组份比例及衬底材料影响YBCO外延膜的光辅助MOCVD法生长特性研究
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摘要
高温超导体YBCO具有高的超导转变温度、低的表面电阻、高的极限电流密度且在磁场下能有很好的超导特性，因而YBCO在超导应用领域被广泛的关注。如将其制备在单晶衬底上可以用来制作超导微波滤波器、SQUID、MRI等器件；如将其制备在金属基带上可以制作成第二代高温超导导线，应用在超导马达、超导限流器等方面。目前制备高温超导材料YBCO外延膜的方法有很多，包括磁控溅射法、脉冲激光沉积法、多源共蒸发法、金属有机物沉积法（MOD）、金属有机化学气相沉积法（MOCVD）等。其中MOCVD法与其他方法相比具有沉积速度快、沉积面积大、制备YBCO外延膜晶体质量高等特点，因而最适合工业化生产。而在光辅助MOCVD法制备YBCO外延膜时，由于光的激化作用，又有本身的特点。其一是以光辅助MOCVD法生长YBCO外延膜的生长速度可以达到0.5μm/min,而传统MOCVD法的生长速度约在0.02-0.2μm/min之间；其二是以光辅助MOCVD法制备的YBCO外延膜具有更高的晶体质量，并且随着膜厚度的增加晶体质量能够保持。本文则围绕光辅助MOCVD法生长YBCO外延膜方面存在的一些问题展开了研究，研究内容主要包括三个方面。首先研究了有机源比例变化对光辅助MOCVD法制备的YBCO外延膜的晶体质量及超导性质的影响。其次研究了光辅助方法退火对YBCO外延膜吸氧及脱氧过程的影响。最后初步研究了在双轴织构Ni带衬底上YBCO外延膜的光辅助MOCVD生长工艺，及衬底材料对光辅助MOCVD法生长YBCO外延膜的影响。
在光辅助MOCVD法制备YBCO外延膜时有机源组份变化影响YBCO外延膜的晶体性质及超导性质研究中。主要讨论了Cu源比例变化对YBCO外延膜晶体及超导性质的影响。发现对于以光辅助MOCVD法制备c轴取向的YBCO外延膜而言，在适当的有机源挥发量比例范围内，适当减少Cu源在挥发混合源中的比例，即(Cu/Y)_(mol)，可有利于抑制外延膜表面颗粒的出现。但是过少的Cu源挥发量比例会给YBCO外延膜表面及内部引入孔洞，使膜质量变差。最终发现在Ba源过量（3<(Ba/Y)_(mol)<5）的情况下(Cu/Y)_(mol)约为2.3时，生长的YBCO外延膜表面平整并且体内整体比较致密。
对样品进行Jc分析发现，随着Cu源比例的变小，YBCO外延膜的Jc值也有变小的趋势。分析其原因，主要因为Cu源比例的变小造成了YBCO外延膜体内的孔洞及其他缺陷变多，非YBCO的杂相增多及微观晶体结构变差，这些因素都会导致YBCO外延膜的Jc降低。总体来看，表面颗粒非常稀少，且膜内部孔洞较少的样品（如A03），其(Cu/Y)_(mol)为2.3，J_c值为2.6MA/cm~2（0T，77K）；而(Cu/Y)_(mol)更小表面没有颗粒但是膜内部孔洞很多的样品A04、A05，J_c值则降至1.8MA/cm~2。在本文讨论的5个样品中，(Cu/Y)_(mol)值最高的样品（如样品A01，为3.55），其Jc值也最高（3.4MA/cm~2），而此样品表面颗粒在种类、数量及尺寸大小等方面也最多、最大。由此可见，如何以外延膜生长参数优化所制备的膜的质量，需视对所生长的膜的需要而定。
在光照退火影响YBCO外延膜脱氧及吸氧特性的研究中。讨论了光照退火及热退火方法对YBCO外延膜脱氧及吸氧速度的影响，以及平衡态氧含量的影响。
首先对YBCO外延膜在460oC和550oC的退火温度下，分别使用光照退火和热退火方法进行吸氧及脱氧退火处理，研究了两种退火方法对YBCO外延膜吸氧及脱氧速度的影响。发现光照对YBCO外延膜的吸氧起到促进的作用，即在光照条件下YBCO外延膜的吸氧速度比无光照条件下的吸氧速度快很多；但是对YBCO外延膜的脱氧起到抑制的作用，即在光照条件下YBCO外延膜的脱氧速度比无光照条件下的脱氧速度慢很多。本论文通过YBCO外延膜吸氧及脱氧过程中光照对氧分子O_2分解成氧原子O的化学反应的影响合理地解释了这两种现象，认为在光照退火时卤钨灯辐射出来的高能光子加速了氧分子O_2分解成氧原子O的化学反应，从而解释了以上实验结果。另外通过比较不同温度及氧分压下退火处理后YBCO外延膜达到平衡状态的氧含量，发现YBCO外延膜在热力学平衡状态时的氧含量只与退火温度T及氧分压P_(O2)等热力学状态函数有关，与是否使用光照退火和吸氧及脱氧速度无关。这也说明光处理电子材料时，光照的作用只会影响反应的速度，而对反应的热力学稳定态不会产生影响。
