High resolution transmission electron microscopy study of the initial growth of diamond on silicon

A polycrystalline diamond film grown by hot filament CVD was ion-milled and thinned to the diamond/silicon-substrate interface and the structures formed during the initial stages of diamond nucleation were studied by high resolution transmission electron microscopy (HRTEM). At the interface, isolated polycrystalline islands (15–35 nm) consisting primarily of mixed phase β-SiC and nanocrystalline diamond could be observed. The β-SiC phase occurred mainly as isolated nano-sized domains with no evidence of a larger micron-scale coalescence. In addition to co-existing with β-SiC in the polycrystalline islands, nanocrystalline diamond was also observed to nucleate in the amorphous carbon matrix. The density of the nanocrystalline diamond in the amorphous carbon matrix was observed to be at least an order of magnitude higher than that in the polycrystalline β-SiC phase. The total nanocrystalline diamond nucleation density was found to be several orders of magnitude higher than the growth density of the micron-sized diamond crystallites that ultimately evolved from the interface at longer growth times.     

First stages of diamond nucleation on iridium buffer layers

During the first stages of epitaxial diamond nucleation by the bias-enhanced nucleation procedure on monocrystalline iridium films the metal surface shows a characteristic roughening. Two different types of furrows and ridges along 〈100〉 and 〈110〉 develop. From atomic force microscopy images a typical structure height of 3 nm is deduced. Transmission electron microscopy indicates {111} faceting. In addition, after 45 min of nucleation 5–10 nm large structures are found with a typical distance of 100–300 nm showing a bright contrast in scanning electron micrographs. While a subsequent 30 min growth step results in a similar density of diamond grains with a size of approximately 100 nm, prolonging the biasing step does not increase the size of these nucleation centers. It is shown that under the present bias-enhanced nucleation conditions large diamond grains are etched. The fact that diamond can nucleate under conditions under which diamond cannot grow has strong implications on the nature of the observed nucleation centers and on theoretical models describing the nucleation process.     

Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp³) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼10¹⁰ cm⁻² were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp³ carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp³ carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.     

Initial stages in the growth of polycrystalline diamond on silicon

Polycrystalline diamond films prepared in a hot filament chemical vapour deposition reactor were investigated with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) in order to identify chemically and structurally distinguishable phases during nucleation and early stages of diamond growth. This was achieved by investigating a series of films grown under identical conditions for 5 min to 4 h. In addition, the interface between a solid diamond film and its silicon substrate was studied after the film had been removed from the substrate. Carbon deposition commences initially with the simultaneous growth of diamond crystallites along scratches and a layer of microcrystalline graphite covering the remainder of the substrate. Small amounts of SiC could also be identified during the first 100 min of deposition. Once the individual diamond crystallites have grown together to form a continuous layer, the graphitic phase in the spectra is replaced by an amorphous carbon phase which we attribute to the grain boundaries between the crystals. Inspection of the film backside revealed that the amorphous carbon had merely overgrown the microcrystalline graphite which was still present as the major component. Only after prolonged growth times (24 h) did the Raman and XPS spectra exhibit the characteristic diamond features free from any other contributions. On the basis of these observations a model for the initial stages of diamond growth on Si is developed.     

Amorphization and graphitization of single-crystal diamond — A transmission electron microscopy study

The amorphization and graphitization of single-crystal diamond by ion implantation were explored using transmission electron microscopy (TEM). The effect of ion implantation and annealing on the microstructure was studied in (100) diamond substrates Si⁺ implanted at 1 MeV. At a dose of 1 × 10¹⁵ cm^− 2, implants done at 77 K showed a damage layer that evolves into amorphous pockets upon annealing at 1350 °C for 24 h whereas room temperature implants (303 K) recovered to the original defect free state upon annealing. Increasing the dose to 7 × 10¹⁵ Si⁺/cm² at 303 K created an amorphous-carbon layer 570 ± 20 nm thick. Using a buried marker layer, it was possible to determine that the swelling associated with the amorphization process was 150 nm. From this it was calculated that the layer while obviously less dense than crystalline diamond was still 15% more dense than graphite. Electron diffraction is consistent with the as-implanted structure consisting of amorphous carbon. Upon annealing, further swelling occurs, and full graphitization is achieved between 1 and 24 h at 1350 °C as determined by both the density and electron diffraction analysis. No solid phase epitaxial recrystallization of diamond is observed. The graphite showed a preferred crystal orientation with the (002)g//(022)d. Comparison with Monte Carlo simulations suggests the critical displacement threshold for amorphization of diamond is approximately 6 ± 2 × 10²² vacancies/cm³.     

