1
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Quantitative analysis of spectroscopic low energy electron microscopy data: High-dynamic range imaging, drift correction and cluster analysis. Ultramicroscopy 2020; 213:112913. [PMID: 32389485 DOI: 10.1016/j.ultramic.2019.112913] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2019] [Revised: 11/13/2019] [Accepted: 11/22/2019] [Indexed: 11/22/2022]
Abstract
For many complex materials systems, low-energy electron microscopy (LEEM) offers detailed insights into morphology and crystallography by naturally combining real-space and reciprocal-space information. Its unique strength, however, is that all measurements can easily be performed energy-dependently. Consequently, one should treat LEEM measurements as multi-dimensional, spectroscopic datasets rather than as images to fully harvest this potential. Here we describe a measurement and data analysis approach to obtain such quantitative spectroscopic LEEM datasets with high lateral resolution. The employed detector correction and adjustment techniques enable measurement of true reflectivity values over four orders of magnitudes of intensity. Moreover, we show a drift correction algorithm, tailored for LEEM datasets with inverting contrast, that yields sub-pixel accuracy without special computational demands. Finally, we apply dimension reduction techniques to summarize the key spectroscopic features of datasets with hundreds of images into two single images that can easily be presented and interpreted intuitively. We use cluster analysis to automatically identify different materials within the field of view and to calculate average spectra per material. We demonstrate these methods by analyzing bright-field and dark-field datasets of few-layer graphene grown on silicon carbide and provide a high-performance Python implementation.
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2
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Quantifying work function differences using low-energy electron microscopy: The case of mixed-terminated strontium titanate. Ultramicroscopy 2019; 200:43-49. [PMID: 30822616 DOI: 10.1016/j.ultramic.2019.02.018] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2018] [Accepted: 02/18/2019] [Indexed: 11/24/2022]
Abstract
For many applications, it is important to measure the local work function of a surface with high lateral resolution. Low-energy electron microscopy is regularly employed to this end since it is, in principle, very well suited as it combines high-resolution imaging with high sensitivity to local electrostatic potentials. For surfaces with areas of different work function, however, lateral electrostatic fields inevitably associated with work function discontinuities deflect the low-energy electrons and thereby cause artifacts near these discontinuities. We use ray-tracing simulations to show that these artifacts extend over hundreds of nanometers and cause an overestimation of the true work function difference near the discontinuity by a factor of 1.6 if the standard image analysis methods are used. We demonstrate on a mixed-terminated strontium titanate surface that comparing LEEM data with detailed ray-tracing simulations leads to much a more robust estimate of the work function difference.
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3
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Kennedy SM, Zheng CX, Jesson DE. Droplet Epitaxy Image Contrast in Mirror Electron Microscopy. NANOSCALE RESEARCH LETTERS 2017; 12:68. [PMID: 28116613 PMCID: PMC5256635 DOI: 10.1186/s11671-017-1837-y] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/07/2016] [Accepted: 01/09/2017] [Indexed: 06/02/2023]
Abstract
Image simulation methods are applied to interpret mirror electron microscopy (MEM) images obtained from a movie of GaAs droplet epitaxy. Cylindrical symmetry of structures grown by droplet epitaxy is assumed in the simulations which reproduce the main features of the experimental MEM image contrast, demonstrating that droplet epitaxy can be studied in real-time. It is therefore confirmed that an inner ring forms at the droplet contact line and an outer ring (or skirt) occurs outside the droplet periphery. We believe that MEM combined with image simulations will be increasingly used to study the formation and growth of quantum structures.
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Affiliation(s)
- S. M. Kennedy
- School of Physics, Monash University, Melbourne, Victoria 3800 Australia
| | - C. X. Zheng
- Department of Civil Engineering, Monash University, Melbourne, Victoria 3800 Australia
| | - D. E. Jesson
- School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA UK
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4
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Jobst J, Kautz J, Mytiliniou M, Tromp RM, van der Molen SJ. Reprint of Low-energy electron potentiometry. Ultramicroscopy 2017; 183:8-14. [PMID: 29103783 DOI: 10.1016/j.ultramic.2017.10.009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2016] [Revised: 04/21/2017] [Accepted: 05/09/2017] [Indexed: 11/28/2022]
Abstract
In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref[1]. We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.
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Affiliation(s)
- Johannes Jobst
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands; Department of Physics, Columbia University, New York, New York 10027, USA.
| | - Jaap Kautz
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands
| | - Maria Mytiliniou
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands
| | - Rudolf M Tromp
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands; IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, P.O. Box 218, USA
| | - Sense Jan van der Molen
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands
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5
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Jobst J, Kautz J, Mytiliniou M, Tromp RM, van der Molen SJ. Low-energy electron potentiometry. Ultramicroscopy 2017; 181:74-80. [PMID: 28527312 DOI: 10.1016/j.ultramic.2017.05.015] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2016] [Revised: 04/21/2017] [Accepted: 05/09/2017] [Indexed: 12/01/2022]
Abstract
In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref[1]. We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.
