Preparation and characterization of high-quality FeSe single crystal thin films

Abstract

Of all iron-based superconductors, FeSe possesses the simplest structure whereas its superconducting critical temperature can be remarkably enhanced. Compared with bulk sample fabrication, the film preparation process is very precise and controllable. Although FeSe monolayer films exhibit a high Tc, they are unstable in air, and ex-situ measurements are very difficult. Therefore, the stable films with~100 nm in thickness can serve as good candidates to explore the mechanisms of iron-based superconductors. There is no doubt that the fabrication of high-quality FeSe thin films is of significance. The pulsed laser deposition (PLD) technique has more advantages in the growth of FeSe thick films than any other film fabrication technology, because of its high efficiency and wide adaptability. In this work, we systematically optimize the growth conditions of FeSe thin film fabricated by PLD. The main results are as follows. 1) The optimal growth temperature is 350℃, where the film has the best crystallinity and the highest Tc. 2) High-quality -FeSe epitaxial thin films with the thickness ranging from 10 to 320 nm have been successfully prepared on twelve types of substrates:CaF2, LiF, SrTiO3, MgO, BaF2, TiO2, LaAlO3, MgF2, Nb-SrTiO3, LSAT, LaSr(AlO4) and MgAl2O4. The Tc for the films on CaF2 with the same thickness of 160 nm can be tuned from 2 K to 14 K. 3) The Tc of the FeSe thick films may be precisely tuned by the Fe/Se ratio which is affected by the proportion of the nominal components of the target, the laser energy density and the ablation off-stoichiometry of target. 4) The surface morphology measurement, cleavability and transferability experiments of films are performed. In addition, it is worth of mentioning that there is a significant positive correlation between Tc, lattice constant c and residual resistivity ratio (RRR), as evidenced through a detailed statistical analysis of the data from more than 1500 samples. Since c and RRR are usually associated with the vacancies or defects, we conclude that the superconductivity of -FeSe thin films is closely related to the ratio of Fe to Se. Moreover, the first principle simulation shows that 0.5% increase of Fe content does lead to a change of 0.05 of c. However, according to the angle-resolved photoelectron spectroscopy experiment, there is no obvious change near the point in the hole energy band, but the energy band changes significantly at the M point. This variation of electronic structures cannot be explained by electron filling which lifts up the Fermi energy. Therefore, the specific relationship among the superconductivity, lattice structure and electronic structure of FeSe thin films remains to be clarified. Such a series of high-quality -FeSe films offers a chance to further explore the nature of FeSe-based superconductors.