Pulsed Laser Deposition (PLD) has become a widely used method to grow single crystal thin films of high quality. Since PLD can take place under controlled oxygen atmosphere, it is especially suited for the growth of thin-film high-Tc superconductors (HTSC^,s), whose properties are highly sensitive to their oxygen content. Although the PLD process is commonly used, the growth and nucleation mechanisms are still not completely understood. In order to gain more insight into these processes, a miniature PLD chamber is being constructed for the purpose of following with x-ray diffraction (XRD) the initial stages of growth of YBa₂Cu₃O_7-δ (YBCO) on SrTiO₃ (STO) substrates, and the subsequent annealing of the film.
A small furnace is constructed to study the structural properties of YBCO films on STO substrates under controlled environment. In the case of thin films grown on substrates, strain is built up in the film due to the lattice mismatch between the former and the latter. Since the lattice mismatch is a function of temperature, it is of interest to study the strain-state of the film at different temperatures.
Here we present the results of in-situ XRD measurements of YBCO thin films at elevated temperatures in controlled oxygen atmosphere, thereby imitating the annealing step as carried out in the PLD process. From the splitting of the XRD reflections due to twinning, the spontaneous strain is determined. This is directly related to the difference in lattice parameters a and b. In 1 bar of oxygen, a gradual transition is seen around 250^oC, indicating strain relaxation.

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