L.C. Nistor

Nanocrystalline diamond films : transmission electron microscopy and Raman spectroscopy characterization

Abstract The structure of fine-grained diamond films with crystallite size as small as 10–50 nm has been studied with cross-section transmission electron microscopy and Raman spectroscopy. The nanocrystalline films of 1 to 2 μm thickness were grown on Si substrates by d.c. plasma chemical vapor deposition in Ar/CH4/H2 gas mixtures, with methane concentration CH4/(H2+CH4) varied from 3 to 100%. The substrates were seeded with 5 nm diamond powder to enhance the nucleation density. Submicron thick nanocrystalline films were also grown as a first layer for successive growth of large-grain films. The films demonstrate a kind of columnar growth even for the case of grains with a size of a few tens of nanometers. The crystallites showed many imperfections, twins on (111) planes being the dominant defect type. The twin density is very high, but often they are only a few atomic layers wide. A buffer layer of β-SiC with a thickness up to 400 nm can be observed at the diamond/silicon interface. In addition to the diamond phase, Raman spectroscopy revealed disordered sp3 and sp2 carbon phases, presumably located at grain boundaries. High intrinsic stresses in the film prevent the observation of the size-induced low-frequency shift of the fundamental diamond peak at 1332 cm−1. The appearance of fingerprints of CN vibrations and distorted diamond in Raman spectra is discussed.

In situ transmission electron microscopy study of Ni silicide phases formed on (001) Si active lines

The formation of Ni silicides is studied by transmission electron microscopy during in situ heating experiments of 12 nm Ni layers on blanket silicon, or in patterned structures covered with a thin chemical oxide. It is shown that the first phase formed is the NiSi2 which grows epitaxially in pyramidal crystals. The formation of NiSi occurs quite abruptly around 400 °C when a monosilicide layer covers the disilicide grains and the silicon in between. The NiSi phase remains stable up to 800 °C, at which temperature the layer finally fully transforms to NiSi2. The monosilicide grains show different epitaxial relationships with the Si substrate. Ni2Si is never observed.The formation of Ni silicides is studied by transmission electron microscopy during in situ heating experiments of 12 nm Ni layers on blanket silicon, or in patterned structures covered with a thin chemical oxide. It is shown that the first phase formed is the NiSi2 which grows epitaxially in pyramidal crystals. The formation of NiSi occurs quite abruptly around 400 °C when a monosilicide layer covers the disilicide grains and the silicon in between. The NiSi phase remains stable up to 800 °C, at which temperature the layer finally fully transforms to NiSi2. The monosilicide grains show different epitaxial relationships with the Si substrate. Ni2Si is never observed.

Microstructural studies of nanocrystalline CeO2 produced by gas condensation

Abstract Nanocrystalline ceria powders prepared by inert gas condensation using thermal evaporation were characterized by high resolution transmission electron microscopy. No significant differences were observed between the powders collected in the different parts of the UHV chamber. The crystallite size distributions are narrow with maxima between 3 and 3.5 nm diameter. The crystallite growth was studied by X-ray diffraction line profile analysis. Transmission electron microscopy shows that particles develop cubeoctahedra shapes during annealing in the temperature range of 400 ° to 800 °C. The crystallites grow individually by a binary coalescence process and only very few grain boundaries were observed. The size of about 25–30% of all crystallites is not affected by sintering at 600 °C. Significant changes occur in the sample annealed at 800 °C when two populations of crystallites develop. The TEM and X-ray diffraction results agree very well. These studies were completed by BET measurements.

Direct evidence of spontaneous quantum dot formation in a thick InGaN epilayer

We report a direct observation of quantum dots formed spontaneously in a thick InGaN epilayer by high resolution transmission electron microscopy. Investigation of a (280 nm thick) In0.22Ga0.78N single layer, emitting in the blue/green spectral region, reveals quantum dots with estimated sizes in the range of 1.5–3 nm. Such sizes are in very good agreement with calculations based on the luminescence spectra of this specimen.

Characterization of {111} planar defects induced in silicon by hydrogen plasma treatments

Microstructural characterization by transmission electron microscopy of the {111} planar defects induced in Si by treatment in hydrogen plasma is discussed. The {111} defects are analyzed by conventional (TEM) and high-resolution transmission electron microscopy (HRTEM). Quantitative image processing by the geometrical phase method is applied to the experimental high-resolution image of an edge-on oriented {111} defect to measure the local displacements and strain field around it. Using these data, a structural model of the defect is derived. The validity of the structural model is checked by high-resolution image simulation and comparison with experimental images.

