Studies Of Growth And In-situ Transport Properties Of FeSe Thin Films

Iron selenide has the simplest lattice structure among iron-based superconductors.Single layer FeSe on surface of SrTiO₃?STO??001?has an amazing high transition temperature?T_c?of 109 K which is much higher than that of bulk FeSe crystal.This enormous difference makes it more attractive in probing the transition of physical natures of FeSe when its thickness varies from one monolayer to multilayer.In this dissertation,we have mainly studied the growth and in-situ transport properties of multilayer FeSe films on STO substrate.During the growth process utilizing Molecular Beam Epitaxy method,the intensity of RHEED patterns oscillate with a“single peak”shape which is agree with the description of“step density model”.Being different from the intensity oscillation of specular spot,the single peak oscillation characteristic of FeSe?02?RHEED strikes is protected from the changing of electron beam grazing angel,which is an ideal calibration of judging the certain thickness of multilayer FeSe samples.We have also sorted out the optimal growth conditions for two layer and multilayer FeSe films and it is found in STM observations that the high-quality multilayer FeSe covers the whole STO surface evenly with a negligible number of small FeSe islands.Under an ultra-high vacuum condition,we measured the in-situ electoral transport property of as grown 80 layer FeSe films,only found that it behaves like a semiconductor at temperatures range from 4 K to 60K and undergoes a semiconductor-metal transition at about 60 K.But,after a proper annealing treatment,the multilayer FeSe film will become a superconductor with a transition temperature of 14 K which is about 70%higher than that of bulk FeSe crystal.The follow-up studies by RHEED and XRD show that,the in-plane lattice constant?a?of the annealed FeSe film has been shrunk into 3.67?and the out-of-plane lattice constant?c?has been elongated into 5.60?.Together with the T_c data got from our experiments and others,we drawn up the a-T_c and c-T_c diagram and found that the T_c values show a well liner relationship with whether a or c.Finally we can draw the conclusion that by modulating the lattice constants via a proper annealing treatment,multilayer FeSe films grown on STO surface will undergo a semiconductor-superconductor transition.These experiences are useful for growing high-quality multilayer FeSe films on STO substrates and these experimental results have important significances for studying the mechanism of high-temperature superconductivity.What's more,this dissertation has given a new way to promote the transition temperature of superconductivity.