Material system composed of different layers or materials, often with tailored electronic properties.A heterostructure is a material made by stacking layers of different semiconductors or other materials on top of one another, usually with atomically sharp interfaces. The layers can differ in chemical composition, doping, or crystal orientation. Because the electronic properties of each layer are different, the junctions between them create tailored potential landscapes for electrons: wells, barriers, and channels that do not exist in any single material. This allows engineers to design the flow of charge and light with great precision. Heterostructures are the foundation of modern optoelectronics—lasers, light-emitting diodes, and high-speed transistors—and they are also a playground for exploring new physics such as two-dimensional electron gases and topological states.
The concept emerged in the 1960s with advances in molecular beam epitaxy and chemical vapor deposition, which allowed atomic-layer control. The first semiconductor heterostructure lasers were demonstrated in 1963 by Herbert Kroemer and by Zh. I. Alferov's group, work that earned them the 2000 Nobel Prize in Physics. In 1974, Esaki and Tsu proposed the superlattice, a periodic heterostructure, and in 1980 von Klitzing discovered the quantum Hall effect in a heterostructure-based two-dimensional electron gas.
When two different semiconductors are grown together, their differing band gaps and electron affinities cause the conduction and valence band edges to jump at the interface. These band offsets create potential steps. By sandwiching a thin layer of a narrow-gap semiconductor between two layers of a wider-gap material, one forms a quantum well that traps electrons or holes in a plane. If the well is undoped but the surrounding material is doped, the electrons fall into the well and form a two-dimensional electron gas—a sheet of mobile charges that can have extremely high mobility because they are separated from the ionized donors. Strain from lattice mismatch can also shift the band structure and even create piezoelectric fields, adding another design knob.
Heterostructures are used in nearly all semiconductor lasers, LEDs, and high-electron-mobility transistors (HEMTs) for cell phones and radar. They are also central to research on quantum computing, spintronics, and topological materials.
The 2000 Nobel Prize in Physics was awarded for developing semiconductor heterostructures; the same year, the Nobel Prize in Chemistry went to work on conducting polymers, showing how layered materials were transforming multiple fields.
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