Island Grains Research Articles

Effects of substrate temperature, deposition pressure, and thickness on the morphology of ultrathin platinum film on SiO 2/Si substrate

The dependence on thickness of the resistance of platinum film deposited by electron-beam evaporation at room temperature onto a thermally grown SiO 2 substrate, shows that the platinum film is in a discontinuous island form when the thickness is less than 1.3 nm. At about 2 nm the film is very continuous electrically but porous. For platinum film deposited at 250 °C, it appears that the morphology is an island form when the thickness is less than 0.5 nm, and in a continuous porous form at about 1.8 nm. As the platinum deposition temperature increases, the film morphology, including the island dimensions or grain size and the gaps between islands or pores, becomes more uniform but has larger grains. Also, it appears that when both the background pressure and deposition temperature are varied, the increase in deposition temperature has a dominant effect in increasing the grain's base area, but that the grain's average height Z rms increases at lower pressure (10 −8 Torr) and tends to decrease at higher pressure (10 −6 Torr). The morphology of an ultrathin metal film on an insulator substrate can also be determined by measuring the electrical properties of the film. For example, measuring the film's impedance at different temperatures, frequencies, or applied bias can reveal that the film is either in discontinuous island form or in continuous form.

Mechanical alloying (MA) of Ti-Mo alloys and contamination with Fe: W-L. Lim et al, (The University, Okayama, Japan), J. Japan Inst. of Metals, Vol 55, No 2, 1991, 125–131, (In Japanese)

Neutron irradiation-induced ductile-brittle transition behaviour of carburized molybdenum polycrystals and single crystals has been studied. These specimens were irradiated at 673, 873 and 1073 K to (7.9–9.8) × 1023 n/m2 (E⩾1 MeV). The results were analyzed as the difference in the behaviour between polycrystals and single crystals, and the effects of irradiation temperature. Three-point bend tests showed that the neutron irradiation-induced shift of ductile-brittle transition temperature (DBTT) at each irradiation temperature was much larger in single crystal specimens than in polycrystal ones. The carburization treatment was found to be more effective in polycrystals rather than in single crystals. This suggests that in polycrystals the grain boundary interface is strengthened due to carbon addition. At higher irradiation temperatures, this shift of DBTT decreased in both the specimens. It is characteristic in post-irradiation fracture behaviour than in single crystal specimens the cracks initiated from the island grains on the specimen surface.

Epitaxial YBa2Cu3O7−x Thin Films: Scanning Tunneling Microscope Study of the Initial Stages of Epitaxial Growth, Growth Mechanism, and Effects of Substrate Temperature

ABSTRACTThe surface microstructure of epitaxial YBa2Cu3O7−x films grown by pulsed laser ablation on (001) MgO and SrTiO3 substrates has been studied at various growth stages, ranging in thickness from eight c-axis perpendicular unit cells to ∼220 nm. On MgO (lattice mismatch ∼9%) even the thinnest films grow unit cell-by-unit cell by an island growth mechanism. However, on SrTi03 (mismatch ∼1%), a transition from a layer-like growth mode to island growth is observed as the film thickness increases. Islands with clear spiral growth structures are observed in even the thinnest films on MgO, but for films grown on SrTiO3 the spiral growth features are found only for film thicknesses slightly greater than the critical thickness for the switch to an island growth mode. The islands consist of stacks of atomically flat terraces whose step heights are multiples of the c-axis lattice parameter. The island density decreases significantly with increasing film thickness, while their diameters range from 50–400 nm, increasing with growth temperature. The terraced island grain morphology causes a surface roughness of from 10 to 30 nm (depending on growth temperature) in films ∼200 nm thick.

A new method for preparing tilt grain boundaries in thin vapor-deposited films

In the past the electron microscope samples used to study tilt grain boundaries and interfaces were prepared from bulk samples grown from the melt which were then cut, polished and thinned by ion milling or chemical jet etching. This technique is also employed routinely in preparing crossectional semiconductor materials and metals. Another technique to prepare tilt boundaries in metals is to fuse two thin single crystal films each on substrates by heating and forming twist boundaries (e.g. Au (100)). Then, the boundary which forms at the composite film midplane, migrates to one of the two films surfaces having many island grains separated by tilt boundaries perpendicular to the foils’ surface. One of the present authors has carried this technique further by being able to perform this process directly in the microscope and observe the boundary migration when the total film thickness, in the case of Au, was ∼250Å. The technique is limited to this composite thickness because small holes develop in thinner films which thin rapidly become discontinuous when heated in-situ.

Bend properties of molybdenum single-crystal sheets produced by the secondary recrystallization method (role of island grains)

The bend properties of molybdenum single-crystal sheets produced by the secondary recrystallization method were measured at temperatures between 20 and −196 °C. It is shown that island grains, which are present at the specimen surface, play an important role in the low temperature fracture of the specimen. The fracture stress increased hyperbolically with decreasing island grain size. The stress was appreciably enhanced by the addition of small amounts of carbon, and no dependence on the size of the island grains was then observed. The ductility of single-crystal sheets, which were surface polished to 0.150-0.200 mm or carburized, was comparable with or higher than the ductility of fine-grained stress-relieved polycrystals.

Orientation dependence of the low temperature bend properties of molybdenum single crystals produced by the secondary recrystallization method

The orientation dependence of the bend properties at −196 °C of molybdenum single crystals produced by the secondary recrystallization method was examined. Specimens with different crystal orientations showed a common apparent fracture behaviour. In the annealed specimens the crack was always initiated from the island grain at the specimen surface. In the carburized specimens, however, the crack was often initiated from other sites at the specimen surface. The fracture stress for the annealed specimens was almost a linear function of c − 1 2 , where c is the crack depth, whereas the stress for the carburized specimens was independent of the size of the island grain. The ratio γ 110 γ 100 of the fracture surface energy for the {110} planes to that for the {100} planes was experimentally estimated to be 2.2.

Electron microscopy of nucleation and growth of indium and tin films

Abstract The results of measurements of island density and grain size variations as a function of film thickness and substrate temperature of vapour-deposited indium and tin films are given. It was observed that the nucleation and growth characteristics of indium and tin films are different even when the substrate temperatures are in the same ratio of their melting points. These differences are ascribed to the differences in interfacial surface energy, nucleation barrier, and critical thickness. The saturation density of indium was found to be 2 × 1011 cm−2 at 85°C while that of tin was 4 × 1011 cm−2 at 130°c. Island density curves systematically illustrate the occurrence of primary, secondary, and tertiary nucleation. This feature is also apparent from transmission and scanning electron micrographs of films deposited at 130°c. The presence of broad peaks in island density curves and dips in average grain size curves is due to the formation and growth of secondary nucleated islands. The spherical shape of...

Elastic Deformation of Polycrystalline Metal :1st Report, Influence of grain shape on distributions of stress and strain in grains

Stress and strain distributions in polycrystalline aggregates during uniaxial elastic deformation were investigated adopting the finite element method. The effect of grain shape on the deformation mode was studied. The distribution of the constraint ratio between grains defined previously was calculated for each grain shape. It was found that the strain is continuous across the grain boundary parallel to the stress axis, while the stress is continuous across the grain boundary perpendicular or inclined to the stress axis. The deformation of island grains was also discussed.