Raphael Tsu

Raphael Tsu
Born	(1932-12-27) December 27, 1932 Shanghai, China
Known for	resonant tunneling diode, Tsu-Esaki Formula
Notable awards	Alexander von Humboldt Award (1975) James C. McGroddy Prize for New Materials (1985)

Raphael Tsu is a Fellow of the American Physical Society and currently serves as a Distinguished Professor of electrical engineering at the University of North Carolina at Charlotte, Charlotte, NC [1].

Biography

Tsu was born to a Catholic family in Shanghai, China. As a child he was inspired by his great uncle who in 1926 was amongst the first six Chinese bishops ever to be consecrated at the Vatican in Rome and as a teenager by his US educated father Adrian and French educated uncle, Louis. His paternal grandfather and great uncle were pioneers in power plant and modern shipyard in Shanghai. While leaving Shanghai, his great uncle, on his death bed told him to remember the old Chinese saying that to succeed requires the right tool. Tsu initially emigrated to the west to study physics in England. He earned a B.S. at the University of Dayton, and M.S. & Ph.D. from the Ohio State University. After several years working as a member of the Technical Staff at Bell Laboratories (BTL) at Murray Hill, NJ, developing an ultrasonic amplifier, a mechanism invented by Dr. D.L. White, Tsu moved to the IBM, T.J. Watson Research Center in Yorktown Heights, NY as an associate to Dr. Leo Esaki beginning a well-known collaboration that yielded a theory of man-made quantum materials, superlattices and quantum wells.

Later, Tsu joined the Amorphous Semiconductors Institute (ASI) and directed energy research at Energy Conversion Devices (ECD) near Detroit, MI, as invited by inventor Stan Ovshinsky. His contribution included the first experimental determination of the volume fraction of crystallinity for conductivity percolation in amorphous silicon and [germanium], and providing experimental proof of the existence of an intermediate order. He discovered experimentally that post annealing with H₂ and O₂ can drastically remove dangling bond defects in amorphous silicon.

During 1985-1987 Tsu served as the amorphous silicon program group leader at the National Renewable Energy Laboratory (then known as SERI, Solar Energy Research Institute) in Golden, CO. His theoretical derivation of the relationship between the optical absorption and disorder in amorphous silicon and germanium in terms of fundamental constants shows that the slope of the Tauc plot is uniquely determined by the oscillator strength of the transition, the deformation potential and the mean deviation of the atomic coordinates obtained from the RDF.

In 1972, Tsu organized a group and was invited by the Chinese Science Academy which resulted in the first report on the technology in China published in Scientific American. This led to his involvement through establishing the first Chinese Scientific delegation visit to the US, which was invited by the US-China Relations Committee of the US Academy of Science. During this visit, he worked with the US State Department for the program and logistics on the East Coast. This effort contributed to the opening of scientific exchange between the United States and China.

It is important to point out that of all his contributions Raphael (Ray) Tsu’s most important impact has been in the invention of spatially modulated or periodically layered materials – the superlattice structures; during the late twentieth century and the superlattice scheme remains a highly productive innovation well into this century. Indeed, Ray Tsu played a pivotal role in the creation, invention and development of synthetic periodic superlattices materials/devices, these artificially fabricated 2-dimensional multiple-quantum well structures while working in Leo Esaki’s Exploratory Device Research Group in the IBM Watson Laboratories. Tsu introduced the idea of alternating layers of A/B with the correct band-edge off set. While at IBM, Ray worked closely with another notable scientist, the late L. L. Chang. Ray’s theoretical analysis at IBM led to the important concept of modulation doping for carrier mobility enhancement independently of and prior to the work of Dingle, et al. at Bell Labs.

These pioneering contributions have led to many current technologies including Terahertz oscillators, NDC or negative differential conductance in the I-V characteristics of superlattice devices, resonant tunneling quantum well (double barrier) structures, of phonon band folding and the related Raman spectra, and the discovery of forbidden phonon modes. Raphael Tsu’s other contributions have impacted a wide range of materials science. A leitmotif in Raphael’s career has been the ubiquitous electron–lattice interactions in materials, another is quantum transport. One of his first publications from the Bell Labs concerned with radiation of phonons by non-accelerating charges. Another from IBM, related to phonons and polaritons. He and Timir Datta have introduced the concept of wave impedance in quantum transport for dissipation free quantum waves, where using the expressions for probability continuity and energy expectation an equation for quantum wave impedance of Schrödinger functions is obtained.

Notable Papers

The following two papers were amongst the 50 most cited articles to appear in the first fifty years of the journal Applied Physics Letters published by the American Institute of Physics (AIP) and were featured as such in the APL's 50th anniversary issue http://apl.aip.org/apl_50th_anniversary .