Quasi-Rheotaxy: Growth of large crystalline grain thin films on quasi-liquid substrates

Abstract

A new technique suitable for growing large grain crystalline thin films on amorphous low cost substrates is described. Such a technique, which we called “Quasi-Rheotaxy”, consists in depositing semiconductor materials on thin metal layers kept at a temperature near to, but lower than their melting point. We used quasi-rheotaxy to prepare several thin films of semiconductors which are considered as good materials for use in solar photovoltaic conversion, such as CdS, CdTe, Zn 3P 2, CuIn x Ga 1− x Se 2 and CuGaSe x Te 1− x . Thin metal layers, such as Sn, Pb, Bi and Tl deposited on stainless steel or sapphire, have been used as substrates. Crystalline grain dimensions larger than 10 μm can be obtained by quasi-rheotaxy. The large grain growth seems to be due to the fact that the metal layers, even if they are kept at a temperature lower than their melting point, exhibit a melted surface to the incoming semiconductor atoms giving them a high surface mobility. Such a high mobility facilitates the formation of clusters which aggregate in large crystalline grains. Quasi-rheotaxy could be used to prepare low cost high efficiency thin film solar cells.