3D Composition of Epitaxial Nanocrystals by Anomalous X-Ray Diffraction: Observation of a Si-Rich Core in Ge Domes on Si(100)

Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at $600°\mathrm{C}$ were obtained from grazing incidence anomalous x-ray scattering data at the Ge $K$ edge. The data were analyzed in terms of a stack of layers with laterally varying concentration. The results demonstrated that the domes contained a Si-rich core covered by a Ge-rich shell and were independently supported by selective etch experiments. The composition profile resulted from substrate Si alloying into the Ge during growth to partially relax the stress in and under the domes.

Figure 1

Diffuse scattering maps of (400) surface reflection for Ge domes using two different x-ray photon energies (a) 11 103 eV (Ge $K$ edge) and (b) 11 005 eV. The intensity color scale is logarithmic. (c) and (d) are angular scans at constant $q_r$ corresponding to the dashed line in (a) and (b). Several fits corresponding to different lateral composition profiles are shown for the scans carried out at 11 103 eV (c) and 11 005 eV (d). The corresponding Ge concentration profiles used in these fits are plotted at the inset.

Figure 2

Angular scans performed on Ge domes. In (a) and (b) the fits were done to extract the Ge composition profile suited to each equal lattice parameter region. Some selected profiles are shown in (c).

Figure 3

(a) Real space chemical composition map for Ge domes; (b) AFM line scans taken on two statistically average Ge domes, before and after 31% ${\mathrm{H}}_2{\mathrm{O}}_2$ selective etch, evidencing the Si-rich core.