Microcracks observed in epitaxial thin films of YBa2 Cu3O7–δ and GdBa2Cu3O7–δ

Abstract

Microcracking of epitaxially c‐oriented YBa2Cu3O7–δ and GdBa2Cu3O7–δ thin films is observed to preferentially occur on buffered, R‐cut sapphire substrates of 3 in diameter in comparison to smaller substrates and other substrate materials such as MgO, SrTiO3, and LaAlO3. The area density and the crystallographic appearance of the crack pattern can vary considerably across a given surface, from sample to sample, and in dependence on the temperature treatment. These dependencies indicate the crack behaviour to be seriously affected by the microstructure. The latter is studied by optical, electron, and atomic force microscopy. Cross‐sectioning TEM reveals cracks to locally separate the HTSC film from the buffer layer. Optical microscopy using transmitted polarized light is found to be a straightforward, sensitive survey method to observe the microcracking behaviour.

Epitaxial CeO2 buffer layer on deliberately miscut sapphire for microcrack-free thick YBa2Cu3O7−δ films

“…Moreover, although Df (1.681) for CeO 2 (2 0 2) was almost unchanged after YBCO deposition, Do (0.271) for the CeO 2 (0 0 2) of the bilayer was about 4 times larger than that of the single CeO 2 layer (about 0.071) [15]. Since the mean thermal expansion coefficient of CeO 2 (11 Â 10 À6 /K) is between that of YBCO (13 Â 10 À6 /K) and sapphire (5À7 Â 10 À6 /K) [5], this distortion could be an effect of the different thermal expansion coefficients of Al 2 O 3 , CeO 2 and YBCO, which leads to extending the strain within the YBCO to the CeO 2 layer during cool-down from deposition temperature to room temperature.…”

“…Especially, as switching elements for resistive fault current limiters, thick YBCO films are necessary to increase the critical current per unit width (i.e., J c t product, here J c is the critical current density and t is the film thickness) and thereby enhance the nominal power [4]. However, it is well known that microcracking occurs in YBCO films deposited on CeO 2 -buffered sapphire when the thickness exceeds a critical value of $300 nm [5,6]. The reason for cracking is attributed to the thermal strain during cooling (from the deposition temperature) since the expansion coefficient of YBCO is about two times larger than that of sapphire.…”

Microwave losses and structural properties of large-area YBa/sub 2/Cu/sub 3/O/sub 7/ films on r-cut sapphire buffered with [001]/(111) oriented CeO/sub 2/

“…Rs(<20K) was below (70 -80) pa. These results obtained for the YBCO films on sapphire, particularly the residual resistances, are competitive with the best high-quality YBCO films grown on La4103 or MgO [1,4,8].…”

“…Sapphire of (1102), or r-cut, orientation buffered with thin Ce02 layer appears to be a very promising substrate for the preparation of large-area YBa2C~307.x (YBCO) thin films suitable for applications in microwave electronics [ 1,2]. On one hand, sapphire combines outstanding crystalline perfection, mechanical strength, low dielectric permittivity and losses with reasonably low costs of large area substrates.…”

Microstructure of YBa2Cu3O7−δ films on deliberately miscut sapphire buffered with CeO2

“…However, it is well known that microcracking occurs in YBCO films deposited on CeO 2 -buffered sapphire when the thickness exceeds a critical value of $300 nm [1,2]. Microcracking in thick YBCO films deposited on CeO 2 -buffered sapphire is a big obstacle towards the use of YBCO films in practical applications.…”