Fourier transform imaging of impurities in the unit cells of crystals: Mn in GaAs

Abstract

The lattice sites of Mn in ferromagnetic ͑Ga,Mn͒As thin films were imaged using the x-ray standing wave technique. The model-free images, obtained straightforwardly by Fourier inversion, disclose immediately that the Mn mostly substitutes the Ga with a small fraction residing on minority sites. The images further reveal variations in the Mn concentrations of the different sites upon post-growth treatments. Subsequent model refinement based on the directly reconstructed images resolves with high precision the complete Mn site distributions. It is found that post-growth annealing increases the fraction of substitutional Mn at the expense of interstitial Mn whereas hydrogenation has little influence on the Mn site distribution. Our study offers an element-specific high-resolution imaging approach for accurately determining the detailed site distributions of dilute concentrations of atoms in crystals.

Fluorescence detection of white-beam X-ray absorption anisotropy: towards element-sensitive projections of local atomic structure

“…While XSW provides information about positions of absorbing atoms relative to a well ordered lattice, XFH is potentially suited for imaging of the local structure around absorbing atoms. In both cases, recorded data are converted to real space either by a holographic reconstruction (Barton, 1988) or by Fourier inversion procedures (Lee et al, 2010).…”

XSW Imaging

“…It is particularly well adapted to adsorbate systems, as shown here, or dopants within the unit cell of host lattices as demonstrated by the 1D imaging of naturally occurring trace impurities in mica 3 or for 3D imaging of Mn doped GaAs. 16 For nanocrystals grown on the surface of a single crystal substrate, XSW imaging can be used to measure the correlation between the two lattices, nanoparticle and substrate, revealing important parameters for describing the interfacial structure. 17 As described in Chapters 5 and 19, the Fourier inversion of XSW data collected at the 0 th -order Bragg peak (or total external reflection) can be used to produce a model-independent 1D atomic density profile of the extended structure above a mirror surface 18,19 with a lower-resolution, but longer length-scale; well beyond the reach of this single crystal Bragg XSW method.…”

Density of Mn interstitials in (Ga,Mn)As epitaxial layers determined by anomalous x-ray diffraction

“…As shown quite recently in Ref. 11, the application of the DAFS method for ͑Ga,Mn͒As is not straightforward; most probably other structural defects like As antisites ͑As atoms in Ga lattice positions͒ affect the fine structure of the diffracted intensity. In the case of anomalous diffraction, the As antisites influence the absolute value of the structure factor F L not changing the shape of its energy dependence.…”

“…[6][7][8] The densities can be also determined from the diffraction intensities ͓i.e., from the absolute values of the structure factor of ͑Ga,Mn͒As͔ that depend sensitively on the presence of Mn interstitials, especially in the case of weak diffractions 002, 006, etc. 8,9 The x-ray standing-wave technique 10,11 and an unit-cell imaging technique using diffuse x-ray scattering 12 determine directly the positions of the Mn ions in the GaAs host lattice while the x-ray absorption spectroscopy 13,14 detects the atomic neighborhood of Mn ions. These methods are direct, however it is quite difficult to obtain reliable numerical values of c sub,int .…”