Thermal stability and electrical conduction behavior of coevaporated WSi2±x thin films

Abstract

The thermal stability of coevaporated amorphous WSi_{2±x(x≂±0.2)} thin films from room temperature to 1000 °C has been studied by in situ resistivity measurements and hot‐stage transmission‐electron microscopy. During continuous heating two consecutive phase transformations were observed to occur via nucleation and growth processes. The first which occurs at ∼420 °C is the crystallization of the amorphous film to a metastable, semiconducting hexagonal phase WSi₂. The second which occurs at ∼620 °C is the transformation of the hexagonal phase to the thermodynamically stable, metallic, tetragonal phase of WSi₂. The hexagonal phase is characterized by an acicular morphology and its formation is associated with a drastic increase in resistivity. The crystallites (grains) of the stable tetragonal phase are equiaxed and their formation is associated with a rapid decrease in resistivity. In order to achieve a low value of resistivity, ∼70 μΩ cm at room temperature, the tetragonal phase must be annealed to the neighborhood of 1000 °C. The activation energy for the hexagonal to tetragonal transformation (∼3 eV) was found to be higher than that for the crystallization (∼2 eV). The mode parameters for both transformations were found to be almost the same, n∼2. The characteristics of both transformations were not greatly influenced by the compositional changes.