Structural and electrical characterizations of crack-free BaSi2 thin films fabricated by thermal evaporation

The BaSi2 semiconductor is a promising candidate for an earth-abundant solar cell absorber. In this study, we have realized a crack-free BaSi2 film by a simple thermal evaporation technique on a CaF2 substrate at a growth temperature of 500 degrees C for electrical characterization. A Si layer preliminarily formed on the substrate by sputtering is a key to obtain stoichiometric BaSi2 film. Detailed structural characterization of the evaporated films with different Si layer thicknesses by X-ray diffraction, scanning and transmission electron microscopy demonstrates that a crack-free 370-nm-thick BaSi2 film is formed by consuming the Si layer. It is observed that the 90-nm-thick bottom part is microcrystalline and contains Ar atoms, which come from Si deposition atmosphere. The surface of the BaSi2 layer is found to be covered by an amorphous Si layer due to Si-rich vapor at the last stage of evaporation. Electrical properties of the BaSi2 film are revealed by Hall measurement and the electron density and mobility are found to be 6 x 10(20) cm(-3) and 0.04 cm(2)/V.s, respectively. Owing to a better crystalline quality contributed by the preliminarily-deposited Si layer, minority-carrier lifetime of the evaporated film (0.6 mu s) is twenty times longer than previous films on glass substrates. (C) 2015 Elsevier B.V. All rights reserved.