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Influence of growth direction on order-disorder transition in (GaAs)1-x(Si2)x and (GaAs)1-x(Ge)2x: alloys.
Hugo Navarro-Contreras, A.G. Rodríguez, and M.A. Vidal,
Instituto de Investigación en Comunicación Optica, Universidad Autónoma de San Luis Potosí,
Alvaro Obregón 64, San Luis Potosí, S.L.P., México 78000.

(GaAs)1-x(Ge)2x metastable alloys were epitaxially grown on (001), (111), (112) and (113) GaAs by rf magnetron sputtering. Additionally, (GaAs)1-x(Si2)x metastable alloys were epitaxially grown on (001), (111), (110) and (112) GaAs. Single crystal alloys were obtained for Si concentrations in the range  0 =< x=< 0.43. At higher concentrations the Si segregated. A different long range order parameter behavior with Ge and Si concentration is observed for each growth direction. This  provides direct evidence that growth direction affects the long range order disorder transition exhibited by these alloys.  The epitaxial growth of these alloys was modeled by a Monte Carlo simulation. The good agreement between the experimental and modeled long range order parameter evidences that atomic ordering in these alloys is ruled mainly by growth direction and not by thermodynamic factors. On the other hand, measurements of the optical gap and Raman scattering in Ge, show that the optical properties are governed by near neighbor correlations and therefore by their short range order. Hence, the substrate orientation and the long range order have negligible effect on the optical properties.

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