Sb-mediated Ge growth on singular and vicinal Si(001) surfaces: A surface optical characterization study

In this work, we apply Raman spectroscopy (RS) and spectroscopic ellipsometry (SE) to surfactant mediated growth (SMG) of Ge on Si(001) surfaces. Under molecular-beam epitaxy conditions, a growth temperature of $400°\mathrm{C}$ and in the absence of a predeposited surfactant (Sb) monolayer, we show that SE can be used to determine the Stranski-Krastanov critical thickness for island formation. In the presence of a predeposited Sb monolayer, SE predicts that layer-by-layer growth is possible up to a Ge coverage of 20 monolayers (MLs). However, no evidence of relaxation through dislocation formation, known to exist in this coverage regime, could be detected. With RS, we present one of the first studies of Sb mediated growth on a vicinal Si(001) substrate. Two Sb surface related phonon peaks are identified, one associated with Sb dimers bonded to the Si(001) substrate $\left(130.5\mathrm{}\hspace{0.5em}{\mathrm{cm}}^{-1}\right),$ the other with Sb dimers on the grown Ge epilayer $\left(141\mathrm{}\hspace{0.5em}{\mathrm{cm}}^{-1}\right).\mathrm{}$ The former disappears and the latter appears gradually with increased Ge coverage up to 3.5 MLs. This indicates that surfactant exchange occurs differently on vicinal Si(001) than on singular Si(001), where exchange has been shown to be complete by deposition of between 0.5–1.0 monolayer (ML). To explain this difference, step bunching must occur during the initial stages of growth on vicinal Si (001), leading to areas of preferred growth on the surface. At a higher Ge coverage of 20 MLs, theoretical predictions of the Raman shift for a strained pseudomorphically grown Ge layer show good agreement with the Raman data presented. This demonstrates that even at 20 ML coverage, some areas of unrelaxed Ge must still exist, pointing to inhomogeneities within the Ge epilayer, as predicted by our model for SMG on a step bunched Si(001) substrate.