Kinetics of Ge growth at low temperature on Si(001) by ultrahigh vacuum chemical vapor deposition

The growth of germanium at low temperature by ultrahigh vacuum chemical vapor deposition on Si(001) is investigated in real time by reflection high-energy electron diffraction. These observations are complementarily checked by atomic force microscopy, Rutherford backscattering spectrometry, transmission electron microscopy, and x-ray diffraction experiments. It can be seen that the currently observed Stranski-Krastanov-related two-dimensional (2D) to three-dimensional transition is avoided at 330 deg. C and that the major part of the relaxation process occurs during the deposition of the first two monolayers. Then, the measured in-plane lattice parameter evolves slowly and approaches that of bulk Ge after deposition of 50 monolayers. The corresponding relaxation equals 83%. The resulting surface is flat, with a rms roughness of 0.55 nm. The relaxation is found to be mainly due to misfits dislocations located at the Ge/Si interface. Regrowth experiments at 600 deg. C show that the low-temperature films are not stable for thicknesses below 27 nm. In spite of the nearly complete relaxation observed for 7.5 nm, a much higher thickness is needed to enable a continuous 2D growth at 600 deg. C. Finally, a 500-nm-thick film, obtained with a low-temperature Ge buffer and with a Ge regrowth at high temperature, exhibits a channeling-to-random Rutherford backscattering spectrometry ratio (χmin) of 4%, which indicates a good crystalline quality