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3C-SiC is a promising material for low-voltage power electronic devices but its growth is still challenging. Heteroepitaxy of 3C-SiC on Si micrometer-sized pillars is regarded as a viable method to achieve high crystalline quality, minimizing the effects of lattice and thermal expansion mismatch. Three-dimensional micro-crystals with sharply-faceted profiles are obtained, eventually touching with each other to form a continuous layer, suspended on the underlying pillars. By comparing experimental data and simulation results obtained by a phase-field growth model, here we demonstrate that the evolution of the crystal morphology occurs in a kinetic regime, dominated by the different incorporation times on the crystal facets. These microscopic parameters, effective to characterize the out-of-equilibrium growth process, are estimated by a best-fitting procedure, matching simulation profiles to the experimental one at different deposition stages. Then, simulations are exploited to inspect the role of a different pillar geometry and template effects are recognized. Finally, coalescence of closely spaced crystals ordered into an hexagonal array is investigated. Two possible alignments of the pattern are compared and the most convenient arrangement is evaluated.

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.

The stacking faults (SFs) in 3C-SiC epitaxially grown on ridges deeply etched into Si(001) substrates offcut towards [110] were quantitatively analyzed by electron microscopy and X-ray diffraction. A significant reduction of SF density with respect to planar material was observed for the {111} planes parallel to the ridges. The highest SF density was found in the (-1-11) plane. A previously observed defect was identified as twins by electron backscatter diffraction.

The growth morphology of epitaxial 3C-SiC crystals grown on hexagonal pillars deeply etched into Si(111) substrates is presented. Different growth velocities of side facets let the top crystal facet evolve from hexagonal towards triangular shape during growth. The lateral size and separation between Si pillars determine the onset of fusion between neighboring crystals during growth at a height tailoring of which is crucial to reduce the stacking fault (SF) density of the coalesced surface. A density of 3400 cm-1 was observed for a coalesced surface, compared to 8100 cm-1 for layers grown on unpatterned area. Intermediate partial fusion of neighboring crystals is shown as well as a surface of fully coalesced crystals.

Monolithic integration of an X-ray absorber layer on a Si CMOS chip might be a potentially attractive way to improve detector performance at acceptable costs. In practice this requires, however, the epitaxial growth of highly mismatched layers on a Si-substrate, both in terms of lattice parameters and thermal expansion coefficients. The generation of extended crystal defects, wafer bowing and layer cracking have so far made it impossible to put the simple concept into practice. Here we present a way in which the difficulties of fabricating very thick, defect-free epitaxial layers may be overcome. It consists of an array of densely packed, three-dimensional Ge-crystals on a patterned Si(001) substrate. The finite gap between neighboring micron-sized crystals prevents layer cracking and substrate bowing, while extended defects are driven to the crystal sidewalls. We show that the Ge-crystals are indeed defect-free, despite the lattice misfit of 4.2%. The electrical characteristics of individual Ge/Si heterojunction diodes are obtained from in-situ measurements inside a scanning electron microscope. The fabrication of monolithically integrated detectors is shown to be compatible with Si-CMOS processing

We report on the mask-less integration of GaAs crystals several microns in size on patterned Si substrates by metal organic vapor phase epitaxy. The lattice parameter mismatch is bridged by first growing 2-mum-tall intermediate Ge mesas on 8-mum-tall Si pillars by low-energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs crystals towards full pyramids exhibiting energetically stable {111} facets with decreasing Si pillar size. The release of the strain induced by the mismatch of thermal expansion coefficients in the GaAs crystals has been studied by X-ray diffraction and photoluminescence measurements. The strain release mechanism is discussed within the framework of linear elasticity theory by Finite Element Method simulations, based on realistic geometries extracted from scanning electron microscopy images.

The fabrication of advanced devices increasingly requires materials with different properties to be combined in the form of monolithic heterostructures. In practice this means growing epitaxial semiconductor layers on substrates often greatly differing in lattice parameters and thermal expansion coefficients. With increasing layer thickness the relaxation of misfit and thermal strains may cause dislocations, substrate bowing and even layer cracking. Minimizing these drawbacks is therefore essential for heterostructures based on thick layers to be of any use for device fabrication. Here we prove by scanning X-ray nanodiffraction that mismatched Ge crystals epitaxially grown on deeply patterned Si substrates evolve into perfect structures away from the heavily dislocated interface. We show that relaxing thermal and misfit strains result just in lattice bending and tiny crystal tilts. We may thus expect a new concept in which continuous layers are replaced by quasi-continuous crystal arrays to lead to dramatically improved physical properties

