Objective
We propose key experiments and theoretical calculations to better understand the nature of defects (trapping centres) in amorphous dielectrics: silicon oxide (SiO2) and nitride (Si3N4). This should help in solving the long-standing problem of the nature of traps in these compounds and give essential information on the role of Si-Si bonds as electrons or hole traps in SiO2. The results will be essential for many applications in solid-state physics an microelectronics. The localisation of carriers in Si3N4 is used in electrically erasable memory devices. The localisation of carriers generated by radiations in solids is widely used in radiation dosimetry.
Pioneering experimental works have suggested the possible Wigner cristallisation of trapped electrons or hole when the number of injected carriers is much weaker than the number of traps. A possible interpretation of the absence of Electron Paramagnetic Resonance in Si3N4 is the pairing of neighbouring localised carriers due to resonant exchange interactions. Experiments have been performed for only one density of localised carriers; they are insufficient to conclude definitively. We propose key experiments to probe this exciting interpretation: at low carrier densities the pairing between neighbours should disappear because exchange interactions are vanishingly small, the Wigner solid is expected to be paramagnetic and one should observe electron paramagnetic resonance. Hence we propose to investigate electron spin resonance at various densities of injected carriers. By increasing the density of injected carriers, we should observe a critical value at which the Wigner solid orders antiferromagnetically and the EPR line disappears. It is possible to vary the density of injected carriers by several orders of magnitude in Si-implanted SiO2 devices. We shall begin by this study. The more difficult experiment of varying the carrier density in Si3N4 will be carried out after.
If the Wigner cristallisation and antiferromagnetic order is confirmed, this should be the first evidence for a three-dimensional Wigner solid with significant exchange interactions.
It will then be useful to place it in the more general context of an almost localised system of identical fermions with exchange interactions. Some similarities should be found with another simpler systems of identical fermions: localised spin 1/2 3He nuclei.
For this reason, part of the project should contribute to answer remaining questions on antiferromagnetic ordering in solid 3He, a fascinating magnetic system that has suscitated a lot of experimental and theoretical works during the last two decades.
We plan to investigate experimentally the unexplored domain concerning the effect of strain on the ordered magnetic phases in bcc 3He. We will use precise sound-velocity and attenuation measurements.
We also propose an original approach for the description of the highly frustrated quantum states, using exact recursion relations that can be found on hierarchical Be the-like lattices that could be reasonable approximations to the real lattices for two-dimensional solid 3He films and three-dimensional bcc 3He.
The whole project could be an important contribution to the fundamental concept of exchange among almost-localised identical fermion formulated by Dirac a long time ago and leading to a generalised Heisenberg model, as observed in solid 3He.
Call for proposal
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91191 Gif-sur-Yvette
France