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A Life In Research: The Impact Of Klaus Ploog, MBE Pioneer

Helmut Jung looks back at the influential career of Klaus Ploog, whose achievements include the invention of the superlattice and the delta-doping technique that are used in today's HEMTs.

Most of us are looking forward to our retirement. It s an opportunity to leave behind the commute, the long hours and the worries about the success of our products, and devote time to the more relaxing things in life.

Academics, however, don t always share this view. Instead, some will continue to work because their research means so much to them. Nobel prize-winning solid-state physicist Sir Neville Mott, for example, carried on writing papers up until his death at 90. Similarly, the German MBE pioneer Klaus Ploog has managed to find a way to continue his research. Although the law of his homeland has forced him retire as director of the Paul-Drude Institute (PDI), Ploog has taken a visiting professorship at Tokyo Institute of Technology, Japan.



Ploog has been involved in MBE research throughout his career and is a legend within that community. Born in 1941 in Klein Kampen, Germany, he studied chemistry at the universities of Kiel and Munich. After graduating in 1967, he started work as a research scientist at Munich University, and gained a PhD with summa cum laude (the highest honour) three years later.



He stayed at Munich as a lecturer for another year, then spent two years as a research associate at the Jülich Research Centre before moving to the Max Planck Institute for Solid-State Research in Stuttgart, where he started to assemble his MBE group. Research in the early 1970s was hampered by a lack of fully equipped commercial MBE tools, and epitaxial effusion cells had to be developed and fabricated in-house. "Often we had to overcome elementary problems," recollects Ploog s colleague Albrecht Fischer. "We had great problems getting boron nitride crucibles for evaporating aluminum."



Despite these difficulties, Ploog built up the expertise required to fabricate high-quality III–V epitaxial structures incorporating dopant atoms. He and his team could then grow semiconductor structures with atomic accuracy on GaAs, and then on AlGaAs; work that led to the fabrication of the first tailor-made doped-superlattice structures.

The high-quality samples produced by Ploog s team during this pioneering period were in high demand from academic groups studying quantum mechanical phenomena throughout the world. These researchers included Klaus von Klitzing, who explored his Nobel prize-winning discovery of the quantum Hall effect further with Ploog s material. "Today nano is one of the most important words in modern research," says von Klitzing. "Klaus Ploog contributed enormously to nanoscience and nanoelectronics. When he pioneered the capabilities of MBE for new electronic devices, the word nanoscience was not in use."



Von Klitzing believes that it is only through the epitaxial growth techniques pioneered by Ploog and others, along with the development of scanning-probe techniques, that today s scientists can control materials with atomic precision and subsequently explore the nano-world.

Ploog s influence stretches beyond academia, however. Today s volume manufacturers of transistors are also indebted to his work. The delta-doped layers used in HEMTs, and other devices that confine doping atoms to an ultra-thin layer, were first developed by his group more than 20 years ago. "Delta doping has become the standard doping technique in numerous devices, due to the inherent advantages of delta-doped structures over uniformly doped ones," explains E Fred Schubert, a former PhD student of Ploog s who is now Wellfleet senior constellation professor, Future Chips, at Rensselaer Polytechnic Institute in Troy, NY. "Since its inception, delta doping has been used in billions of devices."



Ploog s other legacy to HEMT production, from around the same period, is the superlattice buffer. This improves the epitaxial quality of the entire transistor, and is now standard in all HEMT epiwafers.

In 1991 Ploog left Stuttgart to take on the professorship of material science at Darmstadt University, Germany. He didn t stay long, though, and a year later became the director at the newly founded PDI for Solid-State Electronics in east Berlin, at the heart of the freshly reunited Germany. By 1993 he had the additional title of professor of material science at the Humboldt University physics department in Berlin, Germany.
New directions

At the PDI, Ploog continued to push the capabilities of MBE technology, with research that headed in new directions, such as the growth of ferromagnetic semiconductor nanostructures for spintronics and lower-dimensional structures. Developments in all these areas could have major commercial implications in the future. For example, variants of the quantum-wire and quantum-dot structures are now starting to appear in commercial devices such as quantum-dot lasers, while spintronic devices are a hot research topic that could lead to the development of programmable magneto-logic devices.

Ploog has been an active and pioneering developer of wide-bandgap materials based on GaN. When the rest of the community was still focused on polar material, Ploog s team was investigating cubic nitrides and growth on new "non-polar" substrate orientations. His team was also the first to grow GaN by MBE along a non-polar crystal direction.

Non-polar GaN is now a very hot topic. Lasers and LEDs produced using this growth direction promise to have higher efficiencies than their "polar" equivalents, owing to the removal of internal electric fields. There have even been reports this year from UCSB and Rohm of non-polar lasers. HEMTs could also benefit from the non-polar approach, as carriers in the device s channel can be fixed and adjusted by external doping species.




Ploog s research at Stuttgart and the PDI has left a major impression on academia and industry. It has also brought him friends, including many in Japan (see "Ploog fosters international relations"), where he has fostered relations between the Japanese and German III–V semiconductor communities. However, his greatest contribution is helping to establish MBE as a de facto tool for growing high-quality devices. The techniques that he established are now used to produce various III–V transistors at foundries throughout the world, including the one in Europe owned by United Monolithic Semiconductors (UMS), where the author of this article works.



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