Etching Single Crystalline Materials

Bosch process Deep reactive-ion etching (DRIE) Etching single crystalline materials Physical-chemical etching Potassium hydroxide (KOH) etching Silicon etching... 

Si (Canham, 1990), SiC (Shor et al, 1993), GaP (Belogorokhov et al, 1994) and ZnTe (Zenia et ai, 1999) may be taken as examples of etched single-crystalline porous structures. Solar cells have indeed been fabricated with some of these materials, but it appears that the final product is as costly as the original multicrystalline precursors, and the cost advantages may therefore be limited. 

In contrast to isotropic etching, anisotropic etching is a fabrication technique that removes material in specific directions allowing for the production of geometric characteristics such as sharp corners, flat surfaces, and deep cavities. Relative to microfabrication, selectivity can be defined as the ratio of the etch rate of the target material to the etch rate of other materials. The definition can also be applied more specifically to single crystalline materials such as silicon where it would be considered the ratio of the etch rate in the target direction to the etch rate in other directions. 

The formation of etch pits and tunnels on n-Si during anodization in HF solutions was reported in the early 1970 s. It was found that the solid surface layer is the remaining substrate silicon left after anodic dissolution. The large current observed on n-Si at an anodic potential was postulated to be due to barrier breakdown.5,6 By early 80 s7"11 it was established that the brown films formed by anodization on silicon substrate of all types are a porous material with the same single crystalline structure as the substrate. 

Porous photoetching of -type GaP in H2SO4/H2O/H2O2 leads to the formation of macroporous GaP (random networks of single crystalline GaP with pore sizes of 150 nm and porosity of 35 to 50% for porous etched and porous photoetched, respectively), which appears to be the most strongly scattering material in visible light reported to date and localization effects are anticipated [224, 225]. 

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