X-ray diffraction mapping of strain fields and chemical composition of SiGe:Si(001) quantum dot molecules

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X-ray diffraction mapping of strain fields and chemical composition of SiGe:Si(001) quantum dot molecules

M. S. Leite, J. L. Gray, R. Hull, J. A. Floro, R. Magalhães-Paniago, and G. Medeiros-Ribeiro

Phys. Rev. B 73, 121308(R) – Published 31 March 2006

Abstract

A variety of surface morphologies can be formed by controlling kinetic parameters during heteroepitaxial film growth. The system reported is a ${Si}_{0.7} {Ge}_{0.3}$ film grown by molecular beam epitaxy at $550 ° C$ and a $1 Å ∕ s$ deposition rate, producing quantum dot molecule (QDM) structures. These nanostructures are very uniform in size and shape, allowing strain mapping and chemical composition evaluation by means of anomalous x-ray diffraction in a grazing incidence geometry. Tensile and compressed regions coexist inside QDMs, in accordance with the finite-element calculations of lattice relaxation. The Ge content was found to vary significantly within the structures, and to be quite different from the nominal composition.

Received 5 November 2005

DOI:https://doi.org/10.1103/PhysRevB.73.121308

Authors & Affiliations

M. S. Leite^1,2, J. L. Gray³, R. Hull³, J. A. Floro⁴, R. Magalhães-Paniago^1,5, and G. Medeiros-Ribeiro¹

¹Laboratório Nacional de Luz Síncrotron, Caixa Postal 6192, CEP-13084-971 Campinas, São Paulo, Brazil
²Instituto de Física Gleb Wataghin, UNICAMP, 13083-970 Campinas Brazil
³Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22903, USA
⁴Sandia National Laboratories, Livermore, California 94550, USA
⁵Departamento de Física, UFMG, Belo Horizonte 30123-970, Brazil

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Issue

Vol. 73, Iss. 12 — 15 March 2006

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Images

Figure 1
Anomalous x-ray radial scans of the QDM sample at 11.004 and $11.103 keV$ and the pit sample at $11.004 keV$ , showing a significant variation in the scattered intensities. The difference is associated with the Ge content within QDMs. The intensity in the $\log_{10}$ scale is shown as a function of the lattice spacing $a^{'} (Å)$ . For $a^{'}$ larger than Si more Ge is expected. The peak around $5.431 Å$ corresponds to the substrate scattering. The open circles mark the $a^{'}$ corresponding to angular scans used in the strain profile analysis. Insets: AFM images of $250 \times 250 {nm}^{2}$ showing one QDM and one pit. The corresponding line scans (dashed line—pit, solid line—QDM) are along the [010] direction.Reuse & Permissions
Figure 2
(Color online) Cross-section representation of the QDM and SiGe films. $Δ (a^{'})$ is the thickness of each scattering object contributing to the x-ray signal. Graph of the Ge content as a function of the lattice spacing $a^{'} (Å)$ . It is possible to associate the Ge concentration with different QDM regions, shown in the two-dimensional representation. The solid line corresponds to a radial scan on a linear scale, showing the scattered intensity of the Si substrate and the wetting layer. The open circles mark the $a^{'}$ corresponding to angular scans used in the strain profile analysis.Reuse & Permissions
Figure 3
(Color online) (a) Angular scans of the QDM sample measured at $11.004 keV$ . Each curve is associated with a different lattice spacing (the values are indicated). The measurements were performed near the (400) reflection. The solid lines correspond to Gaussian approximations. (b) Graphic of the size of the scattering object $L$ (nm) as a function of the lattice spacing $a^{'} (Å)$ . Each point is determined through the FWHM of the Gaussian fits. Below: top view representation of a single QDM showing the smaller and larger sides— $L_{l}$ and $L_{s}$ —of the islands, which corresponds to different heights, besides the same lattice spacing.Reuse & Permissions
Figure 4
(Color) Relation between the size of each scattering object size (nm) determined by anomalous GIXRD angular scans and the QDM height (nm). The color scale is associated with the lattice spacing variation in the film.Reuse & Permissions
Figure 5
(Color) (a) Atomic force microscopy line scan of one QDM showing the local strain variation (color scale). There is a coexistence of compressive and relaxed materials. (b) Ge content (%) as a function of QDM height (nm) composed by anomalous XRD measurements.Reuse & Permissions

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