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The atomic structure of grain boundaries in Ge micro-crystals grown on Si pillars for the fabrication of a monolithically integrated X-ray detector was studied by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Three different boundaries are found in Ge: Σ 3 { 111 } coherent twin boundaries, Σ 3 { 112 } incoherent twin boundaries, and Σ 9 { 122 } and Σ 27 { 552 } grain boundaries. They are described using the structural unit models containing single columns. Remarkably, we find for the first time a Σ 3 { 112 } incoherent twin boundary exhibiting two different atomic structures; one symmetric and one asymmetric. Their co-occurrence is explained by the presence of a small step in the boundary plane and the introduction of dislocations. Likewise, the atomic structure of junctions formed by the interaction of twin boundaries which result in Σ 9 { 122 } and Σ 27 { 552 } grain boundaries is also revealed. Geometrical phase analysis is applied to map the strain fields at two triple junctions and to uncover the position of the dislocations. more...

In this work we present an innovative approach to realise coherent, highly-mismatched 3-dimensional heterostructures on substrates patterned at the micrometre-scale. The approach is based on the out-of-equilibrium deposition of SiGe alloys graded at an exceptionally shallow grading rate (GR) of 1.5% µm −1 by low energy plasma enhanced chemical vapour deposition (LEPECVD). Fully coherent SiGe/Si crystals up to 6 µm in width were achieved as confirmed by defect etching and transmission electron microscopy (TEM) analyses. The experimental results are supported by calculations of the energy for dislocation formation which indicate that elastic relaxation is energetically favoured over plastic relaxation in the narrower heterostructures. X-ray diffraction measurements show that the SiGe crystals are strain-free irrespective of the stress relieving mechanism which changes from elastic to plastic by increasing their width. The impact of dislocations on the SiGe crystal quality is analysed by comparing the width of X-ray diffraction peaks as a function of the heterostructure size. more...

The local atomic structure of [110] tilt grain boundaries (GBs) formed in ∼100 nm-sized GaAs nanocrystals, which crystallize in the non-centrosymmetric zincblende-type structure with face-centred cubic lattice symmetry, was imaged and analysed by means of high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The nanocrystals were grown by metal–organic vapour phase epitaxy on top of (001) Si nanotips embedded in an oxide matrix. This paper introduces an automatic analysis method and corresponding processing tool for the identification of the GBs. The method comprises (i) extraction of crystallographic parameters,i.e.misorientation angles and transformation matrices for the different crystal parts (grains/twins) observed by HAADF-STEM, and (ii) determination of their common plane(s) by modelling all possible intersections of the corresponding three-dimensional reciprocal lattices. The structural unit model is also used to characterize the GB structures and to validate the data obtained by the developed algorithm. more...

Crystal defects, present in ~100 nm GaAs nanocrystals grown by metal organic vapour phase epitaxy on top of (0 0 1)-oriented Si nanotips (with a tip opening 50–90 nm), have been studied by means of high-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The role of 60° perfect, 30° and 90° Shockley partial misfit dislocations (MDs) in the plastic strain relaxation of GaAs on Si is discussed. Formation conditions of stair-rod dislocations and coherent twin boundaries in the GaAs nanocrystals are explained. Also, although stacking faults are commonly observed, we show here that synthesis of GaAs nanocrystals with a minimum number of these defects is possible. On the other hand, from the number of MDs, we have to conclude that the GaAs nanoparticles are fully relaxed plastically, such that for the present tip sizes no substrate compliance can be observed. more...

The development of new electro-optical devices and the realization of novel types of transistors require a profound understanding of the structural characteristics of new semiconductor heterostructures. This article provides a concise review about structural defects which occur in semiconductor heterostructures on the basis of micro-patterned Si substrates. In particular, one- and two-dimensional crystal defects are being discussed which are due to the plastic relaxation of epitaxial strain caused by the misfit of crystal lattices. Besides a few selected examples from literature, we treat in particular crystal defects occurring in GaAs/Si, Ge/Si and β-SiC/Si structures which are studied by high-resolution annular dark-field scanning transmission electron microscopy. The relevance of this article is twofold; firstly, it should provide a collection of data which are of help for the identification and characterization of defects in cubic semiconductors by means of atomic-resolution imaging, and secondly, the experimental data shall provide a basis for advancing the understanding of device characteristics with the aid of theoretical modelling by considering the defective nature of strained semiconductor heterostructures. more...

