Low temperature synthesis of photoconductive BaSi2 films via mechanochemically assisted close-spaced evaporation

Abstract

BaSi₂ is an emerging photovoltaic material with optimum optoelectronic properties and can be deposited by close-spaced evaporation, which has the advantages of epitaxial growth, high productivity, and scalability to large area deposition. This process is enabled by two reactions: (1) Ba gas generation from a milled BaAl₄–Ni source and (2) BaSi₂ formation from the Ba gas and a Si substrate, and is limited by the temperature of the former reaction. We show that mechanical activation of the BaAl₄–Ni source by ball milling within 60 s lowers the BaSi₂ film synthesis temperature down to 700 °C from 1000 °C required for the source ground using a pestle and mortar. Interestingly, additional milling beyond 60 s reduces the Ba gas generation rate, presumably because Ni is mechanochemically dissolved into BaAl₄. The BaSi₂ films synthesized at 700 °C are free from cracks and are epitaxially grown with (100) orientation on Si(100) substrates. Hall effect measurements reveal low electron concentration below 1.1 × 10¹⁷ cm⁻³. Photoconductance of the BaSi₂ films correlates with film thickness, clearly indicating photocarrier generation in BaSi₂. Thus, the mechanochemically assisted synthesis opens the way for solar cell applications of BaSi₂ films synthesized via close-spaced evaporation.