Prof. Dr. Leo Esaki > Research Profile

by Roberto Lalli

Leo Esaki 

Nobel Prize in Physics 1973 together with Ivar Giaeverand Brian D. Josephson
"for their experimental discoveries regarding tunneling phenomena in semiconductors and superconductors, respectively.

Reiona ‘Leo’ Esaki was born in Osaka, Japan, on 12 March, 1925, in the final stages of the Taishō period, during which Japan experienced an unprecedented industrial and economic growth related to the hegemonic position the Empire of Japan had acquired in East Asia by the end of World War I. Son of the architect Soichiro Esaki, during World War II he attended the Third High School in Kyoto—a renowned educational institution that provided a post-secondary education in western culture and scientific matters. He was fortunate enough not to be gravely affected by the tragic events related to the war and the final surrender of the Empire of Japan to the Allied forces that led to the conclusion of World War II. Esaki continued to study physics, earning his MS degree at Tokyo University in 1947. Initially, Esaki was interested in pursuing research on nuclear physics to work on ultimate questions about the composition and behaviour of matter. However, the devastation of the war he daily witnessed had deep consequences on Esaki’s career. While still a student, he decided to focus on applied physics and industrial research in order to make practical contributions in the postwar reconstruction process of his motherland.
After graduation, Esaki joined the Kobe Kogyo Corporation, where he pursued research on solid-state physics. Just at that time, the field of solid-state physics was in great turmoil because of the recent discovery of the transistor made by three researchers of the AT&T Bell Telephone Laboratory in 1947. The discovery of J. Bardeen, W. Brattain and W. Shockley promised to revolutionize the field of electronic communication thanks to the employment of semiconductors such as doped germanium and silicon. Excited by the new discovery, Esaki immersed himself in the novel field of semiconductor research directed to practical application. After nine years of work at the Kobe Kogyo Corporation, in 1956 he accepted a new position as chief physicist of a small team at Tokyo Tsushin Corporation (renamed Sony in 1958). During the work he did at Sony Corporation, he made the discovery that would gain him the Nobel Prize in Physics in 1973: the tunnelling of electrons in semiconductors.

The Discovery of the Esaki Diode and Earlier Applications
The development of quantum mechanics in the mid-1920s had radically changed many of the views physicists held on the properties of matter. One of the most striking among the new features implied by the quantum formalism was that particles also had wave properties. The particle-wave duality was first proposed by the young French physicist Louis de Broglie in 1923, and was later embodied in the Schrödinger wave equation. The meaning and implication of the wave-particle duality in wave mechanics and its connection with the matrix mechanics put forward by W. Heisenberg in collaboration with M. Born and P. Jordan was controversial. After observations by Clinton Davisson and Lester Germer at the Bell Labs and by G. P. Thomson at the University of Aberdeen had provided persuasive evidence for electron diffraction, de Broglie’s hypothesis on wave-particle duality gained momentum within the physics community.

The new view of particles as endowed with wave properties implied physical phenomena that would have been forbidden if particles behaved according to the laws of Newtonian mechanics. One of these implications was that the wave function of particles allowed a small portion of them to pass through a potential barrier—a phenomenon called quantum tunnelling. While the effect was implicit in the Schrödinger equation, the earliest application of these ideas occurred only in 1928, when some physicists re-interpreted as quantum tunnelling some physical phenomena that had long been known. The most famous of these employments of the tunnelling concept was in nuclear physics, where G. Gamow, and, independently, R. Gurney and E. Condon interpreted alpha decay as the quantum tunnelling of alpha particles through the nuclear potential barrier.
1928 was an important year also for the development of the quantum theory of solids. Within a couple of years after Felix Bloch published his dissertation on the behaviour of electrons in crystals in 1928, all the building blocks of the quantum theory of solids were established, including the theory of energy bands and their connection with electrical and thermal conduction, the Brillouin zones, and the quantum description of magnetic phenomena. Some physicists also tried to apply quantum tunnelling to electrical conduction between contacts of different materials, but the proposed mechanisms did not meet with uncontroversial success.
When Esaki got interested in the junction properties between semiconductors, the field of quantum tunnelling in solids was still in its infancy. No persuasive evidence of quantum tunnelling in solids had ever been provided. In 1956, Esaki began investigating the semiconductor diode—a p-n junction to which an external electric potential is applied—with a special focus on p-n junctions with narrow widths. To diminish the widths, Esaki steadily increased the level of both donors and accepters in a Germanium p-n junction. As a first result, Esaki obtained a diode in which the current in the backward direction—namely, the current that flows when the negative (positive) terminal is connected to the p-side (n-side) of the p-n junction and the applied voltage is sufficiently high to break the depletion zone—was stronger than the current that flew in the forward direction. After further increasing the impurity levels, and consequently narrowing the junction width, Esaki became convinced that there was persuasive evidence that he was observing a tunnelling effect. Moreover, he showed that tunnelling was also responsible for the flow in the forward direction in the low-voltage range. The observed current-voltage characteristic presented clear indication of negative resistance between two values of the applied voltage. Moreover, the occurrence of this effect had a strong dependence on temperature. Esaki interpreted his observations as evidence of the presence of tunnelling currents when the p-n junction width was sufficiently narrow.
Working with his collaborator Y. Miyahara, Esaki also observed that at very low temperature (4.2 K) the current-voltage characteristic presented a fine structure indicating the presence also of inelastic tunnelling. The analysis of the curve led the physicists to realize that the voltages at which the singularities appeared had strong resemblance with well-known energies of the optical absorption spectra of pure silicon. This observation showed that tunnelling currents could also be employed for the study of the interactions of the tunnelling electrons with the vibrational modes of the solid.
Esaki’s discovery of a p-n junction capable of transmitting currents by means of quantum tunnelling resulted in the invention of the related diode: a p-n junction heavily doped on both sides as to reduce the junction width to about 100 Å—a device since then called Esaki diode or tunnel diode. The soon-to-be-called Sony Corporation began to manufacture the device as early as 1957 to make it available for a variety of applications such as rectifiers, oscillators, and amplifiers to employ in switching circuits, frequency converters and detectors. The device was already employed before Esaki communicated his discovery to the physics community by means of a short article published as a letter to the editor in the journal Physics Review.

