ELECTRONIC AND OPTICAL PROPERTIES OF SEMICONDUCTING GLASSES
13-16 June, 2000
Lyon, France
The workshop was held at CECAM in Lyon (France), from June 13 to June 16, 2000. It was organized by David Drabold (Ohio University), Normand Musseau (Ohio University), Gerard Barkema (University of Utrecht) and Pablo Ordejon (C SIC-Barcelona), and sponsored by CECAM and the ESF Programme on "Electronic Structure Calculations for Elucidating the Complex Atomistic Behaviour of Solids and Surfaces" (Psi-k). It was organized in the CECAM style, with 23 invited speakers (plus the organizers). The workshop was intended to be a forum for discussion, and therefore plenty of time after each talk and at focused discussion sessions was available.
The prime goal of the meeting was to bring together active theorists and experimentalists working in the area of amorphous and glassy materials. We wanted to explore the current state of the art in methods of simulation (eg, molecular dynamics, Wooten-Weaire-Winder methods, activation-relaxation technique, etc), strengths and weaknesses of various descriptions of interatomic interactions (ranging from Keating springs to post-LDA). We wanted to determine how to better interact with experimental research in the field - what xperimentalists would most like us to compute. We focused both on the technical issues of these techniques, and on how can they be used to explain and predict the experimental behavior of glasses. More precisely, we wanted to see the following points addressed:
- How important are the details of structure to electronic properties? In which cases can we get away with modeling structure and electronic properties at different levels of accuracy (eg., model potentials for structure and ab initio methods for electrons)?
- How can we get the appropriate structure using the most advanced ab-initio interactions? To obtain a network in agreement with electronic and optical properties, what level of modeling regime (eg. WWW bond-switching, MD, ART...) and Hamiltonian (empirical interatomic potentials to ab initio [LDA, LSDA, GGA...]) is necessary? How does this depend upon the 1?
- Can atomistic level calculations provide useful input to phenomenological theories of transport and defect equilibria (the defect pool model)?
- Can conventional ab initio methods reliably model light-induced structural rearrangements? How accurate must the excited states be?
- Does the adiabatic (Born- Oppenheimer, approximation lead to significant errors in dynamics for photo-excited or doped systems?
- What are the characteristics of a structural model that is able to reproduce band tail states (which are critical to transport and optical processes)?
- What is the mechanism of doping in an amorphous material? Why is a very large concentration (~1 - 10%) of impurities required?
- Experimentally, the method of preparation influences greatly the electronic and optical properties of these glasses - is there any way to replicate such variations numerically?
- Dynamical properties appear to be one of the most promising approaches to characterize these materials; how far can we push the study of the dynamics of electrons?
- Glasses are known to show dynamics on a wide range of time scales. Can we extend the simulation time scales significantly?
All of these issues were discussed over the four days of the meeting. Some issues were resolved and of course others were not. In addition, we believe that the understanding between experimentalists and theorists was much improved - e.g., what can or cannot be measured (or computed) and why. We think that this type of understanding is essential to developing the field, especially one in which the interaction between experimentalists and theorists is so essential.
The meeting ran from approximately 9AM until 6PM on June 13,14,15 and half a day on June 16. The abstracts of talks are included below. Most talks were half an hour long with an additional 10 minutes scheduled for discussion. There were also four one hour review talks given as described below. The tone of the meeting was cordial and we maintained a frank and open discussion about the current understanding of the field, strengths and weaknesses of various theoretical schemes for modeling, and what was needed to improve the interaction between experimental and theoretical research in the field. Two 1.5 hour "roundtables" involving all participants were held to further clarify points raised in the lectures.
A "round table" discussion was led by Mike Thorpe: In search of the perfect amorphous structure. After a good deal of discussion and many helpful comments from the several experimentalists present, it was clear that many open questions remain, and the role of simulation is crucial. One example is the question of whether odd-membered rings in the archetypal amorphous semiconductor a-Si can be directly experimentally measured. It is not clear that this is possible!. A good deal of discussion focused on recent neutron diffraction experiments which suggest a surprisingly low average coordination for a-Si.
Martin Stutzmann led a roundtable discussion on Electronic and optical properties. The discussion focused on electron-phonon coupling, metastability and the need for theory to compute appropriate matrix elements in an attempt to interact more directly with experiments like Raman scattering in amorphous materials. It emphasized the need from the experimentalist for even approximate calculations of a range of quantities that had not been considered by computational researchers.
This workshop was a real success. The considerable amount of time allotted for discussion both after each talk and at the round tables promoted a healthy exchange of ideas. In particular, it was noted that a number of fundamental problems were still pending. Many suggestions and promises regarding how to solve these in the coming years were made and we look forward to progress along these directions and new fruitful collaborations initiated by this workshop.
More details may be found in newsletter 40 from page 64