Obituary of David Redfield (1925-2012)

BIOGRAPHY OF DAVID REDFIELD

David Redfield died June 30, 2012, in the Vi Retirement Community in Palo Alto, California. David was born in 1925 in Brooklyn, NY, the younger son of Herbert and Lillian Rosenfeld. His family moved west in the early 1930’s, where he grew up and went to school in Los Angeles. He entered UCLA and, after training as an Army Air Corps pilot at the end of WWII, he graduated in 1948 with a major in physics and went to work at the National Bureau of Standards (then in Washington, DC) where, among other things, he grew the first single crystal of germanium produced at NBS. In Washington he met and married Barbara Leiken in 1950, beginning a family life that included wide-ranging travels, until her untimely death in 1984. In Washington, studying nights, he earned a MS in physics from the University of Maryland. He then returned to graduate school full time at the University of Pennsylvania where he received a PhD in physics in 1956, working under Park H. Miller on the physics of semiconductors.

In his first subsequent job at the National Carbon Company (a division of the Union Carbide Corporation) in Parma, Ohio, Redfield initiated an experimental study of photoemission from germanium and became interested in the strong electric fields implied by band bending near surfaces, but that had not been explicitly treated because of the prevailing emphasis on potentials. After calculating the distribution and surprising strength of these fields, he next inquired into the analogous fields due to randomly placed charged impurities in solids by applying the methods developed for evaluating the “microfields” in plasmas. These universally present fields in solids turned out to be so strong that Redfield proposed that they were responsible for the exponential broadening of the fundamental optical absorption edge of solids that had come to be known as Urbach’s Rule. In doing so, he invoked the recently developed Franz-Keldysh Effect in which a uniform field allows tunnel-assisted optical absorption at photon energies below that of the band gap. But he never managed to recast the impurity-scattering theory of electrons from the generally-used single-center model to a random-distribution model.

Later, while working at Columbia University with graduate student Martin Afromowitz, Redfield devised an experiment that showed unambiguously that the broadening of the optical absorption edge in doped GaAs is due to charged impurities and not to phonons. This was significant because a competing theory of absorption-edge broadening was based on exciton-phonon interactions. Still later, working at the David Sarnoff RCA Laboratories with John Dow (then at Princeton University), this microfield theory of broadening of absorption edges was extended to include excitonic effects, and further to show that in ionic solids such broadening is due to microfields created by the polar vibrations of the charged host ions. This latter interpretation was confirmed in a series of experiments by R. Bray and his students, using acoustoelectric domains in GaAs showing that only polar vibrations broadened the edge. This explanation of Urbach’s Rule became the first of three international controversies that marked Redfield’s career.

Working with James Wittke and Jacques Pankove, Redfield developed experimental techniques combining time-resolved and steady-state spectra of photoluminescence and photoconductivity from energy band tails in GaAs:Si to show that electrons and holes are necessarily spatially separated, in the same way that later became important in multilayer materials.

While studying the optical properties of strontium titanate and lithium niobate, Redfield and W. J. Burke reinterpreted a published report that two phases of strontium titanate coexisted over a temperature range of 60K around its 105K second-order phase transition. They showed that the observations could be explained without coexisting phases by invoking phonon emission and absorption and suggested an experimental test of this proposal that later confirmed their reinterpretation.

Redfield’s next principal contribution was in “heavy-doping effects” in semiconductors, which for years had prevented several types of devices from reaching their theoretical performance levels. He contended that the prevailing “band-gap shrinkage” explanation was deficient, and proposed an alternative explanation based on the Auger Effect in which high carrier densities shorten the minority carrier lifetime by three-body interactions. This developed into the second major controversy, but eventually became the accepted view.

Building on his experience in doped semiconductors, Redfield went on to study the effects of disorder in general and developed experiments elucidating the role of disorder in producing energy-band tails. These provided the first means of controlling the Fermi energy within a band tail and allowed observation of the energy dependence of the carrier mobility in a tail.

Redfield’s interests then shifted to energy-related problems, a change that prevented him from accepting offers of fellowships from the Cavendish Laboratory at Cambridge University and the von Humboldt Foundation at Frankfurt. Instead he moved into photovoltaics, especially hydrogenated amorphous silicon in which he worked on the light-induced instability that limits its long-term efficiency. This effort continued when he moved to Stanford University in 1985 as a Consulting Professor in the Department of Materials Science and Engineering until his retirement in 1997. In this area he introduced the principle of light-induced recovery of metastable defects and the first prediction of the consequent saturation of the instability. He also disputed a widely held picture of the kinetics of this instability and later, working with Richard Bube and graduate students Lisa Benatar and Michael Grimbergen, showed theoretically and experimentally that the correct interpretation of these kinetics is by a stretched exponential. This proposal developed into the third international controversy. Redfield and Bube published a book Photoinduced Defects in Semiconductors that tied together the metastable defects in amorphous silicon with DX and EL2 centers in compound semiconductors.

Redfield was a founding member of the U.S. Department of Energy’s Solar Photovoltaic Energy Advisory Committee. He is a Fellow of the American Physical Society, was elected to the Executive Committee of its (then) Solid State Division and served on the APS Panel on Public Affairs. He was a senior member of the IEEE and was a member of IEEE Energy Committee and was chairman of its Subcommittee on Solar Energy. He was the founding chairman of the IEEE Photovoltaic Standards Coordinating Committee and vice chairman of the Photovoltaics Subcommittee of the American National Standards Institute. He became the U.S. Technical Expert at the Working Group on Direct Photovoltaic Conversion of Solar Energy of the International Electrotechnical Commission. He was active for years in the Federation of American Scientists and was elected Chairman of its Princeton Chapter.

After his wife Barbara passed away in 1984, David moved to Palo Alto. There he developed a twenty-year close companionship with Estelle Korengold, including international travel as well as local activities. He spent many of those years also doing weekly readings for Learning Ally of Palo Alto (formerly Recording for the Blind and Dyslexic). In 2006 he moved to the Vi retirement community, where he was active in developing a variety of activities and services for residents. He is survived by two sons, Andrew of Weston Massachusetts, and Steven of Chicago. David will be greatly missed by his many friends at Stanford, and around the world.