John F. Mansfield

Crosshatched surface morphology in strained III-V semiconductor films

The correlation between the surface crosshatched morphology and the interfacial misfit dislocations in strained III‐V semiconductor heteroepitaxy has been studied. The surface pattern is clearly seen on samples grown at high temperature (520 °C) and those with small lattice‐mismatched (f<2%) systems. A poorly defined crosshatched morphology was found on layers grown at relatively low temperature (400 °C). As the lattice mismatch of the strained layer becomes larger than 2%, a roughly textured surface morphology is commonly observed in place of actual cross‐hatching. Few threading dislocations are observed in the strained layer when the crosshatched pattern develops. It is also noted that the surface crosshatched pattern develops after the majority of the interfacial misfit dislocations are generated. The result suggests that the surface crosshatch pattern is directly related to the generation of interfacial misfit dislocations through glide processes.

Single Particle Plasmon Spectroscopy of Silver Nanowires and Gold Nanorods

The excitation of surface plasmons in individual silver nanowires and gold nanorods is investigated by means of high-resolution electron energy loss spectroscopy in a transmission electron microscope. The transverse and longitudinal modes of these nanostructures are resolved, and the size variation of the plasmon peaks is studied. The effect of electromagnetic coupling between closely spaced nanoparticles is also observed. Finally, the relation between energy-loss measurements and optical spectroscopy of nanoparticle plasmon modes is discussed.

Determination of lattice parameter, oxidation state, and composition of individual titanomagnetite/titanomaghemite grains by transmission electron microscopy

A transmission electron microscopy (TEM) method, higher-order Laue zone (HOLZ) diffraction, has been applied to measurement of accurate lattice parameters of individual titanomagnetite and titanomaghemite grains. Lattice parameters and cation-composition data are obtained with TEM/analytical electron microscopy (AEM) for individual grains smaller than 0.1 μm in diameter. A suite of Ocean Drilling Program/Deep Sea Drilling Project ocean-floor basalts containing titanomagnetite-titanomaghemite with a wide range of oxidation degree (z), were employed to establish the relationship between lattice parameter (a), z and Ti-content (x) for naturally occurring samples. Standardization was accomplished with measurements on grains sufficiently large to allow the determination of absolute oxygen contents by electron microprobe analyses, which directly yielded z values. The probe data confirm that the maghemitization process in ocean-floor basalts is one involving loss of iron and indicate a nearly linear relationship between lattice parameters and oxidation degrees for a given x value, except for the smallest and largest values of z. The standardized relation permits determination of z for individual submicrometer-sized grains using only measured a-values and cation compositions as determined by HOLZ diffraction and AEM.

X-Ray Microanalysis in the Environmental SEM: A Challenge or a Contradiction?

Abstract. In this paper, the application of X-ray energy dispersive spectroscopy in the environmental scanning electron microscope is reviewed. Various techniques that have been used to remove the effects of the beam spreading in the gaseous environment are discussed, specifically the pressure variation techniques and the beam-stop method. The results of the application of modified versions, developed at the University of Michigan, are also presented. It is shown that quantitative analysis in the environmental SEM, operating at 30 kV, is possible at short working distances (6 mm to 7.2 mm, gas path length 1.2 mm to 2.2 mm) in the 70 to 350 Pa range.

Sub-micron grating fabrication on hafnium oxide thin-film waveguides with focused ion-beam milling

Uniform period sub-micron gratings have been fabricated using focused ion beam milling on hafnium oxide waveguides. Atomic force microscopy indicates that the gratings have smooth and uniform profiles. At the period of 330 nm, the largest peak-to-peak height that was achieved was 85 nm. Scattering at the grating imperfections was found to be at least two orders of magnitude weaker than the intensity of the diffracted order.

On the microstructure and crystallography of directionally solidified MoSi2-Mo5Si3 eutectics

Abstract Directionally solidified MoSi2-Mo5Si3 eutectic rods were produced by the Czochralski method. A script lamellar microstructure was produced and an inverse square root dependence upon the pull rate (39–210 mm/h) was observed for the lamellar spacing. The eutectic rods were textured with [001] of Mo5Si3 and 〈110〉 of MoSi2 parallel to the rod axis and an orientation relationship consisting of [110]MoSi2[110]Mo5Si3 and (1¯10)MoSi2 ||(002)Mo5Si3 was present in all of the samples. The eutectic grows with a lamellar plate morphology inclined at 15° relative to the rod axis and four growth variants are possible. A script lamellar microstructure, as viewed in the transverse plane, was produced by periodic branching of the Mo5Si3 lamellae and this branching was not associated with any change in crystallography. Addition of 0.35 at.% erbium produced a more fibrous microstructure and an erbium-rich compound, Er2Mo3Si4 with space group 21/c, formed congruently with the Mo5Si3-MoSi2 during solidification. A ledge-terrace growth mechanism was proposed to explain the observed microstructures.

