Report to the 1999 General Assembly for 1996-99
The Commission has a regular meeting every second year during the main conference in the field, the International Conference in Semiconductor Physics (ICPS). Thus one meeting took place during ICPS 23 in Berlin July 1996, and another meeting during ICPS 24 in Jerusalem in August 1998. In between these formal meetings correspondence with e-mail between the Commission members is found appropriate to cover discussions and decisions on developing matters.
The general ICPS conferences are important events in the field, and the Commission takes a strong interest in the preparation of this conference. The ICPS 23 conference in Berlin was held in the facilities of the Technical University, which were adequate for the purpose. ICPS 23 attracted about 1200 participants, which is more than the average over the last decade. Participants from 41 countries were registered, and substantial special funds were raised to assist participation from less developed countries. In particular the efforts to support delegates from the Former Soviet Union countries were successful, with 104 participants.
ICPS 24 was held in Jerusalem. Due to the unrest in Israel in the last years it was not clear whether the original planning for this conference could be followed. During the years 1997 and 1998 there were intense discussions within the Commission and with the organizers about alternative solutions, should the personal security for delegates be at risk. Fortunately there was a calm period in 1998, and the Conference in Jerusalem was indeed very successful, with excellent arrangements. The number of participants was lower than before, but still close to 800, which is quite satisfactory. The Young Authors Best Papers Award sponsored by IUPAP was given to nine young researchers giving outstanding contributions at the conference.
During the C8 Commission meeting in Jerusalem it was decided that ICPS 25 would be held in Osaka in September 2000. For ICPS 26 in 2002 Edinburgh is proposed as the site.
There are interesting trends to be noted in the relative importance of various subfields on semiconductor physics, as reflected in the conference program. Heterostructures and superlattices continue to dominate the volume of the program, complemented by a strong growth in the subfield of very small quantum systems. Traditional areas like bulk properties and defects represent a shrinking part of the field, although some of the variation between the recent conferences may reflect the composition of the respective program committee.
The conference also follows the tradition of a proper coverage of the applied developments in semiconductor technology, in important subfields like silicon-based microelectronics and optoelectronics based on III-V materials. The close contact of this conference in basic semiconductor physics with the large community in the broader but closely related materials science area leads to a fruitful cross fertilization, of benefit for both communities. These topics were covered in plenary talks, attended by nearly all delegates, at both ICPS 23 and ICPS 24.
The Commission has elected two new delegates as Associate members of C 8, from C10 (C A Murray) and from C5, (H Ott), recognizing the strong common interests with these two Commissions.
The Commission has also urged its members to use all opportunities at national meetings to spread information about the role of IUPAP.
During the period of fall 1996 to fall 1998 the following international conferences in the C8 field have been sponsored by IUPAP:
New developments in the field.Although there has been a steadily decreasing trend in the interest of bulk semiconductor properties over the last decade (this area being rather mature), a clear revival has occurred recently in the subfield of wide bandgap semiconductors, notably SiC and III-V nitrides. The key to a successful development in these materials systems lies in the important recent advances in crystal growth techniques. The quality of bulk SiC is rapidly improving, and epitaxial SiC of excellent quality can now be prepared at 2000 oC. Important applications in power devices and high temperature devices are strong driving forces for the present developments. For the III-V nitrides the already successful realization of light emitting devices has initiated a strong development of epitaxial techniques to prepare multilayer device structures of lower defect density, e g using lateral epitaxial overgrowth on patterned heterosubstrates. This has proven to be a useful way to produce purple lasers with relevant operating lifetime (> 10000 hrs). The possible outcome of these efforts in a broader long-term perspective is the ability to grow a complicated device structure with a low defect density on virtually any desired substrate. SiC and III-V nitrides represent an unusual class of semiconductors that are extremely tolerant to a harsh environment and extreme operational conditions. Interesting physical properties occur in strongly strained multilayer structures, but most of the physics remains to be explored. In particular defect physics has recently seen a revival due to the strong relevance for the wide bandgap materials. It should also be mentioned, that a very active development has occurred in growth and studies of II-VI based quantum structures during the last years. The material is of very good quality, and interesting physics has been obtained, e g on fractional layer superlattice structures. Light emitting devices have been produced, but for the II-VI materials the degradation problems due to defect production during operation are still serious.
Many advanced device structures have been explored in the history of semiconductor physics. Indeed the properties of excitons in a microcavity, with strong exciton-photon coupling and photon confinement, define an area that is intensely studied in many materials systems, revealing new physics related to both semiconductors and optics. Recently there has been an upsurge in the interest of microcavity devices, i e a device which combines carrier control with photon control. Vertical microcavity lasers have been demonstrated in laboratories, and are expected to reduce the threshold for lasing dramatically, which is of considerable applied interest. For light emitting diode applications the advantage of microcavity structures is a much higher external quantum efficiency, due to the facilitated escape of photons from such structures. Such devices will be commercialized during 1999.
