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Micro-low energy electron diffraction (μLEED) is frequently used in conjunction with low energy electron microscopy (LEEM) to learn about local surface structural features in small selected areas. Scanning μLEED measurements performed with a very small electron beam (250 nm) can provide precise quantitative information about structural variations with high spatial resolution. We have developed the Source Extraction and Photometry (SEP) - Spot Profile Analysis (SPA) tool for evaluating scanning μLEED data with high throughput. The capability to automate diffraction peak identification with SEP-SPA opens up the possibility to investigate systems with complex diffraction patterns in which diffraction peak positions vary rapidly for small lateral displacements on the surface. The application of this tool to evaluate scanning μLEED data obtained for defective graphene on Cu(111) demonstrates its capabilities. A rich rotational domain structure is observed in which a majority of the graphene is co-aligned with the Cu(111) substrate and the significant remainder comprises domains with large rotations and small sizes that are comparable to the small beam size., (Copyright © 2019 Elsevier B.V. All rights reserved.)

We present the extended Fourier Optics (FO) approach for modeling image formation in aberration-corrected low energy electron microscopy (ac-LEEM). The FO formalism is also generalized for image simulations of one or two-dimensional objects in ac and uncorrected (nac) LEEM. A comparison is made of the extended FO approach presented here and the extended contrast transfer function (CTF) approach for ac-LEEM that was developed earlier. The mathematically rigorous extended FO approach gains an advantage under conditions, particularly defocus, that partial coherence of the illumination may compromise the validity of the approximate CTF intensity calculation. The drawback of the FO approach compared to the CTF approach, which is its slow computational speed, is mitigated partly here by the implementation of a multi-core, multi-threading programming architecture. This work broadens our capabilities to understand the origins of LEEM image contrast and to perform quantitative evaluation of contrast observed in an image focal series., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Accurately measuring defocus in cathode lens instruments (Low Energy Electron Microscopy - LEEM, and Photo Electron Emission Microscopy - PEEM) is a pre-requisite for quantitative image analysis using Fourier Optics (FO) or Contrast Transfer Function (CTF) image simulations. In particular, one must establish a quantitative relation between lens excitation and image defocus. One way to accomplish this is the Real-Space Microspot LEED method, making use of the accurately known angles of diffracted electron beams, and the defocus-dependent shifts of their corresponding real-space images. However, this only works if a sufficiently large number of diffracted beams is available for the sample under investigation. An alternative is to shift the sample along the optical axis by a known distance, and measure the change in objective lens excitation required to re-focus the image. We analytically derive the relation between sample shift and defocus, and apply our results to the measurement and analysis of achromats in an aberration-corrected LEEM instrument., (Copyright © 2017. Published by Elsevier B.V.)

A theoretical understanding of image formation in cathode lens microscopy can facilitate image interpretation. We compare Fourier Optics (FO) and Contrast Transfer Function (CTF) approaches that were recently adapted from other realms of microscopy to model image formation in low energy electron microscopy (LEEM). Although these two approaches incorporate imaging errors from several sources similarly, they differ in the way that the image intensity is calculated. The simplification that is used in the CTF calculation advantageously leads to its computational efficiency. However, we find that lens aberrations, and spatial and temporal coherence may affect the validity of the CTF approach to model LEEM image formation under certain conditions. In particular, these effects depend strongly on the nature of the object being imaged and also become more pronounced with increasing defocus. While the use of the CTF approach appears to be justified for objects that are routinely imaged with LEEM, comparison of theory to experimental observations of a focal image series for rippled, suspended graphene reveals one example where FO works, but CTF does not. This work alerts us to potential pitfalls and guides the effective use of FO and CTF approaches. It also lays the foundation for quantitative image evaluation using these methods., (Copyright © 2017 Elsevier B.V. All rights reserved.)

The quantum size effect (QSE) in electron reflectivity from Fe thin films grown on a W(110) surface precovered with a two monolayer Cu film has been investigated using spin polarized low energy electron microscopy. Spin-dependent QSE-induced oscillations in the reflected intensity occur with energy and film thickness. The series of intensity peaks that is observed identifies spin-dependent quantum well resonances in the Fe film that are sensitive to electronic band structure and details of the buried interface. Information about the spin-dependent unoccupied bands of the Fe film in the ΓΝ direction normal to the film plane is obtained by analyzing the observed quantum well resonance conditions. The spin-split bands that are determined are uniformly shifted downward by 1.7 eV compared to bulk-like bands determined previously in Fe films on a bare W(110) substrate by the same method. Evidence is also obtained that the buried interface that defines the thin film quantum well boundary is located one layer above the W(110) surface. These results suggest that the Cu layer in direct contact with the substrate remains largely intact, but the weakly-bound second Cu layer mixes or segregates freely., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Spin polarized low energy electron microscopy has been used to investigate the quantum size effect (QSE) in electron reflectivity from Fe films grown on a pseudomorphic Cu layer on a W(110) surface. Intensity oscillations caused by the QSE as functions of Fe film thickness and incident electron energy identify quantum well resonance conditions in the film. Evaluation of these intensity oscillations using the phase accumulation model provides information on the unoccupied spin polarized band structure in the Fe film above the vacuum level. We also find evidence that the presence of the non-magnetic Cu layer shifts spin polarized quantum well resonances in the Fe layer uniformly downward in energy by 1.1eV compared to Fe/W(110) films without an interface Cu layer, suggesting that the Cu layer gives a small degree of control over the quantum well resonances., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Low energy electron microscopy (LEEM) imaging of strained MnAs layers epitaxially grown on GaAs(001) reveals striped contrast features that become more pronounced and vary systematically in width with increasing defocus, but that are completely absent in focus. Weaker subsidiary fringe-like features are observed along the stripe lengths, while asymmetric contrast reversal occurs between under-focus and over-focus conditions. A Fourier optics calculation is performed that demonstrates that these unusual observations can be attributed to a phase contrast mechanism between the hexagonal α phase and orthorhombic β phase regions of the MnAs film, which self-organize into a periodic stripe array with ridge-groove morphology. The unequal widths of the α and β phase regions are determined accurately from the through focus series, while the height variation in this system can also be determined in principle from the energy dependence of contrast., (Copyright © 2013 Elsevier B.V. All rights reserved.)

We introduce an extended Contrast Transfer Function (CTF) approach for the calculation of image formation in low energy electron microscopy (LEEM) and photo electron emission microscopy (PEEM). This approach considers aberrations up to fifth order, appropriate for image formation in state-of-the-art aberration-corrected LEEM and PEEM. We derive Scherzer defocus values for both weak and strong phase objects, as well as for pure amplitude objects, in non-aberration-corrected and aberration-corrected LEEM. Using the extended CTF formalism, we calculate contrast and resolution of one-dimensional and two-dimensional pure phase, pure amplitude, and mixed phase and amplitude objects. PEEM imaging is treated by adapting this approach to the case of incoherent imaging. Based on these calculations, we show that the ultimate resolution in aberration-corrected LEEM is about 0.5 nm, and in aberration-corrected PEEM about 3.5 nm. The aperture sizes required to achieve these ultimate resolutions are precisely determined with the CTF method. The formalism discussed here is also relevant to imaging with high resolution transmission electron microscopy., (Copyright © 2011 Elsevier B.V. All rights reserved.)