a superlattice film
Think of a layer cake with a strict recipe: sponge, cream, sponge, cream, over and over, each layer only a few atoms thick and every layer the same thickness as its twin down the stack. A superlattice film is exactly that in a crystal — two (or more) different materials grown as alternating ultrathin layers, in a regular repeating stack, on a substrate. The result is a new, larger periodicity: on top of each material's own atomic spacing, the stack repeats with a longer period equal to one bilayer (one layer of A plus one layer of B).
Because there is now a second, much longer repeat distance built into the crystal, the superlattice announces itself in diffraction. A film with a repeat period of, say, 5 nm produces extra satellite peaks clustered around the ordinary Bragg peaks, spaced by 1 over the period — the longer the stacking period, the more closely spaced the satellites. This is a direct, quantitative fingerprint of the layered structure, and it is how the period and the sharpness of the interfaces are measured. Superlattice films are grown by molecular-beam epitaxy or atomic-layer methods that can shutter one material off and another on with near-atomic precision.
The point of stacking materials this way is to engineer a structure, and through it behaviour, that neither material has alone: alternating semiconductors create arrays of coupled quantum wells whose electron minibands are set by the layer thicknesses; alternating a magnetic and a non-magnetic metal gives the giant-magnetoresistance stacks in hard-drive read heads; and straining thin layers against one another builds strained-layer superlattices. In every case the designer's control is structural — the sequence and thickness of the layers — which is why this belongs squarely to nanoscale structure.
A gallium-arsenide / aluminium-arsenide superlattice grown by molecular-beam epitaxy might alternate 4 nm of GaAs with 4 nm of AlAs, hundreds of times over. An X-ray scan shows the ordinary GaAs peaks flanked by a comb of evenly spaced satellite peaks; measuring their spacing gives back the 8 nm bilayer period exactly, confirming the layers came out as designed.
Satellite peaks spaced by 1/period are the diffraction signature of the engineered stacking period.
Do not confuse this compositional superlattice (a stack of chemically different layers) with the ordering superlattice of an ordered alloy, where one crystal's atoms sort onto distinct sites and create extra 'superlattice' reflections. Both add a longer periodicity and extra reflections, but one is a layered film and the other is atomic ordering inside a single crystal.