Thermal strain in lead thin films VI: Effects of oxygen doping

Abstract

The effects of oxygen doping on microstructure and on thermal strain were studied for single- and two-layered lead films. The oxygen-doped single-layered films were prepared by depositing lead onto oxidized silicon substrates in various oxygen partial pressures up to 5×10-5 Torr. The two-layered lead films were prepared by depositing a first layer of lead onto the substrates in an oxygen pressure of 5×10-6 Torr and subsequently evaporating a second layer of lead at a pressure lower than 1×10-7 Torr. The microstructures of the films were observed by transmission and scanning electron microcopy. Compared with lead films deposited at a pressure lower than 1×10-7 Torr, finer grains with less (111) fiber structure were observed in the oxygen-doped films. Also in these films a low density of large inclusions, which could be holes or heterogeneously nucleated precipitates, were observed inside grains and on grain boundaries. The grain boundary widths became larger on increasing the oxygen partial pressure during depositions or on increasing the thickness of the oxygen-doped first layers. The strains, which were introduced into the films upon cooling from 300 to 4.2 K because of the thermal contraction mismatch between the films and the substrates, were measured for films 0.2 and 0.5 μm thick by an X-ray diffraction technique. With increasing oxygen partial pressures during the deposition, or with increasing thicknesses of the oxygen-doped lead first layers, a decrease in the elastically supported strain values was observed for the films 0.2 μm thick and an increase was observed in the films 0.5 μm thick. The strain level was found to be strongly correlated with the average grain size g in a film. For grain sizes g exceeding 1 μm an increase in the strain was observed by reducing g. The increase is believed to be caused by the prevention of dislocation motion by having reduced g. However, for grain sizes g below 1 μm the reverse was observed-the strain was decreased by reducing g. The strain level was found to be proportional to 1/g. A model, which assumes zero stress at grain boundaries and a non-uniform strain distribution parallel to the substrate inside a grain, is proposed to explain the dependence of strain on the grain size. No direct evidence was observed that would indicate that the precipitates in the oxygen-doped films influenced the strain levels. © 1980.