Domain formation in diamond nucleation on iridium

Domain formation in epitaxial diamond nucleation on Ir(001) surfaces using the bias-enhanced nucleation (BEN) procedure has been studied. Bright areas of up to several microns lateral size with negligible topographic contrast are observed by scanning electron microscopy (SEM) after ion bombardment. When a growth step is applied after BEN, these domains develop into islands of identical shape consisting of epitaxial diamond with a high local area density of oriented grains. Outside the domains the nucleation density is either orders of magnitude lower or the grains are completely non-oriented. The diamond nuclei or precursors which are formed during the BEN step proved to be very stable: They still yielded oriented diamond islands when the samples were stored in air for 1 year before the growth step. Electron backscatter diffraction (EBSD) patterns taken from inside and outside the domains immediately after BEN did not show any significant difference. This allows the conclusion that the modification of the iridium crystal lattice accompanied with diamond nucleation is either very faint or only restricted to a very thin layer at the surface. Kelvin probe force microscopy (KPFM) measurements indicate a reduced work function within the domains.     

Atomic force microscopy and Raman spectroscopy study of the early stages of carbon nanowall growth by dc plasma-enhanced chemical vapor deposition

The early stage of carbon nanowall (CNW) growth on Si substrate by dc plasma-enhanced chemical vapor deposition (PECVD) was investigated by means of atomic force microscopy and Raman spectroscopy. First nanodiamond particles with highly defective graphene layers are shown to be formed over the substrate. Subsequently nanographite grains are formed on the nanodiamond film. The density of nanographite grains increases with increasing deposition time, and they coalesce to form a continuous graphite film. Finally CNWs are shown to grow vertically on the graphite film. Such understanding of the interface layers between the substrate and CNWs will be useful for not only the growth control but also device applications.     

Transmission electron microscopy of propene-derived pyrolytic carbon coatings

The microstructures of a series of pyrocarbon coatings prepared from a 20 per cent propene atmosphere in the temperature range 1250 to 2000°C have been examined by transmission electron microscopy. The structures vary markedly with temperature, and the structural changes can be correlated with density and microporosity measurements. At temperatures between 1400 and 2000°C, the structure can be described in terms of two components; the relative amount of each component is dependent on the temperature.     

Interaction of small diamond islands on iridium: A finite element simulation study

Finite element simulations using the code ABAQUS have been performed to model the elastic interaction of small heteroepitaxial diamond islands on iridium. The simulations predict a huge relaxation of the elastic energy density for a thin two-dimensional diamond layer when it splits into individual islands. They show that the interaction of neighbouring islands increases their elastic energy. For the modelled islands with lateral dimensions of 2 nm this mutual rejection works only within a distance of a few nanometers.The results of the simulation allow the following conclusions with respect to potential nucleation models: 1) The repulsive elastic interaction between neighbouring grains rather suppresses nucleation in the vicinity of an existing grain than enhancing it. Thus, it cannot provide an explanation for the agglomeration of the grains in areas of extremely high nucleation density, the so called “domains”. 2) For an alternative model of two-dimensional lateral diamond growth followed by splitting into isolated grains the simulations can explain the splitting and the subsequent repulsion. 3) The rapid drop of the interaction strength with distance indicates that further interaction mechanisms are required to account for the final distance of the grains found experimentally.     

Impurities identification in a synthetic diamond by transmission electron microscopy

Three types of impurities, which were trapped in diamond single crystal during process of the diamond crystal growth under high temperature and high pressure in the presence of iron–nickel solvent catalyst, have been successfully and directly investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Both the chemical composition and the crystal structure of the impurities were identified and determined. The impurities may be derived from the starting materials and the medium (pyrophillite) for transmitting the pressure, they consisted of amorphous graphite, f.c.c. (FeNi)₂₃C₆ and orthorhombic FeSi₂.     

Transmission electron microscopy of supported molybdenum and vanadium oxides

Transmission electron microscopy has been used to characterize dispersions of molybdena and vanadia on titania and silica supports. When silica spheres of controlled morphology were used as support, the dispersed monolayer phase of both oxides could be imaged due to characteristic changes in contrast. In addition to the dispersed phase, we could detect three-dimensional crystallites of V₂O₅ but in the case of MoO₃ only two-dimensional islands were seen. On Degussa P-25 titania, there was no observable contrast change due to the presence of a monolayer of these dispersed oxides. However, exposure to the electron beam caused dramatic changes in the surface texture of the support. Such changes were not seen when blank TiO₂ was similarly irradiated. These e-beam induced changes were more pronounced in the vanadia/titania catalysts leading to formation of 1–3 nm clusters of reduced VO _x . However, on the MoO₃/TiO₂ sample, e-beam exposure caused only a pronounced change in texture but no well defined clusters could be detected.     

