Influence of nanostructure formation on the crystal structure and morphology of epitaxially grown Gd ₂ O ₃ on Si(001)

The influence of growth conditions on the layer orientation, domain structure and crystal structure of gadolinium oxide (Gd ₂ O ₃ ) on silicon (001) has been investigated. Gd ₂ O ₃ was grown at low (250°C) and high (850°C) temperatures with different oxygen partial pressure as well as a temperature ramp up during growth. At low temperature, the cubic bixbyite type of crystal structure with space group was grown at low oxygen partial pressure. The layers consist of two domains oriented orthogonal to each other. The epitaxial relationships for the two domains were found to be Gd ₂ O ₃ (110)[]||Si(001)[110] and Gd ₂ O ₃ (110)[001]||Si(001)[], respectively. Applying additional oxygen during growth results in a change in crystal and domain structures of the grown layer into the monoclinic Sm ₂ O ₃ -type of structure with space group C2/m with () orientation and mainly two orthogonal domains with the epitaxial relationship Gd ₂ O ₃ ()[010]||Si(100)⟨110⟩ and a smooth surface morphology. Some smaller areas have two intermediate azimuthal orientations between these variants, which results in a six-domain structure. The change in crystal structure can be understood based on the Gibbs–Thomson effect caused by the initial nucleation of nanometre-sized islands and its variation in diameter with a change in growth conditions. The crystal structure remains stable even against a temperature ramp up during growth. The layers grown at high temperature exhibit a nanowire-like surface morphology, where the nanowires have a cubic crystal structure and are aligned orthogonal to each other along the ⟨110⟩ in-plane directions. An increase in oxygen supply results in a reduced length and increased number of nanowires due to lower adatom mobility. The results clearly indicate that both kinetic and thermodynamic factors have a strong impact on the crystal structure, epitaxial relationship and morphology of the grown layers.