The Use of Pulsed Electron Beams to Extend TEM Capabilities

Abstract

Due to the high resolutions and high scattering cross-sections accessible using fast electrons, characterization of electronic materials using transmission electron microscopy (TEM) is indispensable in applied materials science and engineering. Modern TEMs are highly versatile, allowing high resolution characterization of structure and morphology, chemical analysis, beam-induced current mapping or even ultrafast structural dynamics at high spatial resolution. This dissertation discusses three ways in which pulsed electron beams can be used to extend these capabilities even further. First, the push to shrink dimensionality, and the subsequent impact of thermal effects, has led to the development of methods capable of being used to quantify nanoscale thermal transport. In order to use pulsed electron beams to determine transient temperatures at atomic length scales, it is possible to rely upon the Debye-Waller (DW) effect, in which the attenuation of Bragg scattering is related to atomic thermal energies. However, other factors, in addition to mean atomic displacements, can affect (and even dominate) the intensity of Bragg reflections, distorting the measurement. In this work, the degree to which structural specimen effects impact thermal measurements are quantitatively studied in order to better facilitate the use of pulsed electron beams to determine transient specimen temperatures. Second, 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. Because the behavior of collective lattice oscillations depends on the structural and electronic properties of the material through which they propagate, coherent acoustic phonons (CAPs) resulting from such properties can be used as an intrinsic, multi-faceted characterization tool. Indeed, optical CAP spectroscopy has been used to access depth-dependent structural properties of materials and buried interfaces, but is limited in its in-plane spatial resolution. In this work, we demonstrate an analogous CAP spectroscopy using UEM, in which CAP phase velocity measured in real space is shown to be sensitive to changes in atomic structure. This facilitates future work in using UEM to access spatially-resolved information about buried structures and optoelectronic properties via high-resolution, real-space measurements. Third, the energy with which electrons propagate in the TEM column leads inevitably to damage, which particularly limits the study of radiation-soft materials such as organic materials, biological specimens, and any other samples containing low-Z atoms. In this work, we show that pulsed electron beams can be used to mitigate the damage caused to organic specimens compared to a stochastically-emitted electron beam. We demonstrate this effect both in the organic crystal hexatriacontane and in the hybrid metal halide perovskite, methylammonium lead iodide. Further, the temporal manipulation of pulsed beams can be leveraged to gain kinetic insights into the processes of beam damage. We show preliminary results in which the disparate damage rates in perovskites potentially elucidate a novel step in the damage mechanism.