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Good morning, Gentlemen!

On behalf of the Nobel Foundation, I should like to welcome you to the Nobel Symposium on "Heterostructures in Semiconductors". It gives me great pleasure to see so many colleagues and old friends from all over the world in the audience and, in particular, to bid welcome to our Nobel laureates, Prof. Esaki and Prof. von Klitzing.

In front of a different audience I would now commend the scientific and technological importance of heterostructures in semiconductors and emphatically emphasise that heterostructures, as an important contribution to microelectronics and, hence, information technology, have changed societies all over the world. I would also mention that information technology is one of the most important global key industries which covers a wide field of important areas each of which bears its own character. Ever since the invention of the transistor, we have witnessed a fantastic growth in semiconductor technology, leading to more complex functions and higher densities of devices. This development would hardly be possible without an increasing understanding of semiconductor materials and new concepts in material growth techniques which allow the fabrication of previously unknown semiconductor structures.

But here and today I will not do it because it would mean to carry coals to Newcastle. I will therefore not remind you that heterostructures were already suggested and discussed in detail a long time before proper technologies were available for the fabrication of such structures. Now, heterostructures are a foundation in science and part of our everyday life. Though this is certainly true, it is nevertheless fair to say that not all properties of heterostructures are yet understood and that further technologies have to be developed before a still better understanding is obtained. The organisers therefore hope that this symposium will contribute not only to improving our understanding of heterostructures but also to opening new perspectives for future applications.

We are most grateful that you agreed with the special format of the symposium which clearly does not follow conven- tional conferences. Allow me to call your special attention once again to two main differences: The presentations are not review papers praising already achieved break-throughs but introductions to a list of open questions and issues for which our understanding is still unsatisfactory. To give such presentations requires courage and scientific integrity. I would like to thank all speakers now already for their willingness to cope with such a difficult task.

We have allocated at least 50 minutes for discussion after each presentation not only for discussing the paper as such but, if possible, to find answers to the open questions. If one or several participants in the audience during the discussion think they can contribute to improving our understanding of heterostructures, they are invited to write their ideas up and, if the referees agree, we are more than happy to publish these ideas in the proceedings.

We admit that the program is rather demanding. For that reason, we plan to have a break on Thursday afternoon by first going to Denmark and touring the Hamlet castle of Kronborg. We then sail back to Sweden and will be hosted by the Krapperup castle where we will have a candle-light dinner and thereafter a baroque music concert featuring the Concerto Copenhagen. All participants, observers, and accompanying spouses are invited and we hope you will all enjoy the excur- sion.

The local organising committee acknowledges with pleasure the financial and all other support received from the Nobel Foundation and the Nobel Institute of Physics as well as the initial initiative taken by the chairman of the Nobel Com- mittee for Physics, Prof. Nordling, who was the first to suggest this Nobel Symposium on "Heterostructures in Semicon- ductors". Special thanks also to the members of the program committee who have been of inestimable value in putting together the program and suggesting speakers and potential participants.

Moreover, I should like to thank Mrs. Medborg, Dr. Olajos, Dr. Mats Kleverman and in particular Dr. Ask who took care of all time-consuming negotiations and details. Without their help, the Symposium would not be what it is. The Nobel Foundation, the Nobel Committee for Physics, the Nobel Institute of Physics and the local organisers hope that you will enjoy the Symposium and I promise you, we will all do our best to make your stay as pleasant as possible and this Symposium a success. Once again welcome to Arild and the Symposium!

The unifying concept underlying all heterostructure physics is the idea that energy gap variations act as quasi-electric forces on the electrons and holes in the structure, even in the absence of a true electric field. They give the device designer a new degree of freedom in designing structures exhibiting phenomena fundamentally unobtainable in homostructures.

A useful classification of heterostructures is by lineup type, distinguishing straddling, staggered and broken-gap lineups. Examples of each type are discussed. The possible combinations of two or more semiconductors into a heterostructure are constrained by bonding considerations, both with regard to bond length compatibility (lattice matching) and valence compatibility. In quantized structures, strain may be a deliberate design element.

The industrial impact of heterostructure devices is likely to be dominated by the large economic system leverage they tend to provide, more than by manufacturing volumes approaching those of mainstream silicon technology - with the possible exception of LEDs.