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Affiliation(s)
- Johannes Jobst
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands; Department of Physics, Columbia University, New York, New York 10027, USA.
| | - Jaap Kautz
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands
| | - Maria Mytiliniou
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands
| | - Rudolf M Tromp
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands; IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, P.O. Box 218, USA
| | - Sense Jan van der Molen
- Huygens-Kamerlingh Onnes Laboratorium, Leiden University, NL-2300 RA Leiden, P.O. Box 9504, Netherlands
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6
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Nataf GF, Grysan P, Guennou M, Kreisel J, Martinotti D, Rountree CL, Mathieu C, Barrett N. Low energy electron imaging of domains and domain walls in magnesium-doped lithium niobate. Sci Rep 2016; 6:33098. [PMID: 27608605 PMCID: PMC5016809 DOI: 10.1038/srep33098] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2016] [Accepted: 08/19/2016] [Indexed: 11/21/2022] Open
Abstract
The understanding of domain structures, specifically domain walls, currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelectric lithium niobate, which presents a large 3.8 eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM – electrons reflected) to Low Energy Electron Microscopy (LEEM – electrons backscattered) gives rise to a robust contrast between domains with upwards (Pup) and downwards (Pdown) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces, and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides experimental evidence for a local stray, lateral electric field.
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Affiliation(s)
- G F Nataf
- SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France.,Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 41 rue du Brill, 4422 Belvaux, Luxembourg
| | - P Grysan
- Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 41 rue du Brill, 4422 Belvaux, Luxembourg
| | - M Guennou
- Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 41 rue du Brill, 4422 Belvaux, Luxembourg
| | - J Kreisel
- Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 41 rue du Brill, 4422 Belvaux, Luxembourg.,Physics and Materials Science Research Unit, University of Luxembourg, 41 rue du Brill, 4422 Belvaux, Luxembourg
| | - D Martinotti
- SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
| | - C L Rountree
- SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
| | - C Mathieu
- SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
| | - N Barrett
- SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
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7
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Zhou ZY, Zheng CX, Tang WX, Tersoff J, Jesson DE. Origin of quantum ring formation during droplet epitaxy. PHYSICAL REVIEW LETTERS 2013; 111:036102. [PMID: 23909340 DOI: 10.1103/physrevlett.111.036102] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/11/2013] [Indexed: 05/16/2023]
Abstract
Droplet epitaxy of GaAs is studied in real time using in situ surface electron microscopy. The resulting movies motivate a theoretical model for quantum ring formation which can explain the origin of nanoscale features such as double rings observed under a variety of experimental conditions. Inner rings correspond to GaAs deposition at the droplet edge, while outer rings result from the reaction of Ga and As atoms diffusing along the surface. The observed variety of morphologies primarily reflects relative changes in the outer rings with temperature and As flux.
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Affiliation(s)
- Z Y Zhou
- School of Physics, Monash University, Victoria 3800, Australia
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8
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Kennedy SM, Hjort M, Mandl B, Marsell E, Zakharov AA, Mikkelsen A, Paganin DM, Jesson DE. Characterizing the geometry of InAs nanowires using mirror electron microscopy. NANOTECHNOLOGY 2012; 23:125703. [PMID: 22397834 DOI: 10.1088/0957-4484/23/12/125703] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Mirror electron microscopy (MEM) imaging of InAs nanowires is a non-destructive electron microscopy technique where the electrons are reflected via an applied electric field before they reach the specimen surface. However strong caustic features are observed that can be non-intuitive and difficult to relate to nanowire geometry and composition. Utilizing caustic imaging theory we can understand and interpret MEM image contrast, relating caustic image features to the properties and parameters of the nanowire. This is applied to obtain quantitative information, including the nanowire width via a through-focus series of MEM images.
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Affiliation(s)
- S M Kennedy
- School of Physics, Monash University, Victoria 3800, Australia
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Kennedy SM, Zheng CX, Tang WX, Paganin DM, Jesson DE. Addendum. Laplacian image contrast in mirror electron microscopy. Proc Math Phys Eng Sci 2011. [DOI: 10.1098/rspa.2011.0204] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
We extend the theory of Laplacian image contrast in mirror electron microscopy (MEM) to the case where the sample is illuminated by a parallel, collimated beam. This popular imaging geometry corresponds to a modern low energy electron microscope equipped with a magnetic objective lens. We show that within the constraints of the relevant approximations; the results for parallel illumination differ only negligibly from diverging MEM specimen illumination conditions.
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Affiliation(s)
- S. M. Kennedy
- School of Physics, Monash University, Victoria 3800, Australia
| | - C. X. Zheng
- School of Physics, Monash University, Victoria 3800, Australia
| | - W. X. Tang
- School of Physics, Monash University, Victoria 3800, Australia
| | - D. M. Paganin
- School of Physics, Monash University, Victoria 3800, Australia
| | - D. E. Jesson
- School of Physics, Monash University, Victoria 3800, Australia
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10
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Caustic imaging of gallium droplets using mirror electron microscopy. Ultramicroscopy 2011; 111:356-63. [DOI: 10.1016/j.ultramic.2011.01.019] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2010] [Revised: 12/22/2010] [Accepted: 01/11/2011] [Indexed: 11/22/2022]
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