Investigation of heterostructure between diamond and iridium on sapphire

Diamond thin film has outstanding physical and chemical properties. Diamond-on-iridium configurations have been prepared by several methods, such as microwave enhanced plasma CVD, direct currency plasma CVD, and hot filament CVD. In this study, an Ir interlayer was deposited on single crystal sapphires (Al₂O₃) with A-planes {1120} by an RF magnetron sputtering method after annealing samples. In addition, a diamond thin film was deposited by a microwave enhanced plasma chemical vapor deposition (MPCVD) method using a mixture of hydrogen and methane gases after a bias enhanced nucleation (BEN) procedure.Ir (001) was grown on the A-plane of sapphire by X-ray pole figure measurement. Diamond thin films were synthesized on each Ir/sapphire substrate and characterized by SEM, Raman spectroscopy. D {100} faces were exhibited in substantial areas of diamond films, and a flat D {100} plane was partially obtained. It is considered that diamond thin films on Ir {100} were mainly grown towards the <100> direction and were epitaxially grown in part.     

Transmission electron microscopy studies on carbon materials prepared by mechanical milling

The effects of mechanical milling on the multiscale organization (structure and microtexture) of various carbon materials were investigated by means of Transmission Electron Microscopy. We show that mechanical grinding generates an increasing amount of disordered carbon at a rate depending on the type of grinding mode used (shear- or shock-type grinding). When the shock-type grinding is used, the triperiodic structure and the lamellar microtexture of the graphite completely break down to give microporous and turbostratic carbons made of misoriented nanometric Basic Structural Units (BSUs). Graphite grinding permits the elaboration of disordered carbons. The involved mechanism is different from a simple reverse graphitization, since not only structure but also microtexture are strongly modified by the grinding. After heat treatment at 2800°C, the graphite organization is not recovered, and a mesoporous turbostratic carbon is mainly obtained. All the carbon precursors studied, submitted to strong grinding, leads to similar microporous carbons. Shear grinding is less effective since remnants of graphitic carbon are still present within the disordered carbon.     

Ultra-high resolution electron microscopy investigation of growth defects in CVD diamond films: twin interactions and fivefold twin centres

An elucidation of the core structure of fivefold twin centres arising from the interaction between less than five first-order twin boundaries in plasma-assisted chemical vapour deposition (CVD) of diamond films is reported. These particular twinning centres have been identified by ultra-high resolution electron microscopy at 0.12 nm resolution, with the help of image calculations. Plausible three-dimensional atomic-scale models are proposed for two specific structural variants, which have been found closely connected to high-order twin boundaries via original heptagonal or octagonal structural units. To our knowledge, this is the first time that such types of fivefold twin centres and associated structural units, which are quite representative of the growth defects observed in CVD diamond, have been reported.     

Heteroepitaxial diamond on iridium: New insights on domain formation

Topography and chemical evolutions of the iridium surface in the successive steps of bias-enhanced nucleation and growth were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), and nano-Auger analyses. This sequential approach, which was performed in localized areas at the nanoscale, provides three new experimental and complementary concepts that can enhance the knowledge of nucleation pathways on iridium. First, SEM imaging at low-acceleration voltage enables the detection of diamond nuclei or stable precursors after the BEN step. Second, domains consist of iridium furrows that are covered by an amorphous carbon overlayer, with a thickness of 6.8 nm, according to AFM and X-ray photoemission spectroscopy data. Third, SEM observations also suggest a close relationship between furrows created under ion bombardment and domains in our study conditions. These results prompted us to propose a scheme that describes the topography and surface chemistry of domains.     

Transmission electron microscopy study of extended defect evolution and amorphization in SiC under Si ion irradiation

The damage induced in 3C-SiC epilayers on a silicon wafer by 2.3-MeV Si ion irradiation for fluences of 10¹⁴, 10¹⁵, and 10¹⁶ cm⁻², was studied by conventional and high-resolution transmission electron microscopy (TEM/HRTEM). The evolution of extended defects and lattice disorder is followed in both the 3C-SiC film and Si substrate as a function of ion fluence, with reference to previous FTIR spectroscopy data. The likelihood of athermal unfaulting of native stacking faults by point defect migration to the native stacking faults is discussed in relation to damage recovery. Threshold energy densities and irradiation doses for dislocation loop formation and amorphous phase transformation are deduced from the damage depth profile by nuclear collisions. The role of electronic excitations on the damage recovery at high fluence is also addressed for both SiC and Si.     

