Self-sustained cyclic tin induced crystallization of amorphous silicon
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Petro Y. Shepeliavyi, Volodymyr O. Yukhymchuk, and Viktor A. Danko Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Nauky Pr., Kyiv 03028, Ukraine
Viktor V. Melnyk and Andrey G. Kuzmich Faculty of Physics, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine (Received 6 March 2015; accepted 3 August 2015)
Experimental evidences for a recently proposed mechanism of tin-induced crystallization of amorphous silicon are presented. The mechanism discusses a crystalline phase growth through cyclic processes of formation and decay of a super-saturated solution of silicon in molten tin at the interface with the amorphous silicon. The suggested mechanism is validated using a nonlinear dynamical model that takes into account the mass diffusion of the components of the system, heat transfer caused by latent (crystallization) heat release and amorphous silicon dissolution events, and concentration nonuniformities created by silicon crystallization. The analysis of a stationary-state solution of the model confirms the existence of periodic solutions for the partial volume of the crystalline phase and other system’s variables. Possible applications of the proposed mechanism in manufacturing of cost-effective nanocrystalline silicon films for the third-generation solar cell technology are discussed.
I. INTRODUCTION
Contributing Editor: Don W. Shaw Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] c) Present address: 21 Nopalitos Way, Aliso Viejo, CA92656, USA DOI: 10.1557/jmr.2015.251
levels within the crystalline Si band gap, doping of Si with Sn within the solubility levels does not affect electrical, optical, and carrier recombination properties of the crystalline Si. At the same time, doping with Sn increases resistance of Si to potentially harmful effects of the thermal treatment18 and ionizing radiation.19 Sn creates the eutectic alloy with Si20 allowing the transformation of amorphous Si into a crystalline phase through e.g., the layer exchange mechanism as was reported recently for the bi-layer structures of Sn and Si films deposited on glass substrates and annealed after the deposition.21,22 Early works on Sn-induced crystallization of a-Si showed that in the deposited from the gas phase Si–Sn mixtures, a-Si crystallization occurs when Sn concentration exceeds a certain threshold value (;2 at.%)23 and crystal formation proceeds within the eutectic Si:Sn melt propagating ahead of the crystallization front into an a-Si region.24 Exceeding the threshold concentration of Sn entails creation of the droplets of metallic Sn, which may indicate surpassing of the solubility limit of Sn in amorphous Si, explaining also the previously reported effect of conductivity switching from the activation to hopping mechanism upon reaching exactly the same concentration of Sn in Si.25,26 Such coincidence of the threshold values for the three different effects (crystallization of a-Si, creation of Sn droplets, and switching of the conductivity mecha
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