Abstract Body

I. Introduction
Pulsed Laser Deposition (PLD) is an effective thin film deposition method for fabricating YBa₂Cu₃O_y (YBCO) epitaxial films. In the PLD method, columnar ablation plasma, called a plume, is observed when a laser is irradiated onto a target. The plume shape is influenced by not only quantitative parameters such as laser energy and oxygen pressure but also non-quantitative parameters such as laser energy fluctuations and shock waves generated by the plume compressing the surrounding gas [1]. This results in a somewhat low reproducibility of superconducting properties even when the oxygen pressure and laser energy are fixed during the deposition of YBCO thin films.

In this study, with the aim of achieving highly reproducible deposition of YBCO thin films by the PLD method, we constructed a system for real-time observation of the plume shape and the color, and then we investigated the effects of various conditions such as oxygen pressure and laser energy on the plume shape and the color.

II. Experimental method
YBCO thin films were fabricated using the PLD method, and the plume during the deposition process was captured using an Intel Realsense Depth Camera D405. For a plume observation system, a Python code was developed to sequentially measure and record the color information around the plume and the plume size. In preparations of YBCO thin films on SrTiO₃(STO) (100) substrates, the substrate temperature was set to 920 °C, and the energy of the Nd:YAG laser (wavelength 266 nm, repetition rate 10 Hz) was varied to 10, 15, and 20 mJ. The oxygen pressure was changed every 10 minutes to 10^-2, 10-1, 1, 13.3, 26.3, 53.2, and 100 Pa to observe the plume shape. The plume size was measured by determining the height and width of the plume based on the three-dimensional coordinate information obtained from the depth camera.

III. Experimental results and discussion
Fig. 1 shows images of the plume formed under different oxygen pressures when the laser energy was 20 mJ. The green rectangle is the bounding box that the plume observation system recognizes as enclosing the plume. This bounding box is obtained by converting the color image of the plume into a grayscale image, binarizing it, and then finding the bounding box of the resulting black-and-white boundary. Fig. 2 shows the plume height for each oxygen pressure and laser energy. From these results, the plume height decreases with increasing oxygen pressure above 13.3 Pa, while no clear trend was observed for oxygen pressures below 1 Pa. Additionally, the plume height decreased with decreasing laser energy at all oxygen pressures. At the presentation, we will discuss the effects of oxygen pressure and laser energy on the aspect ratio (height-to-width ratio) and color information of the plume. We will also report on the construction of a system for feedback control of the plume shape.

References

[1] K.Fukushima, Int’l J. Mod. Phys. B, vol. 9, No. 28, 1995.

Acknowledgment

This research was partly supported by the JST-CREST (JPMJCR2336). We would like to express our sincere gratitude to the Yoshida Laboratory, Department of Electrical Engineering, Nagoya University, for their significant contributions to the construction of the observation system.

Fig. 1 Images of YBCO ablation plumes at different O2 pressure in which the laser energy is fixed to 20 mJ.
Fig. 2 Height of YBCO ablation plume depending on O2pressures and laser energy.

Keywords: Pulsed Laser Deposition, YBa₂Cu₃O_y thin film, ablation plume