Nucleation, Coarsening and Movement of MnAs Precipitates in Wurtzite GaAs Nanowire Shells During In Situ Annealing in transMission Electron Microscope

Here, we investigate thermally induced formation and evolution of nanostructured material with Mn-clusters/MnAs nanocrystals (NCs) emerging in GaAs matrix of untypical wurtzite (WZ) crystal structure (hexagonal symmetry). This nanoscale system can be fabricated as a result of phase transition in <b>WZ (Ga,Mn)As</b>, which can be obtained only in a nanowire (NW) geometry, as an epitaxial shell around WZ NW core [1]. Two types of WZ core-shell MBE NWs are investigated: with one and two (Ga,Al)As separating shells:<br/>1) (Ga,In)As/<u>(Ga,Al)As</u>/<b>(Ga,Mn)As</b>/GaAs,<br/>2) GaAs/<u>(Ga,Al)As</u>/<b>(Ga,Mn)As</b><u>/(Ga,Al)As</u>/GaAs<br/>of several µm length and diameter &gt;200nm with Ga_1-xMn_xAs (x=4,5,6%) shell of ~30nm thickness.<br/><br/>Initial scanning transmission electron microscopy (STEM) investigations, residual strain analysis based on high-resolution STEM images and 4D-STEM of ex-situ annealed (450°C) NWs reveal that (Ga,Mn)As separates to tensely strained MnAs NCs embedded incoherently or semi-coherently in the WZ GaAs matrix. Moreover, SQUID magnetometry shows a remarkable temperature shift of the magneto-structural phase transition from 40°C of bulk MnAs to above 127°C attained for strained MnAs NCs [2], making this system interesting for spintronic applications.<br/><br/>We will present real-time structural changes at high spatial resolution occurring under thermal annealing in a single NW shell by employing an in-situ TEM system. Thanks to appropriate STEM conditions, i.e. using HAADF detector with collection inner semi-angle 20-25mrad, the highest possible contrast for Mn nano-precipitates has been obtained at both formation stages:<br/>a) nucleation and the emergence of coherent WZ clusters with Mn substituting Ga atoms strained in the WZ matrix (r&lt;1 nm)<br/>b) growth (coarsening) of semi-coherent MnAs NCs with NiAs-type crystal structure, partially strained/relaxed in the WZ matrix (r&gt;1nm)<br/>Due to a notable strain effect (electron probe de-channeling) for (a) case and a difference in diffraction (MnAs NCs of crystal structure different than that of the matrix) for (b) case, the excessive signal is generated and registered on HAADF, making Mn nano-precipitates (Mn-clusters and MnAs NCs) appear brighter than the matrix in the acquired images. The experimental results are verified by QSTEM simulations, which show that indeed, in the (a) case the higher intensity (bright contrast) can be obtained for coherently strained Mn-clusters in 20nm depth field of view. Therefore, the precipitation process analysis cannot be fully quantitative, however, by careful image post-processing of image stacks, we are able to observe changes in average size and density of Mn nano-precipitates in the function of time and temperature.<br/><br/>It is found that segregation of the Mn atoms in GaAs starts already at relatively low temperatures ~300°C and is followed by a phase transition to the MnAs NCs at 350-400°C (visible Moiré patterns) and the NCs coarsening at higher temperatures. Moreover, for both types of NWs, the movement of Mn nano-precipitates is observed and occurs perpendicularly to the NW growth axis rather than along it. This is in agreement with DFT calculations for Mn diffusion in the WZ GaAs matrix stating that the jumps along the a-axis require lower energy than those along the c-axis direction. NCs forming in (1) type NWs tend to move towards the NW surfaces, whereas in the (2) type NWs the movement of NCs is observed between two (Ga,Al)As blocking shells.<br/><br/>References:<br/>[1] J. Sadowski, et al., Nanoscale 9, 2129 (2017)<br/>[2] A. Kaleta, et al., Nano Lett. 19, 7324 (2019)