Violation of a variant selection rule in atomic ordering observed in Ga0.5In0.5P with Sb added during growth

A new relation between the direction of the substrate surface normal and th at of triple-period-A (TP-A) ordering was observed in Ga0.5In0.5P grown on a vicinal (001) CaAs substrate misoriented towards the [-1-11]A direction. The crystal was grown with a very small amount of Sb added during metalorga nic vapor phase epitaxy. To our knowledge, this relation of surface-step vs variant is, irrespective of the types of ordering and the kinds of alloys, the first case in which the ordering variant selection rule thus far obser ved is being violated. This indicates a new phase relation at each step bet ween unit cells of a x3 surface reconstruction on the upper terrace and tho se on the adjacent lower terrace, with respect to the (111) direction neare st to the surface normal. The cause of this new step-variant relation is su ggested.