Preface: phys. stat. sol. (c) 4/9
Authors: Landwehr Gottfried , Molenkamp Laurens W. , Umbach Eberhard 
Published: August 2007

II–VI semiconductors were already systematically investigated in the 1930s, however, research in this area faded in the 50s in favor of type IV and III–V semiconductors. II–VI research then was resumed in the 70s. The establishment of extensive II–VI molecular beam epitaxy facilities in Würzburg in the early 90s created ideal conditions for a Collaborative Research Centre on these materials. In 1994, the Würzburg Sonderforschungsbereich (SFB 410) on II–VI physics and chemistry was established by the Deutsche Forschungsgemeinschaft. A variety of powerful characterization methods available in Würzburg were successfully employed, involving (low temperature) optical, magneto‐optical, and transport methods, surface analysis and X‐ray techniques, and, especially in recent years, advanced lithography. The experiments were accompanied by detailed theoretical work, and a close cooperation between all groups was successfully established.

The establishment of SFB 410 allowed for long‐term planning and resulted in continuity. During the 12 years of its existence, a large number of interesting and important results were obtained. In the present special issue a selection of results obtained during the last few years is presented. Some of the contributions in this issue have partly review character, but all of them contain new results. The titles of the 25 contributions to this issue give a good impression of the addressed topics and the employed methods.

While in the early years of the SFB, much of the research supported the application of II–VI semiconductors as lasers in the blue/green spectral range. In recent years the emphasis shifted to compounds containing manganese ions (diluted magnetic semiconductors). The availability of these materials has allowed the SFB to play a leading role in the development of the research area of spintronics (the control of electron spins by an electric field) with the demonstration of II–VI facilitating spin injection and the creation of magnetic resonant tunneling diodes that function as spin filters. Closely related to spintronics is the study of the influence of spin–orbit coupling on quantum transport. The outstanding II–VI material here is HgTe which boosts mobilities as well as spin–orbit splittings that are much larger than those available so far in III–Vs. The development of lithographic techniques tailored for this material has enabled the fabrication of nanostructures exhibiting novel phenomena such as the Aharonov‐Casher phase and the spin Hall effect. Highly interesting (magneto‐)optical properties have been found in quantum dots and micro resonators.

Many of the results were only achieved because of a considerable improvement and thorough, also theoretical, understanding of epitaxial layer and self‐organized quantum dot growth, as well as a detailed control of surfaces and interfaces. Careful surface studies employing a variety of electronic and geometric surface techniques have added considerable knowledge on the two‐dimensional (interface) properties of many II–VI combinations. Ad hoc developed theories on magnetic transport and band structure calculations have largely improved the physical understanding of the experimental results and of electronic, (magneto‐)optical, and (magneto‐)transport properties of some II–VI materials. Finally, a large effort was made to synthesize, model, and, in particular, characterize II–VI nanoparticles, with (or without) organic (or inorganic) shells. A significant effort was made to analyze in detail especially those nanoparticles that are neither molecules nor solids having diameters below three nanometers.

Altogether, this collaborative research effort (“Sonderforschungsbereich”) was very successful in many respects, for example in the number and quality of the research results which accordingly lead to numerous publications and presentations, in various international co‐operations, in the education of many diploma and doctoral students, in the development of several young scientists who got their habilitation and became professors, and in the further development of and improved collaboration between the involved three faculties of Würzburg University. Most of the results would not have been achieved without the extremely well developed co‐operation between all groups of SFB 410.

The three speakers of SFB 410 would like to thank their colleagues for this extraordinary co‐operation, the DFG for 12 years of financial support, the reviewers for their critical monitoring, and the administrative people for their effective involvement.