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The development of three-dimensional architectures in semiconductor technology is paving the way to new device concepts for various applications, from quantum computing to single photon avalanche detectors. In most cases, such structures are achievable only under far-from-equilibrium growth conditions. Controlling the shape and morphology of the growing structures, to meet the strict requirements for an application, is far more complex than in close-to-equilibrium cases. The development of predictive simulation tools can be essential to guide the experiments. A versatile phase-field model for kinetic crystal growth is presented and applied to the prototypical case of Ge/Si vertical microcrystals grown on deeply patterned Si substrates. These structures, under development for innovative optoelectronic applications, are characterized by a complex three-dimensional set of facets essentially driven by facet competition. First, the parameters describing the kinetics on the surface of Si and Ge are fitted on a small set of experimental results. To this goal, Si vertical microcrystals have been grown, while for Ge the fitting parameters have been obtained from data from the literature. Once calibrated, the predictive capabilities of the model are demonstrated and exploited for investigating new pattern geometries and crystal morphologies, offering a guideline for the design of new 3D heterostructures. The reported methodology is intended to be a general approach for investigating faceted growth under far-from-equilibrium conditions.

We present an experimental and theoretical analysis of the formation of nanovoids within Si microcrystals epitaxially grown on Si patterned substrates. The growth conditions leading to the nucleation of nanovoids have been highlighted, and the roles played by the deposition rate, substrate temperature, and substrate pattern geometry are identified. By combining various scanning and transmission electron microscopy techniques, it has been possible to link the appearance pits of a few hundred nanometer width at the microcrystal surface with the formation of nanovoids within the crystal volume. A phase-field model, including surface diffusion and the flux of incoming material with shadowing effects, reproduces the qualitative features of the nanovoid formation thereby opening new perspectives for the bottom-up fabrication of 3D semiconductors microstructures.

The faceting of a growing crystal is theoretically investigated by a continuum model including the incorporation kinetics of adatoms. This allows us for predictions beyond a simple Wulff analysis which typically refers to faceted morphologies in terms of the equilibrium crystal shape for crystals with an anisotropic surface-energy, or to steady-state kinetic shape when the crystals grow with orientation-dependent velocities. A phase-field approach is implemented in order to account simultaneously for these contributions in two- and three dimensions reproducing realistic kinetic pathways for the morphological evolution of crystal surfaces during growth. After a systematic characterization of the faceting determined by orientation-dependent incorporation times, several different crystal morphologies are found by tuning the relative weights of thermodynamic and kinetic driving forces. Applications to realistic systems are finally reported showing the versatility of the proposed approach and demonstrating the key role played by the incorporation dynamics in out-of-equilibrium growth processes.

© 2019, Springer Nature America, Inc. The PLUMED consortium unifies developers and contributors to PLUMED, an open-source library for enhanced-sampling, free-energy calculations and the analysis of molecular dynamics simulations. Here, we outline our efforts to promote transparency and reproducibility by disseminating protocols for enhanced-sampling molecular simulations.

Dislocation networks are one of the most principle sources deteriorating the performances of devices based on lattice-mismatched heteroepitaxial systems. We demonstrate here a technique enabling fully coherent germanium (Ge) islands selectively grown on nanotip-patterned Si(001) substrates. The silicon (Si)-tip-patterned substrate, fabricated by complementary metal oxide semiconductor compatible nanotechnology, features ∼50-nm-wide Si areas emerging from a SiO2 matrix and arranged in an ordered lattice. Molecular beam epitaxy growths result in Ge nanoislands with high selectivity and having homogeneous shape and size. The ∼850°C growth temperature required for ensuring selective growth has been shown to lead to the formation of Ge islands of high crystalline quality without extensive Si intermixing (with 91 atom % Ge). Nanotip-patterned wafers result in geometric, kinetic-diffusion-barrier intermixing hindrance, confining the major intermixing to the pedestal region of Ge islands, where kinetic diffusion barriers are, however, high. Theoretical calculations suggest that the thin Si/Ge layer at the interface plays, nevertheless, a significant role in realizing our fully coherent Ge nanoislands free from extended defects especially dislocations. Single-layer graphene/Ge/Si-tip Schottky junctions were fabricated, and thanks to the absence of extended defects in Ge islands, they demonstrate high-performance photodetection characteristics with responsivity of ∼45 mA W-1 and an Ion/Ioff ratio of ∼103.

