Synthesis and characterization of silicon clathrates of type I Na8Si46 and type II NaxSi136 by thermal decomposition

Type I (Na8Si46) or type II (Nax <= 24Si136) silicon clathrates films with a large 15 x 45 mm(2) surface have been synthesized from p-type and intrinsic c-Si (001) wafers using a two-step process without the need of any glove box. Conditions to selectively obtain either type I or type II silicon clathrates phase have been finely tuned. Optical absorption coefficients are found much larger in the Si clathrates than in diamond silicon in the visible light range. Type II films provide a direct bandgap of around 1.9 eV which is supporting the high absorption coefficient observed. Photovoltaic response of the films has been confirmed using Surface Photovoltage. As prepared type II films show many surface defects, cracks and inhomogeneities which have been drastically reduced thanks to a pressure annealing treatment. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

Type I (Na8Si46) or type II (Nax <= 24Si136) silicon clathrates films with a large surface area have been synthesized using a two-step process. The specific type of silicon clathrates phase can be selectively obtained by finely tuning the synthesis conditions. These films demonstrate higher optical absorption coefficients compared to diamond silicon in the visible light range, with type II films showing a direct bandgap suitable for photovoltaic applications. The surface defects and inhomogeneities of the as-prepared type II films have been significantly reduced through pressure annealing treatment.