Investigation on the origin of preferred a-axis orientation of BaSi2 films deposited on Si(100) by thermal evaporation

BaSi2 is an emerging light-absorbing material for thin-film solar cells. In this study, we have investigated the influence of the preparation conditions on the preferred orientation of evaporated BaSi2 films deposited on Si(100) substrates. X-ray diffraction result shows that the proportion of a-axis-oriented grains increases by raising the substrate temperature to 700°C or by annealing the films at the deposition temperature for 2-5 min, indicating that the a-axis-oriented growth is thermodynamically favorable. Transmission electron microscopy and selected area electron diffraction confirm the epitaxial relationship of BaSi2(100)//Si(100) with BaSi2[001]//Si[01¯1]. It is also found that the melting of the BaSi2 source before deposition significantly enhances the fraction of a-axis orientation, as evidenced by X-ray diffraction patterns. Comparison of the microstructure and composition analyzed by transmission electron microscopy between the BaSi2 films with and without the source pre-melting shows that without pre-melting, the crystal orientation of the bottom part of the film is not transferred to the upper layers and the Ba/Si composition ratio is higher in the film. On the basis of the obtained results, the effects of pre-melting is discussed, which suggests the importance of the epitaxial growth on the initially-deposited Ba-rich layer.