Low temperature wafer bonding of silicon to various materials is a key technology for all applications requiring a low temperature budget, such as monolithic CMOS integrated pixel detectors [1]. Here, we combine wet chemical etching of the native oxide from wafer surfaces with removal of the hydrogen passivation layer by a low-energy plasma prior to bonding in high vacuum (10-8 mbar). The method is applicable to the bonding of a multitude of crystalline semiconductors, and for Si results in conductive interfaces similar to the ones of UHV-bonded defect-free surfaces [2]. more...

Lithium aluminosilicate (LAS) is an interesting material for the electronics industry. The material has near zero water absorption, low dielectric constant, and low loss tangent. Therefore, this material is interesting for the packaging of radio frequency transmitters used under harsh conditions. In this study, LAS glass powders with different particle sizes were dry pressed. Densification, crystallization, microstructure and dielectric properties were studied during thermal treatments. The highest sintered density (about 97%) has been achieved with milled powder with a d90 of about 6 μm. Investigation of the microstructure reveals that flaky rutile has formed in β-spodumene grain boundaries even though spherical rutile is observed inside of β-spodumene. Glass phase in grain boundaries contain mostly SiO2, Al2O3 and ZnO. Impedance analysis showed that the dielectric constant is influenced by phase transformation and density. The formation of hexagonal LiAlSi2O6 increases the dielectric constant while transformation to tetragonal structure decreases the dielectric constant. more...

In this paper we report on the defects formed in Ge crystals grown on Si(001) pillars upon strain relaxation. The analysis is performed by high-angle annular dark-field scanning transmission electron microscopy. The strain relaxation happens by means of 60° and 90° misfit dislocations with Burgers vectors b → = 1 2 ⟨ 110 ⟩ . Misfit dislocations may split forming partial dislocations with Burgers vectors b → = 1 6 ⟨ 112 ⟩ , and are separated by a stacking fault. Besides, intrinsic stacking faults in different {111} planes interact and annihilate each other forming stair rod dislocations. Coherent and incoherent twin boundaries of the Σ 3{111} and Σ 3{112} types are also found in the Ge. more...

Incineration of municipal waste and sewage sludge is becoming an increasingly popular option for the disposal of waste materials and energy generation. The incineration process can concentrate metals in the incineration slags deposited in landfills. Emitted leachates contain a myriad of salts and metals; some of them in (nano)particulate form. In this study we collected the leachate from a Swiss landfill for municipal solid waste incineration (MSWI) residues along with the slags deposited at this site (waste incineration bottom ashes and fly ash, sewage sludge incineration bottom ash) from which simulated leachates were prepared. Basic water quality analysis (pH, DOC, TSS, major ions) and natural, incidental or engineered particles suspended in the leachate were characterized by NanoSight (for general size range), serial filtration with ICP-MS analysis for element specific particle size quantification and TEM/EDX to visualize particle morphology and composition. Special priority was given to those elements that have engineered nanoparticulate counterparts (Ti, Zn, Ag, Cu, Fe and Ce) to give an indication of 1) the current concentration and form of these particles emitted from the landfill, 2) the potential presence of engineered nanoparticles already in the samples, and 3) trends in particle size (change) in the leachate from different slags to provide an indication on particle mobility. Zn, Ag, and Cu had appreciable concentrations associated with small particulate matter (nano and 0.1–0.45 μm size fractions) in natural and laboratory prepared leachates, while Ti (nano)particles were most abundant in the landfill leachate. Multiple sampling dates suggested relatively steady particulate matter in the leachate for most elements, but analysis of differently aged bottom ash slags from municipal waste revealed differences with age, indicating the influence of slag weathering in metal mobilization. MSWI residues are inherently a complex mixture of stable and unstable materials that are subject to continuous and dynamic changes over time. Therefore, in this manuscript we placed an emphasis on understanding the geochemical processes that are associated with MSWI residue weathering and how this may dictate the likelihood of particulate metals leaching into groundwater. more...