We demonstrate that the elastic stress relaxation mechanism in micrometre-sized, highly mismatched heterostructures may be enhanced by employing patterned substrates in the form of necked pillars, resulting in a significant reduction of the dislocation density. Compositionally graded Si1−xGex crystals were grown by low energy plasma enhanced chemical vapour deposition, resulting in tens of micrometres tall, three-dimensional heterostructures. The patterned Si(001) substrates consist of micrometre-sized Si pillars either with the vertical {110} or isotropically under-etched sidewalls resulting in narrow necks. The structural properties of these heterostructures were investigated by defect etching and transmission electron microscopy. We show that the dislocation density, and hence the competition between elastic and plastic stress relaxation, is highly influenced by the shape of the substrate necks and their proximity to the mismatched epitaxial material. The SiGe dislocation density increases monotonically with the crystal width but is significantly reduced by the substrate under-etching. The drop in dislocation density is interpreted as a direct effect of the enhanced compliance of the under-etched Si pillars, as confirmed by the three-dimensional finite element method simulations of the elastic energy distribution.

Nanoscale membranes have emerged as a new class of vertical nanostructures that enable the integration of horizontal networks of III-V nanowires on a chip. To generalize this method to the whole family of III-Vs, progress in the understanding of the membrane formation by selective area epitaxy in oxide slits is needed, in particular for different slit orientations. Here, it is demonstrated that the shape is primarily driven by the growth kinetics rather than determined by surface energy minimization as commonly occurs for faceted nanostructures. To this end, a phase-field model simulating the shape evolution during growth is devised, in agreement with the experimental findings for any slit orientations, even when the vertical membranes turn into multifaceted fins. This makes it possible to reverseengineer the facet-dependent incorporation times, which were so far unknown, even for common low-index facets. The compelling reproduction of the experimental morphologies demonstrates the reliability of the growth model and offers a general method to determine microscopic kinetic parameters governing out-of-equilibrium three-dimensional growth.

Ge vertical heterostructures grown on deeply-patterned Si(001) were first obtained in 2012 (C.V. Falub et al., Science 2012, 335, 1330–1334), immediately capturing attention due to the appealing possibility of growing micron-sized Ge crystals largely free of thermal stress and hosting dislocations only in a small fraction of their volume. Since then, considerable progress has been made in terms of extending the technique to several other systems, and of developing further strategies to lower the dislocation density. In this review, we shall mainly focus on the latter aspect, discussing in detail 100% dislocation-free, micron-sized vertical heterostructures obtained by exploiting compositional grading in the epitaxial crystals. Furthermore, we shall also analyze the role played by the shape of the pre-patterned substrate in directly influencing the dislocation distribution.

We present the morphological evolution obtained during the annealing of Ge strips grown on Si ridges as a prototypical process for 3D device architectures and nanophotonic applications. In particular, the morphological transition occurring from Ge/Si nanostrips to nanoislands is illustrated. The combined effect of performing annealing at different temperatures and varying the lateral size of the Si ridge underlying the Ge strips is addressed by means of a synergistic experimental and theoretical analysis. Indeed, three-dimensional phase-field simulations of surface diffusion, including the contributions of both surface and elastic energy, are exploited to understand the outcomes of annealing experiments. The breakup of Ge/Si strips, due to the activation of surface diffusion at high temperature, is found to be mainly driven by surface-energy reduction, thus pointing to a Rayleigh-like instability. The residual strain is found to play a minor role, only inducing local effects at the borders of the islands and an enhancement of the instability

