A new approach to the diagnostics of nanoislands in Ge$_x$Si$_{1-x}$/Si heterostructures by secondary ion mass spectrometry

A new approach to the diagnostics of Ge$_x$Si$_{1-x}$/Si heterostructures with self-assembled nanoislands, which is based on the method of secondary ion mass spectrometry (SIMS) using secondary Ge$_2$ cluster ions, is discussed. Calibration dependences of the yield of atomic (Ge) and cluster (Ge$_2$) secondary ions on the concentration of germanium in homogeneous Ge$_x$Si$_{1-x}$ have been obtained for a TOF.SIMS-5 setup. It is established that, in contrast to the well-known linear dependence of $^{74}$Ge/$^{30}$Si $\propto x/(1-x)$, the secondary Ge$_2$ cluster ions obey the quadratic relation Ge$_2$/$^{30}$Si $\propto (x/(1-x))^2$. It is shown that the proposed SIMS depth profiling using nonlinear calibration relations for Ge$_2$ cluster ions provides expanded information on multilayer Ge$_x$Si$_{1-x}$/Si heterostructures with nanoislands. By using this approach, without additional a priori data on the sample structure, it is possible to distinguish planar layers and GeSi layers with three-dimensional nanoislands, estimate the height of islands, reveal the presence of a wetting layer, and trace the evolution of islands during their formation in a multilayer structure.