We describe the nanofabrication of the first three-dimensional (3-d) photonic crystals with a forbidden photonic bandgap lying in the near infrared region of the electromagnetic spectrum, 1.1 μm < λ < 1.5 μm, just beyond the electronic band-edge of GaAs. We fabricated these structures by chemically assisted ion beam etching through a triangular hole array mask, defined by electron beam lithography on GaAs. The 3-d forbidden photonic bandgap was spectrally tuned by 2-d lithographic control of the 3-d spatial periodicity. Optical transmission spectra were generally in good agreement with microwave frequency transmission on centimeter scale models. Nevertheless, we find that the mid-gap optical reflectivity is surprisingly sensitive to structural errors in the photonic crystal, degrading the optical rejection from an expected 95% to an observed 80%. We suggest that mid-gap attenuation in the most vulnerable k-space directions, rather than overall thickness, is the relevant Figure-of-Merit for nano-scale photonic crystals.

Single-electron tunneling through zero-dimensional structures formed by lateral confinement of a two-dimensional electron gas is analyzed for both the vertical (current flow perpendicular to the quantum disk) and planar (current flow in the plane of the disk) structure. The discrete energy spectrum of the quantum dot influences drastically both the linear and the nonlinear transport through the dot. It is shown that the properties of a double dot system can be well described on the basis of a quantum molecule. Finally, a new approach for directly grown single-electron transistors is presented.

A short overview is presented on measurements of phase evolution and temporal behavior of electrons in mesoscopic systems. Since ubiquitous conductance measurements provide only limited information about a system (its transmission coefficient), the complex values of all elements of the scattering matrix (transmission and reflection amplitudes), measured via interference experiments, might shed more light on systems (in particular interacting systems) properties. In addition temporal measurements will support our understanding of correlation among the electrons and their dwell times in different systems. These properties are expected to be manifested in observed spectral peaks and a modified behavior of quantum shot noise.

The early history of semiconductor heterostructures and their applications in different electronic devices is described. The article also contains a short historical review of the physics, technology of preparation and applications of quantum wells and superlattices. Recent progress in quantum wires and especially quantum dots structures and future trends and perspectives of these new types of heterostructures are discussed.

This is a brief summary of experimental results on mass and wave vector of composite fermions in two-dimensional electron systems in high magnetic fields. In the spirit of the workshop, it emphasizes the difficulties and inconsistencies inherent in the experimental data and in their interpretation.

Recent work on electron transport through classically chaotic heterostructure systems is briefly reviewed. The use of tunnel-current spectroscopy to map the energy levels of the classically chaotic electron motion in a quantum well at high magnetic field is described in detail. This technique provides the first experimental evidence for the periodic scarring of individual wavefunctions in a chaotic system.

Recent progress in understanding the behavior of condensed excitonic systems is reviewed. The Bose-Einstein condensed excitonic insulator is studied numerically within the mean-field pairing approximation. In particular, we examine in some detail a system of spatially separated, two-dimensional (2D) electrons and holes, as a candidate system for study in semiconductor heterostructures. Based on these calculations, we suggest that it may be possible to create a droplet of high density exciton condensate fluid in semiconductor quantum-well structures. In heterostructures, the recombination of electrons and holes is dipole-allowed, which presents both difficulties and opportunities. The excitonic insulator state should be superradiant, which provides a direct measure of the coherence of the ground state. Additionally there is a gap between the absorption and emission spectrum, and collective modes lying within this quasiparticle gap. Coupling of the condensate to radiative modes of an optical cavity may offer an opportunity to manipulate the condensate and its optical response.

High mobility electron and hole channels have been achieved in selectively doped strained Si and Ge layers, respectively. Electronic transport properties of such structures are discussed.

An overview is given of recent developments in wide bandgap semiconductor lasers. Following a description of the current device status of the ZnSe and GaN-based heterojunction emitters, physics of excitons at high densities and optical microcavities are considered; respectively.

The progress in growth and understanding of semiconductor hetero-structures made the realization of artificial materials like the semiconductor superlattices possible. In the last few years, optical experiments have unambiguously proven that Bloch oscillations exist in semiconductor superlattices. In this paper, the key experiments and their contributions to the understanding of Bloch oscillations are briefly discussed. Then, open questions and possible experiments to answer them are discussed.

