One-step growth of SmFeAs(O,F) films by molecular beam epitaxy using FeF₂ as a fluorine source

Abstract

We report on one-step growth of superconducting SmFeAs(O,F) films without post-growth fluorine diffusion. One-step growth is a prerequisite for multilayer deposition, which is required for fabricating sandwich-type Josephson junctions; however, it has not been achieved because of a lack of a suitable fluorine source. We found that FeF2, a ‘soft’ fluoride, decomposes at relatively low temperatures (∼650 °C) and supplies fluorine as well as Fe to films. The use of FeF2 has enabled us to perform reproducible growth of SmFeAs(O,F) films with a systematic change of the film’s properties by varying growth parameters. Now, single-phase single-crystalline SmFeAs(O,F) films with are routinely prepared. Furthermore, the c-axis lattice parameter of the films are varied in a systematic manner with the FeF2 deposition rate, indicating that the F doping level seems to be controllable. The comparison of one-step growth and our previous two-step growth demonstrates that a more homogeneous F concentration is achieved by one-step growth, whereas higher F concentration is achieved by two-step growth.

In-situ growth of superconducting SmO1−xFxFeAs thin films by pulsed laser deposition

“…2a). The best film has an onset transition temperature, T c , on near 40 K. The c -axis lattice parameters of the PLD grown films (8.63–8.66 Å) are comparable to the reported c -axis lattice parameters for MBE grown films (8.55–8.65 Å)18 but generally larger than 8.495 Å ( x  = 0), 8.428 Å ( x  = 0.1)25, or 8.488–8.498 Å ( x  = 0.2)26, in sintered polycrystalline powder samples and also larger than the c -axis in a single crystal (8.468 Å for x  = 0.14)5.…”

“…In the last five years advances in the growth of oxypnictide thin films were made by means of molecular beam epitaxy (MBE) which should be able to master film growth with volatile elements161718. However, also in MBE F-doping has first been introduced via diffusion from a fluorine containing capping layer or from a fluorine containing substrate.…”

Comparative Review on Thin Film Growth of Iron-Based Superconductors

“…These results indicate that the F content in this film is insufficient. On the other hand, the Sm1111 film on CaF 2 substrate deposited from the same target shows a superconductivity at ∼40 K. The c-axis length is comparable to that of Sm1111 film on CaF 2 fabricated by MBE [38,[41][42][43] and is slightly larger than that of Sm1111 bulk samples [39]. This indicates that the fluorine diffused from the substrate is doped into a film, which induces a superconductivity as is the case of MBE [41].…”

“…In these reports, the extra cap layer is necessary to obtain a superconducting 1111 film, which prevents an application of 1111 film for a sandwich-type junction [36,38]. The one-step growth method which does not need the extra cap layer for obtaining a superconductivity was established by using LnF 3 (Ln = Sm, Nd ) or FeF 2 as a fluorine source [41][42][43].Naito et al pointed out the importance of the fluorine source in the film growth of 1111 and proposed a four kinds of fluorine sources [41]; (1) F diffused from the fluoride substrate (CaF 2 , SrF 2 , BaF 2 ); (2) F from LnF 3 codeposited; (3) F from FeF n (n = 2, 3) codeposited; and (4) molecular F 2 gas. Although method (4) is not realistic, the other three methods are effective for supplying F to the films in the MBE process.…”

“…Although method (4) is not realistic, the other three methods are effective for supplying F to the films in the MBE process. While methods (2) and (3) are shown above [42,43], a superconductivity appeared in the SmFeAsO film on the BaF 2 substrate, in which the fluorine was provided by the diffusion from the substrates [41]. On the other hand, highly textured NdFeAs(O,F) thin films have been grown on ion beam assisted deposition(IBAD)-MgO/Y 2 O 3 /Hastelloy substrates by MBE, which is well known as the one of the most practical methods to obtain the textured film on the metallic substrate [44].…”