The scientific understanding of nanoparticle (NP) release and transformations they undergo during the product life cycle is hampered by the narrow scope of many research endeavors in terms of both breadth of variables and completeness of analytical characterization. We conducted a comprehensive suite of studies to reveal overarching mechanisms and parameters for nanosilver transformations either still adhered to the fabric or when released after washing. Laboratory prepared nanoenhanced fabrics were investigated: three Ag variants and one Au used as an unreactive reference to separate mechanical from chemical releases. Sequential combinations of sunlight irradiation and/or washing in seven different detergent formulations was followed by NP characterization divided into two groups: (1) dissolved and particulate matter in the wash solutions and (2) the fraction that remained on the fabric. Analytical techniques included spICP-MS, XANES, TEM, SEM, and total metals analysis of fabric digests and wash water filtrates. Sunlight irradiation stabilizes metallic Ag upon washing. Detergents containing oxidizing agents assisted with Ag particle release but not Au NPs, inferring additional chemical mechanisms. While particle size played some role, the NP capping agent/fabric binder combination was a key factor in release. When particles were released, little alteration in size was observed. The use of well-controlled fabrics, unreactive reference materials, and a life-cycle based experimental regime are paramount to understanding changes in Ag speciation and release upon use of nanoenhanced textiles. Refereed/Peer-reviewed more...

In the present study, powder metallurgy processed unmilled AlMg5 (AlMg5-UM), milled AlMg5 (AlMg5-M), and milled AlMg5–0.4 vol.% Al2O3 (AlMg5–0.4Al2O3) nanocomposite were successfully spark plasma sintered at 550 °C. The microstructural analysis of spark plasma sintered AlMg5-UM revealed equi-axed grains with average size of 20 ± 5 μm. The microstructural analysis of spark plasma sintered AlMg5-M and AlMg5–0.4Al2O3 revealed a bimodal grain structure with both fine-coarse grains of size 700 nm–8 μm and 300 nm–8 μm, respectively. The spark plasma sintered AlMg5-UM exhibited a yield strength (YS) of 132 ± 3 MPa, ultimate tensile strength (UTS) of 296 ± 2 MPa and % elongation (% El) of 22 ± 2 comparable to the conventional Al 5083 alloy — O temper (YS of 145 MPa, UTS of 281 MPa and % El of 16). The spark plasma sintered AlMg5-M and AlMg5–0.4Al2O3 exhibited YS of 242 ± 9 MPa and 274 ± 5 MPa, UTS of 376 ± 4 MPa and 417 ± 8 MPa and % El of 19 ± 1 and 10 ± 1, respectively. The lower strength (YS and UTS) of AlMg5-UM can be ascribed to its coarse grains. Among the milled samples, the AlMg5–0.4Al2O3 composite exhibited high strength and it can be ascribed to its fine grain size (Hall–Petch effect), higher dislocation density and Orowan strengthening effect due to Al2O3 reinforcements. Keywords: Spark plasma sintering, Nano-composites, Microstructure, Hardness, Mechanical properties more...

Modern direct injection gasoline engines (GDI) generate considerably higher soot particulate matter (PM) emissions than conventional, port fuel gasoline, as well as diesel cars with particle filter. Soot PM generated by a typical state-of-the-art GDI (Euro VI) vehicle during two standard international driving cycles (NEDC, WLTC), as well as during a short experimental sub-cycle was investigated by electron microscopy. The study reveals primary particles between ∼4–55 nm in diameter, the majority being more...

The potential hazard posed by nanomaterials can be significantly influenced by transformations which these materials undergo during their lifecycle, from manufacturing through to disposal. The transformations may depend on the nanomaterials’ own physicochemical properties as well as the environment they are exposed to. This study focuses on the mechanisms of transformation of cerium oxide nanoparticles (CeO2 NPs) in laboratory experiments which simulate potential scenarios in which the NPs are exposed to phosphatebearing media. We have experimented with the transformation of four different kinds of CeO2 NPs, in order to investigate the effects of nanoparticle size, capping agent (three were uncapped and one was PVP capped) and oxidation state (two consisted mostly of Ce4+ and two were a mix of Ce3+/Ce4+). They were exposed to a reaction solution containing KH2PO4, citric acid and ascorbic acid at pH values of 2.3, 5.5 and 12.3, and concentrations of 1mM and 5mM. The transformations were followed by UV-vis, zeta potential and XRD measurements, which were taken after 7 and 21 days, and by transmission electron microscopy after 21 days. X-ray photoelectron spectroscopy was measured at 5mM concentration after 21 days for some samples. Results show that for pH 5 and 5mM phosphate concentration, CePO4 NPs were formed. Nanoparticles that were mostly Ce4+ did not dissolve at 1mM reagent concentration, and did not produce CePO4 NPs. When PVP was present as a capping agent it proved to be an extra reducing agent, and CePO4 was found under all conditions used. This is the first paper where the transformation of CeO2 NPs in the presence of phosphate has been studied for particles with different size, shapes and capping agents, in a range of different conditions and using many different characterisation methods., peer-reviewed more...