同时在该部分还讨论了使用XRD测试的方法确定YBCO外延膜氧含量的可行性。这种确定YBCO外延膜氧含量相对变化的方法主要依据是由YBCO粉末材料的XRD-2θ测试和碘滴定法以确定的c轴晶格长度和O含量的线性关系。但是相对于粉末材料，在薄膜材料中，会存在衬底与外延膜的晶格失配、热缺陷、相纯度及界面态等因素，很明显这些因素会影响YBCO外延膜的晶格常数。因此相同氧含量下YBCO外延膜的晶格常数会与YBCO粉末材料的有所差别，所以YBCO外延膜的氧含量与c轴晶格长度的关系与粉末材料所确定的关系会有所差别。在本文的研究中为了排除掉外延膜中晶格失配、热缺陷、相纯度及界面态等因素对c轴晶格长度带来的影响，使用相同生长条件下制备的YBCO外延膜样品进行氧含量的比较。并且由氧含量为6.0及6.9的YBCO外延膜样品所对应的c轴晶格长度确定了实验中所使用的外延膜样品的c轴晶格长度和O含量的线性关系，利用该线性关系计算同类YBCO外延膜样品中氧含量的相对变化。认为该方法虽然不能直接确定YBCO外延膜的绝对氧含量，但是可以较精确地确定YBCO外延膜氧含量的相对变化。
最后在围绕使用光辅助MOCVD法在双轴织构Ni带衬底上生长YBCO外延膜初步研究中，讨论了卤钨灯光照加热方法对Ni带衬底温度的影响。通过比较Ni带衬底与LAO衬底上生长的YBCO外延膜讨论了衬底的热导率对YBCO外延膜生长速度的影响。同时还从热力学角度分析了YBCO外延膜a轴取向及c轴取向生长的原因。
通过对c轴取向YBCO外延膜的生长温度区间的实验发现在Ni带衬底上生长c轴取向YBCO外延膜的最佳设定温度约为770oC，而在LAO衬底上生长c轴取向YBCO外延膜的最佳设定温度在800oC左右。其主要原因有两点，其一是在卤钨灯光照加热衬底时，由于Ni带衬底与LAO衬底的透光特性不同，而造成了衬底与托盘之间的导热特性不同。对于LAO衬底而言，其加热方式为衬底托盘对LAO导热进行加热；而对于Ni衬底而言，则是被卤钨灯直接加热，并且还需衬底托盘与其有良好的接触来进行均匀散热。这一区别造成了相同设定温度下Ni衬底的实际温度远高于LAO衬底的实际温度。其二是因为Ni是一种良好的导体，而LAO是一种介电材料，Ni衬底的导热性能比LAO要高很多。因此分别以两种材料为衬底生长YBCO外延膜时，在相同的设定生长温度下，Ni衬底表面的反应物原子具有更高的活性，因而更容易形成c轴取向生长。
同时发现在Ni带衬底上c轴取向YBCO外延膜的生长速度远大于LAO衬底上的生长速度。通过两种衬底的热导率不同分析了生长速度不同的原因，认为就Ni衬底及LAO衬底而言，由于其热导率不同，因而表面吸附分子的迁移及反应速度也会不同。Ni相对于LAO具有更强的导热能力，因此更容易给表面吸附分子传递能量。所以在Ni衬底表面，被吸附源分子的迁移及反应速度也更快，从而导致在Ni衬底上的YBCO外延膜生长速度更快。
另外本论文从热力学角度分析了YBCO外延膜a轴取向及c轴取向生长的原因，认为YBCO外延膜的c轴取向和a轴取向生长为两种不同的热力学稳定态。a轴取向YBCO外延膜生长所需的生长温度低于c轴生长温度这一现象表示形成a轴取向YBCO外延膜所需要克服的激活能低于c轴取向生长所需要克服的激活能。然而在a轴取向YBCO外延膜的高温退火实验中也发现，长时间的高温退火可以导致a轴取向外延膜向c轴取向转变，但是c轴取向外延膜不会向a轴取向转变，这一现象说明YBCO的c轴取向比a轴取向具有更低的自由能。
High temperature superconductor (HTS) YBCO possesses high critical current density(J_c), high critical transition temperature (Tc), low surface resistance (Rs), and goodsuperconducting properties under magnetic fields. So it is very popular in superconductingapplication. For low current applications, YBCO films are usually grown on apropriatecrystaline wafers. These YBCO epitaxial films can be ultilipd for microwave filters, SQUID,MRI, etc.. For high current applications, YBCO films are usually coated on opportune metalsubstrate. These coated conductors can be used in power transmission, superconductingcurrent limiter, superconducting magnetic energy storage (SMES), etc. Various filmpreparation techniques, such as laser ablation, magnetron sputtering, co-evaporation, andmetalorganic chemical vapor