*3.4. Compliance Substrates: Inverted Silicon Pyramids (ISP)*

One of the most interesting attempts to grow a high-quality 3C-SiC epilayer on a silicon substrate was done by creating a structured substrate. The structure came from the following consideration: the SFs lie on {111} planes and can interact with each other, stopping the propagation. Consider two SFs laying, for example, in the (111) and (11-1) planes; they can cross and the structure is able to stop the propagation of one or even both SFs. This clearly improves the crystalline quality of the film surface because the SFs remain buried in the epilayer. The rate of SF annihilation is inversely related to SF density, however, by means of the inverted silicon pyramid (ISP) compliant substrate, allowing for a significant drop in SF concentration just within a few microns.

Its unique shape can concentrate SFs in tiny areas, enhancing the phenomenon of SF annihilation [48].

In Figure 8a, we show a schematic cross-section view of the effect of this compliant substrate. Silicon and silicon carbide are drawn as black and white regions. Blue lines are SFs which either generate an X-shaped defect known as the forest dislocation or selfannihilate, resulting in a system known as the Lomer lock, or end on an existing SF, producing a so-called "λ-shaped" defect. In Figure 8b,c, SEM images of the ISP structure are shown in plane cross-view and plane-view. The four (111) planes of the pyramid are shown, as well as the (001) zone among the two pyramids.

**Figure 8.** (**a**) Schematic cross-section view of the effect of the ISP compliant substrate on the SFs. Silicon and silicon carbide are drawn as black and white regions. Blue lines are SFs. (**b**) Cross-view SEM image of the ISP structure. (**c**) Plane-view SEM image. The four (111) planes of the pyramid are shown, as well as the (001) region among the two pyramids (adapted from Reference [17]).

The drawback of the use of this substrate is the formation of APB due to the different polarities of the (111) faces of the SiC [17]. Nevertheless, it is well known that the grain boundary density can be greatly decreased through the enhancement of the film thickness.

The APBs coverage with respect to layer thickness is depicted in Figure 9a. Despite the fact that the substrate design yields APBs, their concentration was rapidly reduced. Some tenth of microns of the SiC layer is enough to largely reduce the density of APBs. The ISP morphology also induces the formation of buried voids in the epilayer because the (111) face has a slower growth rate than the (100) face. These voids are observed in Figure 9b in which a cross-view TEM image of the 12 µm-thick epitaxial 3C-SiC layer is shown. The generation of voids may be advantageous in reducing the defectiveness of the epilayer. Voids can annihilate SFs and reduce the residual stress in the layer. SFs that cross the void are not able to propagate into the epilayer, reducing the defectivity. It is also feasible to manage the void height by adjusting the growth rate and conditions. In such a way, it is possible to modulate the concentration of SFs arriving on the surface [17].

**Figure 9.** (**a**) Anti-phase boundaries (APBs) covered-area percentages for different thicknesses of the epitaxial growth. (**b**) Cross-section SEM image of 12 um-thick epitaxial 3C-SiC layer grown on ISP (adapted from Reference [17]).
