A route to realize strain engineering in weakly bonded heterostructures is presented. Such heterostructures, consisting of layered materials with a pronounced bond hierarchy of strong and weak bonds within and across their building blocks respectively, are anticipated to grow decoupled from each other. Hence, they are expected to be unsuitable for strain engineering as utilized for conventional materials which are strongly bonded isotropically. Here, it is shown for the first time that superlattices of layered chalcogenides (Sb 2 Te 3 / GeTe) behave neither as fully decoupled two-dimensional (2D) materials nor as covalently bonded three-dimensional (3D) materials. Instead, they form a novel class of 3D solids with an unparalleled atomic arrangement, featuring a distribution of lattice constants, which is tunable. A map to identify further material combinations with similar characteristic is given. It opens the way for the design of a novel class of artificial solids with unexplored properties.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201705901.dichalcogenides (e.g., MoSe 2 , WSe 2 , TiTe 2 , etc.), and quintuple layered V 2 -VI 3 chalcogenides (e.g., Sb 2 Te 3 , Bi 2 Te 3 , and Bi 2 Se 3 ). Even septuple, nontuple, and more complex layered systems can be created when alloying the latter class of materials with IV-VI chalcogenides like GeTe, SnTe, or PbTe.The weak interlayer interaction-which causes the 2D nature of these materials-is both a blessing and a curse. On the one hand, it allows the growth of heterostructures and superlattices of dissimilar 2D materials without epitaxial guidance (vdW epitaxy). [7] Yet, it also creates adverse side effects such as poor adhesion [7] and wetting. [8] More importantly, the weak coupling impedes strain engineering. [9][10][11][12] Clearly, in the limit of zero coupling across vdW gaps, it should be impossible to introduce any strain in the growing 2D film. Yet, if enough coupling prevails across these gaps, strain engineering should be possible, too. The engineering of strain is an elegant concept to tailor physical properties without changing composition. Heteroepitaxial growth provides a versatile platform to create such strained films on appropriately chosen substrates. A manifold of novel devices have been realized by strain control, such as MOSFET transistors with strained Si, leading to 80-120% gains in electron mobility, [13,14] or quantum cascade lasers, where the growth of thicker defect-free quantum wells shortens the operation wavelengths. [15] Hence, strain engineering is also subject of numerous publications dealing with 2D systems. [16][17][18][19][20][21][22] It requires an understanding and possibly tailoring of the coupling across vdW gaps. With this goal in mind, we have investigated GeTe/Sb 2 Te 3 superlattices (SLs). These layered systems are currently attracting significant scientific interest for next-generation data storage media based on phase-change materials (interfacial pha...

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...

Articles you may be interested inIn situ observation of the elastic deformation of a single epitaxial SiGe crystal by combining atomic force microscopy and micro x-ray diffraction

High-resolution x-ray diffraction has been performed on strained SiGe nanoscale islands grown coherently on Si͑001͒. Reciprocal space maps show a widely extended ''butterfly''-shaped island reflection and strong diffuse scattering around the substrate reflection. From such intensity maps the Ge content and its distribution inside the islands are evaluated. This is done by simulation of diffuse scattering for a variety of island models. The island shape is known from atomic force and scanning electron microscopy. The only free parameter was the Ge distribution, here approximated by a vertical concentration profile. With an abrupt increase of Ge content at about one third of the island height a rather good agreement with the experimental results is achieved. The strain distribution in the islands is then given by the finite element calculations, which are part of the simulation algorithm.

Epitaxial $ {\rm Ge}_{2} {\rm Sb}_{2} {\rm Te}_{5} $ thin layers were successfully grown in the metastable cubic phase on both slightly lattice‐mismatched (GaSb) and highly lattice‐mismatched (Si) templates. The higher quality of the films grown on (111)‐oriented substrates is attributed to the tendency to form layered structures in the stable bulk phase as well as to the nature of distortion in the metastable cubic phase. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)