Selective chemical wet etching of Si1-xGex versus Si in single-layer and multi-layer with HNO3/HF mixtures

A gate-all-around device, considered as a key structural design for improving performance and density, requires selective etching to obtain nanostructure channels. We investigated the wet etching of epitaxial Si1-xGex films with various HNO3/HF-based chemical etchants to elucidate the mechanism of selective etching of Si1-xGex versus pure Si. Blanket etching was performed on Si1-xGex single-layer and lateral etching was performed on Si/Si1-xGex/Si multi-layer with three different Ge concentrations (x = 0.13, 0.24, and 0.30). In both cases of etching, our results show that the Si1-xGex layer with higher Ge concentration was etched faster. We found that the Si1-xGex etch rate decreased consistently with decreasing HF in both cases, while the slope of the etch rate lowers as the Ge concentration decreases with the blanket etching of Si1-xGex. We postulated that the decrease in the slope of the Si1-xGex etch rate with low Ge concentration occurs due to the difficulty of hole injection into the surface of Si1-xGex, which has a lower valence band offset. In contrast, for the case of lateral etching of Si1-xGex, we found that the slopes of the etch rate are similar to each other irrespective of Ge concentrations. We suggest that the similarity of the slopes is due to the sufficiency of holes, which is supplied not only by HNO3 but also from adjacent Si layers into the Si1-xGex layer. Furthermore, we investigated the effect of structural factors on the lateral etching of Si1-xGex to apply a selective etching process in Si/Si1-xGex/Si multi-layer to obtain Si nanostructures. We found that the lateral etch rate of Si1-xGex in Si/Si1-xGex/Si multi-layers can be reduced owing to the oxide of adjacent Si layers in the etched tunnel, which is not completely removed by HF during the etching process.