Analysis of strain and intermixing in single-layer $\mathrm{Ge}∕\mathrm{Si}$ quantum dots using polarized Raman spectroscopy

The built-in strain and composition of as-grown and Si-capped single layers of $\mathrm{Ge}∕\mathrm{Si}$ dots grown at various temperatures $(460–800°\mathrm{C})$ are studied by a comparative analysis of the Ge-Ge and Si-Ge modes in the polarized Raman spectra of the dots. A pronounced reduction of the strain and Ge content in the dots after deposition of the cap layer at low temperatures is observed, indicating that strain-induced Si diffusion from the cap layer is occurring. For large dots grown at $700–800°\mathrm{C}$ the observations are in agreement with a model of the $\mathrm{Ge}∕\mathrm{Si}$ dot consisting of a Si-rich boundary region and a Ge-rich core.

Figure 1

AFM image of $\mathrm{Ge}∕\mathrm{Si}\left(001\right)$ dots grown at different temperatures: (a) $525°\mathrm{C}$, (b) $600°\mathrm{C}$, and (c) $800°\mathrm{C}$; scale bars of 0.25, 0.5, and $1.0\mu \mathrm{m}$, respectively, are shown. (d) Schematic of electric vector orientation in incident and scattered channels during polarized Raman measurements.

Figure 2

(a) Raman spectra collected in $VH$ polarization of Si substrate (thin line) and samples grown at $460°\mathrm{C}$ (1, without Si cap layer, and 2, with $20\text{−}\mathrm{nm}$ Si cap layer), and (3) at $550°\mathrm{C}$ with $20\text{−}\mathrm{nm}$ Si cap layer. (b) Differential Raman spectra for single-layer $\mathrm{Ge}∕\mathrm{Si}$ dots grown at different temperatures with (thick line) and without (thin line) a Si cap layer. The dependence of the peak position of (c) Ge-Ge and (d) Si-Ge phonon modes vs growth temperature for as-grown (◻) and coated with Si cap layer (∎) $\mathrm{Ge}∕\mathrm{Si}$ dots.

Figure 3

Dependence of (a) Ge content $x$ and (b) biaxial strain $\epsilon$ on growth temperature for $\mathrm{Ge}∕\mathrm{Si}$ dots.