Anisotropic surface diffusion and strain are used to explain the formation of three-dimensional (In,Ga)As quantum dot lattices. The diffusion characteristics of the surface coupled with the elastic anisotropy of the matrix, provides an excellent opportunity to influence the dot positions. In particular, quantum dots that are laterally organized into long chains or chessboard two-dimensional arrays vertically organized with strict vertical ordering or vertical ordering that is inclined to the sample surface normal are accurately predicted and observed.PACS 68.35.Fx,81.15.Hi During the last decade semiconductor quantum dots (QDs) have attracted increasing attention because of potential applications as novel semiconductor devices [1,2]. Besides time consuming techniques using electron beam lithography and subsequent etching to fabricate QDs, selforganized growth techniques have captured research interest [2][3][4][5][6]. In the Stranski-Krastanow growth mode, while the growth conditions can be optimized to produce nanostructures of near identical size and shape, often only a random spatial distribution of the QD is observed for a single layer of QDs [7]. However, for multiple layers a range of different results, from near perfect QD chains to threedimensional (3D) lattices, have been reported and discussed [8][9][10][11]. In this case, it has been suggested that the anisotropy in surface diffusion for (In,Ga)As QDs on GaAs (100), which is mainly caused by the (2x4) surface reconstruction with dimer rows running along [0][1][2][3][4][5][6][7][8][9][10][11], is responsible for the formation of QD chains along the [0-11]-direction [9,12]. In particular, the surface diffusion length along [0-11] is larger than along [011]. This leads to greater strain relaxation along the [0-11]-direction, producing an elliptical strain relief that is transferred to succeeding layers. This, eventually causes an asymmetric separation between neighboring dots and consequently leads to QD chain structures. Another example of the outcome of multiple layers of QD growth is the PbSe/PbEuTe system where a nearly perfect 3D lattice is reported along with the suggestion that the self-organized result is caused by anisotropic strain transfer from QD layerto-QD layer [10,11]. In each case the explanation is qualitative and a quantitative understanding of the role of diffusion and strain and the corresponding ability to design 3D QD structures is still lacking.In this letter, we report on experiments that use natural surface steps on high index substrates to further uncover the role of both surface diffusion and strain in producing 3D ordering of QDs. In particular we examine the formation and development of 3D square-like lattices of (In,Ga)As QDs in a GaAs matrix. The square lattices are created by vertically stacking QD layers while simultaneously introducing surface steps in each layer in order to vary and control the symmetry of the diffusion and strain pattern in each layer. Our findings show that by using different high index substrates...

A systematic study is presented in which multilayers of different composition (W/Si, Mo/Si, Pd/B(4)C), periodicity (from 2.5 to 5.5 nm) and number of layers have been characterized. In particular, the intrinsic quality (roughness and reflectivity) as well as the performance (homogeneity and coherence of the outgoing beam) as a monochromator for synchrotron radiation hard X-ray micro-imaging are investigated. The results indicate that the material composition is the dominating factor for the performance. By helping scientists and engineers specify the design parameters of multilayer monochromators, these results can contribute to a better exploitation of the advantages of multilayer monochromators over crystal-based devices; i.e. larger spectral bandwidth and high photon flux density, which are particularly useful for synchrotron-based micro-radiography and -tomography.

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Ex situ observed growth stages of LPE-SiGe/Si͑001͒ Stranski-Krastanow islands with a germanium content of 10% give clear evidence of a rapid shape transition at one third of the final island height. The island shape changes from a lenslike type without a top facet to truncated pyramids with ͕111͖ side facets and an ͑001͒ top facet. High-resolution x-ray diffraction has been applied to islands with higher germanium content of about 30%. Experimental results are compared with respective kinematical scattering simulations based on finite element calculations for the strain field. From these simulations the three-dimensional germanium composition profile inside the islands can be extracted and it substantiates a similar growth scenario with a distinct shape transition at one third of the final island height also for this germanium concentration range. We attribute the observed finite island size to a distinct nucleation problem at the island bottom caused by exceptional high strain energy around the island corners in combination with a strain driven wetting layer depression.

An experimental and simulation study of the full strain tensor and of strain-induced magnetocrystalline anisotropies in arrays of lithographically patterned (Ga,Mn)As on GaAs(001) is performed using a coherent diffraction lensless microscopy technique. We demonstrate the ability of our technique to get an insight into the strain field propagating in the crystal part belonging to the substrate. The experimentally reconstructed strain fields are in good agreement with those obtained from simulations based on elasticity theory.