Laudatio for Klaus H. Ploog

It is a great honor for me to write this laudatio for Klaus Ploog, one of the pioneers of molecular beam epitaxy (MBE) in Europe. I met Klaus for the first time in 1975 when looking for a postdoc position at the Max Planck Institute for Solid State Research (MPI‐FKF) in Stuttgart. In those days, he made a great effort to get the first GaAs MBE machine operational. I was fascinated by this new crystal growth technique, which allowed the fabrication of layered semiconductor structures with atomic precision, and I was seeking a close collaboration with Klaus and started characterizing his epitaxial layers with spectroscopic methods. This led to some key publications in the late 1970s and early 1980s. Klaus Ploog studied chemistry at the Christian Albrechts University in Kiel and Ludwig Maximilians University (LMU) in Munich, from where he graduated as a chemist in 1967. He continued with post graduate studies at the LMU, working on boron and borides, and received a PhD degree with honors in 1970, followed by a two years' lecturer position. After this period, he spent another two years as a postdoc at the Jülich Research Center and the University of Bonn, before he started in 1974 as a senior researcher and head of the MBE group at the newly founded Max Planck Institute for Solid State Research in Stuttgart. Building up a high‐quality MBE system for semiconductor research was a great challenge at that time, as this technique was still in its infancy. Together with his collaborator Albrecht Fischer, he managed to handle and solve all technological problems and they were soon able to fabricate high‐quality GaAs/AlGaAs heterostructures. A first publication on in‐situ characterization of MBE grown GaAs and (Al,Ga)As by Ploog and Fischer appeared in 1977 in Applied Physics. The early times of MBE in Stuttgart were characterized by learning what the best preparation method of GaAs surfaces for epitaxy is – epi‐ready wafers were far from being available at that time –, the understanding of the growth mechanisms, and the control of n‐ and p‐type doping. Already in the 1970s, he hosted guest scientists from Japan, namely Y. Horikoshi, and started several collaborations with Japanese laboratories, especially with NTT. The excellent relations to Japanese researchers were extremely fruitful, and Klaus continued and expanded these connections throughout his scientific carrier. The detailed analysis of MBE growth processes performed by Klaus and his group enabled them soon to fabricate complicated and demanding electrically and optically active heterostructures. In 1979, we had a joint publication in Physical Review Letters on the first realization of two‐dimensional electron systems at GaAs/(Al,Ga)As heterointerfaces by selective (remote) doping. This work was the basis for the development of the so‐called High Electron Mobility Transistor (HEMT). Another pioneering work of that time was the development of periodic doping structures, so‐called nipi‐superlattices, together with G. H. Döhler, H. Künzel and others. The ability to fabricate high‐quality semiconductor heterostructures and superlattices stimulated a huge interest in the early 1980s, both with respect to basic physics of low‐dimensional systems as well as towards the development of novel devices for information technology. Klaus started numerous collaborations and provided specific MBE grown samples to many groups world‐wide. The number of publications increased nearly exponentially in the coming years, reaching a peak value of nearly 120 publications in 1992. Many research programs relied on the excellent samples designed and fabricated at the MPI‐FKF in Stuttgart. The large number of very productive cooperation partners of Klaus Ploog includes K. von Klitzing, M. Cardona, R. Cingolani, D. Heitmann, H. T. Grahn, R. J. Haug, E. O. Göbel, W. Rühle, T. Elsässer, K. Fujiwara, and many more. Klaus and his team worked in the 1980s to continuously improve the MBE process and to fabricate highly complicated and demanding hetero‐ and nanostructures based on III–V semiconductor compounds, which were the basis for studying various quantum effects in low‐dimensional electronic systems. Many of his students made use of their deep knowledge of MBE technology for their new tasks in research and development at other institutions. Among those are Harald Künzel, Wolfgang Stolz, Klaus Köhler, and various others. In 1992, Klaus Ploog accepted the directorship of the Paul Drude Institute (PDI) in Berlin concomitant with a professorship in the Department of Physics at Humboldt University in Berlin. His main scientific interest continued to be the direct synthesis of semiconductor quantum structures by MBE. However, he expanded his activities to new materials systems like the wide‐gap semiconductor GaN and related compounds as well as ferromagnetic layers on GaAs, aiming for optoelectronic and spintronic applications. Another challenge was the control of self‐assembled nanostructures as well as in‐situ characterization of MBE growth by synchrotron radiation at BESSY. His productivity remained very high also during his time in Berlin and the average number of publications reached on the average about fifty per year. This has continued until now. Among his main coauthors from the PDI are O. Brandt, R. Nötzel, A. Trampert, R. Hey, L. Däweritz, M. Ramsteiner, K.‐J. Friedland, P. Santos, W. Braun, and many more. Klaus Ploog has led the Paul Drude Institute to an internationally well known institute in semiconductor materials science, with many visitors and high visibility worldwide. Among the international recognitions, which Klaus received for his outstanding achievements, are the following: The Technology Transfer Award of the German Ministry of Research and Technology in 1983, the Award of the Italian Physical Society, together with R. Cingolani in 1989, the Philip Morris Research Award together with nine colleagues of the Max Planck Institute in 1990, the IBERDROLA Ciencia y Technologia Award in 1998, the Max Planck Research Award for International Cooperation in 1999, and the Welker Award in 2003. Finally he received in 2007 the Eugen and Ilse Seibold Prize of the German Research Foundation, which acknowledges his extensive collaborations with Japanese research institutions. His excellent relations with Japanese researchers are also evident from his many appointments as a visiting professor in Japan (eleven times since 1984). He was also invited as a Visiting Professor at Stanford, Eindhoven, and Madrid and was appointed a Fellow of the Institute of Physics in the UK in 2004 and a Distinguished Lecturer at the Australian Research Council Nanotechnology Network in 2005. Klaus has served the semiconductor physics and materials society community as an organizer or member of program and advisory committees at numerous international conferences, workshops and schools, and his clear opinion and advice has been often decisive. Klaus Ploog is undoubtedly one of the most productive scientists working in semiconductor materials science. He has coauthored more than 1500 publications, which have been cited more than 30,000 times. With this he is among the most often cited scientists worldwide. The impact of his research is also demonstrated by the fact that 53 of his publications have been cited more than 100 times. His achievements have found world‐wide recognition and as a pioneer of molecular beam epitaxy in Germany and Europe, he has laid the foundation for the high‐quality semiconductor research, which could be performed here in the past 30 years. We hope that he will continue to show up in the scientific community after his retirement, now as a friend, collaborator, and excellent adviser, and we wish him all the best for the future. Garching, July 2007