Formation of alkaline-earth template layers on Ge(100) for oxide heteroepitaxy: Self-organization of ordered islands and trenches
The first step required for oxide heteroepitaxy with atomically abrupt interfaces on Si and Ge(100) is the formation of an alkaline-earth template layer. The atomic structure of this template layer on Ge(100) was characterized using scanning tunneling microscopy and electron diffraction. At elevated temperatures, Sr immediately roughens the surface; a transition that can be associated with Sr displacing Ge from the surface. With increasing Sr coverage a series of ordered (3 × 4), (3 × 2), (9 × 1), and (6 × 1) phases were observed. Transitions between these phases were accompanied by morphological changes: formation of the (3 × 4) phase smoothed surface; transition to a local (3 × 2) ordering was accompanied by trench formation; ordering of the trenches led to the (9 × 1) structure; and finally, the (6 × 1) structure was characterized by atomic rows. For both Sr and Ba, highly ordered arrays of one-dimensional islands could be produced with double-height steps preventing orthogonal domain formation. We associate the morphological transitions with strain relief of the surface phases and interactions of step ledges.