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Title: Contrast and resolution versus specimen thickness in low energy scanning transmission electron microscopy

Abstract

A theoretical and experimental investigation of contrast and resolution versus specimen thickness in scanning transmission electron microscopy at low energy is reported. Due to absence of postspecimen imaging lenses it is possible to have images with a resolution defined by the probe size using very wide collection angles and independent of the energy loss of the transmitted electrons. The fundamental limitation in observable specimen thickness is represented by the signal to noise ratio, i.e., the intensity of the beam current. The investigated specimens are semiconductor multilayers and Sb precipitates in a Si implanted specimen. The observations of layers crossing the whole specimens parallel to the electron beam point out that only a small portion of them, the one close to the surface, causes the image contrast, while the portion below, where the probe diameter, as a consequence of the broadening, is larger than the layer itself, reduces the contrast. A similarity with recent results, achieved in scanning transmission electron microscopy at high energy, where the layers are represented by atomic columns, is pointed out. The image contrast depends on the angular distribution of the transmitted electrons, and for thick specimens it is always of bright field type, independent of themore » collection angle of the transmitted electrons. The observation of Sb precipitates, distributed along the specimen thickness, evidences the role of beam broadening on the resolution and contrast.« less

Authors:
;  [1]
  1. CNR-IMM, Sezione di Bologna, via Gobetti 101, 40126 Bologna (Italy)
Publication Date:
OSTI Identifier:
20979430
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 101; Journal Issue: 11; Other Information: DOI: 10.1063/1.2745333; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALUMINIUM COMPOUNDS; ANGULAR DISTRIBUTION; ANTIMONY; BEAM CURRENTS; ELECTRON BEAMS; ELECTRONS; ENERGY LOSSES; IMAGES; LAYERS; PRECIPITATION; RESOLUTION; SEMICONDUCTOR MATERIALS; SIGNAL-TO-NOISE RATIO; THICKNESS; TRANSMISSION ELECTRON MICROSCOPY

Citation Formats

Morandi, Vittorio, and Merli, Pier Giorgio. Contrast and resolution versus specimen thickness in low energy scanning transmission electron microscopy. United States: N. p., 2007. Web. doi:10.1063/1.2745333.
Morandi, Vittorio, & Merli, Pier Giorgio. Contrast and resolution versus specimen thickness in low energy scanning transmission electron microscopy. United States. doi:10.1063/1.2745333.
Morandi, Vittorio, and Merli, Pier Giorgio. Fri . "Contrast and resolution versus specimen thickness in low energy scanning transmission electron microscopy". United States. doi:10.1063/1.2745333.
@article{osti_20979430,
title = {Contrast and resolution versus specimen thickness in low energy scanning transmission electron microscopy},
author = {Morandi, Vittorio and Merli, Pier Giorgio},
abstractNote = {A theoretical and experimental investigation of contrast and resolution versus specimen thickness in scanning transmission electron microscopy at low energy is reported. Due to absence of postspecimen imaging lenses it is possible to have images with a resolution defined by the probe size using very wide collection angles and independent of the energy loss of the transmitted electrons. The fundamental limitation in observable specimen thickness is represented by the signal to noise ratio, i.e., the intensity of the beam current. The investigated specimens are semiconductor multilayers and Sb precipitates in a Si implanted specimen. The observations of layers crossing the whole specimens parallel to the electron beam point out that only a small portion of them, the one close to the surface, causes the image contrast, while the portion below, where the probe diameter, as a consequence of the broadening, is larger than the layer itself, reduces the contrast. A similarity with recent results, achieved in scanning transmission electron microscopy at high energy, where the layers are represented by atomic columns, is pointed out. The image contrast depends on the angular distribution of the transmitted electrons, and for thick specimens it is always of bright field type, independent of the collection angle of the transmitted electrons. The observation of Sb precipitates, distributed along the specimen thickness, evidences the role of beam broadening on the resolution and contrast.},
doi = {10.1063/1.2745333},
journal = {Journal of Applied Physics},
number = 11,
volume = 101,
place = {United States},
year = {Fri Jun 01 00:00:00 EDT 2007},
month = {Fri Jun 01 00:00:00 EDT 2007}
}
  • Abstract A long-standing goal of electron microscopy has been the high-resolution characterization of specimens in their native environment. However, electron optics require high vacuum to maintain an unscattered and focused probe, a challenge for specimens requiring atmospheric or liquid environments. Here, we use an electron-transparent window at the base of a scanning electron microscope’s objective lens to separate column vacuum from the specimen, enabling imaging under ambient conditions, without a specimen vacuum chamber. We demonstrate in-air imaging of specimens at nanoscale resolution using backscattered scanning electron microscopy (airSEM) and scanning transmission electron microscopy. We explore resolution and contrast using Montemore » Carlo simulations and analytical models. We find that nanometer-scale resolution can be obtained at gas path lengths up to 400μm, although contrast drops with increasing gas path length. As the electron-transparent window scatters considerably more than gas at our operating conditions, we observe that the densities and thicknesses of the electron-transparent window are the dominant limiting factors for image contrast at lower operating voltages. By enabling a variety of detector configurations, the airSEM is applicable to a wide range of environmental experiments including the imaging of hydrated biological specimens andin situchemical and electrochemical processes.« less
  • Structural properties of the clean Si(001) surface obtained as a result of low-temperature (470-650 Degree-Sign C) pre-growth annealings of silicon wafers in a molecular-beam epitaxy chamber have been investigated. To decrease the cleaning temperature, a silicon surface was hydrogenated in the process of a preliminary chemical treatment in HF and NH{sub 4}F aqueous solutions. It has been shown that smooth surfaces composed of wide terraces separated by monoatomic steps can be obtained by dehydrogenation at the temperatures Greater-Than-Or-Equivalent-To 600 Degree-Sign C, whereas clean surfaces obtained at the temperatures <600 Degree-Sign C are rough. It has been found that there existsmore » a dependence of structural properties of clean surfaces on the temperature of hydrogen thermal desorption and the process of the preliminary chemical treatment. The frequency of detachment/attachment of Si dimers from/to the steps and effect of the Ehrlich-Schwoebel barrier on ad-dimer migration across steps have been found to be the most probable factors determining a degree of the resultant surface roughness.« less
  • We report the observation of porous structures in laser-ablation-deposited Y{sub 2}O{sub 3}:Eu thin films and their correlation with luminescent properties by a combination of transmission electron microscopy and Z-contrast scanning transmission electron microscopy (Z-STEM). Depending on growth conditions, a large density of voids is incorporated into the films, which leads to a much increased surface area. Cathodoluminescence imaging in the STEM directly reveals a 5 nm ''dead layer'' around each void, which is responsible for the observed reduction in luminescence efficiency. (c) 2000 American Institute of Physics.