Lateral ordering of heteroepitaxial islands can be conveniently achieved by suitable pit-patterning of the substrate prior to deposition. Controlling shape, orientation, and size of the pits is not trivial as, being metastable, they can significantly evolve during deposition/annealing. In this paper, we exploit a continuum model to explore the typical metastable pit morphologies that can be expected on Si(001), depending on the initial depth/shape. Evolution is predicted using a surface-diffusion model, formulated in a phase-field framework, and tackling surface-energy anisotropy. Results are shown to nicely reproduce typical metastable shapes reported in the literature. Moreover, long time scale evolutions of pit profiles with different depths are found to follow a similar kinetic pathway. The model is also exploited to treat the case of heteroepitaxial growth involving two materials characterized by different facets in their equilibrium Wulff’s shape. This can lead to significant changes in morphologies, such as a rotation of the pit during deposition as evidenced in Ge/Si experiments.

We simulate the morphological evolution of Ge microcrystals, grown out-of-equilibrium on deeply patterned Si substrates, as resulting from surface diffusion driven by the tendency toward the minimization of the surface energy. In particular, we report three-dimensional phase-field simulations accounting for the realistic surface energy anisotropy of Ge/Si crystals. In Salvalaglio et al. (2015) [10] it has been shown both by experiments and simulations that annealing of closely spaced crystals leads to a coalescence process with the formation of a suspended film. However, this was explained only by considering an isotropic surface energy. Here, we extend such a study by showing first the morphological changes of faceted isolated crystals. Then, the evolution of dense arrays is considered, describing their coalescence along with the evolution of facets. Combined with the previous results without anisotropy in the surface energy, this work allows us to confirm and assess the key features of the coalescence process.

Our recent experimental results of Ge nanoheteroepitaxy (NHE) on Si nanopillars (NPs) are reviewed to confirm the possibility of relaxed Ge growth on Si without misfit dislocations (MDs) formation by elastic deformation. Selective Ge growth is performed by using reduced pressure chemical vapor deposition (CVD) on two types of Si NPs with thermal SiO2 or CVD SiO2 sidewalls and on Si nanoislands (NIs) on SiO2. By using thermal SiO2 sidewall, compressive strain is generated in the Si pillar and fixed by the thermal SiO2. This results in an incoherent Ge growth on Si NPs due to MD formation. By using CVD SiO2 sidewall, tensile strain formation due to thermal expansion during prebake for Ge epi process is observed. However, strain in Si due to Ge growth is not dominant. By introducing a Si0.5Ge0.5 buffer layer, no MD and stacking faults are observed by cross section TEM. The shape of Ge on Si NPs becomes more uniform due to improved crystal quality. On Si NIs on SiO2, a clear compliance effect is observed after Ge growth. Coherent growth of Ge on Si is also realized on Si NIs by using Si0.5Ge0.5 buffer.

The structure, interface abruptness and strain relaxation in InAs/GaAs nanowires grown by molecular beam epitaxy in the Ga self-catalysed mode on (111) Si have been investigated by transmission electron microscopy. The nanowires had the zincblende phase. The InAs/GaAs interface was atomically and chemically sharp with a width around 1.5 nm, i.e. significantly smaller than previously reported values. This was achieved by the consumption of the Ga droplet and formation of a flat top facet of the GaAs followed by the growth of InAs by splitting the depositions of In and As. Both elastic and plastic strain relaxation took place simultaneously. Experimental TEM results about strain relaxation very well agree with linear elasticity theory calculations by the finite element methods.

Semiconductor heteroepitaxy involves a wealth of qualitatively different, competing phenomena. Examples include three-dimensional island formation, injection of dislocations, mixing between film and substrate atoms. Their relative importance depends on the specific growth conditions, giving rise to a very complex scenario. The need for an optimal control over heteroepitaxial films and/or nanostructures is widespread: semiconductor epitaxy by molecular beam epitaxy or chemical vapour deposition is nowadays exploited also in industrial environments. Simulation models can be precious in limiting the parameter space to be sampled while aiming at films/nanostructures with the desired properties. In order to be appealing (and useful) to an applied audience, such models must yield predictions directly comparable with experimental data. This implies matching typical time scales and sizes, while offering a satisfactory description of the main physical driving forces. It is the aim of the present review to show that continuum models of semiconductor heteroepitaxy evolved significantly, providing a promising tool (even a working tool, in some cases) to comply with the above requirements. Several examples, spanning from the nanometre to the micron scale, are illustrated. Current limitations and future research directions are also discussed.

