Title: Evidence for structural transition in crystalline tantalum pentoxide films grown by RF magnetron sputtering,
Abstract: We investigate the effect of annealing temperature on the crystalline
structure and physical properties of tantalum-pentoxide films grown by radio
frequency magnetron sputtering. For this purpose, several tantalum films were
deposited and the Ta$_2$O$_5$ crystalline phase was induced by exposing the
samples to heat treatments in air in the temperature range from (575 to
1000)$^\circ$C. Coating characterization was performed using X-ray diffraction,
scanning electron microscopy, Raman spectroscopy and UV-VIS spectroscopy. By
X-ray diffraction analysis we found that a hexagonal Ta$_2$O$_5$ phase
generates at temperatures above $675^\circ$C. As the annealing temperature
raises, we observe peak sharpening and new peaks in the corresponding
diffraction patterns indicating a possible structural transition from hexagonal
to orthorhombic. The microstructure of the films starts with flake-like
structures formed on the surface and evolves, as the temperature is further
increased, to round grains. We found out that, according to the features
exhibited in the corresponding spectra, Raman spectroscopy can be sensitive
enough to discriminate between the orthorhombic and hexagonal phases of
Ta$_2$O$_5$. Finally, as the films crystallize the magnitude of the optical
band gap increases from 2.4 eV to the typical reported value of 3.8 eV.