Metastable zinc-blende MgS structure: Combined experimental and theoretical study

C. Bocchi, A. Catellani, F. Germini, L. Nasi, J. K. Morrod, K. A. Prior, and G. Calestani

Phys. Rev. B 79, 235310 – Published 5 June 2009

Abstract

MgS/ZnSe/GaAs multilayers with the MgS thickness ranging from 20 to 140 nm were grown at $300 ° C$ by molecular-beam epitaxy on [001] GaAs substrates. The samples were studied by using several x-ray methods and transmission electron microscopy. The coexistence of metastable zinc-blende (ZB) and rocksalt MgS structural phases was evidenced and discussed. The analysis of reciprocal space maps of the x-ray intensity distribution around asymmetrical reciprocal-lattice nodes allowed us to determine the strain status of the MgS layers and to show that the ZB-MgS phase was pseudomorphic also in the case of the thickest film. The lattice parameter of the pure ΖΒ-MgS phase ranging between $0.563 33 < a_{MgS} < 0.563 67 nm$ was obtained by extrapolation from x-ray diffraction data and predicted ab initio elastic constants, taking into account that there was a Zn incorporation during the MgS growth estimated in the range $0.005 \leq x_{Zn} \leq 0.02$ .

Received 9 January 2009

DOI:https://doi.org/10.1103/PhysRevB.79.235310

Authors & Affiliations

C. Bocchi^*, A. Catellani, F. Germini, and L. Nasi

CNR-IMEM Institute, Parco Area delle Scienze 37/A, I-43100 Parma, Italy

J. K. Morrod^† and K. A. Prior

School of Engineering and Physical Sciences, Brewster Building, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom

G. Calestani

Department of Chemistry, Parma University, Parco Area delle Scienze 17/A, I-43100 Parma, Italy

^*bocchi@imem.cnr.it
^†Present address: Physical Sciences, University of Cambridge, 25 Trumpington Road CB2 1QA, UK.

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Vol. 79, Iss. 23 — 15 June 2009

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Images

Figure 1
TEM plan view micrographs of sample 1 taken close to the $45 °$ -inclined [110] zone axis with (a) $g = 1 - 11$ and (b) $g = 200$ . The bright regions in Fig. 1b correspond to the RS grains at the crossing stacking faults.Reuse & Permissions
Figure 2
(a) Sample 1: logarithm in base 10 of the 002 $Cu K α_{1}$ diffracted intensity vs the deviation from the substrate Bragg-peak angular position $(Δ θ)$ collected in $ω − 2 θ$ scan mode along the [001] direction. (b) RSM collected around the 002 reciprocal-lattice node; the dashed line marked in the figure is coincident with the [001] direction. The markers (solid lines) perpendicular to the [001] direction give the ZnSe, GaAs, ZB-MgS, and RS-MgS 002 nodes position, respectively. The intensity broadening along the marker direction is due to the mosaic misorientation.Reuse & Permissions
Figure 3
Schematic of the change in phase occurring at the intersection between stacking faults lying on different (111) glide planes. (a) RS-MgS cores grow initially limited by a pyramid-shaped sites; (b) RS phase maintains the [001] orientation of the original ZB lattice.Reuse & Permissions
Figure 4
Asymmetrical $- 224$ $Cu K α_{1}$ RSM of sample 1 shown in reciprocal space coordinates. The intensity distribution is positioned along a direction perpendicular to the surface (parallel to [001]). This means that the (ZB) ${Zn}_{X} {Mg}_{1 - X} S$ layer $(0.02 \leq x \leq 0.03)$ is pseudomorphic. The position of the $- 224$ node on the [112] direction in the case of unstrained (relaxed) lattice is also shown.Reuse & Permissions
Figure 5
Logarithm in base 10 of 004 $Cu K α_{1}$ diffracted intensity (spot) of all the samples and relating fittings (solid line) obtained with the trends of the mismatch perpendicular component $(Δ d / d)$ vs depth shown in the figures. The figures are relative: (a) sample1; (b) sample 2; (c) sample3; (d) sample 4.Reuse & Permissions
Figure 6
Comparison of structural properties from ab initio calculations: (a) relative total-energy curve of the different crystal structures for MgS [closed (open) circle for rocksalt (zinc blende)] and predicted equilibrium lattice parameter for a 3% virtual-crystal ${Zn}_{X} {Mg}_{1 - X} S$ alloy; (b) stress as a function of strain for the rocksalt (closed symbols) and zinc-blende (open symbols) metastable phases and predicted $σ_{11}$ for the 3% virtual-crystal ${Zn}_{X} {Mg}_{1 - X} S$ alloy; here $σ_{11} = δ E / u_{x x}$ and $σ_{12} = δ E / u_{x y}$ are obtained from ab initio total-energy (e) calculations, varying the lattice parameter in the corresponding directions.Reuse & Permissions

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