Understanding the thickness-dependent effective lifetime of crystalline silicon passivated with a thin layer of intrinsic hydrogenated amorphous silicon using a nanometer-accurate wet-etching method

• Published in: Journal of applied physics 2016-06, Vol.119 (23)
• Main Authors: Deligiannis, Dimitrios, Marioleas, Vasileios, Vasudevan, Ravi, Visser, Cassan C. G., van Swaaij, René A. C. M. M., Zeman, Miro
• Format: Article
• Language: English
• Subjects: Amorphous silicon; Applied physics; Bulk density; Dependence; Energy gap; Etching; Passivity; Photovoltaic cells; Silicon substrates; Silicon wafers; Thickness; Thin films; Valence band
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.4954069

This work studies the dependency of the effective lifetime on the a-Si:H layer thickness of c-Si substrates passivated with intrinsic a-Si:H. This is experimentally investigated by using a soft wet-etching method that enables accurate control of the a-Si:H layer thickness. In this way, variations in the effective lifetime down to thicknesses of a few nanometers are studied, while excluding effects originating from the deposition conditions of a-Si:H when samples of different thicknesses are fabricated. For thin passivation layers, results show a strong thickness dependency of the effective lifetime, which is mainly influenced by the recombination at the external a-Si:H surfaces. For thicker passivation layers, the effective lifetime is predominantly determined by the bulk a-Si:H and/or c-Si defect density. During the etching of the a-Si:H passivation layers, a gradient in the Cody gap for our samples is observed. This gradient is accompanied by a stronger decrease in the effective lifetime and is attributed to a decrease in the a-Si:H band gap and valence band offset. The observed changes in lifetime with a-Si:H layer thickness are supported with AFORS-HET simulations. When a gradient in the a-Si:H passivation layer band gap is used, simulations can reproduce the experimental results.