Nucleation and surface morphology evolution of ferroelectric SrBi₂Ta₂O₉ films studied by atomic force microscopy

Nucleation stages and surface morphology evolution of epitaxial SrBi₂Ta₂O₉ thin films on SrTiO₃ (001) substrates have been studied by ultra-high vacuum atomic force microscopy. The SrBi₂Ta₂O₉ nucleates and grows half-unit-cell by half-unit-cell as well-defined 2D islands at low coverage. After 2D growth stage, which proceeds up to several half unit-cell layers, a transition from 2D to 3D island growth is observed. After coalescence of small 3D islands into larger islands, clear columnar structures, consisting of spirals, emerge. The surfaces of films with their thickness above 1500 A exhibit round hills consisting of curved terraces with sizes of steps ranging from 6 Å to 12.5 Å.