superlattices

Superlattices are periodic structures composed of alternating layers of two or more materials with thicknesses typically in the nanometer range. The stacking direction provides a one-dimensional periodic potential, often realized by epitaxial growth on a common substrate. By choosing materials with different electronic, optical, or lattice properties, researchers engineer the overall behavior of the stack.

The constituent layers are kept thin to produce quantum confinement and band structure modifications. Layer thicknesses range from about a few monolayers up to tens of nanometers, and a repeat unit called a period is repeated many times to form the superlattice. Lattice matching and strain management are important to minimize defects at interfaces.

Electronic and optical consequences: The periodic potential causes Brillouin zone folding and the formation of minibands and minigaps in the conduction and valence bands, altering effective masses, carrier mobility, optical transition energies, and density of states. In phonons, reduced thermal conductivity can occur, with potential for thermoelectric applications.

Fabrication: Epitaxial techniques such as molecular beam epitaxy and metal-organic chemical vapor deposition enable precise control of layer thickness and sharp interfaces. Oxide and compound semiconductor systems are common; interfacial quality determines device performance.

Applications: quantum cascade lasers, multi-quantum well infrared detectors, high-electron-mobility transistors, and thermoelectric materials; magnetic superlattices investigate spin-dependent transport and giant magnetoresistance; oxide superlattices explore emergent interfacial phenomena such as two-dimensional electron gases and superconductivity.

Examples: GaAs/AlAs and InAs/GaSb are classic semiconductor superlattices; Si/Ge superlattices; oxide systems such as LaAlO3/SrTiO3, or SrTiO3/LaTiO3; magnetic superlattices include Fe/Cr or Co/Cu stacks studied for spintronic effects.