Electrical resistivity change in amorphous Ta42Si13N45 films by stress relaxation

In a first experiment, a reactively sputtered amorphous Ta₄₂Si₁₃N₄₅ film about 260 nm thick deposited on a flat and smooth alumina substrate was thermally annealed in air for 30 min and let cooled again repeatedly at successively higher temperatures from 200 to 500 °C. This treatment successively and irreversibly increases the room temperature resistivity of the film monotonically from its initial value of 670 μΩ cm to a maximum of 705 μΩ cm (+5.2 %). Subsequent heat treatments at temperatures below 500 °C and up to 6 h have no further effect on the room temperature resistivity. The new value remains unchanged after 3.8 years of storage at room temperature. In a second experiment, the evolution of the initially compressive stress of a film similarly deposited by reactive sputtering on a 2-inch silicon wafer was measured by tracking the wafer curvature during similar thermal annealing cycles. A similar pattern of irreversible and reversible changes of stress was observed as for the film resistivity. Transmission electron micrographs and secondary ion mass profiles of the film taken before and after thermal annealing in air establish that both the structure and the composition of the film scarcely change during the annealing cycles. We reason that the film stress is implicated in the resistivity change. In particular, to interpret the observations, a model is proposed where the interface between the film and the substrate is mechanically unyielding.

Influence of sample matrix on the alkaline extraction of Cr(VI) in soils and industrial materials

An accurate and efficient determination of the highly toxic Cr(VI) in solid materials is important to determine the total Cr(VI) inventory of contaminated sites and the Cr(VI) release potential from such sites into the environment. Most commonly, total Cr(VI) is extracted from solid materials following a hot alkaline extraction procedure (US EPA method 3060A) where a complete release of water-extractable and sparingly soluble Cr(VI) phase is achieved. This work presents an evaluation of matrix effects that may occur during the hot alkaline extraction and in the determination of the total Cr(VI) inventory of variably composed contaminated soils and industrial materials (cement, fly ash) and is compared to water-extractable Cr(VI) results. Method validation including multiple extractions and matrix spiking along with chemical and mineralogical characterization showed satisfying results for total Cr(VI) contents for most of the tested materials. However, unreliable results were obtained by applying method 3060A to anoxic soils due to the degradation of organic material and/or reactions with Fe2+-bearing mineral phases. In addition, in certain samples discrepant spike recoveries have to be also attributed to sample heterogeneity. Separation of possible extracted Cr(III) by applying cation-exchange cartridges prior to solution analysis further shows that under the hot alkaline extraction conditions only Cr(VI) is present in solution in measurable amounts, whereas Cr(III) gets precipitated as amorphous Cr(OH)3(am). It is concluded that prior to routine application of method 3060A to a new material type, spiking tests are recommended for the identification of matrix effects. In addition, the mass of extracted solid material should to be well adjusted to the heterogeneity of the Cr(VI) distribution in the material in question.