Phase transformations in alloy and bilayer thin films of vanadium and silicon

Abstract

Phase transformations in coevaporated amorphous vanadium-silicon thin alloy films and bilayer vandium/silicon films have been studied as a function of heat treatment by in situ electrical resistivity measurement together with Rutherford backscattering spectrometry, Seeman-Bohlin glancing angle incidence x-ray diffraction, and scanning and transmission electron microscopy. In the as-deposited state the amorphous alloy films were silicon rich, having an atomic ratio of 1:3 for vanadium and silicon, respectively. Upon heat treatment a sharp decrease in resistivity occurs at approximately 250 °C, which has been determined to be a transformation from the amorphous to crystalline VSi 2 phase. The kinetics of the transformation have been obtained by isothermal treatment over the temperature range of 184-220 °C. The transformation is described by a Johnson-Mehl-Avrami-type equation with an apparent activation energy of 1.30±0.06 eV. Subsequent heat treatment causes a gradual decrease in resistivity up to 850 °C. Upon cooling, a monotonic decrease in resistivity was observed. Heat treatment at high temperatures (900 °C) promotes the growth of nonuniformly distributed silicon grains. For the bilayer vanadium/silicon films, the sheet resistance increases gradually upon heat treatment up to 500 °C, then a sharp decrease is observed, which is due to the formation of VSi2. Further heat treatment at higher temperatures (850 °C) promotes a monotonical decrease in the resistance. The cooling behavior is similar to that of the crystallized alloy specimens except for having a slightly lower resistivity value. In a model for the two thin films connected electrically in parallel, the growth kinetics of VSi2 in the bilayer films has been found to be linear in time over the temperature range of 500-535 °C with an activation energy of 2.23±0.09 eV. The microstructure of films at various stages of annealing have been studied by x-ray diffraction and transmission electron microscopy. Correlation between the resistivity and microstructure is given and discussed. In situ resistivity of annealed films below room temperature has been measured. Crystalline VSi2 thin films do not become superconductive down to 2 K.