GROWTH-MORPHOLOGY OF TETRAGONAL ABCO(4) COMPOUNDS - THEORY AND OBSERVATIONS ON CZOCHRALSKI-GROWN CRYSTALS

Tetragonal ABCO(4) compounds, where A = Sr, Ca, B = rare earth element s and C = Ga or Al, are potential substrate candidates for high-freque ncy superconducting films. The Hartman-Perdok theory (HPT) explains th e relation between crystal structure and morphology and provides the a tomic surface topology of the crystalline interface. Theoretical growt h forms can be constructed from computed attachment energies, E(a)(hkl ) which is assumed to be directly proportional to the growth rate for F faces. HPT has been applied to CaYAlO4 as a model for all ABCO(4) co mpounds with a K2NiF4 crystal structure. F forms are {002}, {101}, {10 3}, {110}, {112}. {200}, {211} and {213}. The theoretical growth form is planar following {001} with {101} and {110} as lateral forms. At lo wer effective charges on oxygen, q(O), the growth forms are still tabu lar, but the order of importance of lateral forms changes as function of q(O). When the ions on the slice boundaries are ordered, {110} will be absent for the model with the formal charges and replaced by {112} in the case of models with less negative oxygen charges. As-grown cry stals show often inhomogeneities in color parallel to the {110} interf ace. This can be explained by the surface topology of {110}.