What Are Semiconductor Superlattices?

Semiconductor superlattices are periodic heterostructures composed of alternating ultrathin layers of two or more semiconductor materials, typically with individual layer thicknesses between one and tens of nanometers and overall periods repeated ten to a hundred or more times. When the layer thicknesses approach the de Broglie wavelength of electrons, quantum mechanical interference couples the energy states of adjacent quantum wells, splitting the discrete well levels into quasi-continuous bands called minibands separated by minigaps. This engineered band structure, tunable through choice of materials and layer thicknesses, produces electronic and optical properties unavailable in any homogeneous bulk semiconductor.

The concept was proposed by Leo Esaki and Raphael Tsu in 1970 and first realized shortly afterward using molecular beam epitaxy, which provided the atomic-layer deposition control required to define the period precisely. Today, superlattices are grown by both MBE and metal-organic chemical vapor deposition across III-V, II-VI, and group-IV material systems.

Band Structure Engineering and Minibands

The defining feature of a superlattice is the formation of minibands in the energy direction perpendicular to the layer interfaces. In an isolated quantum well, carriers occupy discrete energy levels. When wells are separated by thin barriers, the wavefunctions tunnel through the barriers and couple, broadening each level into a miniband whose width depends on barrier thickness and composition. Wider minibands arise from thinner or lower barriers, while narrower minibands (closer to isolated well levels) require thicker or higher barriers. The miniband widths and the minigaps between them are engineered to match specific device requirements, such as achieving a large gain transition oscillator strength in a laser or setting the transport bandwidth in an infrared photodetector. The Springer chapter on quantum wells, superlattices, and band-gap engineering gives a systematic treatment of the envelope-function methods used to compute miniband dispersions as a function of layer parameters.

Electronic Transport

Electron transport perpendicular to the superlattice layers occurs by miniband conduction at low electric fields, where carriers travel coherently through the periodic structure analogously to electrons in a conventional crystal band. At intermediate fields, Wannier-Stark localization breaks the minibands into field-tilted discrete levels, confining carriers to a few periods and producing negative differential conductance, the signature Esaki and Tsu originally predicted. At high fields, sequential resonant tunneling through individual quantum wells dominates, the mechanism underlying quantum cascade laser operation. In the quantum cascade laser, electrons injected at the top of the conduction band cascade down through a designed sequence of intersubband transitions, emitting a photon at each transition, before being re-injected into the next active region. Because the emission wavelength is set by the intersubband spacing rather than the material bandgap, quantum cascade lasers cover mid-infrared wavelengths from about 3 to beyond 25 micrometers by design. The IEEE Xplore paper on high-performance superlattice quantum cascade lasers documents how superlattice active regions improve current-carrying capacity and optical power compared with isolated quantum well active regions.

Optical Properties and Infrared Applications

The intersubband transitions of semiconductor superlattices occur at energies well below the material bandgap, covering mid-infrared and terahertz spectral ranges where few efficient semiconductor emitters or detectors existed before superlattice engineering. Quantum well infrared photodetectors (QWIPs) exploit transitions between the ground and first excited state of a quantum well, with the absorption wavelength tuned by well width. Superlattice infrared detectors offer absorption across a broader miniband rather than a single narrow intersubband resonance, improving uniformity and detectivity across the target waveband. Second-harmonic generation and other nonlinear optical processes are enhanced in asymmetric superlattice designs that break the inversion symmetry of conventional semiconductors, enabling engineered nonlinear susceptibilities orders of magnitude larger than in bulk III-V materials. The Physical Review B analysis of gain in quantum cascade lasers provides the quantum transport framework underlying gain calculations in superlattice active regions.

Applications

Semiconductor superlattices have applications in a wide range of fields, including:

• Quantum cascade lasers for gas sensing, medical breath analysis, and infrared countermeasures
• Quantum well infrared photodetectors in thermal imaging and night-vision systems
• Thermoelectric modules, where superlattice barriers reduce thermal conductivity without degrading electrical conductivity
• Avalanche photodiodes with engineered impact ionization ratios for low-noise optical receivers
• Terahertz generation and detection in spectroscopy and security imaging