The scientific community rapidly recognized the importance of Esaki’s achievement. His was the first persuasive evidence of electron tunnelling in solids, and it opened a new field of investigation with important technological applications. On the basis of this research, Esaki earned a PhD in physics at the University of Tokyo in 1959. After further experimental and theoretical development, including the discovery of tunnelling in superconductors by Ivar Giaever and B. Josephson’s theoretical study leading to the discovery of Josephson effect, it was broadly recognized that quantum tunnelling in solids had passed from the status of theoretical possibility to the stage of wide technological applicability in a handful of years after Esaki’s discovery.
In 1973, Leo Esaki shared with Giaever one half of the Nobel Prize in Physics “for their experimental discoveries regarding tunnelling phenomena in semiconductors and superconductors, respectively.” The other half was awarded to Josephson “for his theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effects.” As the Press Release of the 1973 Nobel Prize stressed, the three research endeavours were closely related although they had been pursued independently from one another: “Esaki's pioneering work in 1958 provided the basis for Giaever's tunnel experiments with superconductors in 1960. In turn, Giaever's work created the basis and stimulus for Josephson's theoretical discoveries in 1962.” To Esaki, of course, went the merit to have been the first to open this novel research field.

Semiconductor Research at IBM
After having obtained his PhD for his research on the tunnel diode, Esaki was invited to join as consultant the International Business Machines (IBM) in the United States. Although the working collaboration was initially planned to last just one year, Esaki became a permanent member of the IBM research staff till his retirement in 1992. Because of his important achievement, Esaki was granted a fellowship to continue his studies on semiconductors at the IBM Thomas J. Watson Research Center. One of the motivations Esaki exposed for his decision to leave Japan for working in the United States was that he did not appreciate some contradictory features of Japan’s approach to science where technological development was not really appreciated as a valuable scientific enterprise. In particular, Esaki lamented that his discovery was underrecognized in his own country, whilst it was highly valued in the United States.
At IBM, Esaki continued to perform research on tunnel currents in various kinds of junction as well as explore the properties of manufactured semiconductors. In 1966 and 1967, he headed a team of IBM experimenters in the investigation of tunnelling in metal-oxide-semiconductor junctions. The junctions were made of polycrystalline materials, whilst the p-n junction diodes had monocrystalline structure. Esaki showed that in metal-oxide-semiconductor junctions one observed a similar kind of tunnelling effect as observed in p-n junctions.
Further reasoning led Esaki to pioneer investigations on the quantum properties of semiconductor superlattices from 1969 onward. In 1951, employing the Wentzel-Kramers-Brillouin (WKB) approximation, David Bohm deduced that at certain kinetic energies of the incident electrons the transmission coefficient through a double barrier is equal to one. This hypothesised phenomenon—called resonant transmission—could have important applications, and Esaki set up a research project devoted to study this effect in material systems.

