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Using UEM to Probe Buried Structures in Real Space: An Analog to Phonon Spectroscopy

The process by which photoexcited semiconductors return to the ground state consists of a series of strongly-correlated, many-body interactions which overlap in space and time. Such behaviors have both fundamental and practical implications, which include insights into the quantum nature of matter and control of device and materials behaviors in electronic and optoelectronic applications. Accordingly, extensive work has been done to illuminate the sequence of hot-carrier excitation, carrier-carrier scattering, carrier-lattice scattering and thermalization, recombination, and lattice heat diffusion [13]. Linked to carrier-driven dynamics is the excitation of coherent acoustic phonons (CAPs). Because the behavior of these collective lattice oscillations depends on the structural and electronic properties of the material through which they propagate, CAPs can be used as an intrinsic, multi-faceted characterization tool. Indeed, CAP spectroscopy, in which transient reflectivity measurements are used to track CAPs as they travel into a material, has been used to access depth-dependent structural properties of materials and buried interfaces [4]. In addition to all-optical methods, ultrafast scattering techniques are well-suited for the study of CAP behaviors owing to the associated lattice distortions. For example, CAPs have been directly imaged with ultrafast electron microscopy (UEM), and the ability to spatiotemporally resolve discrete phonon behaviors, such as phase velocity and time-varying dispersion, has been demonstrated [5]. While CAP spectroscopy benefits from 10-nm spatial resolution in the zdirection, there are many systems where the structural or optoelectronic properties under investigation vary in plane, warranting additional study with high spatial resolution in the xy plane. It is possible, then, to imagine the utility of probing in-plane CAP behaviors with the high spatiotemporal resolutions possible with UEM, thus enabling real-space inference of spatially heterogeneous structural and optoelectronic properties via correlation with time-domain phonon behaviors.