The Radon transform as a tool for 3D reciprocal-space mapping of epitaxial microcrystals.

This work presents a new approach suitable for mapping reciprocal space in three dimensions with standard laboratory equipment and a typical X-ray diffraction setup. The method is based on symmetric and coplanar high-resolution X-ray diffraction, ideally realized using 2D X-ray pixel detectors. The processing of experimental data exploits the Radon transform commonly used in medical and materials science. It is shown that this technique can also be used for diffraction mapping in reciprocal space even if a highly collimated beam is not available. The application of the method is demonstrated for various types of epitaxial microcrystals on Si substrates. These comprise partially fused SiGe microcrystals that are tens of micrometres high, multiple-quantum-well structures grown on SiGe microcrystals and pyramid-shaped GaAs/Ge microcrystals on top of Si micropillars.

Growth and Coalescence of 3C-SiC on Si(111) Micro-Pillars by a Phase-Field Approach.

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.

A comparative study of defect formation in GaAs nanocrystals selectively grown on nanopatterned and flat Si(001) substrates.

Crystal defects present in GaAs nanocrystals ∼15-50 nm in diameter and grown by metal organic vapor phase epitaxy on top of two different nanopatterned Si(001) substrates (nanopillars and nanotips with ∼40-80 nm openings embedded in a SiO2 matrix) and on a planar substrate, have been investigated by means of atomic-resolution aberration-corrected scanning transmission electron microscopy. Conditions of their formation are discussed. The defect analysis of the three GaAs/Si systems reveals a higher defect density in the GaAs crystals grown on nanopillars as compared to those grown on nanotips and the planar substrate, possibly concomitant to the atomic-scale irregularities identified at the patterned Si(001) nanopillars. It is concluded that the misfit strain in the GaAs nanocrystals is fully plastically relaxed while no noticeable substrate compliance effects are observed on any of the studied substrates.

Selective Nucleation of GaAs on Si Nanofacets.

The early growth stage of GaAs by metal organic vapor phase epitaxy on a novel kind of Si substrate is investigated. The substrate consists of nanotips (NTs) fabricated on a Si(001) wafer by means of lithography and reactive ion etching. 3D GaAs nanocrystals are found to nucleate with a probability of 90% on the (n0m), (-n0m), (0nm), and (0-nm) facets (n, m integers) of these NTs. Additionally, in terms of nucleation yield, an average of 2 GaAs nanocrystals in each of those facets is observed. By contrast, facets of type {±nnm} remain virtually free of any 3D nuclei. A simple model based on the kinetics of the growth is used to explain the facet selective 3D nucleation. The model is consistent with a similar selectivity observed on micrometer-sized substrate features.

Structural and optical characterization of GaAs nano-crystals selectively grown on Si nano-tips by MOVPE.

We present the nanoheteroepitaxial growth of gallium arsenide (GaAs) on nano-patterned silicon (Si) (001) substrates fabricated using a CMOS technology compatible process. The selective growth of GaAs nano-crystals (NCs) was achieved at 570 °C by MOVPE. A detailed structure and defect characterization study of the grown nano-heterostructures was performed using scanning transmission electron microscopy, x-ray diffraction, micro-Raman, and micro-photoluminescence (µ-PL) spectroscopy. The results show single-crystalline, nearly relaxed GaAs NCs on top of slightly, by the SiO2-mask compressively strained Si nano-tips (NTs). Given the limited contact area, GaAs/Si nanostructures benefit from limited intermixing in contrast to planar GaAs films on Si. Even though a few growth defects (e.g. stacking faults, micro/nano-twins, etc) especially located at the GaAs/Si interface region were detected, the nanoheterostructures show intensive light emission, as investigated by µ-PL spectroscopy. Achieving well-ordered high quality GaAs NCs on Si NTs may provide opportunities for superior electronic, photonic, or photovoltaic device performances integrated on the silicon technology platform.

Dislocation-free Ge Nano-crystals via Pattern Independent Selective Ge Heteroepitaxy on Si Nano-Tip Wafers.

The integration of dislocation-free Ge nano-islands was realized via selective molecular beam epitaxy on Si nano-tip patterned substrates. The Si-tip wafers feature a rectangular array of nanometer sized Si tips with (001) facet exposed among a SiO2 matrix. These wafers were fabricated by complementary metal-oxide-semiconductor (CMOS) compatible nanotechnology. Calculations based on nucleation theory predict that the selective growth occurs close to thermodynamic equilibrium, where condensation of Ge adatoms on SiO2 is disfavored due to the extremely short re-evaporation time and diffusion length. The growth selectivity is ensured by the desorption-limited growth regime leading to the observed pattern independence, i.e. the absence of loading effect commonly encountered in chemical vapor deposition. The growth condition of high temperature and low deposition rate is responsible for the observed high crystalline quality of the Ge islands which is also associated with negligible Si-Ge intermixing owing to geometric hindrance by the Si nano-tip approach. Single island as well as area-averaged characterization methods demonstrate that Ge islands are dislocation-free and heteroepitaxial strain is fully relaxed. Such well-ordered high quality Ge islands present a step towards the achievement of materials suitable for optical applications.

Highly Mismatched, Dislocation-Free SiGe/Si Heterostructures.

Defect-free mismatched heterostructures on Si substrates are produced by an innovative strategy. The strain relaxation is engineered to occur elastically rather than plastically by combining suitable substrate patterning and vertical crystal growth with compositional grading. Its validity is proven both experimentally and theoretically for the pivotal case of SiGe/Si(001).

Engineered Coalescence by Annealing 3D Ge Microstructures into High-Quality Suspended Layers on Si.

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.

Perfect crystals grown from imperfect interfaces.

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.

Unexpected dominance of vertical dislocations in high-misfit ge/si(001) films and their elimination by deep substrate patterning.

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.

Scaling hetero-epitaxy from layers to three-dimensional crystals.

Quantum structures made from epitaxial semiconductor layers have revolutionized our understanding of low-dimensional systems and are used for ultrafast transistors, semiconductor lasers, and detectors. Strain induced by different lattice parameters and thermal properties offers additional degrees of freedom for tailoring materials, but often at the expense of dislocation generation, wafer bowing, and cracks. We eliminated these drawbacks by fast, low-temperature epitaxial growth of Ge and SiGe crystals onto micrometer-scale tall pillars etched into Si(001) substrates. Faceted crystals were shown to be strain- and defect-free by x-ray diffraction, electron microscopy, and defect etching. They formed space-filling arrays up to tens of micrometers in height by a mechanism of self-limited lateral growth. The mechanism is explained by reduced surface diffusion and flux shielding by nearest-neighbor crystals.

Nanolithographic templates using diblock copolymer films on chemically heterogeneous substrates.

The orientation of the lamellae formed by the phase separation of symmetric diblock copolymer thin films is strongly affected by the wetting properties of the polymer blocks with respect to the substrate. On bare silicon wafers the lamellae of polystyrene-b-polymethylmethacrylate thin films tend to order parallel to the wafer surface, with the polymethylmethacrylate block preferentially wetting silicon. We have developed a methodology for inducing the arrangement of lamellae perpendicular to the substrate by using chemically modified substrates. This is done by chemisorbing a self-assembled monolayer of thiol-terminated alkane chains on thin gold films deposited on silicon wafers. We also show that it is possible to spatially control the perpendicular orientation of the lamellae at sub-micron length scales by using simple chemical patterns and etch them, in order to produce nanolithographic templates. This method may be of great technological interest for the preparation of well-defined templates using block copolymer thin films.