Effect of template on the photoresponsivity of BaSi2 films grown on Ge(111) substrates by molecular beam epitaxy

Epitaxial growth of BaSi2 films on Si(111) has demonstrated that the BaSi2 template can serve as a seed crystal for BaSi2 overlayers by molecular beam epitaxy (MBE) and shows high photoresponsivity, but not yet on Ge(111) substrates. We have investigated the effect of various templates on the photoresponsivity of BaSi2 films grown on Ge(111) substrates. Samples with MBE-grown templates exhibited high a-axis orientation, but the surface was uneven, leading to partial oxidation of the BaSi2 film. On the other hand, the sample without such templates showed a smoother surface but was confirmed to be polycrystalline. When a template formed by the combination of solid phase epitaxy (SPE) and subsequent annealing for 30 min was used, a-axis-oriented BaSi2 together with 111-oriented Si appeared in the θ–2θ x-ray diffraction (XRD) patterns. The fact that no crystalline Si was detected by the surface-sensitive Raman spectroscopy and that the full width at half maximum of the XRD BaSi2 600 peak was increased to 2.686° in such samples suggests that Si aggregates were at the BaSi2/Ge interface. The photoresponsivity of the epitaxial BaSi2 film with MBE-grown templates reached 0.14 A W−1 at a wavelength of 790 nm. This is the highest photoresponsivity reported so far for BaSi2 films on Ge substrates. Even for samples without the MBE-grown templates, SPE-grown templates significantly increased the photoresponsivity up to 0.10 A W−1. These results confirm that the template has a significant impact on the photoresponsivity of BaSi2 films on Ge(111) substrates.