Reduction of threading dislocation density beyond the saturation limit by optimized reverse grading

The threading dislocation density (TDD) in plastically relaxed Ge/Si(001) heteroepitaxial films is commonly observed to decrease progressively with their thickness due to mutual annihilation. However, there exists a saturation limit, known as the geometrical limit, beyond which a further decrease of the TDD in the Ge film is hindered. Here, we show that such a limit can be overcome in SiGe/Ge/Si heterostructures thanks to the beneficial role of the second interface. Indeed, we show that ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/\mathrm{Ge}/\mathrm{Si}\left(001\right)$ films display a TDD remarkably lower than the saturation limit of Ge/Si(001). Such a result is interpreted with the help of dislocation dynamics simulations. The reduction of TDD is attributed to the enhanced mobility acquired by preexisting threading dislocations after bending at the new interface to release the strain in the upper layer. Importantly, we demonstrate that the low TDD achieved in ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/\mathrm{Ge}/\mathrm{Si}$ layers is preserved also when a second, relaxed Ge layer is subsequently deposited. This makes the present reverse-grading technique of direct interest also for achieving a low TDD in pure-Ge films.

Figure 1

TDD as a function of Ge thickness in Ge/Si(001) samples. The inset shows an exemplary (10 × 10) μm² AFM image of the sample with 2.3 μm of Ge. Next to the AFM image, the corresponding vertical scale bar is reported. The rms roughness value obtained from the image is 0.44 nm.

Figure 2

(a) SEM image of the 1.2 μm ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/1.2\mu \mathrm{m}\mathrm{Ge}/\mathrm{Si}\left(001\right)$ sample after 15 min Secco etch: etch pits are clearly visible. (b) (10 × 10) μm² AFM image of the same sample as in (a). The vertical scale bar is displayed below the image on which a rms roughness value of 0.41 nm is measured. (c) XRD RSM of the asymmetric ($\overline{2}\overline{2}4$) reflections of SiGe and Ge measured on the same sample. The $Q_z$-axis is parallel to the [001] direction, while the $Q_x$-axis is oriented along the $\left[\overline{1}\overline{1}0\right]$ direction. (d) TDD (blue dots) and relaxation degree $R$ (green squares) as a function of the thickness of ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}$ films grown on 1.2 μm Ge/Si(001). (e) TDD and $R$ values obtained for different postannealing processes on the 1.2 μm ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/1.2\mu \mathrm{m}\mathrm{Ge}/\mathrm{Si}\left(001\right)$ sample.

Figure 3

TDD as a function of the heterostructure thickness $t_{\mathrm{tot}}$ (obtained by varying only the Ge layer thickness) for Ge/Si(001) samples (semitransparent red squares) and for 1.2 μm ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/\mathrm{Ge}/\mathrm{Si}\left(001\right)$ samples (blue dots). The green diamond marker corresponds to a double Ge/SiGe interface featuring the following module structure: 1.2 μm Ge/1.2 μm ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/\mathrm{Ge}/\mathrm{Si}\left(001\right)$ (see the text).

Figure 4

Simulated images obtained from dislocation dynamics simulations depicting TD motion over time in heterostructures consisting of 1.2-μm-thick (a) Ge and (b) ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}$ on top of 4.5-μm-thick Ge/Si(001) VS. (c) Behavior of TDD as a function of the simulation time for the heterostructures in (a) (red curve) and (b) (blue curve).

Figure 5

STEM images of samples featuring a 1.2-μm-thick Ge/Si(001) stack on top of which a 0.5-μm-thick layer of (a) Ge and (b) ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}$ is deposited. In panels (c) and (d), STEM images of the ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/\mathrm{Ge}$ interface are shown, evidencing (c) a TD arm along the heterointerface (marked by the red arrow) which “bends” into the Ge buffer layer, and (d) the effect of a TD arm meeting a MD at the heterointerface. Simulated TEM images based on DD simulations corresponding to a 0.5-μm-thick (e) ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}$ and (f) Ge layer on top of 1.2-μm-thick Ge/Si(001).