Although the theoretical treatment of these effects was well established, the applicative implications had not actually been pursued. Recent technical advances in the deposition of crystalline overlayers on a crystalline substrate—a technique called molecular beam epitaxy—allowed Esaki and his group to construct semiconductor superlattices by inserting thin layers of AlAs (or other materials with similar properties) into an n-type Germanium doped with Arsenic. The introduction of these layers created sharp potential barriers within the semiconductor. Thanks to the enormous precision of the employed techniques, Esaki could accurately estimate the energies necessary to obtain a resonant transmission in a manufactured double potential. When he went to Stockholm for the Nobel Prize ceremony, this research was still in progress, although he had already published important results including the observation of the resonant transmission as expected according to his calculations. In 1973, Esaki, in collaboration with R. Tsu and L. L. Chang, calculated that resonance could be detected not only in coefficient transmission, but also in the current-voltage characteristic, and experimentally observed this theoretical prediction. In his Nobel Prize lecture, Esaki stated that he was in the middle of extending this work to the periodic barrier structure—a venture that he pursued in the following years. This area of research is still at the forefront of technological development for the application of resonant-tunnelling diodes (RTD) for the construction of high-frequency oscillators and switching devices.
Role in education across national boundaries
After he was awarded the Nobel Prize and in view of his important contributions, Esaki’s professional status within the IBM rapidly changed. In 1976, Esaki became the director of IBM Japan, which also meant more administrative and organizational duties that left him less and less time to pursue pure research endeavours. After his retirement from IBM in 1992, Esaki returned to Japan to become the president of Tsukuba University in Ibaraki.
Esaki found the academic challenge appealing for two main reasons. First, the university was part of the urban project Tsukuba Science City, which in the 1960s was planned to become a large environment for researchers in order the increase the speed of scientific discovery and technological innovation. The aim of the entire project was dear to Esaki who had always stressed the fundamental relevance of technological innovations. The second reason was related to the intention of the university to experiment with more modern styles of teaching with respect to the highly traditional way of learning in which Japanese students were asked to refer to authority and discipline. Esaki had long been advocating the relevance of creativity, peer-to-peer communication, and open discussions not invalidated by power structures, and he was willing to introduce these practices in the teaching of various disciplines at Tsukuba University.
Esaki served as President of this university from 1992 to 1996 in which he worked to build stronger relationships between the university and industrial firms. He also encouraged a closer collaboration with both Japanese and non-Japanese institutions, by creating exchange programs with universities in Europe and United States. After 1996, Esaki has continued to have a preeminent role in Japanese policy about scientific matters and to communicate to the academic world his own experience as a researcher who worked in an industrial environment and in a different country for most of his professional life.

Bibliography

Brown, R. G., & Pike, E. R. (1995) A History of Optical and Optoelectronic Physics in the Twentieth Century. In Brown, L., Pippard, B., & Pais, A. (Eds.) Twentieth Century Physics (Vol. 3). AIP, New York, pp. 1385-1504.

Esaki, L. (1973) Nobel Lecture: Long Journey into Tunnelling. In Stig Lundqvist (eds.) (1992) Nobel Lectures, Physics 1971-1980. World Scientific Publishing Co., Singapore, pp. 126-133.

Esaki, Leo. Encyclopedia of World Biography. 2005. Retrieved January 11, 2015 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3435000068.html

Giaever I. (1973) Nobel Lecture: Electron Tunneling and Superconductivity. In Stig Lundqvist (eds.) (1992) Nobel Lectures, Physics 1971-1980. World Scientific Publishing Co., Singapore, pp. 137-153.

Hoddeson, L., Braun, E., Teichmann, J., & Weart, S. (eds.) (1991) Out of the Crystal Maze: Chapters from the History of Solid-State Physics. Oxford University Press, New York.

Josephson, B. D. (1973) Nobel Lecture: The Discovery of Tunnelling Supercurrents. In Stig Lundqvist (eds.) (1992) Nobel Lectures, Physics 1971-1980. World Scientific Publishing Co., Singapore, pp. 157-164.

Leo Esaki - Biographical. Nobelprize.org. Nobel Media AB 2014. Retrieved 9 January 2015. http://www.nobelprize.org/nobel_prizes/physics/laureates/1973/esaki-bio.html

Pippard, B. (1995) Electrons in Solids. In Brown, L., Pippard, B., & Pais, A. (Eds.) Twentieth Century Physics (Vol. 3). AIP, New York, pp. 1279-1383.

Press Release: The 1973 Nobel Prize in Physics. Nobelprize.org. Nobel Media AB 2014. Retrieved 10 January 2015. http://www.nobelprize.org/nobel_prizes/physics/laureates/1973/press.html