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Séminaire MCBT

Wednesday 20 november 2019 à 14h00
Salle Louis Weil, E424

Orateur : Frances Hellman, University of California, Berkeley
"Ideality and Tunneling Two Level Systems (TLS) in Amorphous Silicon Films"


Low temperature specific heat and internal friction of covalently bonded amorphous silicon (a-Si) vapor deposited films with and without hydrogen show that low energy excitations commonly called tunneling or two level systems (TLS) can be tuned over nearly 3 decades, from below detectable limits to high in the range commonly seen in glassy systems. This tuning is accomplished by growth temperature, thickness, growth rate, light soaking or annealing. We see a strong correlation with atomic density in a-Si and in literature analysis of other glasses, as well as with dangling bond density, sound velocity, and bond angle distribution as measured by Raman spectroscopy, but TLS density varies by orders of magnitude while these other measures of disorder vary by less than a factor of two. Additionally, a strong correlation is found between an excess T3 term (well above the sound velocity-derived Debye contribution) and the linear term in heat capacity, suggesting a common origin. The lowest TLS films are grown at temperatures near 0.8 of the theoretical glass transition temperature Tgof Si, similar to results on polymer films and suggestive that the high surface mobility even at 0.8Tgproduces materials close to an ideal glass, with higher density, lower energy, and low TLS due to fewer nearby configurations with similarly low energy.

The TLS measured by specific heat and internal friction are strongly correlated for pure a-Si for high density films, but not for lower density a-Si (thinner films grown at lower temperature) or for hydrogenated a-Si, suggesting that the standard TLS model works for high density a-Si, but that low density a-Si and a-Si:H possess TLS that are decoupled from the acoustic waves measured by internal friction. These data require unusual TLS relaxation mechanisms ("anomalous" TLS) or an unexpectedly low TLS-phonon coupling constant γ. In the standard tunneling model, the TLS-phonon coupling constant γ is the deformation potential. We suggest this requires modification : the deformation potential varies from TLS site to site, while the coupling constant represents an average γ. Comparisons to results on other vapor deposited insulating materials will be made.

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