deposition (MOCVD), have been utilized for epitaxial growthof high quality YBCO films for coated conductors. Among them, MOCVD has certainunique advantages over others, such as uniform growth of films over large areas, thecapability to grow high-quality YBCO films with high growth rate and beingsingle-crystal-like. So it is attractive for industrial applications. For preparation of a c-axisYBa2Cu3O6.9film by photo-assisted MOCVD, there are two characteristics because of photoactivation. Firstly, photo-assisted MOCVD often can have much higher growth rate (>500nm/min) than traditional MOCVD technique (<200nm/min); secondly, the c-axis YBCOfilms grown by photo-assisted MOCVD often can have very high crystalline quality. Mythesis work is done with three core issues that in the preparation of YBCO films byphoto-assisted MOCVD. Firstly, morphology and superconducting properties of photo-assisted MOCVD processed YBCO films as variation of precursors’ proportion wasstudied. Then we discussed the effects of photo-assisted and thermal annealing on oxidationand de-oxidation of YBCO films. Finally, the preparation of YBCO film on biaxiallytextured Ni substrates and the effects of thermal conductivity of different substrates, onwhich YBCO films grown by photo-assisted MOCVD were studied.
Firstly, crystalline quality and superconducting properties of photo-assisted MOCVDprocessed YBCO films as variation of sublimation of the Cu-based precursor was studied. Itwas found that by decreasing appropriately the sublimated gas ratio of the Cu-based precursor,as specified by the_(mol)e ratio of (Cu/Y)_(mol), and practically performed by gradually loweringthe sublimation temperature Tsubof the Cu-based precursor, surface growth particles on c-axisoriented YBCO epi-films prepared by photo-assisted MOCVD could then be effectivelysubdued. But the holes and other defects would be happened in YBCO film if the (Cu/Y)_(mol)value was too small. Finally, it is found that with the condition that Ba precursor in excessamount, YBCO film sample A03has it (Cu/Y)_(mol)as2.3, with essentially no outgrowths on itssurface.