The development of cutting-edge opto- and micro-electronic devices requires novel fabrication techniques, able to deliver high-quality materials, monolithically integrable into Si-based technology. Theoretical models and simulations supporting the experimental activities are highly needed to fully understand the growth physics at the nano- and micro-scale and tailor semiconductor heterostructures for technological applications. In this work, the modeling of the plasticity onset and of the morphological evolution for Ge/Si vertical heterostructures is introduced, fostered by the peculiar features of such systems with respect to the standard heteroepitaxy. Indeed, the aim of this thesis is to understand the main properties of systems with large height-to-base aspect-ratios, in order to offer new solutions for the realization of heterostructures with unprecedented material quality. Continuum models are selected to describe length scales ranging from a few nanometers to microns, and time scales of minutes (or even more). By means of the linear elasticity theory equations, solved by Finite Element Method (FEM) simulations, the competition between elastic and plastic relaxation in vertical Ge/Si systems is investigated. The critical parameters for the insertion of dislocations are determined for a single-layer structure, made of a SiGe layer on a Si pillar, and then generalized to multilayer configurations. Moreover, the possibility to achieve coherent structures at any size is demonstrated, provided that a proper grading of the Ge content during the growth is considered. A recipe for the calculation of such a grading of the Ge content is also introduced. Several comparisons with experiments show the generality of the proposed investigation for heterostructures at the nanoscale, and the versatility of the developed method. Moreover, thanks to dedicated experiments stimulated by the theoretical predictions, dislocation-free structures are proven to be feasible also at the

A phase‐field model allowing for the simulation of heteroepitaxial growth in semiconductors is developed. Both material deposition, mimicking Molecular Beam Epitaxy conditions, and surface diffusion, driven by the thermodynamic tendency toward free‐energy minimization, are taken into account. The typical Stranski‐Krastanow growth is investigated by considering the balance between surface energy, misfit strain and wetting effects. Additional contributions including substrate patterning, anisotropic surface energy and intermixing are introduced for a more realistic treatment. The finite element method is exploited for an accurate numerical solution. Simulation results are compared with experimental data.

We present a new concept applicable to the epitaxial growth of dislocation-free semiconductor structures on a mismatched substrate with a thickness far exceeding the conventional critical thickness for plastic strain relaxation. This innovative concept is based on the out-of-equilibrium growth of compositionally graded alloys on deeply patterned substrates. We obtain space-filling arrays of individual crystals several micrometers wide in which the mechanism of strain relaxation is fundamentally changed from plastic to elastic. The complete absence of dislocations at and near the heterointerface may pave the way to realize CMOS integrated SiGe X-ray detectors.

A method for growing suspended Ge films on micron-sized Si pillars in Si(001) is discussed. In [C.V. Falub et al., Science 335 (2012) 1330] vertically aligned three-dimensional Ge crystals, separated by a few tens of nanometers, were obtained by depositing several micrometers of Ge using Low-Energy Plasma-Enhanced Chemical Vapor Deposition. Here a different regime of high growth temperature is exploited in order to induce the merging of the crystals into a connected structure eventually forming a continuous, two-dimensional film. The mechanisms leading to such a behavior are discussed with the aid of an effective model of crystal growth. Both the effects of deposition and curvature-driven surface diffusion are considered to reproduce the main features of coalescence. The key enabling role of high temperature is identified with the activation of the diffusion process on a time scale competitive with the deposition rate. We demonstrate the versatility of the deposition process, which allows to switch between the formation of individual crystals and a continuous suspended film simply by tuning the growth temperature.

