Abstract Body

Introduction
We have studied the vapor-liquid-solid (VLS) growth technique to fabricate REBa₂Cu₃O_y (REBCO, RE = rare earth: Y, Sm, etc.) thin films [1]. The VLS growth technique is one of solution growth methods, which can fabricate thicker REBCO films with a higher deposition rate and good crystalline orientation than films prepared by conventional pulsed laser deposition (PLD) method, which is one of vapor phase epitaxies. To improve J_c in magnetic field, although BaMO₃(BMO, M = metal: Hf, Sn, etc.)-doped REBCO films were grown by the VLS technique, the films include three regions, (1) a segregated BMO region, (2) a REBCO region without BMO and (3) a REBCO region including BMO nanorods. These separations are not included in films grown by conventional PLD method, however there are few reports on a mechanism of BMO self-organization in the VLS growth technique. To understand the mechanism, we have been developing and performing a crystal growth simulation code concerning to the VLS growth technique. Our previous results of the simulation have suggested the presence of a liquid phase can form two regions, (1) a segregated BMO region and (2) a REBCO region without BMO [2]. This texture differs from the experimental results as mentioned above. A possible reason in the experiments is that droplets consisting of liquid in the VLS growth mode move on a growth surface during the film deposition [3].In this paper, we considered the possibility of moving droplets at the surface during the film growth.  Under the droplets, growth mode becomes VLS growth mode, on the other hand, a crystal surface without the droplets becomes conventional vapor phase epitaxy (VPE). Therefore, we developed a crystal growth simulation code which can switch between the VLS growth and the VPE during a film growth.

Simulation method
In this study, we used a three-dimensional Monte Carlo simulation for crystal growth. This simulation takes into account solvation energy of REBCO in a liquid phase and one of BMO, to simulate a solution growth [2]. To consider droplets motion, we developed the simulation which can switch growth modes of VLS growth and VPE.

Experimental results and discussion
Fig. 1 shows one of results for the alternate growth mode at substrate temperature (T_S ) = 1113 K, and 5 vol. % BMO-doped REBCO. Fig. 1(a) shows BMO nanorods, while the nanorods in (b) were cut off or became thicker due to the growth mode changing to the VLS growth. In fact, similar nanostructures were observed in TEM images of a BSO-doped YBCO film [4]. These results suggest that the switch on growth mode between VLS growth and VPE during the film deposition. We will report results of nanostructures simulated at vaious growth conditions such as T_S , growth rate and solvation energy.

References

[1] T. Ito, et al., “Effect of Surface Liquid Layer during Film Growth On Morphology of BaHfO3in YBa2Cu3OyCoated Conductors Fabricated by Pulsed Laser Deposition” IEEE. Trans. Appl. Supercond., vol. 31, no. 5, Aug. 2021, Art. No. 6601205.
[2] T. Arita, et al., The 70thJSAP Spring Meeting, 16p-D215-5 (2023).
[3] R. Takahashi, et al., “Ceramic liquid droplets stabilized in vacuum”, J. Appl. Phys.101.033511(2007).
[4] S. Ito et al., The 36thInternational Symposium on Superconductivity (2023).

Acknowledgment

This work was partly supported by JSPS-KAKENHI(20H02682, 21H01872), NEDO, The Kazuchika Okura Memorial Foundation, and The Hibi Science Foundation.