Si/SiGe superlattice
Beginner
A stack of very thin layers that take turns: silicon, then silicon mixed with germanium, then silicon again. Nanosheet transistors start from it.
Novice
Alternating crystal layers of silicon and silicon-germanium, each a few nanometers thick, grown on the wafer one atomic layer after another. The silicon layers become the nanosheets; the silicon-germanium is later etched away.
Expert
Epitaxial stack (one published flow used about 30% Ge and layers about 9 nm thick). Sheet thickness and spacing are set by layer growth rather than by etching, as a fin’s width is.
Explained in Fins and nanosheets (Transistors).
See also: Channel release, Inner spacer.