Cited By :6; Export Date: 30 September 2019QC 20191029

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Phase change materials are the active compounds in optical disks and in nonvolatile phase change memory devices. These applications rest on the fast and reversible switching between the amorphous and the crystalline phases, which takes place in the nano domain in both the time and the length scales. The fast crystallization is a key feature for the applications of phase change materials. In this work, we have investigated by means of large scale molecular dynamics simulations the crystal growth of the prototypical phase change compound GeTe at the interface between the crystalline and the supercooled liquid reached in the device upon heating the amorphous phase. A neural network interatomic potential, markedly faster with respect to first-principles methods, allowed us to consider high-symmetry crystalline surfaces as well as polycrystalline models that are very close to the actual geometry of the memory devices. We have found that the crystal growth from the interface is dominant at high temperatures while it is competing with homogeneous crystallization in the melt at lower temperatures. The crystal growth velocity markedly depends on the crystallographic plane exposed at the interface, the (100) surface being kinetically dominant with respect to the (111) surface. Polycrystalline interfaces, representative of realistic conditions in phase change memory devices, grow at significantly slower pace because of the presence of grain boundaries.

We present the fabrication of axial InAs/GaAs nanowire heterostructures on silicon with atomically sharp interfaces by molecular beam epitaxy. Our method exploits the crystallization at low temperature, by As supply, of In droplets deposited on the top of GaAs NWs grown by the self-assisted (self-catalyzed) mode. Extensive characterization based on transmission electron microscopy sets an upper limit for the InAs/GaAs interface thickness within few bilayers (≤1.5 nm). A detailed study of elastic/plastic strain relaxation at the interface is also presented, highlighting the role of nanowire lateral free surfaces.

Several crystalline structures are metastable or kinetically frozen out-of-equilibrium states in the phase space. When the corresponding lifetime is sufficiently long, typical equilibrium features such as regular and extended faceting can be observed. However, interpreting the extension of the facets and the overall shape in terms of a standard Wulff analysis is not justified. Here, we introduce a convenient general formulation of the anisotropic surface energy density, combined with a suitable phase-field model of surface diffusion. This allows for the investigation of the evolution toward equilibrium of realistically shaped nanostructures, describing an actual kinetic path and including the proper faceting. Numerical solution by the finite element method allows for efficient simulations even for the so-called strong anisotropy condition. After illustrating applications yielding equilibrium crystal shapes (corresponding to the Wulff construction), we focus our attention on faceting of structures in long-lived metastable states. The generality and numerical robustness of the approach is proven by a few applications to crystalline systems of great importance (quantum dots, quantum wires, patterned substrates) in present materials science.

The move from dimensional to functional scaling in microelectronics has led to renewed interest toward integration of Ge on Si. In this work, simulation-driven experiments leading to high-quality suspended Ge films on Si pillars are reported. Starting from an array of micrometric Ge crystals, the film is obtained by exploiting their temperature-driven coalescence across nanometric gaps. The merging process is simulated by means of a suitable surface-diffusion model within a phase-field approach. The successful comparison between experimental and simulated data demonstrates that the morphological evolution is driven purely by the lowering of surface-curvature gradients. This allows for fine control over the final morphology to be attained. At fixed annealing time and temperature, perfectly merged films are obtained from Ge crystals grown at low temperature (450 °C), whereas some void regions still persist for crystals grown at higher temperature (500 °C) due to their different initial morphology. The latter condition, however, looks very promising for possible applications. Indeed, scanning tunneling electron microscopy and high-resolution transmission electron microscopy analyses show that, at least during the first stages of merging, the developing film is free from threading dislocations. The present findings, thus, introduce a promising path to integrate Ge layers on Si with a low dislocation density.

