Identification of a large amount of excess Fe in superconducting single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ films

Single-layer FeSe films grown on ${\mathrm{SrTiO}}_3$ (STO) substrates have attracted much attention because of their record high superconducting critical temperature $T{}_C$. It is generally believed that the composition of the epitaxially grown single-layer FeSe/STO films is stoichiometric; that is, the ratio of Fe to Se is 1:1. Here we report the identification of a large amount of excess Fe in superconducting single-layer FeSe/STO films. By depositing Se onto superconducting single-layer FeSe/STO films, we find by in situ scanning tunneling microscopy the formation of second-layer FeSe islands on top of the first layer during the annealing process at a surprisingly low temperature ($\sim 150{}^{\circ }\mathrm{C}$) which is much lower than the usual growth temperature ($\sim 490{}^{\circ }\mathrm{C}$). This observation is used to detect excess Fe and estimate its quantity in single-layer FeSe/STO films. The amount of excess Fe detected is at least 20%, which is surprisingly high for superconducting single-layer FeSe/STO films. The discovery of such a large amount of excess Fe should be taken into account in understanding the high-$T{}_C$ superconductivity and points to a likely route to further enhance $T{}_C$ in superconducting single-layer FeSe/STO films.

Figure 1

Preparation of the single-layer FeSe/STO film. (a) Schematic of the FeSe film grown on the STO substrate. (b) STM topography ($700\times 700$ ${\mathrm{nm}}^2$) of the atomically flat STO(001) surface. (c) The as-grown 1-ML FeSe film on the STO(001) substrate with some second-layer FeSe patches (yellow) as marked by the white arrow. (d) A perfect 1-ML FeSe/STO film after annealing the film in (c) at $530{}^{\circ }\mathrm{C}$ for 4 h and (e) its zoomed-in image and (f) the atomically resolved image.

Figure 2

High-temperature superconductivity in the single-layer FeSe/STO film. (a) STM topography ($700\times 700$ ${\mathrm{nm}}^2$) of a perfect 1-ML FeSe/STO film after annealing at $530{}^{\circ }\mathrm{C}$ for 4 h. (b) Fermi surface of the annealed single-layer FeSe/STO film in (a), measured at 20 K. The carrier concentration is $\sim \text{0.116}$ electrons/Fe atom as determined from the area of the electron pockets near $M$. (c) Band structure of the single-layer FeSe/$\mathrm{STO}$ film along a momentum cut near $M_2$ as marked in (b). (d) Symmetrized photoemission spectra (energy distribution curves, EDCs) at the Fermi momentum $k_R$ marked in (c) measured at different temperatures. The superconducting gap size is $\sim \text{15}$ meV at 20 K and it closes around $\sim 60$ K.

Figure 3

Se deposition on the single-layer FeSe/STO film and evolution of the surface morphology with annealing. (a1)–(a8) STM topography ($700\times 700$ ${\mathrm{nm}}^2$) after each annealing sequence. In the first five sequences [(a1)–(a5)], the islands grow in size, and their total area decreases with annealing. The islands completely disappear after annealing sequence 6 [(a6)]. Then Se is deposited again on the single-layer FeSe film [(a6)]. (a7) and (a8) show STM images after annealing sequences 7 and 8, respectively. (b) The annealing sequence after depositing Se onto the 1-ML FeSe thin film. For convenience, we use 1–8 to denote eight consecutive annealing conditions. (c) The coverage of islands on the 1-ML FeSe film after each annealing sequence. Images of (d1) an atomically flat STO surface and the surface morphology after deposition of Se on a STO surface followed by (d2) annealing at $150{}^{\circ }\mathrm{C}$ for 0.5 h and (d3) another annealing at $150{}^{\circ }\mathrm{C}$ for 2 h.

Figure 4

STM characterization of the islands on the single-layer FeSe/STO film. (a) STM image ($100\times 100$ ${\mathrm{nm}}^2$) after the third annealing sequence [Fig. 3]. It consists of two single-layer FeSe terraces and several islands (in yellow) on the top of the surface. (b1)–(b3) Line profiles along cuts 1, 2, and 3, respectively, as marked by white lines in (a). (c) Expanded view of the region marked by a black square in (a). It shows an atomically resolved STM image ($10\times 10$ ${\mathrm{nm}}^2$) consisting of both the single-layer FeSe [terrace 2 in (a)] and an island. (d) Line profiles along the two cuts marked by the white lines in (c). The upper blue curve represents the profile along the cut on the island surface, while the lower black curve is for the cut on the surface of single-layer FeSe.

Figure 5

Detection of excess Fe in FeSe/STO films with different thickness. STM images ($300\times 300$ ${\mathrm{nm}}^2$) of (a1) the as-grown 0.45-ML FeSe/STO film, (a2) 0.75-ML FeSe/STO film, (a3) 1-ML FeSe/STO film, and (a4) 1.5-ML FeSe/STO film. (b1) STM image of the 0.45-ML FeSe/STO film after Se deposition and annealing at $250{}^{\circ }\mathrm{C}$ for 2 h. (b2) STM image of the 0.75-ML FeSe/STO film after Se deposition and annealing at $250{}^{\circ }\mathrm{C}$ for 2 h. (b3) STM image of the 1-ML FeSe/STO film after the as-grown FeSe film was annealed at $530{}^{\circ }\mathrm{C}$ for 4 h, followed by Se deposition and annealing at $150{}^{\circ }\mathrm{C}$ for 2 h. (b4) STM image of the 1-ML FeSe/STO film after the as-grown 1.5-ML FeSe film was annealed at $530{}^{\circ }\mathrm{C}$ for 9 h, followed by Se deposition and annealing at $150{}^{\circ }\mathrm{C}$ for 2 h. (c) Normalized coverage of the second-layer FeSe islands for FeSe/STO films with different initial thicknesses.