Quantitative Untersuchungen der Zusammensetzung von kubischen III/V-Verbindungshalbleitern mittels HAADF-STEM
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Philipps-Universität Marburg
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Abstract
The use of an annular dark field (ADF) detector for measuring electrons scattered in high angles (HAADF) by a usually thin crystalline specimen in a scanning transmission electron microscope (STEM) allows for observation of chemically sensitive intensity. Therefore, this HAADF-STEM method is also known as Z-contrast, since the measured intensity of atoms or atomic colums, illuminated by a convergent electron beam, is proportional to the mean atomic number Z of the material.
Simulations of the HAADF-intensity can be made in multislice (MS) algorithm considering either an undisturbed virtual crystal (VCA), approximating the real structure of various technologically relevant compound III/V semiconductors, or taking into account static displacements (SDs) of atoms. These are introduced by incorporation of atoms with different sizes into one crystal sublattice, like Bi, B and N in GaAs or As, B and N in GaP. Several different random configurations of crystal lattices containing such SDs in frozen phonons (FP) are used to give a reliable representation of the interacting specimen.
By direct quantification of the measured intensities, instead of evaluating their ratio, one can achieve spatially resolved information on local thickness and composition. This is realized by comparison of results from experiment and simulation on an absolute intensity scale and requires knowledge of experimental parameters such as the electron probe semi-convergence angle and the angular range of the ADF-detector in a STEM like the JEM2200FS. Since the measured intensity depends on the efficiency of the detector scintillator, typically P or YAG:Ce, a direct scan of the detector is used to provide the detector sensitivity to account for in the simulations as well as the intensities of the background and the incident beam for corrected normalization of experimental intensities to the absolute scale of simulations.
For the two different ADF detectors in the uncorrected JEM2200FS microscope used in this work, such detector scans were achieved and evaluated for the needed factors.% The lower ADF detector with inhomogeneous P as scintillator, used at a cameralength of 8cm, ranges from 24.2mrad to 73mrad, and the upper one with more homogeneous YAG:Ce from 43.3mrad to 208mrad at 60cm cameralength, the outer detector edge determined by a differential pumping aperture. According sensitivity curves were determined and used as weighting factor for simulated scattering intensities of GaAs based Ga(BiAs), (BGa)As and Ga(NAs) as well as GaP based Ga(AsP), (BGa)P and Ga(NP).
Corresponding sensitivity curves were determined and used as weighting factor for scattering intensities, simulated in MS-FP approach in VCA and with SDs. Investigations of size and distribution of SDs show a marginal influence of As on the lattice of GaP but increasing dominance by Bi in GaAs. According to that, only the latter case shows significant changes in the HAADF-contrast to be expected, since a disturbed lattice qualifies less for channelling of the electron beam but produces additional popular Huang-scattering. The portion in Ga(BiAs)/GaAs contrast reaches one half for particular detector ranges.
The influence of that scattering further increases for B and N in GaP and GaAs which induce comparatively huge SDs generating Huang-scattering dominating the expectable ADF contrast quantitatively and qualitatively.
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Created: 2014Issued: 2014-09-08Updated: 2014-09-08
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Fachbereich Physik
Publisher
Philipps-Universität Marburg
Language
ger
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DoctoralThesis
Keywords
STEMIII-VsemiconductorcompositionHAADF
DFG-subjects
QuantifizierungHalbleiterphysikRaster-Transmissions-ElektronenmikroskopieZusammensetzungElektronenmikroskopieVerbindungshalbleiter
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500
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Fritz, Rafael (1054792720): Quantitative Untersuchungen der Zusammensetzung von kubischen III/V-Verbindungshalbleitern mittels HAADF-STEM. : Philipps-Universität Marburg 2014-09-08. DOI: https://doi.org/10.17192/z2014.0381.
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This item has been published with the following license: In Copyright