With the thickness increasing to 2,100 nm, the rectangular-shaped outgrowths are overlapped together. Some gaps are left between the grains. This will certainly lower the GdBCO films’ density and decrease the J c value with increasing film thickness. The surface roughness for our samples is measured by AFM, which is shown in Figure 4. The RMS value for the 200-nm-thick film is 23.6 nm. As the film thickness increases to 1,030 nm, the RMS value is 64.6 nm. For further increase of the
film thickness to 1,450 nm, there is a little RMS value increasing from 64.6 to 68.7 nm. It is believed that the appearance of a-axis grains for the 1,450-nm-thick film results in a slower increase of the RMS value. It is found that films grown with pure a-axis grains at low temperature in another experiment show selleck products a rather flat surface morphology. The RMS value goes up to 73.5 nm in the case of the 2,100-nm-thick film. Roughness measurement is in agreement with the observation of SEM (Figure 3). It is believed that the biggest RMS value for the 2,100-nm-thick film arises from the gaps between a-axis grains, as shown in Figure 3d. Figure 4 Surface morphologies of GdBCO films CUDC-907 ic50 with various thicknesses.
(a) 200 nm. (b) 1,030 nm. (c) 1,450 nm. (d) 2,100 nm. Stress analysis by means of the Williamson-Hall method Up to now, the stress effect for the GdBCO films has not been discussed yet by us. In reality, the Williamson-Hall method is an old and effective new method to analyze film internal strain ϵ by XRD measurement . The relationship of the internal strain ϵ and the integral breadth β value of each (00L) peak of the GdBCO film is as the expression: (1) where θ is the Bragg angle position of each (00L) peak, λ is the value
of X-ray wavelength (λ = 1.5418 Å). Figure 5 shows β 2cos2 θ variation as a function of sin2 θ for the GdBCO film with different thicknesses. Using the obtained linear fit slopes in Figure 5, the residual stresses calculated using Equation 1 are 0.101, 0.076, 0.086, and 0.091 for the four GdBCO films, respectively. The corresponding film thicknesses are 200, 1,030, 1,450, and 2,100 nm, respectively. It is concluded that the thinnest film has the highest residual stress while the 1,030-nm-thick film has the lowest residual stress. With further increase of the film thickness, the film residual stresses increase again. Figure 5 Williamson-Hall plot for GdBCO films with different thicknesses. In this image, β is the Bragg angle position of each (00L) peak. The internal strain ϵ can be obtained by the slope of this fitting of the data points. The Williamson-Hall method has a disadvantage that it cannot make a distinction between compressive stress and tensile stress. To get further insight into the stress behavior of the GdBCO films, more studies are needed. Because the cubic lattice constant of the GdBCO (a = 3.831 nm, b = 3.893 nm, from JCPDS card no.