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Title: High-resolution analogue of time-domain phonon spectroscopy in the transmission electron microscope

Journal Article · · Philosophical Transactions of the Royal Society. A, Mathematical, Physical and Engineering Sciences

Femtosecond photoexcitation of semiconducting materials leads to the generation of coherent acoustic phonons (CAPs), the behaviours of which are linked to intrinsic and engineered electronic, optical and structural properties. While often studied with pump-probe spectroscopic techniques, the influence of nanoscale structure and morphology on CAP dynamics can be challenging to resolve with these all-optical methods. Here, we used ultrafast electron microscopy (UEM) to resolve variations in CAP dynamics caused by differences in the degree of crystallinity in as-prepared and annealed GaAs lamellae. Following in situ femtosecond photoexcitation, we directly imaged the generation and propagation dynamics of hypersonic CAPs in a mostly amorphous and, following an in situ photothermal anneal, a mostly crystalline lamella. Subtle differences in both the initial hypersonic velocities and the asymptotic relaxation behaviours were resolved via construction of space-time contour plots from phonon wavefronts. Comparison to bulk sound velocities in crystalline and amorphous GaAs reveals the influence of the mixed amorphous-crystalline morphology on CAP dispersion behaviours. Further, an increase in the asymptotic velocity following annealing establishes the sensitivity of quantitative UEM imaging to both structural and compositional variations through differences in bonding and elasticity. Lastly, implications of extending the methods and results reported here to elucidating correlated electronic, optical and structural behaviours in semiconducting materials are discussed.

Research Organization:
Univ. of Minnesota, Minneapolis, MN (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division; National Science Foundation (NSF)
Grant/Contract Number:
SC0018204; DGE-1348264
OSTI ID:
1686107
Journal Information:
Philosophical Transactions of the Royal Society. A, Mathematical, Physical and Engineering Sciences, Vol. 378, Issue 2186; ISSN 1364-503X
Publisher:
The Royal Society PublishingCopyright Statement
Country of Publication:
United States
Language:
English

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