A brief review is given of the current status of ab initio theory for calculating bulk and interface properties of materials. This is followed by suggestions and speculations for creating new types of superlattice and heterojunction systems based on standard semiconductor superlattices subjected to pressure and on nanotubes.

In this review the status and future of heterostructure devices is discussed. The author concentrates on III/V and Si/SiGe. Performance and applications are folded to the data and expectations of the micro- and opto-electronic market and to the traditional Si-mainstream. New trends, i.e. the SIA-roadmap, are checked how heterodevices can fit in. Only the most attractive candidates for applications are considered, i.e. the heterobipolar-, the hetero field effect-transistors, the resonant tunnel diode and to a less extent, some optoelectronic devices. Considered figures of merit are frequencies, transconductance, noise at high and low frequencies, threshold voltage, power delay, threshold current and quantum efficiencies. It is pointed out how to optimize those by material and design. Extrapolations to the future potential of heterodevices are made, just taking the claimed scaling of lateral dimensions into consideration. Field of applications are presented, where heterodevices offer exclusive qualities, i.e. high frequency transmission and sensors, and new mixed systems. In the case of logic the trend goes to nanoscaled devices and ICs targeting nanoelectronics beyond traditional electronics. Heterostructure layers allow a vertical nanoscaling and thus give an additional degree of freedom for designing and optimation. It is an attractive challenge for scientists and engineers to solve the related technological problems like thin, low thermal budget oxides, like defect free buffer layers etc. Special attention is put on Si/SiGe, which is now on an upswing in electronics and photonics.

The advent of semiconductor heterostructures has opened many opportunities for novel physics fields, or has enabled to study with precision many concepts which were difficult to apprehend in bulk materials. How can we try to define the future developments in the physics of heterostructures? We identify four main (overlapping) directions: (i) the physics of the heterostructures materials system (fabrication processes, materials issues, bandgap discontinuities, approximations and their breakdown at small dimensions...); (ii) the follow-on of existing physics topics now carried out in heterostructures; (iii) the novel physical concepts and experiments which might be carried out in the future in heterostructures (a bottom-up approach to new physics driven by the capacity of heterostructures to act as microlaboratories for demonstrating new physical concepts, even originating from other fields of physics); (iv) the novel physics to be done in heterostructures in order to answer new demands, be it on general motivations (i.e. advances towards the limits of computing or communications) or on evaluating the physical limits of existing heterostructure concepts (i.e. defining the physical limits of electrical to optical energy conversion in (thresholdless?) lasers or in LEDs).

We then describe some of the future physics on five examples: (i) the phonon relaxation issues in quantum dots; (ii) the search for non-epitaxial deposition techniques (an heterostructure "paint"), with its obvious solution through polymer/molecular electronics, considering organic molecules as ultimate heterostructure systems; (iii) the basic physics items and the wide applications stemming from the design of wavelength-scale structures such as microcavities and photonic bandgap materials; (iv) the development of materials and structures for a full quantum control of electron or photon dynamics; (v) the search for ultimate solutions based on novel physics for computation or telecommunications.

This paper does not intend to be a full-fledged document on the future of heterostructure physics. It should rather be thought as trying to provoke thinking on the many opportunities offered by heterostructures. An examination of the numerous past achievements of semiconductor heterostructures induces one to be very modest in predicting the future: many of the new physical concepts appeared by surprise; Within the predicted physics concepts or applications, most did hardly work, and some of the most prominent ones today work for other reasons than predicted!

Quantum cascade lasers are new unipolar light sources based on electron resonant tunneling and optical transitions between quantized conduction band states. Quantum engineering of the electronic energy levels, wavefunctions, matrix elements and scattering rates is used to design population inversion and optimize laser gain and overall performance. The wavelength can be selected over a broad range of the infrared spectrum using the same heterostructure material by tailoring the active layer thickness. High power room temperture operation at 5 and 8.5 μm has been achieved. These are the first semiconductor lasers to operate at 300K and above in this wavelength region.

The stimulated emission of strongly confined heterostructures, namely highly polar II-VI quantum wells, is discussed in terms of a phase transition between the exciton gas and the free-carrier gas. The extent to which the transition has proceeded under the operation condition of the device (high temperature and high injection density) depends on the many-body renormalization of the exciton binding energy. The general applications of the model are finally discussed.