A particular epitaxial Si1-yCy alloy growth mode on Si(001) evidenced by cross-sectional transmission electron microscopy

Si1-yCy alloys have been grown on Si(001) by Si molecular beam epitaxy (MBE) under a low pressure of C2H4 at 600 degrees C. In spite of relatively high carbon concentrations, estimated by X-ray photoelectron spectroscopy and secondary ion mass spectrometry (SIMS), a good epitaxial relationship with the substrate is obtained as proved by cross-sectional transmission electron diffraction observations (XTED), reconstructed 2x1 low-energy electron diffraction (LEED) diagrams and Si2p X-ray photoelectron diffraction (XPD) modulated polar patterns for the alloys with the cubic symmetry of the Si substrate. Nevertheless the simultaneous absences of relevant Cls XPD features, significant strain induced X-ray diffraction (XRD) shifts and clear C-related local infrared (IR) or Raman modes, give rise to the question of the incorporation mode of C, different here from the often invoked substitutional mode in other Si1-yCy alloys. Inspection by cross-sectional transmission electron microscopy (XTEM) of these samples with the lowest C-content shows regularly spaced contrast variations (striations), roughly parallel to the Si/alloy interface with a periodicity of about 9 nm. High resolution micrographs reveal between the contrasted strips a nearly perfect and unstrained Si lattice, free of dislocations or SiC nanocrystal inclusions. With increasing C-concentrations, extended defects such as twins or stacking faults appear in the layer, generating roughness at the surface and the striations finally disappear. The striations may tentatively be accounted for pseudomorphic growth of new C-rich phases, theoretically suggested by Rucker et al. [H. Rucker, M. Methfessel, E. Bugiel and H.J. Osten, Phys. Rev. Lett. 72 (1994) 3578], matching the C-induced strain and allowing subsequent strain free Si-growth.