Lattice-mismatched heteroepitaxial growth is nowadays a key step involved in the fabrication of a multitude of devices. However, full control over the different, sometimes competing phenomena taking place during deposition [1] has yet to be reached. Simulations can be precious in limiting the growth-parameter space to be sampled in actual experiments when searching for the desired system morphology. In this work we present a continuum approach able to tackle heteroepitaxy while matching typical experimental sizes and time scales. Starting from a suitable free-energy functional describing both elastic and surface energy , evolution is described based on a surface-diffusion model including an external flux. A convenient and general description of surface-energy anisotropy (allowing us to simulate faceting also in the “strong anisotropy” regime) is introduced [2] and several illustrative applications to semiconductors are described, exploiting both Phase-Field and sharp-interface approaches. Successful comparison with experiments is demonstrated for qualitatively different systems. We also discuss our attempt to simultaneously tackle elastic and plastic relaxation. In particular, we show how the stress field associated with an assigned distribution of misfit dislocations can be computed on the fly, its contribution to the surface chemical potential deeply influencing the growth mode. It must be noted that for miscible systems, such as Ge/Si, the above set of phenomena is not yet sufficient to yield a realistic description of the growth process. At high growth temperatures, indeed, entropy of mixing can lead to significant surface exchanges between deposited and surface atoms. Tackling intermixing is possible, but it requires for a considerable extension of the formalism [3,4]. [1] F. Montalenti, D. Scopece, and Leo Miglio, Comptes Rendus Physique 14 (7), 542-552 (2013). [2] M. Salvalaglio , R. Backofen , R. Bergamaschini , F. Montalenti , and Axel Voigt, Faceting of eq

Integration of different materials, such as GaAs or Ge, on Si via heteroepitaxy is nowadays a key step involved in the fabrication of a multitude of devices, some already in the market. However, full control over the different, sometimes competing phenomena taking place during deposition [1] has yet to be reached. Simulations can be precious in limiting the growth-parameter space to be sampled in actual experiments when searching for the desired system quality and morphology. In this work we present a continuum approach able to tackle heteroepitaxy while matching typical experimental sizes and time scales. A convenient and general description of surface-energy anisotropy is introduced [2] and several illustrative applications to semiconductors are described, exploiting both Phase-Field and sharp-interface approaches. Successful comparison with experiments is demonstrated for qualitatively different systems. We also discuss how to simultaneously tackle elastic and plastic relaxation. In particular, we show how the stress field associated with an assigned distribution of misfit dislocations can be computed on the fly, its contribution to the surface chemical potential deeply influencing the growth mode and/or the morphology of the growing front. [1] F. Montalenti, D. Scopece, and Leo Miglio, Comptes Rendus Physique 14 (7), 542-552 (2013). [2] M. Salvalaglio , R. Backofen , R. Bergamaschini , F. Montalenti , and Axel Voigt, Cryst. Growth Des. (2015); (DOI:10.1021/acs.cgd.5b00165)

The move from dimensional to functional scaling in microelectronics has recently revamped the interest towards integration of high-quality Ge on Si [1]. However, due to the 4% lattice misfit, this is quite a technological challenge. In ref. [2] we showed the possibility to grow vertically-aligned micrometric Ge crystals, separated by nanometric gaps, on top of Si pillar patterned substrates, without dislocations at the top region. Herein we show by simulation-driven experiments, how these structures can evolve into a suspended Ge film by simply exploiting their temperature-driven coalescence. By closely comparing experimental data and simulation results based on a phase-field model of surface diffusion [3], we demonstrate that the massive morphological evolution is essentially driven by the lowering of surface-curvature gradients. Scanning Tunneling Electron Microscopy (STEM) and High Resolution - Transmission Electron Microscopy (HR-TEM) measurements indicate that the resulting film has very high crystal quality. In particular, dislocations are not observed to nucleate when the crystals start to coalesce, as confirmed by etch-pit analysis. Our approach thus provides a viable path for the integration on Si of Ge layers with very low defect density. [1] R. Pillarisetty, Nature 479, 324 (2011); [2] C.V. Falub et al., Science 335, 1330 (2012); [3] M. Salvalaglio, R. Backofen, R. Bergamaschini, F. Montalenti, A. Voigt, Cryst. Growth Des, in press (doi: 10.1021/acs.cgd.5b00165)

