Exploring the growth of planar hexagonal germanium on commercially available wurtzite substrates

Although silicon (Si) and germanium (Ge) dominate the semiconductor industry, their potential in next-generation optoelectronic devices remains limited due to their indirect bandgap. While direct bandgap emission has been achieved from SiGe alloys with the hexagonal (hex-) crystal structure, it has not yet been realized in a configuration suitable for scalable chip fabrication. Here, we take the first step toward practical implementation by realizing planar hex-Ge using molecular beam epitaxy on wurtzite cadmium sulfide (CdS) substrates. We observe that hexagonal phase purity of the film is strongly dependent on growth temperature with an optimum between 250 and 275°C for (1−100) oriented substrates and approximately 225°C for (11−20) substrates. Initially, higher temperatures enhance the phase purity; however, beyond 250°C for the (1−100) oriented substrate and 225°C for the (11−20) oriented substrate, the Ge/CdS interface starts to thermochemically degrade. We show that a zinc sulfide buffer layer enhances the interface stability, thereby broadening the growth window. By using a planar configuration, this study provides a pathway toward planar hex-Ge for scalable optoelectronic and quantum technologies.