> directions. This is shown in the plan view of n-type silicon near the propagation tip (pores go into the plane of the picture). The geometry of the pores is square and aligned to the (110) planes. The etch domain of each pore is also clearly seen as a square with the dendrites propagating outwards from the central pore. Pores also form preferentially at defect sites. The large square in the center is a dislocation defect and is clearly seen rotated from the other smaller pores.
Plan view of roughness generated under pore formation. Porous silicon regions have been selectively etched away leaving only the underlying silicon substrate.
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