S. M. Yalisove

Surface and grain boundary segregation in relation to intergranular fracture: Boron and sulfur in Ni3Al☆

Unlike most solutes that segregate to grain boundaries, we have observed that boron segregates more strongly to grain boundaries than to free surface in Ni/sub 3/Al. This observation, along with the previously reported beneficial effect of boron segregation on grain boundary strength, is in qualitative agreement with a theory of grain boundary cohesion first put forward by Rice. If similar studies of other beneficial grain boundary segregants bear out this initial agreement with Rices theory, our understanding concerning the nature of segregation effects on grain boundary cohesion could be significantly enhanced.

Growth anisotropy and self-shadowing: A model for the development of in-plane texture during polycrystalline thin-film growth

The development of a preferred crystallographic orientation in the plane of growth, an in-plane texture, is addressed in a model that incorporates anisotropic growth rates of a material and self-shadowing. Most crystalline materials exhibit fast growth along certain crystallographic directions and slow growth along others. This crystallographic growth anisotropy, which may be due to differences in surface free energy and surface diffusion, leads to the evolution of specific grain shapes in a material. In addition, self-shadowing due to an obliquely incident deposition flux leads to a variation in in-plane grain growth rates, where the “fast” growth direction is normal to the plane defined by the substrate normal and the incident flux direction. This geometric growth anisotropy leads to the formation of elongated grains in the plane of growth. Neither growth anisotropy alone can explain the development of an in-plane texture during polycrystalline thin-film growth. However, whenever both are present (i.e., oblique incidence deposition of anisotropic materials), an in-plane texture will develop. Grains that have “fast” crystallographic growth directions aligned with the “fast” geometric growth direction overgrow grains that do not exhibit this alignment. Furthermore, the rate of texturing increases with the degree of each anisotropy. This model was used to simulate in-plane texturing during thin-film deposition. The simulation results are in excellent quantitative agreement with recent experimental results concerning the development of in-plane texture in sputter deposited Mo films.The development of a preferred crystallographic orientation in the plane of growth, an in-plane texture, is addressed in a model that incorporates anisotropic growth rates of a material and self-shadowing. Most crystalline materials exhibit fast growth along certain crystallographic directions and slow growth along others. This crystallographic growth anisotropy, which may be due to differences in surface free energy and surface diffusion, leads to the evolution of specific grain shapes in a material. In addition, self-shadowing due to an obliquely incident deposition flux leads to a variation in in-plane grain growth rates, where the “fast” growth direction is normal to the plane defined by the substrate normal and the incident flux direction. This geometric growth anisotropy leads to the formation of elongated grains in the plane of growth. Neither growth anisotropy alone can explain the development of an in-plane texture during polycrystalline thin-film growth. However, whenever both are present (i.e.,...

Multilayer rippled structure of the NiAl(110) surface: A medium energy ion scattering study

The NiAl(110) surface structure has been studied with medium energy ion scattering in a channeling and blocking (MEIS/CB) experiment. A high sensitivity to the first layer ripple (10% ripple with the Al on top) is reported and a multilayer model is proposed. The first layer Ni atoms are contracted relative to the second layer Ni atoms by 7%, and the second layer Ni atoms are expanded relative to the third layer Ni atoms by 1%. The first layer Al atoms are expanded relative to the second layer Al atoms by 5%, and the second layer Al atoms are contracted relative to the third layer Al atoms by 1%. The model assumes bulk-like positions of the third layer for both Ni and Al atoms. The present results are in excellent agreement with a recent single layer LEED intensity analysis.

Epitaxial orientation and morphology of thin CoSi2 films grown on Si(100): Effects of growth parameters

CoSi2 can be grown epitaxially on Si(100) with two dominant epitaxial orientations: CoSi2(100) on Si(100) where [100]CoSi2∥ [100]Si with [011]CoSi2∥ [011]Si , and CoSi2(110) on Si(100) where [011]CoSi2 ∥[100]Si with [011]CoSi2 ∥[011]Si . A rotated variant of the CoSi2(110) orientation is also observed where [011]CoSi2 ∥[100]Si with [011]CoSi2 ∥[011]Si. These films require a template technique in UHV conditions to grow high‐quality material. By varying the growth conditions, films which are >95% of either orientations have been obtained. The relationships between these growth parameters and the epitaxial orientation of the resulting thin film are discussed and a possible model is suggested for some particular growth conditions.

