Au-mediated low-temperature solid phase epitaxial growth of a Si_xGe_1-x alloy on Si(001)

The evolution of microstructure during Au-mediated solid phase epitaxial growth of a Si_xGe_1-x alloy film on Si(001) was investigated by in situ sheet resistance measurements, x-ray diffraction, conventional and high-resolution transmission electron microscopy, energy dispersive x-ray spectroscopy, and Rutherford backscattering spectrometry. Annealing amorphous-Ge/Au bilayers on Si(001) to temperatures below 120 °C caused changes primarily in the microstructure of the Au film. Near ≈130 °C, Ge from the top layer diffused and crystallized along the grain boundaries of Au. The Ge that had reached the Au/Si (001) interface mixed with Si from the substrate, to form epitaxial Si_xGe_1-x islands on Si (001). Si from the substrate had dissolved into Au before entering the growing epitaxial islands. Meanwhile, the Au that was displaced by Ge that filled the Au grain boundaries, diffused into the top layer along columnar voids in the amorphous Ge film. With increasing temperature, more Au was displaced to the top by the flux of Ge towards the substrate, facilitating further epitaxial growth and the coalescence of epitaxial Si_xGe_1-x islands. At 310 °C, the initial Au film was displaced completely to the top by a laterally continuous Si_xGe_1-x epilayer of uniform composition (x≈0.15). The epilayer thickness was limited by that of the initial Au film. Twins and residual amounts of Au trapped near the Si_xGe_1-x/Si(001) interface were the predominant defects observed in the completely strain-relaxed Si_xGe_1-x epilayer.