Growth of platinum ultrathin films on Al2O3(0001)

The early stages of Pt growth on Al2O3(0001) are investigated by means of electron microscopy and X-ray diffraction. We deposit Pt ultrathin films of thicknesses ranging from 0.5 nm to 10 nm using DC sputtering deposition at 650 °C and 750 °C. We demonstrate that the growth is of Volmer-Weber type and show that epitaxy of islands could be reached at elevated temperature. The island morphology is governed by surface energy minimization, leading to well defined islands whose size depends on the nominal thickness. At 750 °C, the appearance of two island populations reveals that although the two studied growth temperatures are quite close, the growth kinetic is static at 650 °C and dynamic at 750 °C. Due to the lattice misfit between Pt and Al2O3, X-ray and electron diffractions reveal that the islands undergo an in-plane compression accompanied with an out-of-plane tension and such strains relax with thickness to reach the bulk lattice parameter above 10 nm when the Pt film becomes continuous. Electron microscopy in high resolution mode allows measuring the strain in isolated islands and corroborates X-ray diffraction measurements.