Buffer Effect And Ion-exchange Preparation Of FeSe_1-xTe_x Superconducting Thin Films

Superconductors have been widely used in the field of strong and weak electric applications and show attractive prospects,which is because the special properties of zero resistance,perfect diamagnetism and Josephson effect.For enhancing the critical temperature of superconductor and expanding the applications of superconductivity,it is important to understand the superconducting mechanism.However,there has still been a debate about the explanation of superconducting mechanism so far.The discovery of iron-based superconductor provides an important opportunity for the research of superconductivity because of the extremely high upper critical fields and potential high superconducting critical temperature of this new high-temperature superconductor.Furthermore,the"11"type iron-based superconductor(FeSe_1-xTe_x)has attracted much attention because of the simple structure and non-toxic property.Compared with bulk material,FeSe_1-xTe_x films have special two-dimensional characteristics and interface effects,which leads to the unique transmission characteristics and controllable physical properties.In this thesis,we conduct two experiments about FeSe_1-xTe_x superconducting thin films.One is to prepare SrTiO₃/Ce O₂/FeSe_0.5Te_0.5 and SrTiO₃/Ti O₂/FeSe_0.5Te_0.5thin films with different thickness of Ce O₂ and Ti O₂ buffer layer by using pulse laser deposition,followed by investigating their properties.The other is to develop a ion exchange method for growth of high-quality FeSe_1-xTe_x thin films.The major achievements are summarized as follows:?1?By using pulsed laser deposition,we first grew 0-60 nm Ce O₂?or 0-220 nm Ti O₂?as a buffer layer,then deposited the FST films with a thickness of 65 nm.The results of structural characterization show that both the buffer films and FST films have a perfect c-axis growth orientation and excellent crystalline quality,with smooth interface/surface and almost no lattice defects.The electrical resistance measurements show that inserting a buffer layer between SrTiO₃ substrate and FST film can significantly enhance the superconductivity.With a proper thickness of buffer layer,the SrTiO₃/buffer/FST film shows a superconducting transition temperature of higher than 17 K.In addition,both T_c and the c-axis lattice constant of FeSe_0.5Te_0.5 films increase first and then decrease with the buffer film thickness,each exhibiting its maximum at a particular buffer film thickness.The reason is that the change in the buffer thickness can adjust the lattice matching between FST film and substrate,which affects the strain of FST film.Finally,we found that there is an almost linear correlation between the c-axis lattice constant and T_c in FST thin films,which will provide valuable information for FST superconducting mechanism exploration.In this work,in replacement of the traditional method of trying different substrates,we use a new idea of varying the buffer thickness to study the superconductivity of FST films,which avoids introducing multiple variables and is more continuously adjustable.?2?we developed a new method of preparing high-quality FeSe_1-xTe_x thin film based on ion-exchange process.By selecting Fe Te films as precursor matrixes and annealing them in Se vapor,the weakly bonded Te anions will break away from crystal lattice and be replace by Se anions under the thermal perturbation.In the ion-exchange process,the layered structure remain unchanged,so FST films are formed through Te-Se ion exchange.By adjusting the annealing parameters of ion exchange,i.e.annealing temperature and annealing time,we have successfully prepared the FST films with high superconducting transition temperature of 16.5 K and smooth surface.What's more,the diffusion mechanism of ion exchange technology is explored in our work.It is found that Se ions can diffuse into the lattice layer-by-layer along the c-axis in the ion exchange process,which is due to the fact that the structure of Fe Te film only consists of the Fe Te layer and has a short c-axis lattice constant.The ion exchange method in this thesis not only provides an indirect way to prepare target materials for easily decomposed or volatile compounds,but also provides a new approach of composition controlling and sample storage.