Double epitaxy of tetragonal and hexagonal phases in the FeSe system

Abstract

Epitaxial films which contain more than one crystallographic phase or orientation are of interest due to the possibility of altered magnetic, electrical, and optical properties. Thin films of FeSe have been grown on single-crystal MgO substrates under conditions that produce the simultaneous, epitaxial growth of tetragonal and hexagonal phases. We show that this double epitaxy is characterized by phase domains with a well-defined epitaxial relationship to each other and that the relative phase fraction can be controlled. For growth temperatures of 350-450 • C, the (001)-oriented tetragonal phase (β-FeSe) grows with its unit cell aligned with the cubic substrate, while a (101)-oriented hexagonal phase (Fe 7 Se 8 ) shows domains with two different in-plane orientations separated by 45 • . Additionally, the β-FeSe phase can be chosen to be (001)-or (101)-oriented with respect to the substrate, with the (101) orientation containing three rotated domains.Keywords: A1. Double epitaxy, B1. Iron selenide, B1. Cu-doped iron selenide, B1. Fe 7 Se 8 , B1. FeSe, B2. Iron-based superconductor ✩

Chemical Composition Control at the Substrate Interface as the Key for FeSe Thin-Film Growth

“…Higher substrate temperatures (500 -550ºC) could prevent the growth of Fe7Se8 but supported the growth of (001)-or (101)-oriented tetragonal FeSe with different in-plane rotated domains. 18 Important for our study is that for the tetragonal FeSe phase, DME with a ratio of m/n = 8/7 was considered to reduce the residual strain εr (εr = maFeSe/naMgO -1) 17 to 0.35%. 18 The detailed structure of the FeSe/MgO heterointerface remained, however, largely unclear, because high-resolution transmission electron microscopic (HR-TEM) investigations were rare: Although Chen et al 15 provided evidence of an atomically sharp interface of a 400 nm thin FeSe film grown on MgO at TS = 320ºC, the origin of the cube-on-cube epitaxy despite the large mismatch between FeSe and MgO remained unsolved.…”

X-ray diffraction data and analysis to support phase identification in FeSe and Fe7Se8 epitaxial thin films

“…In the related research article [1], epitaxial thin films were grown by pulsed laser deposition (PLD) using a target formed of a mixture of β-FeSe (22%) and 3 c -Fe 7 Se 8 (78%) whose X-ray diffraction (XRD) is shown in Fig. 1.…”

“…Based on the θ-2θ XRD scans in Fig. 1 of [1], the orientation of the Fe 7 Se 8 phase was found to take on two different orientations with (101) and (001) planes oriented parallel to the substrate surface, using Miller indices referred to the setting of the fundamental NiAs-type structure of Fe 7 Se 8 . This convention of indexing the Fe 7 Se 8 lattice planes and reflections with respect to its fundamental NiAs-type structure is adopted throughout this paper, unless otherwise noted, and is necessary whenever it is not possible to specify which Fe vacancy superstructure (3 c or 4 c ) is present, which is our case.…”

“…Camata, Double epitaxy of tetragonal and hexagonal phases in the FeSe system, J. Cryst. Growth, 514, 2019, 54–59 [1].…”

Challenges for Pulsed Laser Deposition of FeSe Thin Films

“…Iron-based superconductor FeSe thin films haven been grown by different methods [1,2]: pulsed laser deposition (PLD) [3][4][5][6], molecular-beam epitaxy (MBE) [7], radio-frequency (RF) sputtering [8,9], selenization [10], and mechanical exfoliation [11]. So far, high-superconducting transition temperature (Tc) monolayers as well as superconducting ultrathin FeSe films could be obtained in an MBE approach, for example, one-unit-cell FeSe/SrTiO3(STO) with Tc = 64-75 K [12][13][14][15][16], or 1-5 nm thin FeSe films on bilayer graphene with Tc = 3-8 K [7].…”