Monolithic integration of dissimilar materials with Si microelectronics by hetero-epitaxial growth, a process by means of which all the components are manufactured on the same piece of Si substrate, has been an active research topic for many years. The epitaxial growth of layers differing in lattice parameter and thermal expansion coefficients poses, however, a number of problems detrimental to device processing, yield and performance, such as dislocations, wafer bowing or even layer cracks. The latter become especially relevant at larger layer thicknesses, required for a number of applications, such as high-brightness LEDs, power electronic devices, multi-junction solar cells. We eliminated these disadvantages by a new approach in which layers are partitioned into arrays of disconnected single crystals by a mechanism of self-limited lateral growth. Our approach, tested so far for group IV epitaxy, consists of the following steps. First a Si substrate is patterned into high-aspect ratio pillars or ridges by conventional photolithography and deep reactive ion etching, to expose limited areas on which to grow Ge and Si 1-xGe x layers. Subsequent epitaxial growth by low energy plasma enhanced chemical vapor deposition (LEPECVD) takes advantage both of geometric shielding of the growth species arriving at the patterned substrate surface, as well as of growth parameters (e.g. low temperature, high growth rate) designed to limit the surface diffusion length, thus favoring vertical over lateral growth. This results in a uniform space-filling array of three dimensional epitaxial crystals. Surprisingly, the observed self-limited lateral expansion leaves an air gap between the neighboring crystals of the order of just several tens of nanometers, thus preventing their coalescence. The formation of these arrays was found to be remarkably independent of the thickness of the deposit and the details of the substrate patterns. It does not even depend on the lattice and thermal mism

Nanocrystalline silicon (nc-Si) already attracted a considerable attention as a promising material for photovoltaic applications, while its full optoelectronic potentiality is still under investigation, due to the relatively poor knowledge of the correlations between growth conditions, microstructure and physical properties. This paper aims at the illustration of the preliminary results of a 3 years project, addressed at the demonstration of the multifunctionality of nanocrystalline silicon, via the understanding of the quantitative correlations between growth process parameters and the structural, optical, electrical and physico-chemical properties of nc-silicon. The main topics foreseen and actively pursued were:•The development of a gas phase kinetic model to be used for the simulations of the plasma enhanced chemical deposition (PECVD) process, the development of a kinetic model relative to the interaction of radicals present in the plasma phase at the substrate interface, and last but not least, the development of models relative to the subsequent nc-Si growth.•The growth of undoped and doped nc-Si layers on convenient substrates by the low energy plasma enhanced chemical deposition (LEPECVD) process using silane, phosphine, diborane and hydrogen mixtures.•The quantitative experimental determination of the correlation among the crystallinity fraction, the film microstructure, the grain size/shape/orientation, the hydrogen content, the density of the recombination centres, the optical absorption coefficient the strain/stress state and the carrier mobility and diffusion length in undoped and n-type and p-type films, in view of the optimization of the minority carrier generation and carrier collection. The preliminary results already demonstrate that a proper modelling of the growth process is of great help in the selection of optimised growth procedures for nc-Si films and that a full range experimental characterisation is needed to get the proper inputs to the m

This work deals with the characterization of nanocrystalline (nc) silicon films, grown using the plasma enhanced chemical vapour deposition (PECVD) process based on a lowvoltage– high-current arc discharge plasma named LEPECVD (low-energy PECVD). The structural, electrical and chemical properties of the LEPECVD grown films have been studied as a function of the deposition parameters (substrate temperature, growth rate, silane dilution). The results show that the films consist of elongated nanocrystals along the /11 1Sdirection, embedded in an amorphous matrix. The crystallite size along the /1 11S direction is in the range of 9-20 nm. The volume fraction of crystallinity (wc) varies between 51% and 78%, depending on preparation conditions. Conductivity values of the order of 10 6O 1 cm 1 for the layers were measured, making the material suitable for the p–i–n junction application.

By comparing the scanning tunnel microscope images of the morphological evolution in Ge/Si(001) during Si overgrowth with the results of classical molecular-dynamic simulation and ab initio calculations for Si distribution in the wetting layer, we conclude that the disappearance of the (MxN) reconstruction is determined by Si-Ge intermixing in the third and fourth layers in the Ge wetting layer. The subsequent evolution of the (2xN) pattern, where N increases with further Si deposition, is shown to be produced by Si-Ge intermixing in the second layer. We suggest that this process corresponds to a crossover between thermodynamic behavior and kinetic behavior, which eventually leads to the Si capping.

Low-energy plasma-enhanced chemical vapor deposition experiments indicate that strain relaxation in thin virtual SiGe layers grown abruptly on a Si substrate may occur more efficiently than in thick graded layers. We provide one possible explanation using molecular dynamics simulations based on Tersoff potentials, showing that the mobility of shuffle dislocation segments is enhanced with increasing misfit in Ge/SiGe(001) alloys. A low strain regime favors the formation of partialized glide segments. The simulated mobility of the perfect shuffle core, in the case of strong compressive stress, appears to be extremely high. Despite being qualitative in nature, these results suggest an intriguing interpretation of the experimental observation. © 2004 Elsevier B.V. All rights reserved.