From the J_c analysis, it also found that as the (Cu/Y)_(mol)value decreasing, a trend of J_creduce can be found. The main causes of the jc lower trend is possibility of formation ofholes in quantity and size with the film becomes higher with the (Cu/Y)_(mol)value decreasing.The holes will cause lower J_cto the YBCO film obviously. Overall, the sample, preparedunder condition that (Cu/Y)_(mol)=2.3, with some bits of small size Cu-O outgrowths onsurface (sample A03), has a J_c value as2.6MA/cm~2（0T，77K）. Moreover for sampleA04and A05prepared under much lower (Cu/Y)_(mol), there is no outgrowths on top but many holes,have the J_c value of1.8MA/cm~2. For the5YBCO film samples used in this study, sampleA01has the highest J_c, which value is3.4MA/cm~2. However it also has the outgrowths withmost types, maximum quantity, and larger size. Thus, how to optimize the YBCO filmsepitaxial parameters should be guisted by the need of applications.
Then effects of photo-assisted and thermal annealing on oxidation and de-oxidation ofYBCO films were studied. In this study, the effects of photo-assisted annealing and thermal annealing on oxidation and de-oxidation rates and the equilibrium oxygen content of YBCOfilm were discussed.
The YBCO film samples were annealed at460oC and550oC photo-assisted andthermally for oxidation and de-oxidation. Then effects of these two annealing processes onoxidation/de-oxidation rate YBCO were analyzed. It is found that the photo-activation isclearly more effective for oxidation of YBCO films than thermal activation. The oxidationrate of the YBCO film is obviously higher by photo-assisted annealing than thermalannealing. However, the photo-assisted annealing is less effective for de-oxidation of YBCOfilms than thermal annealing. It indicates that the de-oxidation rate of YBCO film isobviously lower by photo-assisted annealing than thermal annealing. These two results canbe explained by the fact that in oxidation and de-oxidation processing of YBCO films, O_2molecules adsorbed on surface can be dissociated into oxygen atoms more easily byphoto-assisted process than thermally. We think that the reaction rate of O_2moleculesdissociating into oxygen atoms can be speed up by high energy photons provided by thehigh power halogen tungsten lamps. From this the experimental results cited above can beexplained. On the other hands, from the comparison study of equilibrium oxygen content ofYBCO film processed at different annealing temperature and oxygen partial pressure, it isfound that the stabilized on equilibrium state oxygen-content x of a c-axis oriented YBCOfilm can be specified by thermodynamic variables such as annealing temperature T, oxygenpartial pressure PO_2, irrespective of whether annealed by photo-assisted or thermalactivation process. This means that in case of photo-assisted processing the photo activationcan have effect on reaction rates, but no effect on thermodynamic equilibrium conditions forthe material.
At the same time the feasibility of the technique to evaluate the oxygen content x in aYBCO film indirectly from values of the c-axis lattice parameter determined by XRD2θ-scan patterns was discussed in this part. Adoption of this indirect method is based on a“linear relationship” between c-axis parameter determined by XRD2θ-scan data andoxygen content of powder samples of YBCO obtained by the standard iodometric titration. However, comparing with the powder samples of YBCO, effects such as stresses andgradients as well as interfacial layers happened in films of YBCO must be concerned.Certainly, these effects will cause differences in c-axis values resulted between powder andfilm samples of YBCO. Thus the c-axis parameter values will be different for the YBCOfilms and powder that have the same oxygen content. So the linear relationship betweenc-axis parameter and oxygen content will also be different for the film sample and powdersample of YBCO. In this part the all small pieces of YBCO film samples used for thiscomparison study (about2×5mm2) were cut from a2" c-axis oriented epitaxial YBCO film.So the effects from stresses and gradients as well as interfacial layers are similar for all ofthose YBCO film samples. Then a similar line was drawn based on the c-axis parameters ofYBCO film samples that with oxygen content of6.0and6.9, just as the “linear relationship”found between c-axis value and oxygen content x of powder YBCO samples as cited earlier.Then based on this line, all the relative changes of oxygen content values at various c-axisparameters of YBCO film samples used in this study can be estimated by the relativechanges of c-axis values. Thus, we think that the relative changes of oxygen content x of afilm sample can be estimated reliably from the changes of the respective c-axis parametersof these YBCO film samples.
Finally, a preliminary research on preparation of YBCO film on biaxially textured Nisubstrates by photo-assisted MOCVD was presented. In this issue, the effects ofphoto-assisted on substrate temperature of biaxially textured Ni were discussed. The effect ofthermal conductivity of substrates on YBCO film growth rate was analyzed from this study.At the same time, the reason of c-axis and a-axis orientation growth of YBCO film wasanalyzed from the point of thermodynamics.
