Effect of oxygen and carbon on the formation and stability of A-15 crystal structure chromium thin films

Abstract

The effect of oxygen and carbon impurities on the crystal structure, chemical binding states and phase transformation of r.f. sputter-deposited Cr thin films was investigated. It was found that when the O and C concentrations in the films were each within the range of 8–15 at.% (total impurity concentration between 15 and 30 at.%) the crystal structure of the Cr films was cubic A-15. Body-centered cubic (b.c.c.) Cr films formed when the total impurity concentration was <15 at.% or >30 at.%. The binding energy states of Cr, O and C were identical in the A-15 and low-impurity b.c.c. Cr films. Cr and O were found to be predominantly in an elemental or unbound state while the binding energy of C was indicative of a chromium carbide compound. Studies of the A-15 to b.c.c. Cr phase transformation were done by vacuum annealing samples for 1 h at temperatures in the 200–500 °C range. Characterization of the annealed Cr films demonstrated that the transformation occured without a measurable change in either the atomic concentration of the binding energy states of the elements in the films. X-ray diffraction studies of the A-15 → b.c.c. phase transformation indicated that complete transformation of A-15 Cr into b.c.c. Cr required an annealing temperature between 400 °C and 500 °C. Differences in the annealing temperature required to fully transform A-15 Cr into b.c.c. Cr were attributed to differences in impurity concentration among the as-deposited films; the higher the impurity concentration in the film, the higher the temperature needed to completely transform an A-15 structure into a b.c.c. structure. Vacuum heat treatments of 25 nm thick Cr films done in situ in a transmission electron microscope revealed that the A-15 to b.c.c. transformation occured by nucleation and growth of b.c.c. particles from the A-15 matrix. Therefore, it was concluded that Cr films with an A-15 crystal structure are an impurity-locked, metastable phase that transforms into a b.c.c. crystal structure at T T m ≈0.3 by a nucleation and growth mechanism.