Effect of hydrogen on surface roughening during Si homoepitaxial growth

Hydrogen is shown to have a strong influence on the evolution of surface morphology during low temperature (310 °C) Si(100) homoepitaxy. Molecular beam epitaxy growth in the presence of deuterium shows a surface roughness within the epitaxial film that increases rapidly until the Si film exhibits a crystalline to amorphous transition. The rate at which the surface roughens depends critically on the partial pressure of deuterium. Although the kinetics of growth are sensitive to small pressures (4×10−8 Torr) of D, it appears that the breakdown of epitaxy does not result from a ‘‘critical’’ D concentration at the surface. This work suggests that the crystalline to amorphous transition, instead, results from increased roughening during epitaxy.

Analysis of thin film stress measurement techniques

Abstract Residual stresses in several magnetron sputtered Mo thin films, with thicknesses from 100 nm to 1.60 μm, were determined using double-crystal diffraction topography (DCDT), sin 2 ψ , and the high-resolution X-ray diffraction technique (HRXRD). The Mo films had a range of microstructures that included random and polycrystalline, textured out-of-plane, and textured in-plane. When the average biaxial stresses over the entire film thickness were determined for the films using the aforementioned techniques, the results were comparable in magnitude. However, the stresses determined with the substrate curvature technique, DCDT, were consistently smaller than those obtained with the sin 2 ψ and HRXRD techniques. The difference may arise for several reasons. For example, the HRXRD and sin 2 ψ measurements of a textured film may not be indicative of the mean film stress, and thus may differ from the curvature measurement. Also, substrate curvature techniques measure extrinsic stresses, or stresses that arise solely from the presence of the substrate. The techniques which analyze the film directly, such as sin 2 ψ and HRXRD, measure the extrinsic stresses and the intrinsic stresses that arise from defects or morphology changes within the film. The additional information that can be obtained from the depth-sensitive HRXRD technique concerning stress variations within thin films was also highlighted.

Femtosecond pulsed laser direct write production of nano- and microfluidic channels

Nano- and microfluidic channels were produced by selectively delaminating 1200nm thermally grown oxide films (SiO2) films from Si(100) substrates using a femtosecond pulsed laser. Single pass channels exhibiting bell-like cross sections with widths of 24μm and heights of 355nm were directly written at a speed of 1cm∕s, while larger channels (320μm in width and ∼15μm in height) were produced by laterally overlapping single pass channels. The results of an investigation of the interior surfaces of the channels via atomic force microscopy and scanning electron microscopy are presented.

Surface roughening during low temperature Si(100) epitaxy

Reflection high energy electron diffraction (RHEED) was used to investigate surface roughening during low temperature Si(100) homoepitaxy. The use of RHEED allowed in situ real-time collection of structural information from the growth surface. RHEED patterns were analyzed using a simple kinematic diffraction model which related average surface roughness and average in-plane coherence lengths to the lengths and widths of individual RHEED diffraction features, respectively. These RHEED analyses were quantified by calibrating against cross-section transmission electron microscopy (TEM) analyses of surface roughening. Both the RHEED and TEM analyses revealed similar scaling of surface roughness with deposited thickness, with RHEED analyses resulting in roughness values a factor of ∼2 times lower than those obtained from TEM analyses. RHEED was then used to analyze surface roughening during Si(100) homoepitaxial growth in a range of temperatures, 200–275 °C. Initially, surface roughness increased linearly with...

Pulsed laser ignition of reactive multilayer films

Nanostructured Al∕Pt multilayer films were ignited by single pulse irradiation from a Ti:sapphire femtosecond laser system. Critical ignition fluences (0.9–22J∕cm2) required to initiate a self-propagating reaction were quantified for different multilayer designs. Multilayers with smaller bilayer thickness required relatively lower fluence for ignition. Ignition threshold fluence was also found to be 1.4–3.6 times higher for Al-capped multilayers than for Pt-capped multilayers. Ablation threshold fluences were measured for Al (860±70mJ∕cm2) and Pt (540±50mJ∕cm2) and related to the observed difference in ignition fluences for Al- and Pt-capped multilayers.

Homoepitaxial growth of CoSi2 and NiSi2 on (100) and (110) surfaces at room temperature

Homoepitaxial growth of NiSi2 and CoSi2 on (100) and (110) surfaces is demonstrated at room temperature. Codeposition of stoichiometric silicide, by molecular beam epitaxy, onto thin, preannealed silicide layers on Si (100) and (110) leads to single‐crystal growth. High quality NiSi2 and CoSi2 films with ion channeling χmin<4% have been fabricated. The epitaxial orientation and the interfacial defect structures of the original silicide templates are maintained in the overgrown silicide. The high temperatures usually required for the formation of NiSi2 and CoSi2 are related to the nucleation and mass transport processes. It is concluded that the reaction of disilicide takes place at room temperature.