Impurity segregation and ordering in $\text{Si}/{\text{SiO}}_2/{\text{HfO}}_2$ structures

We use first-principles calculations and experimental data to demonstrate that impurity segregation at heterointerfaces is governed by several factors. In particular, Hf impurities avoid the ${\text{Si-SiO}}_2$ interface when present in the ${\text{SiO}}_2$ side, might segregate if present in the Si side, but do not cross into ${\text{SiO}}_2$. Substitutional Hf impurities in ${\text{SiO}}_2$, as revealed by a through-focal series of $Z$-contrast images, act as markers for Si sites, suggesting ordering in the first two Si planes of the amorphous ${\text{SiO}}_2$. Finally, we show that dopants in Si segregate at the interface by adopting several distinct configurations and also do not cross into ${\text{SiO}}_2$.

Figure 1

(Color online) $Z$-contrast STEM image of the $\text{Si}/{\text{SiO}}_2/{\text{HfO}}_2$ structure. Stray Hf atoms inside the ${\text{SiO}}_2$ interlayer are encircled. The vertical lines represent the location of the ${\text{Si-SiO}}_2$ interface plane.

Figure 2

Minimum energy configurations for Hf atoms (large gray spheres) in ${\text{SiO}}_2$ close to the ${\text{Si-SiO}}_2$ interface: (a) substitutional and [(b) and (c)] interstitial defects. Si (O) atoms are depicted as larger (smaller and dark) spheres. The arrows mark undercoordinated Si atoms. Only parts of the actual unit cells are displayed [here and in Figs. 3, 4, 5, 6].

Figure 3

Formation energies of substitutional impurities and dopants in ${\text{Si-SiO}}_2$.

Figure 4

Hf-Hf interstitial pair in Si.

Figure 5

(Color online) Histograms representing the number of Si atoms as a function of distance to the ${\text{Si-SiO}}_2$ interface plane as extracted from the experimental data set and determined for the theoretically derived structures.

Figure 6

Pair configurations of dopants at the subinterface region: (a) nearest-neighbor and (b) second-neighbor dopant pairs. The arrows denote displaced atoms.