Interpretation of meV Resolution Phonon EELS Data (original) (raw)
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Microscopy and Microanalysis
Phonon transport is the dominant factor in determining thermal conductivity in semiconductors and is greatly modulated by nanostructures, interfaces, and defects but the physics of the nanoscale process has hardly been explored by experimental means. The phonon resistance provided by these structures are often a result of a mismatch between the local phonon density of states (LDOS) [1], studies of which have eluded optical measurement techniques due to their insufficient spatial resolution. Additionally, optical, and other conventional phonon spectroscopies have no way of investigating the phonon dynamics of individual interfaces and as a result, cannot aid in the physical understanding of nanoscale thermal transport. However, recent developments in electron microscopy have enabled the acquisition of vibrational spectra at sub-nanometer resolutions via monochromated electron energy loss spectroscopy (EELS) [2]. Armed with this technical capability, we demonstrate a mapping of phonons revealing interface dynamics at the nanometer scale. Utilizing momentum-averaged and resolved EELS, we image phonon reflections from nanostructure interfaces revealing the dynamics of phonon propagation and reflection.
The phonon contribution to high-resolution electron microscope images
Ultramicroscopy, 2003
The amount of phonon scattering as a function of specimen thickness is determined for a clean silicon sample, free from amorphous surface layers, by measuring the diffuse scattering in energy-filtered convergent-beam diffraction patterns. It is found that for a 25 nm thick sample, only 7.5% of the intensity scattered to less than 18 nm À1 is phonon scattered. This means that in a typical high-resolution sample most of the diffuse scattering is caused by surface amorphous layers rather than phonon scattering.
2004
The contribution of electrons that have been phonon scattered to the lattice fringe amplitude and the background intensity of a high-resolution electron microscope (HREM) image is addressed experimentally through the analysis of a defocus series of energy-filtered off-axis electron holograms. It is shown that at a typical specimen thickness used for HREM imaging approximately 15% of the electrons that contribute to an energy-filtered image have been phonon scattered.
Advances in Momentum Resolved EELS
Microscopy and Microanalysis
Vibrational modes affect conduction of heat and sound in solids, and are altered by local structure such as defects and interfaces. Angle Resolved Electron Energy Loss Spectroscopy (AR-EELS) within the Scanning Transmission Electron Microscope (STEM) provides a way to probe the four dimensional phonon band dispersion relation, S(qx, qy, qz, ω), with nanometer spatial resolution [1]. The technique has benefited from significant advances in recent years, including increased efficiency by parallel acquisition using slot-shaped spectrometer entrance apertures and the introduction of low-noise high dynamic range direct detectors for EELS [2]. Together with high-brightness electron sources, brightness-preserving monochromators, and next-generation spectrometers, these improvements have reduced the acquisition time for phonon band dispersion diagrams from hours [3] to minutes [2]. Even with these advances, phonon band structure measurement in the STEM has been limited to ideal systems containing light elements with Z=5-7 (e.g. boron nitride, or carbon), where the phonon structure spans roughly 0-200 meV energy loss. Phonon band structures in materials with heavier atoms, where the energy range can be significantly smaller, have been elusive until now.
Electron-phonon interaction in two-dimensional systems: A microscopic approach
Superlattices and Microstructures, 1991
We present a calculation of the electron optical-phonon scattering rates in GaAs/AlAs quantum wells, based on a very accurate microscopic description of the phonon spectra. The results show that-besides the contribution of confined modes-a very large contribution originates from interface phonons of both GaAs-like and AlAs-like character. We then compare our results for phonon displacements and potentials, as well as for scattering rates, with those obtained from several macroscopic phonon models. We are therefore able to provide indications for selecting the model which allows the most appropriate simplified description of vibrations at the wavevectors relevant to the interaction with carriers. I. Introduction The electron-optical-phonon (e-ph) interaction plays a crucial role in the transport properties of two dimensional (2D) semiconductor systems. The cooling rate of hot carriers in quantum wells (QW's) and superlattices (SL's), as well as the room temperature mobilities of modulation doping structures are determined by the strength of the electron coupling to the optical vibrations. With respect to the three dimensional bulk case, the nature of the e-ph interaction in 2D systems is drastically modified by the presence of the heteroin-terfaces between the different materials. Despite the growing interest in the field, and the availability of ultrafast spectroscopies providing direct information on the peculiarity of such processes, from the theoretical point of view a long-standing controversy is still open on the effect of the reduced dimensionality on the eph interaction'. The discussion, concerning specifically the correct description of the phonon modes which are relevant to the electronic scattering, comes in spite of the large amount of information existing on the vibrational properties of SL's2, and points out the little exchange that has occurred sofar between the researchers
Electron–phonon coupling and lifetimes of excited surface states
Surface Science, 2005
Many important chemical and physical phenomena are influenced by inherent dissipative processes, which involve energy transfer between the electrons (electron-electron scattering) and between the electrons and the ionic motion (electron-phonon scattering). The non-adiabatic interaction between the valence electrons and the ion motion in a solid reveals the break down of the Born-Oppenheimer approximation. To pin down the influence of the electron and phonon structure on these scattering processes the two-dimensional surface states are ideal both from an experimental and theoretical point of view. Several experimental techniques presently in use are able to give information about the lifetime of an excited electron or hole in the surface state band. With help from advanced theoretical calculations it is possible to sort out the relative importance of the electron-electron and electron-phonon scattering processes responsible for the quenching of the excitation and to point out the key parameters of the electron and phonon structure.