Hanjong Paik, University of Oklahoma, Norman
We focus on exploring the intricate phase diagram of the Sr-Ge-O thin film system using molecular-beam epitaxy. Under ultra-high vacuum (UHV) conditions, we employed a suboxide GeO-flux and elemental Sr-flux to successfully grow single-crystalline complex strontium germanates on LaAlO₃ (001) substrates. The growth of these films in a single phase is highly dependent on the ratio of Sr-flux/GeO-flux and substrate temperature. Through in-situ RHEED observation, we observed a transition in growth mode from an initial complicated 3D nucleation-dominant growth to a smoother 2D layer growth, which was consistent with Atomic Force Microscopy (AFM) observations. XRD measurements confirmed the presence of single-phase Sr₂GeO₄-like thin films with an out-of-plane d-space of 4.05 Å, indicating a strain-relaxed growth mechanism as evidenced by wider ω-rocking curves. Large area reciprocal space mapping revealed a unique domain structure. STEM imaging further demonstrated the epitaxial relationship between the thin films and LaAlO₃ (001) substrate, exhibiting a relatively sharp interface structure. This complex germanates system holds promise as a potential candidate for high-mobility oxide semiconductors in power electronics applications. We are currently conducting further studies involving optical and electrical doping to expand our understanding of this system's properties.