Commensurate and Incommensurate Phases at Reconstructed (In,Ga)As(001) Surfaces: X-Ray Diffraction Evidence for a Composition Lock-in

Surface chemical ordering of group III elements in the ternary alloy ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$, stabilizing the $2\times 3$ reconstruction at the surface composition I${\mathrm{n}}_{0.67}$G${\mathrm{a}}_{0.33}$As, is demonstrated on the basis of x-ray diffraction data. An incommensurate $2\times n$ reconstruction is observed for lower In surface concentrations, achieved by burying an InAs monolayer under a number of GaAs layers and letting the In surface segregation process operate. A quantitative account of the intensity measured in the incommensurate phase is obtained by using a probabilistic distribution of Ga- and In-rich structural elements.