“…The inset of Figure b shows the variations in the grain size of the films estimated from the two XRD peaks using the Scherrer equation ( I = k λ/βcos θ, k = 0.9 for arbitrary particles. λ, β, and θ are the wave length, full width at half maximum, and diffraction angle, respectively) . It can be understood that the grain size is relatively small and constant up to the critical O 2 ratio, but abruptly increases when the O 2 ratio increases over that value, which can be understood from the lower growth rate under this condition.…”