Fine tuning of crystal morphology is nowadays an important step for a large variety of applications, such as the performance improvement in cutting-edge electronic devices. However, full control over the different, sometimes competing phenomena, is far from trivial and has yet to be reached. Numerical simulations are precious in limiting the parameter space to be sampled in actual experiments when searching for the desired system morphology, and also in the design of future, innovative structures. Here, we present a phase-field (PF) model of surface diffusion [1], able to describe the morphological evolution by means of a real kinetic pathway, driven by the minimization of the surface energy. A convenient and general description of surface-energy anisotropy, allowing us to simulate faceting also in the “strong anisotropy” regime, is introduced [2] and several illustrative applications to semiconductor structures are described. Complex phenomena, involving even topological changes, can be readily tackled in the PF framework, and their occurrence in the processing of technology-relevant microstructures [3] is also shown as an example, demonstrating the predicting power of our approach. Finally, we discuss preliminary but encouraging results, revealing the possibility to extend our model including further key phenomena such as misfit-strain relaxation, anisotropic fluxes, and orientation-dependent growth velocity. 1] B. Li, J. Lowengrub, A. Rätz and A. Voigt. Commun. Comput. Phys. 6, 433 (2009). [2] M. Salvalaglio, R. Backofen, R. Bergamaschini, F. Montalenti and A. Voigt, Accepted, Cryst. Growth Des. 15, 2787 (2015). [3] C. V. Falub, H. von Känel, F. Isa, R. Bergamaschini, A. Marzegalli, D. Chrastina, G. Isella, E. Müller, P. Niedermann and L. Miglio. Science 335, 1330 (2012).

Future progress in Silicon (Si) micro- and nanoelectronics depends on further miniaturization ("More Moore") and/or diversification ("More than Moore") of existing Si CMOS - based technologies. In both cases, the integration of alternative semiconductors (i.e. Ge, III-V & II-VI) into the Si mainstream technology platform presents an important approach, simply because the optoelectronic properties of alternative semiconductors promise to outperform those of Si. However, lattice and thermal mismatch with respect to Si(001) limit achievable optoelectronic properties of heteroepitaxial micro- and nanostructures due to structural defects and chemical impurities. In consequence, advanced Si CMOS compatible heteroepitaxy concepts with precisely adjusted thermal growth and annealing budgets are of central importance. © 2014 IEEE

We present a theoretical investigation of plasticity onset and strain relaxation in Ge on Si pillar-like, vertical heterostructures (VHEs). By means of linear elasticity theory solved by Finite Element Methods, we determine the critical thickness h(c) for the insertion of a 60 degrees dislocation in Si1-xGex/Si VHEs as a function of their lateral extension. Then, we quantify the effect of inserting one or more buffer layers in further delaying plasticity when growing a Ge-pure layer on top of the VHEs. The presence of intermediate layers of suitable Ge content allows for the formation of fully coherent structures up to the micron scale. The optimal thickness of one or multiple buffers to avoid dislocations is also discussed.

We investigate the structural properties of Ge nanostructures selectively grown on Si. Defect-free nanostructures with a lateral size of 100 nm and surrounded by a thick (∼20 times larger than the coherent-film limit) Ge layer are achieved as demonstrated by transmission electron microscopy. As demonstrated by modeling based on elasticity theory solved by finite element methods, the peculiar combination of morphology and chemical composition of the nanostructures allows for a very efficient elastic relaxation of the heteroepitaxial strain. We demonstrate that, despite the relatively large size of the nanostructures, even a single dislocation would raise the energy of the system. A direct comparison between the strain field predicted by modeling and measured by energy-dispersive synchrotron-radiation grazing incidence x-ray diffraction shows substantial agreement. © 2014 American Physical Society.

We report micron-sized Ge crystal arrays grown on deeply patterned Si substrates that yield a surge of the interband photoluminescence intensity by more than 2 orders of magnitude with respect to that typical for epitaxial layers directly grown on planar substrates. This finding is ascribed to the strongly modified internal quantum efficiency induced by controlling the nonradiative recombination at dislocations and to the improved light extraction offered by the array architecture. By spectrally resolving the interband and the dislocation-related luminescence, we address the parasitic activity of extended defects and its impact on the optical properties of the heterosystem. Such results are then exploited along with band gap engineering to design SiGe reflectors and Ge quantum wells that are effective in further amplifying the emission yield