Molecular dynamics simulations with the Tersoff potential of the strain distribution around 60degrees misfit dislocation in a heteroepitaxial SiGe film confirm that highly compressed and expanded, cylindrical nanometer-sized regions appear on opposite sides of the (111) glide plane. Such a configuration is suggested to generate opposite chemical potential gradients for Si and Ge diffusion and, as verified by a Monte Carlo simulation, in the formation of Si-rich and Ge-rich nanowires along the dislocation core. This model is supported by photoluminescence measurements as a function of annealing temperature and time. (C) 2004 American Institute of Physics.

We show by molecular dynamics simulations on a scale comparable to experimental dimensions that a peculiar surface reconstruction of the {105} facets is responsible for the stability of Ge pyramids on Si(001). This finding is confirmed by ab initio total energy calculations for competing surface reconstructions and a very satisfactory comparison of the corresponding charge density maps to scanning tunneling microscopy measurements of the facets, both for filled and empty states

In ballistic electron emission microscopy (BEEM) the propagation of hot carriers in thin metal films has long been treated using a free electron model. While the model explains many experimental findings, it cannot account for the lateral resolution observed for both electrons and holes on epitaxial CoSi2/Si(111), where interfacial point defects of atomic size appear as small as 1.3 nm, even below a 5.6 nm thick film. We present ab initio calculations explaining this high resolution in terms of conduction (valence) band structure focusing of electrons (holes), according to a recent Green's function approach to the BEEM process

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

An innovative strategy in dislocation analysis, based on comparison between continuous and tessellated film, demonstrates that vertical dislocations, extending straight up to the surface, easily dominate in thick Ge layers on Si(001) substrates. The complete elimination of dislocations is achieved by growing self-aligned and self-limited Ge microcrystals with fully faceted growth fronts, as demonstrated by AFM extensive etch-pit counts

A theoretical approach to the lattice dynamics of Si1-xGex alloys within a valence force field framework is discussed. A modified Keating model, the anharmonic Keating, was employed to perform supercell calculations in order to investigate accurately the three Raman active optical phonon modes. Theoretical results on both relaxed and strained SiGe samples are shown. A detailed study of the biaxial strain-induced shift of phonon modes of epitaxial SiGe alloys has been performed in all the composition range. Furthermore, a comparison of experimental data with the model results on phonon strain shift coefficients will be also discussed. (C) 2006 Elsevier Ltd. All rights reserved

This work deals with the properties of nanocrystalline silicon films, which have been grown using a Low Energy Plasma Enhanced Chemical Vapor Deposition (LEPECVD) process. This process permits to increase the intensity of the plasma discharge in the growth region and thus to achieve high growth rates while avoiding ion-induced surface damage of the deposited films. The structural, chemical, electrical and optical properties of the LEPECVD grown films were studied in detail as a function of the deposition parameters. As a result of this work we were able to show that the grown films present higher crystallinity and were obtained at higher deposition rate than with standard PECVD techniques. The electrical and structural properties indicate that the films are promising for application as an intrinsic layer in pin solar cell production. Further optimisation work is needed for optoelectronic applications. (C) 2005 Elsevier B.V. All rights reserved.

The morphological transition from pyramid to dome islands during the growth of Ge on Si(001)was investigated using high resolution scanning tunneling microscopy (STM). It was observed that the pyramids were grown from top to bottom and that from a critical size on incomplete facets were formed. It was also observed that the bunching of the steps delimiting the facets evolved into the steeper dome facets. It was found that a microscopic model was developed based on first principles and Tersoff-potential calculations for the onset of the morphological transition.

By using scanning tunneling microscopy we found that the surface reconstruction of Ge/Si(001) epilayers evolves from (MxN) to (2xN), and eventually to (2x1), during exposure to a Si flux. This sequence appears to be just the inverse of that observed during the growth of Ge or SiGe alloys on Si(001). However, molecular dynamics simulations supported by ab initio calculations allow us to interpret this morphological evolution in terms of Si migration through the epilayer and complex Si-Ge intermixing below the top Ge layer.

Epitaxially stabilized iron monosilicide films having the CsCl structure (c-FeSi) have been investigated by conversion electron Mossbauer spectroscopy and transmission electron microscopy. The (57)Fe Mossbauer parameters (isomer shift delta, linewidth Gamma, and quadrupole splitting Delta) are reported and discussed in terms of the local surrounding of the Fe nucleus. High statistical accuracy and resolution allowed a detailed investigation of the effects of strain and of the structural phase transformation from the epitaxially stabilized to the bulk stable phase, The phase transformation was found to proceed in a rather surprising layer by layer mechanism with smooth interfaces between the epitaxially stabilized, the bulk stable, and a third phase. Results from a molecular-dynamics simulation at constant pressure and temperature of the structural phase transition are presented and compared with the experimental findings. The isomer shift and the electric-field gradient at the Fe nucleus in the strained c-FeSi and in the third phase have been calculated using the ab initio full potential linear muffin-tin orbital method. The Mossbauer parameters of some relevant point defects in c-FeSi have likewise been calculated within this framework. [S0163-1829(99)01302-8].