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Epitaxial growth of (111) thin films in the compositional range around the Heusler alloy
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color online) Schematic diagrams of composition spread samples. Cross-sectional view of (a) full binary sample of the type with and (b) partial binary sample with . (c) Partial ternary sample and (d) composition range of the partial ternary sample (shaded region) within the full ternary phase diagram. The trilayers of Si–Co–Mn submonolayer wedges and “partial” wedges are typically repeated 200 times.

Image of FIG. 2.
FIG. 2.

(Color online) Evolution of surface structure and morphology of grown on Ge (111) at and at a coverage of 310 Å. (a) and (b) Characteristic RHEED patterns along the azimuth at Co concentrations of (a) 50 at. % and (b) 63 at. %. The respective binary values are 0.65 and 0.82. The RHEED beam is aligned such that the specular reflection from the 2D streaky patterns (b) is positioned away from the Bragg reflection exhibited by the 3D transmission patterns (a) in the out-of-plane [111] direction (the down direction in the images). (c) Composition dependence of integrated RHEED intensity (close circles) and peak position (open circles) of the zeroth order spot along the out-of-plane [111] direction. The labels indicate the respective RHEED patterns shown in (a) and (b). The dotted lines indicate the roughening transitions discussed in the text.

Image of FIG. 3.
FIG. 3.

(Color online) Evolution of the surface morphology for . (a) Image of integrated RHEED intensity and graph of roughening transition (symbols and lines) vs binary concentration and coverage for growth at . The close and open symbols correspond to the abrupt roughening transition and the subtle transition from smoother quasi-3D surface to rougher 3D surface, respectively [the two dotted lines in Fig. 2(c)]. (b) Roughening transition at three growth temperatures of 150 (triangles), 225 (circles), and (squares). The points correspond to the midpoints between the two transition points at the same coverages. The solid lines are guide for the eyes.

Image of FIG. 4.
FIG. 4.

(Color online) Epitaxial phase diagram of a ternary combinatorial film grown on Ge (111) at . The nominal film thickness is 370 Å. (a) Crystallographic phases vs composition, where thick black lines delineate three distinct regions, (1) cubic fcc, (2) mixed cubic and hexagonal, and (3) disordered nanocrystalline. The concentration contours that correspond to the concentrations of Co, Mn, and Si, respectively, were obtained from XRF measurements. The white circle within the fcc region indicates the composition of the Si Heusler. The horizontal dashed line indicates the approximate compositions for the binary samples, whereas the arrow on the left denotes the direction of shown in Figs. 2, 3, and 6. (b) Characteristic out-of-plane off-axis XRD scans along [in hexagonal notation (Ref. 15)] from each of the regions, fcc (open circles), mixed phase (close circles), and nanocrystalline (crosses). The respective XRD scans were taken at compositions of , , and . The peak at is from the Ge substrate.

Image of FIG. 5.
FIG. 5.

Threefold symmetry of RHEED patterns for fcc (111) surface. Top row: RHEED patterns of the Ge (111) surface along various azimuths; bottom row: those of the Heusler alloy along the same respective azimuths of the Ge. The direction labels in the bottom images are indexed with respect to the crystal structure of the alloy.

Image of FIG. 6.
FIG. 6.

(Color online) Composition dependent XRD analysis of grown at and 540 Å thick. (a) Integrated intensities along the out-of-plane direction of (022), (002), and (111) cubic reflections. The intensities for the latter two superstructure reflections are multiplied by 5. (b) In-plane and out-of-plane FWHMs of the fundamental (022) reflection. (c) In-plane and out-of-plane strains with respect the substrate. For clarity, the in-plane strain is multiplied by 5. The vertical dashed lines indicate the region with zero in-plane strain (coherent growth). The scatter in the points at Co concentration below 55% and above 95% is the result of low diffraction intensity (polycrystalline regions).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Epitaxial growth of CoxMnySiz (111) thin films in the compositional range around the Heusler alloy Co2MnSi