Metastable surface ordering in strain relaxed Si0.5Ge0.5 epitaxial layers grown at high temperature

We have studied compositional ordering in the near surface region of 3500 A ngstrom thick unstrained Si0.5Ge0.5(001) samples grown by chemical vapor de position. Measuring asymptotic Bragg scattering along integer and half-inte ger truncation rods, we found a type of metastable ordering at this surface which is characterized by integer/half-integer reflections along the integ er order truncation rods. We show unambiguously that those scattering featu res originate from a thin layer at the surface. Annealing at 750 degrees C extinguished these reflections irreversibly, while the reflections of the R S3 bulk structure were not affected. Anomalous scattering at the Ge K edge also confirmed the existence of a new structure in the near surface region. (C) 1999 American Institute of Physics. [S0003-6951(99)04804-4].