For VASP your "supercell" is just "cell". So, you will get a layered structure of Si and HfO2, periodic in the z direction (as well as in x and y directions)

Great. Thanks Stepan. Do you know what would happen in this case if the ISYM is set anulated (ISYM=-1)? Should I do this?

Greetings

For the type of system you describe you typically wouldn't want symmetries. I think you can save a small amount of time by setting ISYM=-1 (skipping even doing an analysis of the symmetry), but the most important thing is probably to make sure that you don't have accidental symmetry present, so slightly distorting some atomic positions at the interface may be a good idea.

Thank you very much Torbjörn, but Why would it be important to avoid accidental symmetry?

Greetings

Geometrical symmetries generally lead to forces being zero on some atoms and in some directions, despite the energy minimum being away from this configuration, you can have zero forces also at an energy maximum.

A simple illustration is  if you compress something atomically thin, like graphene. What should happen is that the graphene buckles, but if you stick with a flat, high-symmetry configuration, you will never see this, since forces perpendicular to the plane will be zero by symmetry. In this case it is of no consequence what you do with your ISYM tag, since the forces are zeroed by the geometry, not by anything in the computational procedure.

Therefore it can be good to slightly perturb your system to get away from the risk of accidentally being at a local maximum.