Epitaxial relations in Ca_xSr_1-xF₂ films grown on GaAs {111} and Ge(111) substrates

Epitaxial relations in Ca_xSr_1-xF₂ (0≤x≤1) films grown on GaAs(111), (1̄1̄1̄), and Ge(111) substrates were investigated by He⁺ ion channeling analysis and transmission electron microscopy. Though the lattice constants of GaAs and Ge are nearly the same, the relations were found to be completely different. That is, the lattice-matched fluoride film on GaAs exhibited an orientation identical to that of the substrate (type A orientation), whereas the orientation of the film on Ge was rotated by 180°about the surface normal 〈111〉 axis (type B orientation). It was also found that the type A growth of the Ca _x Sr_1-x F₂ films is maintained in the whole range of x on the As face of GaAs, while the growth types are partially or totally inverted in the lattice-mismatched fluoride films grown on the Ga face of GaAs and on Ge.