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. more...

We present an innovative approach to integrate arrays of isolated, strain-free GaN crystals on patterned Si substrates. First, micrometer-sized pillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5 μm Si substrates are deposited by low-energy plasma-enhanced chemical vapor deposition, forming crystals mostly bounded by {1 1 1}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beam epitaxy is then used for GaN deposition. GaN crystals with slanted {0 0 0 1} facets having a root-mean-square surface roughness of 0.7 nm are obtained for a deposited material thickness of >3 μm. Microphotoluminescence measurements performed at room and cryogenic temperature show no yellow luminescence and a neutral donor-bound A exciton transition at 3.471 eV (10 K) with a full width at half-maximum of 10 meV. Microphotoluminescence and micro-Raman spectra reveal that GaN grown on Si pillars is strain-free. Our results indicate that the shape of GaN crystals can be tuned by the pattern periodicity and that a reduction of threading dislocations is achieved in their top part. more...

Engineered nanoparticle (ENP) life cycles are strongly dependent on the life-cycle of the nanoenhanced products in which they are incorporated. An important phase for ENP associated with textiles is washing. Using a set of liquid and powdered commercially available detergents that span a wide range of different chemistries, washing studies were performed with one "standard" nanoparticle suspended in wash solution to systematically investigate (changes to) particle size distribution, dissolution, reprecipitation (i.e., "new" particle formation), and complexation to particulate matter. Au ENPs were used as a "tracer" through the system. TEM and EDX analysis were performed to observe morphological and chemical changes to the particles, and single-particle ICP-MS was used to build a size distribution of particles in solution. Varying the washing solution chemistry was found to dictate the extent and rate of dissolution, particle destruction, surface chemistry change(s), and new particle formation. Detergent chemistry, dominated by oxidizing agents, was a major factor. The detergent form (i.e., powder vs liquid) was the other decisive factor, with powder forms providing available surfaces for precipitation and sorption reactions. Control experiments with AgNO3 indicated metallic Ag particles formed during the washing process from dissolved Ag, implying not all Ag-NPs observed in a textile washing study are indicative of released Ag-ENPs but can also be the result of sequential dissolution/reduction reactions. more...

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Numerous nanometrology techniques have been developed in recent years to determine the size, concentration, and a number of other characteristics of engineered nanomaterials (ENM) in environmental matrices. Among the many available techniques, nanoparticle tracking analysis (NTA) can measure individual particles to create a size distribution and measure the particle number. Therefore, we explore the possibility to use these data to calculate the particle mass distribution. Additionally, we further developed the NTA methodology to explore its suitability for analysis of ENM in complex matrices by measuring ENM agglomeration and sedimentation in municipal solid waste incineration landfill leachates over time. 100 nm Au ENM were spiked into DI H2O and synthetic and natural leachates. We present the possibility of measuring ENM in the presence of natural particles based on differences in particle refractivity indices, delineate the necessity of creating a calibration curve to adjust the given NTA particle num... more...

Scanning electron microscopy and transmission electron microscopy were applied to investigate the morphology, mode of occurrence and chemical composition of metal particles (diesel ash) in the exhaust stream of a small truck outfitted with a typical after-treatment system (a diesel oxidation catalyst (DOC) and a downstream diesel particulate filter (DPF)). Ash consists of Ca-Zn-P-Mg-S-Na-Al-K-phases (lube-oil related), Fe, Cr, Ni, Sn, Pb, Sn (engine wear), and Pd (DOC coating). Soot agglomerates of variable sizes (0.5-5 μm) are abundant upstream of the DPF and are ash-free or contain notably little attached ash. Post-DPF soot agglomerates are very few, typically large (1-5 μm, exceptionally 13 μm), rarely0.5 μm, and contain abundant ash carried mostly from inside the DPF. The ash that reaches the atmosphere also occurs as separate aggregates ca. 0.2-2 μm in size consisting of sintered primary phases, ca. 20-400 nm large. Insoluble particles of these sizes may harm the respiratory and cardiovascular systems. The DPF probably promotes breakout of large soot agglomerates (mostly ash-bearing) by favoring sintering. Noble metals detached from the DOC coating may reach the ambient air. Finally, very few agglomerates of Fe-oxide nanoparticles form newly from engine wear and escape into the atmosphere. more...

