The Structure, Stability and Origin of beta-Phase Ta

Tantalum thin films exhibit two crystalline phases, bcc ($\alpha $-phase, the bulk structure of tantalum) and metastable tetragonal $\beta $-phase, which differ in both mechanical and electrical properties. In order to understand the stability of the $\beta $ phase and the origin of the $\beta $-to-$\alpha $ phase transformation, molecular dynamics simulations have been performed on tantalum clusters. Molecular dynamics simulations show that the $\beta $ phase is stable with a very high melting point. No phase transformation was observed for pure $\beta $-Ta clusters from room temperature to the melting point. Simulations of Ta clusters with mixed $\alpha $ and $\beta $ phases revealed that inclusion of a small $\alpha $-Ta cluster inside a $\beta $-Ta cluster induces $\beta $ to bcc transformation at a temperature far below its melting point, depending on the cluster size and $\alpha $ to $\beta $ atom ratio. These results suggest that the observed $\beta $ to bcc transformation results from the presence of small $\alpha $-phase grains within the $\beta $-Ta films. The growth of $\beta $-Ta on substrate as a result of strain is being evaluated.