Speaker
Description
SrTiO₃ (STO), a functional perovskite oxide, is dielectric at room temperature but can become ferroelectric under strain or doping. Here, STO thin films are grown epitaxially on MgO substrates with vicinal surfaces flat terraces, with RMS roughness on the order of tens of picometers. The lattice mismatch between MgO (~4.2 Å) and STO (~3.9 Å) results in a tensile strain of approximately 7.8%, which is expected to induce strain-driven modifications of the material properties, potentially including ferroelectricity. STO films with different thicknesses were deposited, and the influence of RF power and deposition pressure was systematically investigated. AFM topography shows STO films replicate the vicinal MgO substrate structure, confirming epitaxial growth. XRD measurements detect the main peak of STO which is shifted due to the strain. The calculated d-spacing indicates an increase in the out-of-plane lattice parameter, which is unexpected for a purely tensile strain. Furthermore, survey XPS spectra confirm the correct stoichiometry of the STO films. Film thickness was determined by XRR and TEM imaging. Films deposited at low RF power (10 W) and high pressure (40 mTorr) resulted in ultra-thin (~6 nm) layers with exceptionally smooth surfaces. Higher power and pressure led to thicker films, together with increased surface roughness. At the most extreme deposition conditions corresponding to 20 W of deposition power, features that can be associated to deposition and resputtering by negatively charged oxygen ions. At intermediate conditions (15 W, 20–40 mTorr), small surface holes appear. These features are likely initiated in two steps: 1) by re-sputtering of atoms from sites with the lowest binding energy, such as step edges; 2) surface relaxation that occurs in these heavily strained regions. The results indicate that strain plays a significant role in the evolution of surface morphology. We highlight how fine-tuning deposition parameters can control film thickness, surface roughness, and strain-driven surface features.
| Keyword-1 | Epitaxial thin films |
|---|---|
| Keyword-2 | Strain engineering |
| Keyword-3 | Re-sputtering |