In the fabrication GaN-based devices, several growth orientations are currently under investigation in order to exploit material properties which are inaccessible using layers grown along the c-axis. However, such procedures rely on foreign substrates with large misfits. Therefore, complex crystallographic defects form in the epitaxial layers and have been the subject of extensive studies. They include threading dislocations and stacking faults, which can be within basal or prismatic planes. Out of the c-axis, depending on the growth orientation, the glide planes of perfect dislocations may be no longer available, complicating the relaxation processes. more...

We present a detailed experimental and theoretical analysis of the epitaxial stress relaxation process in micro-structured compositionally graded alloys. We focus on the pivotal SiGe/Si(001) system employing patterned Si substrates at the micrometre-size scale to address the distribution of threading and misfit dislocations within the heterostructures. SiGe alloys with linearly increasing Ge content were deposited by low energy plasma enhanced chemical vapour deposition resulting in isolated, tens of micrometre tall 3D crystals. We demonstrate that complete elastic relaxation is achieved by appropriate choice of the Ge compositional grading rate and Si pillar width. We investigate the nature and distribution of dislocations along the [001] growth direction in SiGe crystals by transmission electron microscopy, chemical defect etching and etch pit counting. We show that for 3 μm wide Si pillars and a Ge grading rate of 1.5% μm−1, only misfit dislocations are present while their fraction is reduced for higher Ge grading rates and larger structures due to dislocation interactions. The experimental results are interpreted with the help of theoretical calculations based on linear elasticity theory describing the competition between purely elastic and plastic stress relaxation with increasing crystal width and Ge compositional grading rate. more...

An experiment has demonstrated that truly dislocation free SiGe crystals with widths of at least 5μm can be epitaxially grown on Si substrates by applying the compositional grading developed for layers to 3D structures. At such crystal widths, the epitaxial growth of SiGe alloys with a constant high Ge content inevitably leads to the injection of dislocations. The reason is that at dimension far above the nanoscale the low aspect ratio of the SiGe/Si system during the initial stages of the growth does not allow for sufficient elastic strain relaxation. The findings suggest that there is no limit to the final Ge content, and that the grading rate can be suitably tailored in order to give access to even larger dislocation-free structures. Indeed, preliminary experimental results indicate similar absence of dislocations in SiGe crystals graded to a fi nal Ge content of 80%. Combining this method with layer transfer techniques makes it possible to form dislocation-free SiGe patches many micrometers in size acting as virtual substrates with lattice parameters intermediate to those of Si and Ge on an Si substrate. more...

The microscopic characteristics of soot particulate matter (PM) in gas turbine exhaust are critical for an accurate assessment of the potential impacts of the aviation industry on the environment and human health. The morphology and internal structure of soot particles emitted from a CFM 56-7B26/3 turbofan engine were analyzed in an electron microscopic study, down to the nanoscale, for ∼ 100%, ∼ 65%, and ∼ 7% static engine thrust as a proxy for takeoff, cruising, and taxiing, respectively. Sampling was performed directly on transmission electron microscopy (TEM) grids with a state-of-the-art sampling system designed for nonvolatile particulate matter. The electron microscopy results reveal that ∼ 100% thrust produces the highest amount of soot, the highest soot particle volume, and the largest and most crystalline primary soot particles with the lowest oxidative reactivity. The opposite is the case for soot produced during taxiing, where primary soot particles are smallest and most reactive and the soot amount and volume are lowest. The microscopic characteristics of cruising condition soot resemble the ones of the ∼ 100% thrust conditions, but they are more moderate. Real time online measurements of number and mass concentration show also a clear correlation with engine thrust level, comparable with the TEM study. The results of the present work, in particular the small size of primary soot particles present in the exhaust (modes of 24, 20, and 13 nm in diameter for ∼ 100%, ∼ 65% and ∼ 7% engine thrust, respectively) could be a concern for human health and the environment and merit further study. This work further emphasizes the significance of the detailed morphological characteristics of soot for assessing environmental impacts. more...

Nano-heteroepitaxial growth of GaAs on Si(001) by metal organic vapor phase epitaxy was investigated to study emerging materials phenomena on the nano-scale of III-V/Si interaction. Arrays of Si nano-tips (NTs) embedded in a SiO2 matrix were used as substrates. The NTs had top Si openings of 50-90 nm serving as seeds for the selective growth of GaAs nano-crystals (NCs). The structural and morphological properties were investigated by high resolution scanning electron microscopy, atomic force microscopy, electron backscatter diffraction, x-ray diffraction, and high resolution scanning transmission electron microscopy. The GaAs growth led to epitaxial NCs featuring a bi-modal distribution of size and morphology. NCs of small size exhibited high structural quality and well-defined {111}-{100} faceting. Larger clusters had less regular shapes and contained twins. The present work shows that the growth of high quality GaAs NCs on Si NTs is feasible and can provide an alternate way to the integration of compound semiconductors with Si micro- and opto-electronics technology. more...