Surface study of iridium buffer layers during the diamond bias enhanced nucleation in a HFCVD reactor

The BEN nucleation of diamond on iridium substrates has been studied in a hot filament reactor. Without a prior BEN stage, no diamond nucleation could be detected. Nucleation is promoted only if a BEN step is applied before the CVD growth with nucleation densities up to 5 10⁹ cm⁻². The present study focuses on the early stages of BEN to better understand its specific role. In this way, samples have been in situ characterized using electron spectroscopies (XPS, AES, ELS) and further investigated by HR-SEM, AFM, Nano-Auger and Raman spectroscopy. A very different behaviour in the interface formation has been observed, as compared to silicon. First, a substrate roughening takes place during the cleaning step. Second, the formation of a graphite layer was systematically observed, with or without the BEN stage, in the early stages of CVD synthesis. Its crystallinity has been studied from the Raman experiments. The study of the XPS Ir 4f peaks supports a weak chemical bonding between graphite and iridium. Finally, after the BEN stage, spatially resolved Nano-Auger and Raman measurements revealed the presence of diamond nanocrystals.     

Transmission electron microscopy study of the microstructure of unidirectional C/C composites fabricated by catalytic chemical vapor infiltration

Unidirectional carbon/carbon (C/C) composites were fabricated by catalytic chemical vapor infiltration, using electroless Ni–P as catalyst. Transmission electron microscopy (TEM) investigations indicate that the catalyst particles (100–800 nm) in the pyrocarbon (PyC) matrix are composed of Ni₃P and Ni phases, but only the Ni₃P phase was observed in the tiny catalyst particles (<50 nm) in carbon fibers. The catalyst particles in the matrix were encapsulated by high-textured PyC shells, in which openings were observed. The thicknesses of the medium-textured PyC in the composites (720–850 nm) are greater than in conventional C/C composites (660–740 nm), but have no significant difference in texture degree. Catalysts were partially extruded out of the PyC shells and migrated into the carbon fibers, leading to the catalytic graphitization of the carbon fibers, and their structural homogeneity was destroyed. Based on the TEM observation, a dissolution/precipitation mechanism was proposed for the catalytic graphitization of carbon fibers, and a dissolution/precipitation/encapsulation/fracture/extrusion mechanism was proposed for the encapsulation of catalyst particles.     

Transmission electron microscopy study of the microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers

Introduction of carbon nanofibers (CNFs) into carbon/carbon (C/C) composites is an effective method to improve the mechanical properties of C/C composites. In situ grown CNFs reinforced C/C composites as well as conventional C/C composites without CNFs were fabricated by chemical vapor infiltration. Transmission electron microscopy investigations indicate that the entangled CNFs (30–120 nm) formed interlocking networks on the surface of carbon fibers (CFs). Moreover, a thin high-textured (HT) pyrocarbon (PyC) layer (～20 nm) was deposited on the surface of CFs during the growth of CNFs. We find the microstructure of C/C composites depends strongly on the local distribution density (LDD) of CNFs. In regions of low CNF LDD, a triple-layer structure was formed. The inner layer (attached to CF) is HT PyC (～20 nm), the middle layer (150–200 nm) is composed of HT PyC coated CNFs (HT/CNFs) and medium-textured PyC, and the outmost layer (several microns) is composed of HT/CNFs and micropores. In regions of high CNF LDD, a double-layer structure was formed. The inner layer is HT PyC (～20 nm), and the outer layer is composed of HT/CNFs, isotropic PyC and nanopores. However, only medium-textured PyC and micropores were found in the matrix of the conventional C/C composites.     

Investigation of diamond etching and growth by in situ scanning tunneling microscopy

Recently we published a new experimental set-up for the investigation of dynamic processes under diamond deposition conditions. In the present paper the first measurements of diamond etching and growth with this set-up are presented. Etching experiments at 500°C substrate temperature reveal no etching of the pre-grown diamond film used as sample, whereas at 530°C rapid etching of a (111)-faceted crystal and slow etching of a (100)-faceted crystal is visible. Similar, at 500°C no growth could be detected on a boron doped (100)-single crystal, but at 530°C growth was monitored with a rate comparable with that of the etching rate of the (100)-plane at the same substrate temperature (about 10 nm h⁻¹).