Crystallographic aspects related to the high pressure–high temperature phase transformation of boron nitride

Crystallographic relations between different forms of boron nitride (BN) appearing at the high pressure–high temperature structural phase transformation have been revealed by high-resolution transmission electron microscopy (HRTEM). As starting materials, crystalline hexagonal BN (hBN) with different degrees of crystallinity, or with defects intentionally introduced, were used. Cubic BN (cBN) is formed only as a minor component, the rest consisting of different forms of sp 2 bonded BN: hBN, compressed, monoclinic deformed hBN, or turbostratic BN (tBN). The small cBN crystallites (300–400 nm) contain many defects such as twins, stacking faults and nanoinclusions of other BN forms: tBN, rhombohedral BN (rBN) and wurtzite BN (wBN). The cBN phase grows epitaxially on the basal plane of hBN. The nucleation sites for cBN are revealed by HRTEM. They consist of nanoarches (sp 3 hybridized, highly curved nanostructures), frequently observed at the edges of the hBN crystallites in the starting materials. Based on HRTEM observations of specimens not fully transformed, a nucleation and growth model for cBN is proposed which is consistent with existing theoretical and experimental models.

Microstructure and spectroscopy studies on cubic boron nitride synthesized under high-pressure conditions

High-resolution electron microscopy (HREM) studies of the microstructure and specific defects in hexagonal boron nitride (h-BN) precursors and cubic boron nitride (c-BN) crystals made under high-pressure high-temperature conditions revealed the presence of half-nanotubes at the edges of the h-BN particles. Their sp3 bonding tendency could strongly influence the nucleation rates of c-BN. The atomic resolution at extended dislocations was insufficient to allow us to determine the stacking fault energy in the c-BN crystals. Its mean value of 191 ± 15 mJ m−2 is of the same order of magnitude as that of diamond. High-frequency (94 GHz) electron paramagnetic resonance studies on c-BN single crystals have produced new data on the D1 centres associated with the boron species. Ion-beam-induced luminescence measurements have indicated that c-BN is a very interesting luminescent material, which is characterized by four luminescence bands and exhibits a better resistance to ionizing radiation than CVD diamond.

Annealing of hydrogen-induced defects in RF-plasma-treated Si wafers: ex situ and in situ transmission electron microscopy studies

The smart-cut™ process is based on inducing and processing structural defects below the free surface of semiconductor wafers. The necessary defects are currently induced by implantation of light elements such as hydrogen or helium. An alternative softer way to induce shallow subsurface defects is by RF-plasma hydrogenation. To facilitate the smart-cut process, the wafers containing the induced defects need to be subjected to an appropriate thermal treatment. In our experiments, (0 0 1) Si wafers are submitted to 200 and 50 W hydrogen RF-plasma and are subsequently annealed. The samples are studied by transmission electron microscopy (TEM), before and after annealing. The plasma-introduced defects are {1 1 1} and {1 0 0} planar-like defects and nanocavities, all of them involving hydrogen. Many nanocavities are aligned into strings almost parallel to the wafer surface. The annealing is performed either by furnace thermal treatment at 550 °C, or by in situ heating in the electron microscope at 450, 650 and 800 °C during the TEM observations. The TEM microstructural studies indicate a partial healing of the planar defects and a size increase of the nanometric cavities by a coalescence process of the small neighbouring nanocavities. By annealing, the lined up nanometric voids forming chains in the as-hydrogenated sample coalesced into well-defined cracks, mostly parallel to the wafer surface.

Transmission electron microscopy study of silicon nitride amorphous films obtained by reactive pulsed laser deposition

Abstract In situ decomposition of silicon nitride films was observed by high-resolution electron microscopy. The films, which were produced by reactive pulsed laser deposition from Si wafer targets at 50–100 Pa ammonia pressure, had a prevalent content of hydrogen-doped, amorphous, non-stoichiometric silicon nitride. A layered morphology of the film, consisting of a density variation in the amorphous structure, developed under electron beam irradiation. This morphology became evident only in cross-sectional observation and was related to the stress-relaxation effect, based on a rearrangement of the bonds. We presume that the stress field anisotropy in the amorphous structure must be related to the presence of some bond texture in the as-grown, amorphous film.

Modeling of Geometry Design in Wire Drawing Using Cassette Roller Die

The most important activity of wire drawing technology is the correct design of stages geometry. The wire drawing within a roller die is described by an irregular deformation on wire width, strongly connected by the gauge shape between the rollers and cross section wire surface. Free lateral wire surface that has no contact with the roller on the deformation zone obstructs the wire elongation. There is a biunique correspondence between the shape and dimension of wire that enters between the rollers, and the free space between the rollers, corresponding to a specific stage of drawing. This has a great effect on dimensional accuracy of final wire. A drawing zone feature based model, allowed the examination of interaction evolution between wire and rolls geometry. This analysis helps the user of this technology to make a proper change in the arrangement and position of wire entering between rolls, or in the rolls configuration.