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Received Nov 11, 2017; Revised Jan 7, 2018; Accepted Jan 22, 2018
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1. Introduction
Layered composites utilizing nanocrystalline silicon dispersed within an amorphous Si matrix are considered to be very promising for the next generation of quantum dots-based solar cells [1]. Such composites (referred to as nc-Si in the text below) pose a number of properties important for the photoelectric conversion of solar radiation, for example, quasi-direct bandgap mechanism of light absorption, dependence of the energy bandgap on the nanocrystals size, stability against the Stabler-Vronsky effect, and a possibility of manufacturing on flexible substrates.
Employing nc-Si as the base material allows enhancing significantly the efficiency of solar cells due to formation of polymorphic cascade heterostructures [2, 3] and decreasing their production cost due to advantages of the thin-film roll technology [4, 5]. Among the hurdles that block wide-scale utilization of nc-Si advantages in practice is poorly developed methods of control of the nanocrystals’ size and concentration at economically relevant rates of the composite film formation. That is why despite a large number of already developed methods of nc-Si fabrication, still much attention is paid and efforts are exerted to improve the existing methods and to develop new ones (see, e.g., [6–12]).
A promising path in this respect relies on the effect of metal-induced crystallization (MIC) of the amorphous Si (α-Si) [13–17]. In particular, efficient formation of Si nanocrystals with the sizes of 2–7 nm and partial volume of a crystalline phase of up to 80% was demonstrated in the recent experiments on low-temperature crystallization of α-Si with Tin (Sn) [18–20]. The experimental findings of those referenced works were modeled and explained in [20–22] by suggesting the crystallization mechanism that is essentially different from those known for Si crystallization assisted by other metals [13, 15–17]. In accordance with the proposed mechanism, Si nanocrystals are formed during self-sustained cyclic (repeatable) processes of formation and decay of the supersaturated solution of silicon in tin within a thin eutectic layer at the interface of α-Si and Sn. Further development of the proposed mechanism confirmed that Sn-induced...