Y. Kuru

Crystallite size dependence of the coefficient of thermal expansion of metals

The coefficients of thermal expansion (CTEs) of polycrystalline Ni and Cu thin films have been investigated by employing temperature-dependent x-ray diffraction measurements of lattice parameters. Great care has been taken to exclude effects of, in particular, microstructural relaxation and mechanical stresses on the dependences of the lattice parameters on temperature. The CTEs determined in the as-deposited condition, characterized by grain sizes in the range of 25–35nm, are considerably (about 10%) larger than the corresponding literature values of bulk materials. Heat treating the specimens at moderate temperatures induced grain growth and decrease of the crystalline imperfection. After the heat treatment, the CTEs determined for the thin films had reduced considerably and had become equal to (Ni) or approached (Cu) the corresponding literature data for bulk materials.

Large excess volume in grain boundaries of stressed, nanocrystalline metallic thin films: Its effect on grain-growth kinetics

The excess volumes per unit grain-boundary area of nanocrystalline Pd and Ni thin films were measured by an efficacious method based on real time in situ x-ray diffraction measurements. The obtained large values for the grain-boundary excess volume reveal the background of surprising, yet unexplained observations of grain growth in nanocrystalline materials.

Interdiffusion and stress development in thin film diffusion couples

Thin film diffusion couples (Pd-Cu, individual layer thicknesses: 50 nm) have been prepared by DC-magnetron sputtering on silicon substrates coated with amorphous inter-layers (Si3N4 on top of SiO2). The microstructural development, phase formation and the stress evolution during diffusion annealing have been investigated employing Auger-electron spectroscopy in combination with sputter depth profiling, ex-situ and, in particular, in-situ X-ray diffraction measurements. Upon annealing at relatively low temperatures (175°C to 250°C) for durations up to 100 hours, considerable diffusional intermixing occurs. Interdiffusion is accompanied by the sequential formation of a new phase (Cu3Pd). The detected stress changes are discussed in terms of possible mechanisms of stress generation.

Coexistence of colossal stress and texture gradients in sputter deposited nanocrystalline ultra-thin metal films

This paper demonstrates experimentally that ultra-thin, nanocrystalline films can exhibit coexisting colossal stress and texture depth gradients. Their quantitative determination is possible by X-ray diffraction experiments. Whereas a uniform texture by itself is known to generally cause curvature in so-called sin2ψ plots, it is shown that the combined action of texture and stress gradients provides a separate source of curvature in sin2ψ plots (i.e., even in cases where a uniform texture does not induce such curvature). On this basis, the texture and stress depth profiles of a nanocrystalline, ultra-thin (50 nm) tungsten film could be determined.

Grain growth in nanocrystalline copper thin films investigated by non-ambient X-ray diffraction measurements

The microstructure evolution (crystallite size and microstrain) as well as the residual stress of Cu thin films of various thicknesses (250 nm, 500 nm, and 1 μm) on passivated Si substrates during isochronal annealing was investigated by in situ X-ray diffraction measurements in the temperature range between 25 °C and 250 °C. Before annealing, the thermoelastic behavior was investigated excluding the occurrence of thermally activated relaxation processes occurring above ambient temperature by in situ stress measurements below ambient temperature. On this basis, above ambient temperature, effects of stress relaxation and emerging secondary stresses (due to grain growth and annihilation of crystal defects, giving rise to a considerable tensile stress contribution development) could be identified for all three layers in the temperature regime between ambient temperature and 250 °C. Grain growth in the nanocrystalline thin films started at much lower temperatures as compared to coarse-grained materials. The results were discussed in terms of the effects of different driving forces and grain-boundary mobilities acting in nanocrystalline materials.

Tuning the Magnetic and Electronic Properties of Manganite Thin Films by Epitaxial Strain

Electrical and magnetic properties of manganites are governed by a delicate balance between several mechanisms such as charge, orbital, and spin ordering superimposed to lattice effect that can cause mesoscopic phase separation. Manganites have generally a rich phase diagram, and their properties are very sensitive to external perturbations (e.g., electrical and magnetic fields, X-ray illumination, hydrostatic pressure, and epitaxial strain), which can cause phase separation at a given temperature. The growing interest in the manganites, in both, bulk and thin film form, is due to their possible device applications and, particularly, the new physics, based on strong electron–electron interaction. The observed peculiarities like colossal magnetoresistance (CMR) and metal to insulator (MI) transitions may serve as examples. In this work, first an overview about the general properties of manganites, important mechanisms controlling phase separation, and some of the key observations about the modification of electrical and magnetic properties by external effects is given. Subsequently, the consequence of epitaxial strain is elaborated in more detail and results regarding the epitaxial La0.5Ca0.5MnO3(LCMO) thin films, grown on planar (100), (111) SrTiO3(STO), (001) SrLaGaO4(SLGO) substrates by pulsed laser deposition technique (PLD), are presented.