MOCVD Preparation Of YBCO Thin Films On Single Crystal

Compared with conventional conductor filters,high-temperature superconducting(HTS)filters can increase filter order,reduce insertion loss,and improve out-of-band suppression.Among the HTS materials,YBa₂Cu₃O_7-?(YBCO)is relatively mature in application.In order to promote its application in microwave,it is necessary to achieve high-performance,large-scale and low-cost batch preparation of YBCO thin films.Thus,laboratory independently designed a metalorganic chemical vapor deposition(MOCVD)system,which can improve the preparation efficiency,reduce the cost and ensure the high quality of the YBCO film.In this thesis,based on this self-developed new deposition system,the factors affecting the growth mechanism and properties of YBCO thin films were studied.Firstly,parameters of the new MOCVD system were explored.When the oxygen partial pressure is 800-950sccm and the deposition temperature is 840?,the YBCO thin film based on La Al O₃(LAO)substrate can achieve better crystallization.Under this condition,a composition interval was obtained:the Y content is 21%-24%,the Ba content is 28%-31%,and the Cu content is 46%-53%.In this range,the critical current density of the YBCO film is greater than 2.0 MA/cm~2,and the microwave surface resistance is less than 0.5 m?,which meets the practical needs of the YBCO film in microwave application.The preparation of YBCO film on MgO by MOCVD needs to overcome the problem of mixed growth mode.There are two growth modes for YBCO thin films on MgO substrates:one is the 0°growth mode of cubic-on-cubic,and the other is the 45°growth mode by in-plane rotating 45°.Therefore,the effects of temperature and composition on the in-plane matching of YBCO and MgO were investigated separately.When the temperature is 805°C,the film grows symmetrically along 0°;and the temperature is 810or 825°C,the film achieves pure 45°growth.It shows that the single growth mode of YBCO on MgO substrate can be realized by controlling the temperature.For the mixed growth at 820°C,the regulation of YBCO growth matching was also achieved by adjusting the composition of Y content.As the Y content is 15%,the film grows along 0°.When it reaching to 36%,the film grows along 45°.While Y content is between the 15%to 36%,YBCO mainly grows at 45°,accompanying with Y-Cu-O and Y₂O₃ impurity phases existing.Furthermore,The strength of 0°orientation gets weaker with the weakening of Y-Cu-O phase.In this thesis,a HTS filter for GSM900 frequency band was fabricated using the YBCO thin film prepared by MOCVD method on LAO substrate.The frequency response exhibit its having filter characteristics,but post-tuning is required to overcome the uneven dielectric constant of LAO.It is proved to be feasible to realize microwave circuit using the YBCO film prepared by MOCVD method,which is beneficial to promote the practical process of HTS films on the microwave in the future.