superlattice

A superlattice is a periodic structure composed of alternating thin layers of two or more materials, with thicknesses on the nanometer scale, designed to produce a new periodicity that modifies electronic, optical, or magnetic properties relative to the constituent materials. The individual layers act as quantum wells or barriers, and the overall stack exhibits miniband formation and Bragg-type reflections when the period is comparable to the carrier wavelength. Superlattices are commonly created by epitaxial growth on a crystalline substrate, using methods such as molecular beam epitaxy, metal-organic chemical vapor deposition, or pulsed laser deposition.

Types and materials: The most studied are semiconductor superlattices such as GaAs/AlGaAs, but oxide, magnetic, and ferroelectric superlattices are also explored. The choice of materials and interface quality controls strain, lattice mismatch, and coupling between layers, which in turn shape electronic and magnetic interactions.

Electronic and optical properties: The coupling between adjacent quantum wells splits discrete energy levels into minibands, altering the effective band structure and carrier transport. Quantum confinement can shift optical transition energies, enabling tailored photoluminescence and absorption spectra. Bragg reflections from the periodic structure can give rise to photonic effects in the visible to infrared range.

Applications and examples: Semiconductor superlattices underpin devices such as quantum cascade lasers, resonant tunneling diodes, and infrared detectors. They also inform thermoelectric materials, spintronic structures, and high-electron-mobility platforms. A classic system is GaAs/AlGaAs, with other widely studied pairs including Si/Ge and oxide/oxide stacks like SrTiO3/LaAlO3.

History and challenges: The concept was developed in the 1970s by Esaki and Tsu, who predicted new band structures in periodically layered materials. Realization faces challenges from interface roughness, lattice mismatch, and strain that can introduce defects. Careful control of layer thickness, composition, and growth conditions is required to achieve the desired miniband structure and device performance.