Research ArticleMATERIALS SCIENCE

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys

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Science Advances  07 Jun 2017:
Vol. 3, no. 6, e1700270
DOI: 10.1126/sciadv.1700270

Figures

  • Fig. 1 Calculated mixing enthalpy ΔHm(x) and resulting T(x) phase diagram with binodal (blue) and spinodal (red) lines for three different cases of alloys.

    (A and B) Conventional case of an isostructural alloy, In1−xGaxN, in the wurtzite (WZ) structure with the regular solution interaction parameter Ω = 0.26 eV from the study of Ho and Stringfellow (19). f.u., formula unit. (C and D) Heterostructural alloy formed from two materials with incommensurate lattices and Mn1−xZnxO formed from rock salt (RS) MnO and WZ ZnO. (E and F) Heterostructural alloy formed from materials with commensurate lattices and Sn1−xCaxS formed from orthorhombic (ORC) SnS and RS CaS. The spinodal gap is suppressed relative to the binodal gap in heterostructural alloys, producing a wider metastable region compared to the isostructural alloys.

  • Fig. 2 Evolution of the structural properties of heterostructural alloys as a function of composition.

    XRD patterns of (A) incommensurate Mn1−xZnxO alloys exhibiting a discontinuous change of the structure with a two-phase region in the interval 0.2 < x < 0.4 for the growth temperature of 297°C and (B) commensurate Sn1−xCaxS alloys grown at 240°C, showing a continuous change in the structure. a.u., arbitrary units.

  • Fig. 3 Experimentally determined nonequilibrium phase diagrams.

    XRD derived nonequilibrium phase diagram of (A) Mn1−xZnxO and (B) Sn1−xCaxS overlaid on their respective calculated thermodynamic phase diagram (cf. Fig. 1). The circles show the single-phase boundary points obtained from the disappearing phase analysis of the XRD data and were used to determine the single-phase regions (shaded areas beneath dashed lines). For Sn1−xCaxS in the Sn-rich range x < 0.25, the single-phase boundary is estimated (see the main text). The diamonds indicate (x,T) combinations of samples grown to test the decomposition mechanism (see below), and the bars shown at the higher temperature indicate the composition variation, as determined by scanning transmission electron microscopy (STEM) with energy dispersive spectroscopy (EDS).

  • Fig. 4 Cross-sectional STEM-EDS spectral images of the heterostructural alloys grown at different substrate temperatures.

    (A and C) A homogeneous single-phase film is confirmed for the lower growth temperatures. At the higher temperature outside the single-phase region (cf. Fig. 3), a (binodal) phase separation is observed for Mn0.5Zn0.5O (B), but a spinodal decomposition is observed for Sn0.68Ca0.32S (D). One-dimensional linescan compositional profiles extracted from the two-dimensional (2D) spectral images are shown in the bottom panels, from which the quantified data in Fig. 3 were determined.

  • Fig. 5 Evolution of the structural parameters and optical absorption spectra of Sn1−xCaxS alloys as a function of the composition.

    The calculated lattice parameters (A) and cation coordination numbers (B) illustrate the continuous evolution of global and local lattice symmetries associated with the displacive-type phase transformation in commensurate heterostructural alloys. Note that the ORC structure of SnS can be derived from a √2 × √2 × 2 supercell of the conventional RS cell. Hence, the ideal c/√(ab) ratio is 2√2, and the c lattice parameter corresponding to this ideal ratio (cRS-ideal) is shown by the blue dashed line in (A). The commensurate alloys therefore show a gradual change in the (C) optical properties due to the continuous evolution of the lattice symmetry. This example demonstrates the coupled utilization of composition-structure and structure-property relationships for materials design in heterostructural alloys.

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