Short-range order structures of self-assembled Ge quantum dots probed by multiple-scattering extended x-ray absorption fine structure

Multiple-scattering extended x-ray absorption fine structure (MS-EXAFS) has been used to investigate the local structures around Ge atoms in self-assembled $\mathrm{Ge}–\mathrm{Si}$ quantum dots (QDs) grown on Si(001) substrate. The MS effect of Ge QDs is dominated by the scattering path ${\mathrm{Ge}}_0\to {\mathrm{B}}_1\to {\mathrm{B}}_2\to {\mathrm{Ge}}_0\left(\mathrm{DS}2\right)$, which contributes a signal destructively interfering with that of the second shell single-scattering path (SS2). MS-EXAFS analysis reveals that the degree of $\mathrm{Ge}–\mathrm{Si}$ intermixing for $\mathrm{Ge}–\mathrm{Si}$ QDs strongly depends on the temperature at which the silicon cap layer is overgrown. It is found that the interatomic distances ($R_{\mathrm{Ge}–\mathrm{Ge}}$ and $R_{\mathrm{Ge}–\mathrm{Si}}$) within the third nearest-neighbor shells in $\mathrm{Ge}–\mathrm{Si}$ QDs indicate the compressively strained nature of QDs. The present study demonstrates that the MS-EXAFS provides detailed information on the QDs strain and the $\mathrm{Ge}–\mathrm{Si}$ mixing beyond the nearest neighbors.

Figure 1

The diamond structure of crystalline Ge with the representative Ge atom in the first three shells. The number 0 refers to the central absorber Ge atom. Numbers 1 to 3 identify corresponding coordination shells atoms and number $1^{\prime }$ denotes another first coordination shell atom different from atom 1. Two different DS paths DS1 and DS2 are also described.

Figure 2

Contributions of the individual scattering paths to the total EXAFS oscillation function for c-Ge: (a) in $R$-space, and (b) in $k$-space. The dark dashed line in Fig. 2a shows the fitting result by using all 10 scattering-paths, and empty circles represent the fitting results by using only four paths (SS $\text{paths}+\mathrm{DS}2$).

Figure 3

Ge $K$-edge EXAFS $\chi \left(k\right)$ oscillation functions for the Ge QDs samples, c-Ge, and ${\mathrm{Ge}}_{0.006}{\mathrm{Si}}_{0.994}$ dilute solution.

Figure 4

The radial structural function by Fourier transforming $k^2$-weighted $\chi \left(k\right)$ functions: experiment (solid) and multiple-scattering fit (open circle).

Figure 5

Schematic cross-sectional geometry of the Ge islands and the Ge wetting layer grown on Si(001).