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Issue 37, 2010
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A solid-state NMR and DFT study of compositional modulations in AlxGa1−xAs

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We have conducted 75As and 69Ga Nuclear Magnetic Resonance (NMR) experiments to investigate order/disorder in AlxGa1−xAs lift-off films with x ∼ 0.297 and 0.489. We were able to identify all possible As(AlnGa4−n) sites with n = 0–4 coordinations in 75As NMR spectra using spin-echo experiments at 18.8 Tesla. This was achieved by employing high rf field strengths using a small solenoid coil and an NMR probe specifically designed for this purpose. Spectral deconvolution, using an evolutionary algorithm, complies with the absence of long-range order if a CuAu based order parameter is imposed. An unconstrained fit shows a deviation of the statistics imposed by this type of ordering. The occupational disorder in the Ga and Al positions is reflected in a distribution of the Electric Field Gradients (EFGs) experienced at the different arsenic sites. We established that this can be modelled by summing the effects of the first coordination sphere and a Czjzek type distribution resulting from the compositional variation in the Al/Ga sub-lattice in the higher coordination spheres. 69Ga 3QMAS and nutation data exclude the presence of highly symmetric sites and also show a distribution in EFG. The experimentally obtained quadrupolar interactions are in good agreement with calculations based on Density Functional Theory (DFT). Using additivity of EFG tensors arising from distant charge perturbations, we could use DFT to model the EFG distributions of the n = 0–4 sites, reproducing the Czjzek and extended Czjzek distributions that were found experimentally. On the basis of these calculations we conclude that the 75As quadrupolar interaction is sensitive to compositional modulations up to the 7th coordination shell in these systems.

Graphical abstract: A solid-state NMR and DFT study of compositional modulations in AlxGa1−xAs

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Publication details

The article was received on 25 Feb 2010, accepted on 17 May 2010 and first published on 02 Aug 2010

Article type: Paper
DOI: 10.1039/C003624B
Citation: Phys. Chem. Chem. Phys., 2010,12, 11517-11535

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    A solid-state NMR and DFT study of compositional modulations in AlxGa1−xAs

    P. J. Knijn, P. J. M. van Bentum, E. R. H. van Eck, C. Fang, D. L. A. G. Grimminck, R. A. de Groot, R. W. A. Havenith, M. Marsman, W. L. Meerts, G. A. de Wijs and A. P. M. Kentgens, Phys. Chem. Chem. Phys., 2010, 12, 11517
    DOI: 10.1039/C003624B

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