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. more...

Semipolar (11-22 ) GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are studied by transmission electron microscopy (TEM). The layers exhibit numerous defects with different geometries in comparison to the growth along the c-axis, they are identified as mostly partial dislocations, basal and prismatic stacking faults. The dislocations density is in the order of 5x10 9 cm -2 , the corresponding Burger vectors are b = 1/6 , b= 1/3 and a small fraction of perfect a type dislocations with b = 1/3 has been observed. The basal stacking fault density is in the order of 1x10 6 cm -1 . In an attempt to reduce the defect density, Si x N y interlayers have been used as nanomasks for epitaxial lateral overgrowth, our analysis shows that this leads to a quite small reduction of the defects as compared to the starting layer. more...

International audience; Semipolar (1122 ) GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are studied by transmission electron microscopy (TEM). The layers exhibit numerous defects with different geometries in comparison to the growth along the c-axis, they are identified as mostly partial dislocations, basal and prismatic stacking faults. The dislocations density is in the order of 5x109 cm-2, the corresponding Burger vectors are b = 1/6 < 2023 >, b = 1/3 < 1010 > and a small fraction of perfect a type dislocations with b = 1/3 < 1120 > has been observed. The basal stacking fault density is in the order of 1x106 cm-1. In an attempt to reduce the defect density, SixNy interlayers have been used as nanomasks for epitaxial lateral overgrowth, our analysis shows that this leads to a quite small reduction of the defects as compared to the starting layer. more...

International audience; InAlN/GaN layers grown by metalorganic vapor phase epitaxy have been investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is shown that the surface morphology and the crystallinity of the InAlN layers critically depend on the In composition. AFM analysis points out that step flow growth may not be easily attained in this system: when the InAlN or AlN interlayer thickness is increased, the growth mode becomes three dimensional. However, the formed islands are hundred of nanometers apart, and were not observed in the transmission electron microscope. From the TEM investigation, it is pointed out that v-shaped defects are present at the surface of some layers and they may be as deep as the whole ternary alloy, they are connected to threading dislocations originated from the underlying template layers. As expected the measured residual strain is minimum when the ternary layer composition is close to 16-17% In. more...

Threading dislocations (TDs) in germanium (Ge) crystals epitaxially grown on a patterned (001)- silicon (Si) substrate are investigated using transmission electron microscopy (TEM) techniques. Analysis of dislocations performed on the Ge crystals reveals 60_ and edge TDs with Burgers vector b =1/2. High-angle annular dark-field scanning TEM (HAADF-STEM) is used to observe the core of the edge TDs at atomic scale. Pairs of TDs with b =1/2 are present in the material running parallel at small distances between them (0.5–1.5 nm). The observation of such parallel dislocation pairs in Ge has not been documented before. The interaction between the edge dislocation pairs is obtained experimentally from the high-resolution HAADF-STEM images by applying geometrical phase analysis. The experimental strain maps are compared to analytical calculations based on the anisotropic elastic theory demonstrating a good match between them. more...

International audience; The defect structures in semipolar (11¯22)-GaN, AlN layers grown on m-sapphire by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are characterized by transmission electron microscopy. The epitaxial relationships are identified as [10¯10]GaN ǁ [11¯20]sap and [1¯213]GaN ǁ [0001]sap. Defects are identified as mostly partial dislocations, I1-basal and prismatic stacking faults. The density of dislocations is of the order of 5.5 × 109 cm−2. They are Frank–Shockley partial dislocations with b = 1/6〈20¯23〉 (90%), Shockley partial dislocations with b = 1/3〈10¯10〉 (8%) and perfect dislocations of a-type with b = 1/3〈11¯20〉 (2%). This is in contrast with the growth in c- or a-orientations, where the large majority of extended defects consists of perfect dislocations. Upon MBE regrowth of GaN on MOVPE GaN, no additional defects are generated, although the defects in the substrate propagate through the overgrown layer. However, in the case of MBE deposition of AlN on MOVPE GaN, new threading dislocations of the type b = 1/3〈11¯23〉 are generated taking stepped and curved structures along their lines. more...