Epitaxial ZnGeP₂ Thin Films on Si and GaP by Reactive Combinatorial Sputtering in Phosphine

Modern optoelectronic devices are constrained to a fixed collection of band gap and lattice parameter combinations by the limited number of semiconductors that can be epitaxially integrated with high crystal quality. II-IV-V2 compounds are promising materials to break this paradigm as changes to the cation lattice site disorder can modify the band gap without a substantial change to the lattice parameter. ZnGeP2 is a particularly interesting member of this group as it is lattice-matched to Si and GaP, but substantial work is needed to understand and improve the epitaxial growth of ZnGeP2. In this paper, we report on the growth of epitaxial ZnGeP2 on Si and GaP substrates via reactive combinatorial sputtering in phosphine gas. Reciprocal space maps revealed that films on both GaP and Si have high crystalline quality, matching that of the substrate. The out-of-plane lattice parameter was found to increase with increasing Ge content, displaying an alloy-like behavior. Films deposited on Si displayed a much larger range for the (004) peak full width at half maximum (FWHM) than those deposited on GaP. Due to the growth of a lower-symmetry material, ZnGeP2, on a higher-symmetry substrate, Si, it is likely that the films grown on Si have antiphase domains and larger threading dislocation densities than those on GaP. Electron channeling contrast imaging revealed the films on GaP to be largely dislocation-free. In the films deposited on Si, the optical absorption onset energies trended toward lower energies with larger (004) FWHM values. These results suggest that the defects in the films on Si that result in a broadened (004) FWHM cause sub-band gap absorption. This work provides the first combinatorial study of epitaxial ZnGeP2 on Si and GaP and demonstrates the strong potential for the growth of high-quality epitaxial ZnGeP2 with future work optimizing synthesis conditions and substrate preparation.