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.