Physical properties of (GaAs)(1-x)(Ge-2)(x): Influence of growth direction- art. no. 115328

(GaAs)(1-x)(Ge-2)(x) metastable alloys were epitaxially grown on (001), (11 1), (112), and (113) GaAs by rf magnetron sputtering. A different long-rang e order parameter behavior with Ge concentration is observed for each growt h direction. This provides a direct evidence that growth direction affects the long-range order-disorder transition exhibited by these alloys. The epi taxial growth of these alloys was modeled by a Monte Carlo simulation. The good agreement between the experimental and modeled long-range order parame ter evidences that atomic ordering in these alloys is ruled mainly by growt h direction and the avoidance of the formation of "wrong" atomic pairs of A s-As and Ga-Ga, and not by thermodynamic factors. On the other hand, measur ements of the optical gap and Raman scattering, show that the optical prope rties are governed by near-neighbor correlations and therefore by their sho rt-range order. Hence, the substrate orientation and the long-range order h ave negligible effect on the optical properties. Fitting the experimental d ata of the optical gap, we obtained linear expressions that show the fundam ental gap behavior with Ge concentration of some of these alloys at room te mperature. For 0<x<0-3 and 0.3<x<1 the empirical expressions are E-o(x) = 1 .43 - 2.99x eV and E-o(x) = 0.45 + 0.35x eV, respectively.