Particulate Generation on Surface of Iron Selenide Films by Air Exposure

Hiramatsu, Hidenori; Hanzawa, Kota; Kamiya, Toshio; Hosono, Hideo

doi:10.1007/s10948-019-5020-9

Cited by 3 publications

(3 citation statements)

References 43 publications

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“…Electronic structures were observed directly by angle-resolved photoemission spectroscopy (ARPES) excited by monochromatic He Iα light with a photon energy of 21.2180 eV from an MBS L-1 discharge lamp (MB Scientific AB, Sweden). Before the ARPES measurements, we constructed an allin situ sample transfer system [36] (i.e., to prevent sample exposure to any gasses such as air, pure N 2 , or Ar), in which the growth chamber, ARPES measurement chamber, and sample carrier transfer chambers were connected in an ultrahigh vacuum of <1×10 −7 Pa because of the serious sensitivity of FeSe surfaces to air [37]. The ARPES measurements were carried out under an ultrahigh vacuum of <4×10 −8 Pa and at a low temperature of ~10 K. We scanned along the Γ−M line using a Scienta DA30 photoelectron analyzer (Scienta Omicron Inc., Germany).…”

Section: Methodsmentioning

confidence: 99%

“…Small particles with lateral sizes of ~40 nm and heights of ~10 nm in Fig. 2(c) originated from surface degradation of the samples induced by air exposure during AFM observation[37]. For all the films, we evaluated the crystallinity along the c axis from Δω of the FeSe 001 diffraction and surface roughness from R rms .Figure 2(e) plots these values as a function of T s .…”

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Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy

et al. 2019

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This paper reports that ~10-nm-thick iron selenide (FeSe) thin films exhibit insulator-like behavior in terms of the temperature dependence of their electrical resistivity even though bulk FeSe has a metallic electronic structure that has been confirmed by photoemission spectroscopy and first-principles calculations. This apparent contradiction is explained by potential barriers formed in the conduction band. Very thin FeSe epitaxial films with various atomic composition ratios ([Fe]/[Se]) were fabricated by molecular beam epitaxy and classified into two groups with respect to lattice strain and electrical properties. Lattice parameter a increased and lattice parameter c decreased with increasing [Fe]/[Se] up to 1.1 and then a levelled off and c began to decrease at higher [Fe]/[Se]. Consequently, the FeSe films had the most strained lattice when [Fe]/[Se] was 1.1, but these films had the best quality with respect to crystallinity and surface flatness. All the FeSe films with [Fe]/[Se] of 0.8-1.9 exhibited insulator-like behavior, but the temperature dependences of their electrical resistivities exhibited different activation energies E a between the Se-rich and Fe-rich regions; i.e., E a were small (a few meV) up to [Fe]/[Se]=1.1 2 but jumped up to ~25 meV at higher [Fe]/[Se]. The film with [Fe]/[Se]=1.1 had the smallest E a of 1.1 meV and exhibited an insulator-superconducting transition at 35 K with zero resistance under gate bias.The large E a of the Fe-rich films was attributed to the unusual lattice strain with tensile in-plane and relaxed out-of-plane strains. The large E a of films with [Fe]/[Se]>1.1 resulted in low mobility with a high potential barrier of ~50 meV in the conduction band for percolation carrier conduction compared with that of the [Fe]/[Se]=1.1 film (~17 meV). Therefore, the Fe-rich films exhibited remarkable insulator-like behavior similar to a semiconductor despite their metallic electronic structure. The high potential barrier of Fe-rich films is tentatively attributed to the presence of large amounts of excess Fe, which could plausibly cause a broad superconducting transition without zero resistance under gating.

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Section: Methodsmentioning

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Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy

et al. 2019

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“…Therefore, carriers are doped in the channel but the channel thickness and/or surface state changed during the measurements. Possible reasons for the changes in the channel during cycling may be electrochemical etching by the ionic liquid [15] and surface degradation, similar to the case for t-FeSe [14,37]. Thus, further improvement of film quality and the fabrication process of the EDLT is necessary to induce superconductivity in t-FeS EDLT structures.…”

Section: Resultsmentioning

confidence: 97%

Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition

Hanzawa

Sasase

Hiramatsu

et al. 2019

Supercond. Sci. Technol.

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Pulsed laser deposition, a non-equilibrium thin-film growth technique, was used to stabilize metastable tetragonal iron sulfide (FeS), the bulk state of which is known as a superconductor with a critical temperature of 4 K.Comprehensive experiments revealed four important factors to stabilize tetragonal FeS epitaxial thin films: (i) an optimum growth temperature of 300 °C followed by thermal quenching, (ii) an optimum growth rate of ~7 nm/min, (iii) use of a high-purity bulk target, and (iv) use of a single-crystal substrate with small in-plane lattice mismatch (CaF 2 ).Electrical resistivity measurements indicated that none of all the films exhibited superconductivity. Although an electric double-layer transistor 2 structure was fabricated using the tetragonal FeS epitaxial film as a channel layer to achieve high-density carrier doping, no phase transition was observed. Possible reasons for the lack of superconductivity include lattice strain, off-stoichiometry of the film, electrochemical etching by the ionic liquid under gate bias, and surface degradation during device fabrication.

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Growth, Properties, and Device Fabrication of Iron-Based Superconductor Thin-Films

Hiramatsu

Hosono

2019

Superconductivity

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Particulate Generation on Surface of Iron Selenide Films by Air Exposure

Cited by 3 publications

References 43 publications

Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy

Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy

Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition

Growth, Properties, and Device Fabrication of Iron-Based Superconductor Thin-Films

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