An interesting recent development in solid state optics has been photonic lattices. So far these have essentially been studied experimentally at longer wavelengths (microwaves) in metallic systems, but recently it has been possible to realize and study 2 dimensional photonic lattices in semiconductor systems, with photonic bandgaps in the range of the electronic bandgaps. These systems are potentially very interesting for photon control in optical semiconductor devices, such as microcavity light sources and integrated optoelectronic structures.
Semiconducting organic polymers have attracted increased attention recently. The basic work to understand the electronic structure of various polymers has laid the ground for fabrication of device structures for light emitting diodes in the visible spectral range, and recently "plastic lasers" have also been demonstrated. Degradation problems still exist for devices, but the recent achievements do promise commercial devices soon.
Another area in fast development for semiconductors since more than a decade is the dynamic properties on a very short time scale (femtoseconds and picoseconds). This is particularly important for quantum structures, where the distribution of electron energy states can be tailored at wish. Coherent oscillations related to interference between different electron states can be studied in real time, and ir optical emission is observed on the fs time scale related to such oscillations. Nonlinear phenomena are very strong in this time domain, promising interesting future device applications. The development of spin dynamics in such systems has also been studied in real time. It has recently been demonstrated that spin transport can be directly monitored, possibly promising development of spin electronics. Other applications include the recent development of photoemission studies on an fs time scale, allowing real time studies of the electronic processes on surfaces, including e g relaxation phenomena in surface structures of semiconductors. Real time studies of ballistic transport in semiconductor structures have been demonstrated. Device studies on this time scale evidence hot carrier phenomena directly from optical transients monitored in silicon integrated circuits.
Hydrogen-related properties have been studied in semiconductors for nearly two decades, and are found to have great practical significance, since hydrogen is easily incorporated inadvertently into most semiconductors. The ability of hydrogen to passivate dopants and defects was recognized early, but the structure of hydrogenrelated centers has only recently been understood. It has recently been discovered that hydrogen in many materials is a negative U defect. It may then control the Fermi level position and strongly interfere with doping, and in fact sometimes facilitate desired incorporation of dopants.
An area of great fundamental interest is the behavior of two-dimensional electrons in semiconductors in a high magnetic field. The idea of composite particles of Boson or Fermion character, involving fractional charge and attached magnetic flux quanta provides an explanation of the fractional quantum numbers observed in transport experiments. This is a fascinating field of basic physics, and attracts many physicists although the ideas were presented more than 10 years ago. Experiments are also refined to demonstrate directly the fractional charge. The classical developments in this field were honored with the Nobel Prize in Physics to R Laughlin and H Störmer in 1998.
Nanostructures involving semiconductor materials provide a fascinating area, which has now occupied a large fraction of semiconductor physicists for a decade. New techniques invented to produce such materials with nm scale features go hand in hand with the development and use of new techniques to characterize the individual features of such a structure with nm resolution microscopy and spectroscopy. Part of this development is intimately connected with other areas of condensed matter physics, where nm size structures are intensely studied, such as e g quantum transport in metals. Very analogous effects are seen in semiconductors, and single electron transfer processes are detected and studied both electrically and optically (near field spectroscopy). The Coulomb blockade processes in semiconductor quantum dots were recently studied. An active area is the study of noise in electrical transport data, related to fundamental fluctuations in the charge structure in ultrasmall systems. Another interesting topic has been the demonstration of single electron processes at a single defect, both in optical measurements and in electrical transport. Single point defects have also been visualized directly via STM in a semiconductor surface area. New instrumentation is being developed rapidly in this field; the atomic force microscope (AFM) is now already a standard technique in most laboratories. The ultimate goal now is to combine the high spatial resolution in optical and electrical measurements (single atom processes) with magnetic resonance techniques, which sense the spin quanta of single electrons or nuclei.
Mesoscopic physics and nanostructures nowadays dominate the menue at the basic semiconductor physics conferences. Indeed there will be more interesting discoveries to come in this field. So far, the possibilities to use these nanostructured materials in devices remain uncertain, it is not clear that the improvements which are predicted for some device properties warrant the advanced processing steps that are necessary to develop in order to move into industrial production.
On the theory side the fast development of computer capacity leads to increasingly reliable calculations of both geometrical and electronic structure of semiconductors. In particular studies of ultrasmall systems have been strongly developed recently, correlated with the experimental situation mentioned above. An encouragingly strong activity is taking place in the area of surface structure calculations. Also, diffusion processes in general, and surface diffusion in particular, have attracted much attention recently, and reliable results seem to appear. These results are of vital importance, e. g. for the understanding of mechanisms of crystal growth, in particular epitaxy where the growth often occurs via step flow processes, which can now be theoretically modeled. Interface studies constitute another area where recently substantial progress has been made. The detailed structure of semiconductor hetero-interfaces is extremely important, and can now be precisely predicted, along with other related parameters such as band offsets. These studies include properties like spontaneous polarization and piezoelectric effects induced at such interfaces. Needless to say, this is an area of strategic importance for the semiconductor-based industry.
B. Monemar (Secretary)