Formation and coarsening of Ga droplets on focused-ion-beam irradiated GaAs surfaces

We have investigated the formation and coarsening of Ga droplets on focused-ion-beam (FIB) irradiated GaAs surfaces. To separately examine formation and coarsening, Ga droplets were fabricated by Ga+ FIB irradiation of GaAs substrates with and without pre-patterned holes. We determined the droplet growth rate and size distribution as a function of FIB energy following irradiation. The data suggest a droplet formation mechanism that involves Ga precipitation from a Ga-rich layer, followed by droplet coarsening via a combination of diffusion and Ostwald ripening or coalescence via droplet migration (dynamic coalescence).

Effects of Systemic Fluoride and in vitro Fluoride Treatment on Enamel Crystals

Systemically administered fluoride at a concentration of 75 ppm increases the surface roughness of developing enamel crystals in rats, which may be significant in advancing our understanding of the biological mechanism of fluorosis. Thus, the aim of this study was to investigate whether the increased surface roughness may be a result of surface restructuring by the direct action of fluoride at the crystal surface. We examined the fluoride dose-dependent roughening of enamel crystal surfaces in vivo, in the rat, and whether this roughening could be mimicked by the in vitro treatment of rat enamel crystals with neutral pH fluoride solutions. Our results showed that enamel crystal surface roughness increased after treatment with increasing fluoride ion concentrations, whether applied in vitro or administered systemically. This suggests a mechanism, alongside others, for the increased surface roughness of crystals in fluorotic enamel.

A quantitative model for the intergranular precipitation of M7X3 and M23,X6 in Ni-16Cr-9Fe-C-B

A thermodynamic model has been constructed to explain the details of grain boundary chemistry in Ni-16 Cr-9 Fe doped with carbon and boron. In this model, Gibbs free energy of the precipitates is calculated using an ideal solution model for both the metallic and non-metallic sub-lattices in M,,X, and M,X,, and the activities of the five species in the matrix are calculated using a Kohler equation formalism involving only binary terms. The parameters required by the model are based solely on data on the constituent binary and ternary subsystems. The model yields correct results for the compositions of intergranular, Ni + B-rich M,,X, (M > 80 at.% Ni, X x 100 at.% B) as well as Cr + C-rich M,C-rich M,C, at 873 K and 973 K. The result that Mz3)(h is rich in Ni and B only occurs when the chemical activities in the matrix are those for a grain boundary in equilibrium with Cr, C,, which implies that Ni + B-rich M,,X, develops as a consequence of intergranular Cr,C, and the resultant depletion of Cr at the grain boundary. The model does not successfully explain the precipitation of B rich M?3X6 and M,X, at 1173 or 1273 K.

In-situ elevated temperature imaging of thin films with a microfabricated hot stage for scanning probe microscopes

Abstract A microfabricated hot stage for a scanning probe microscope (SPM) has been developed to enable in-situ investigations of thin film specimens at elevated temperatures. With this hot stage, a SPM can now examine and test materials at high magnifications under conditions that closely resemble their true service temperature. The hot stage is capable of operating from ambient room temperature up to 800°C without damage to the microscope. With this device, topographical images of platinum supported titanium films, which are important for catalytic reaction studies, thin film gas sensor technology, and microelectronic applications, have been acquired at temperatures between 25–400°C. The average roughness of these films remained constant at 12.4±1.9 nm. The surface of the hot stage can be equipped with electrodes enabling four point probe measurements of conducting specimens as the temperature is increased and the surface is imaged. In-situ imaging of the titanium underlayer diffusing through the platinum film has been observed at 375°C. Titanium migration to the surface near this temperature is also shown on 35 A Pt/65 A Ti films with X-ray photoelectron spectroscopy (XPS). This stage can be retrofitted to any existing SPM to expand its current capabilities to include high temperature analysis of a wide diversity of materials, from biological samples, to polymers, and metals.