The Effects of Post-Annealing on Ferecrystals

Ferecrystals are metastable turbostratically-disordered misfit layer compounds that are useful in probing fundamental questions in solid state chemistry. Previous studies have shown that the electrical properties of semiconducting (PbSe)_0.99WSe₂ ferecrystals, can be improved dramatically by annealing in a Se-rich environment. In this study, we sought to investigate the effects of Se vapor postannealing treatments on the structures and electrical properties of metallic (PbSe)_1+δNbSe₂, (SnSe)_1+δNbSe₂, and (BiSe)_1+δNbSe₂ ferecrystals. We used X-ray reflectivity and diffraction to characterize the thickness and structure of each sample before and after vapor annealing in a Se-rich atmosphere at 350 °C, and acquired room-temperature electrical resistivity and Hall coefficient measurements as a function of vapor annealing time. The thickness of all of the samples increased slightly after vapor annealing. Some samples also showed incorporation of secondary selenium-rich phases, indicating that excess Se may have been incorporated into the films. Additionally, the room temperature carrier concentrations for all samples decreased. This suggests that electrically active defects in metallic ferecrystals are reduced by a Se vapor postannealing treatment. Our results have implications for manipulating film structure and composition and for improving the electrical properties of metallic ferecrystals.