In 1969, research on “man-made crystals” was initiated with Esaki and Tsu’s invention of a semiconductor superlattice. The superlattice period here should be designed to be substantially longer than the lattice constant of the host crystal to create mini Brillouin zones and distinct Wannier-Stark ladders under an electric field, but shorter than the electron phase-coherent length to satisfy the quantum condition. This invention was perhaps the first proposal to advocate the engineering of a new semiconductor material by applying the advanced growth technique of MBE, after designing the structure with the principles of quantum theory in such a way as to exhibit extraordinary optical and transport properties. Since this offered a new degree of freedom in research, making a “gedanken-experiment” a reality, many ingenious studies were inspired, leading to new physics and unprecedented electronic devices.

Esaki and his coworkers’ pioneering research on superlattices and quantum wells in the 1970s and 1980s triggered a wide spectrum of experimental and theoretical investigations resulting in not only the observation of a number of intriguing new phenomena such as differential negative resistance, high electron mobilities, large excitonic binding energies, large Stark shifts, distinct Wannier-Stark ladders and Bloch oscillations due to electric quantization, but also the emergence of a new class of transport and optoelectronic devices
such as high electron-mobility transistors (HEMT), high-speed resonant tunnel diodes, high-performance injection lasers with quantum wells, and high-power cascade superlattice lasers.

Since the superlattice periods or the quantum well-widths are on the
nanometer scale, the studies served as the precursor to a variety of
nanostructures.