Leo Esaki

Modern Alchemy: Engineered Quantum Structures


Abstract

In 1969, research on “man-made crystals“ was initiated with Esaki and Tsu`s invention of a semiconductor superlattice. The lattice constant of this superlattice is of the order of 10 nanometers, substantially longer than that of the host crystal but shorter than the electron phase-coherent length. 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 in accordance with the principles of quantum theory in such a way as to exhibit extraordinary optical and transport properties. This approach could justifiably be called the alchemy of the present era, for it is intended to transform “common“ semiconductors to “super“ semiconductors. Since this offered a new degree of freedom in research, rather like making a “Gedanken-experiment“ a reality, many ingenious studies were inspired, leading to the surprising outcame.

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

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.


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