High-resolution analogue of time-domain phonon spectroscopy in the transmission electron microscope
High-resolution analogue of time-domain phonon spectroscopy in the transmission electron microscope
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DOI：
10.1098/rsta.2019.0598
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发表时间：
2020-10
期刊：
影响因子：
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通讯作者：
Elisah J. VandenBussche;D. Flannigan
中科院分区：
文献类型：
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作者：
Elisah J. VandenBussche;D. Flannigan
半导体材料的飞秒光激发会导致相干声学声子（CAP）的产生，其行为与固有的和工程的电子、光学和结构性质有关。虽然经常使用泵浦-探测光谱技术进行研究，但纳米结构和形态对CAP动力学的影响可能具有挑战性，难以用这些全光学方法解决。在这里，我们使用超快电子显微镜（UEM）来解决CAP动力学的差异所造成的结晶度在制备和退火的GaAs薄板。在原位飞秒光激发后，我们直接成像的高超声速CAP的产生和传播动力学在一个主要是无定形的，并在原位光热退火后，主要是结晶片层。通过从声子波前构造时空轮廓图，解决了初始高超音速速度和渐近松弛行为的细微差异。晶体和非晶GaAs中的体声速的比较揭示了混合非晶-晶体形态对CAP色散行为的影响。此外，退火后的渐近速度的增加建立了定量UEM成像的灵敏度，通过不同的结合和弹性的结构和组成的变化。扩展的方法和结果在这里报道阐明相关的电子，光学和半导体材料的结构行为的影响进行了讨论。这篇文章是讨论会议议题“动态原位显微镜与结构和功能”的一部分。
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. Implications of extending the methods and results reported here to elucidating correlated electronic, optical and structural behaviours in semiconducting materials are discussed. This article is part of a discussion meeting issue ‘Dynamic in situ microscopy relating structure and function'.