【作者】 李国兴；

【导师】 杜国同； 全宝富；

【作者基本信息】 吉林大学 ， 微电子与固体电子学， 2008， 博士

【摘要】 针对微波滤波器等微波无源器件对高温超导薄膜的需求,本论文使用光辅助MOCVD法,围绕大面积YBCO外延膜的制备进行了一系列的初步研究工作。本论文设计了不锈钢反应室的光辅助MOCVD系统,解决了系统中影响薄膜制备重复性的多个问题。其中包括铜和钇固态金属有机源在源炉盖上易凝结导致的称量不准确的问题;多个卤钨灯之间相互炙烤造成的卤钨灯使用寿命短的问题;由于腔体过高造成的转轴摆动和大量固态有机源在腔体下部沉积的问题。此外,独立进行了多个卤钨灯产生的光辐射照度的理论计算和计算机模拟。使用此光辅助MOCVD系统,进行了大量YBCO薄膜的制备实验,对制备高质量c轴向YBCO外延膜的系统参数进行了优化。讨论了MOCVD法制备YBCO外延膜的生长机理,并基于此解释了a轴向和c轴向YBCO外延膜的典型形貌。成功制备了各方面指标比较均匀的直径1”YBCO外延薄膜(约600 nm),厚度差异在10%以内,具有优异的超导性能(J_c＞2 MA/cm~2(0T,77K))和微波传导性能(R_s＜0.4mΩ(10GHz,77K))。成功制备了直径2"的高质量YBCO外延薄膜,具有优越的超导能(J_c＞1 MA/cm~2(0T,77K))和良好的微波传导性能(R-s≈0.5 mΩ(10GHz,77K))。更多还原

【Abstract】 For the sake of the excellent microwave transmission properties of high temperature superconductor（HTS） YBa₂Cu₃O_7-x（YBCO） epitaxial thin films, YBCO has been one of the perfect materials,which can highly reduce the microwave devices size and enhance its performance.The microwave devices, e.g.microwave filters,made of YBCO working in liquid nitrogen at atmospheric pressure possess high quality factor（Q-factor）,low insert loss, small size,light weight,etc.Therefore,YBCO epitaxial thin films were researched intensively and deeply.The research works of our rap group were aiming the microwave filters’ great demand for high quality YBCO epitaxial thin films.We always applied ourselves to the preparing of double-sided 2″YBCO epitaxial films by photo-assisted MOCVD method.In this paper,the photo-assisted MOCVD system for the growth of one-sided 2″YBCO epitaxial films was designed and installed.And then,a series of research were done about the growth technological process and the uniformity of larg area YBCO films。Comparing with quartz chamber,stainless steel chamber has multi-advantages,such as good sealing,mature mechanical workout, non-frangibility,and Flexible design etc.Thereby,based on some work on photo-assisted MOCVD with quartz chamber,this paper designed a new photo-assisted MOCVD system with a stainless reactor.In this system, precursor ovens with novel seal structure was designed,it can prevent the condensation of precursor vapor（Y and Cu） onto the upper cove.The susceptor and substrate was heated by more large power tungsten-halogen lamps,which were placed inside the reactor chamber.Novel and efficient water-cooled lampshade was designed and manufactured.It can prevent the lights from baking to each other,and lead to the life extension of lamps. Besides,based on some experiments,a new short stainless reactor was redesigned to solve the serious problems such as the swing of rotating shaft and the condensation,caused by too high of chamber,of precursor vapor on the lower part of reactor wall.As a result,repeatable preparation of YBCO epitaxial films by the photo-assisted MOCVD system designed and assembled by us was realizedA "substrate temperature window"（T_S window） for purely c-axis oriented YBCO films was found,based on many times repeated experiments with the same airflow and pressure.This T_S window is 800～830℃at total pressure of 4.6 torr and oxygen pressure of 2.8 torr.Lower temperature will lead to a-axis crystal grains,and higher temperature will lead to film decomposed or hardly depositon.Therefore,during the large area film deposition,the uniformity of illumination and accurate temperature control is a key factor who directly impact on the crystal quality uniformity of the films. Thereby,computer simulation of illumination intensity distribution for parallel line light sources was adopted.The arrangement of lights was optimized through computer-assisted.This improved the optical power density distribution lighting on 2″substrate surface.Experiments proved that this kind of word is very important for 2″and larger area film deposition.The airflow and air density near and upper the substrate is the key factor affecting the film growth rate there for most CVD reactors.The thickness uniformity of large area film is base on the airflow and air density distribution. In this paper,thickness distribution of 1″YBCO film growth on LAO substrate irrotated.Combining with the computer simulation of airflow and air density distribution,the thickness uniformity of large area YBCO film fabricated by photo-assisted MOCVD was discussed,and the basic method was proposed.During this study,1″purely c-axis YBCO epitaxial films were successfully fabricated by photo-assisted MOCVD system,on rotary LaAlO₃ （100）（LAO） substrate with well crystalline quality uniformity.The thickness can be controlled from 0.2μm to 2μm,with difference of±10%.For 1″thin YBCO films（about 0.6μm）,repeated experiments proved that J_C of whole film are larger than 1 MA/cm²（0T,77K）.The J_C value of typical samples were 1-3 MA/cm²,and R_S value were less than 0.4 mΩ. For 1″YBCO thick films（1.5μm）,a preliminary result indicated that J_C values were about 0.18MA/cm²（0T,77K）,R_S value is about 21 mΩ（10GHz, 77K）。By the same photo-assisted MOCVD system,c-axis YBCO films with well crystalline quality uniformity on 2″LAO substrates were also succesfully grown.The feasibility for large area YBCO deposition was adequately proved.The thikness distribution of 2″YBCO films was nonuniform caused by the nonuniform airflow distribution in this reactor. Film thicknesses at the edge of 2″YBCO films were thiner（about 300 nm） than that at the center（about 500 nm）.And J_C values at the edge were higher [3 MA/cm²（0T,77K）]than that at the center[1.8 MA/cm²（0T,77K）].The R_S value at the center of this YBCO sample was about 0.5 mΩ（10GHz,77K）.更多还原