Reduce of reflection and recombination losses by surface texturization

Homogeneous pyramid formation is very sensitive to residual contaminants and crystal defects. Surface texturing is conventionally achieved by anisotropic wet-chemical etching of Si(100) wafers in alkaline solutions containing isopropyl alcohol (IPA) (is 70 - 80 °C ) as an additive. IPA easily evaporates resulting in a continuous change of the etching solution concentration. In order to avoid these disadvantages IPA can be substituted by an IPA-free additive, i.e. Alkatex ZeroTM (GP Solar GmbH). This feature promotes a more homogeneous surface structure and allows tuning the pyramid size distribution.

The texture etching time has to be optimised to reduce reflection as well as recombination losses.

Reflection losses can be reduced with decreasing fraction of small pyramids. Both the depth of the saw damage etching (SDE) and the duration of the texturization etching have crucial influence on the resulting pyramid size distribution.

dit alkatex

Fig. 3b Dit(E) on random pyramids prepared in KOH + IPA (curves 1, 2) and in KOH + Alkatex (curves 3, 4) after RCA + HF (curves 1 and 3) and additional wet-chemical smoothing in H2SO4:H2O2 + HF (curves 2, 4).


Interface recombination behaviours result from 2 opposed effects during longer etch times: (i) the further decrease of saw damage enhances carrier life time but (ii) higher fraction of small pyramids increases number of defects. By intermediate saw damage removal and texture etching times in (IPA)-free KOH solution the densities of electronic interface states were found to be strongly decreased (Dit,min < 5 10-11 cm-2eV-1), in comparison to pyramids prepared in IPA containing solutions.

references

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