Chemical etching to dissolve dislocation cores in multicrystalline silicon

• Published in: Physica. B, Condensed matter Condensed matter, 2012-08, Vol.407 (15), p.2970-2973
• Main Authors: Gregori, N.J., Murphy, J.D., Sykes, J.M., Wilshaw, P.R.
• Format: Article
• Language: English
• Subjects: Aspect ratio; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Defect; Defects and impurities in crystals; microstructure; Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.); Dislocation; Dislocations; Etching; Exact sciences and technology; Materials science; Multicrystalline; Photovoltaic; Photovoltaic cells; Physics; Silicon; Solar; Solar cells; Structure of solids and liquids; crystallography; Surface treatments; Tube; Tubes; Wafers
• ISSN: 0921-4526; 1873-2135
• DOI: 10.1016/j.physb.2011.07.049

Multicrystalline silicon wafers are used for approximately half of all solar cells produced at present. These wafers typically have dislocation densities of up to ∼106cm−2. Dislocations and associated impurities act as strong recombination centres for electron–hole pairs and are one of the major limiting factors in multicrystalline silicon substrate performance. In this work we have explored the possibility of using chemical methods to etch out the cores of dislocations from mc-Si wafers. We aim to maximise the aspect ratio of the depth of the etched structure to its diameter. We first investigate the Secco etch (1K2Cr2O7 (0.15M): 2HF (49%)) as a function of time and temperature. This etch removes material from dislocation cores much faster than grain boundaries or the bulk, and produces tubular holes at dislocations. Aspect ratios of up to ∼7:1 are achieved for ∼15μm deep tubes. The aspect ratio decreases with tube depth and for ∼40μm deep tubes is just ∼2:1, which is not suitable for use in bulk multicrystalline silicon photovoltaics. We have also investigated a range of etches based on weaker oxidising agents. An etch comprising 1I2 (0.01M): 2HF (49%) attacked dislocation cores, but its etching behaviour was extremely slow (