From the experiments, it is found that the growth temperature is about770oC for c-axisYBCO film on biaxially textured Ni substrates, and about800oC for that grown on LAOsubstrates. There are two reasons for this difference between different substrates. The firstreason is that, because the light transmittance is different for Ni substrate and LAO substrate,so the heat transfer behavior between the susceptor and these two substrates is different. For substrates as LAO, the substrate is heated by susceptor as heat conducted from susceptor tosubstrate. However, for substrate as Ni tape, the substrate is heated by tungsten halogen lamp.More over it should keep well contact between substrate and susceptor to dissipate heatenergy equally from the substrate. For this case, the actual temperature of Ni substrate will bemuch higher than that of LAO substrate at the same set temperature. The second reason isthat, because Ni is a good conductor, but LAO is a dielectric material, so the thermalconductivity of Ni substrate is higher than that of LAO substrate. Thus the heat can beconducted more easily from Ni to reactant atoms than from LAO to reactant atoms.
At the same time, it is found the growth rate of YBCO films on Ni substrates is higherthan that on LAO. The reason of this different of growth rate can be got form the differenceof thermal conductivity between these tow substrates. Because the thermal conductivity isdifferent from Ni to LAO, so the mobility and reaction rate of the molecule on surfaces ofsubstrates are different. The heat can be conducted more easily from Ni to reactant atomsthan from LAO to reactant atoms. Thus the mobility and reaction rate of the molecule onsurfaces of Ni substrate are higher than that on LAO substrate. And then the growth rate ofYBCO film on Ni substrate is higher than that on LAO substrate.
On the other hand, the reason of c-axis and a-axis orientation growth of YBCO film wasanalyzed from the point of thermodynamics. We think that, they are two differentthermodynamical equilibrium states for c-axis and a-axis orientated YBCO films. It isknown, the growth temperature of a-axis YBCO film is lower than that of c-axis YBCO film.This means the activation energy for growth of a-axis YBCO film is lower than c-axisYBCO film. We also find from the experiment of annealing a-axis YBCO at highertemperature, the a-axis YBCO film can change to be c-axis at a high anneal temperature forlong time, but the c-axis YBCO film can not change to be a-axis. This means the free energyof the c-axis YBCO film is lower than that of a-axis YBCO film.
在光辅助MOCVD法制备YBCO外延膜时有机源组份变化影响YBCO外延膜的晶体性质及超导性质研究中。主要讨论了Cu源比例变化对YBCO外延膜晶体及超导性质的影响。发现对于以光辅助MOCVD法制备c轴取向的YBCO外延膜而言，在适当的有机源挥发量比例范围内，适当减少Cu源在挥发混合源中的比例，即(Cu/Y)_(mol)，可有利于抑制外延膜表面颗粒的出现。但是过少的Cu源挥发量比例会给YBCO外延膜表面及内部引入孔洞，使膜质量变差。最终发现在Ba源过量（3<(Ba/Y)_(mol)<5）的情况下(Cu/Y)_(mol)约为2.3时，生长的YBCO外延膜表面平整并且体内整体比较致密。
对样品进行Jc分析发现，随着Cu源比例的变小，YBCO外延膜的Jc值也有变小的趋势。分析其原因，主要因为Cu源比例的变小造成了YBCO外延膜体内的孔洞及其他缺陷变多，非YBCO的杂相增多及微观晶体结构变差，这些因素都会导致YBCO外延膜的Jc降低。总体来看，表面颗粒非常稀少，且膜内部孔洞较少的样品（如A03），其(Cu/Y)_(mol)为2.3，J_c值为2.6MA/cm~2（0T，77K）；而(Cu/Y)_(mol)更小表面没有颗粒但是膜内部孔洞很多的样品A04、A05，J_c值则降至1.8MA/cm~2。在本文讨论的5个样品中，(Cu/Y)_(mol)值最高的样品（如样品A01，为3.55），其Jc值也最高（3.4MA/cm~2），而此样品表面颗粒在种类、数量及尺寸大小等方面也最多、最大。由此可见，如何以外延膜生长参数优化所制备的膜的质量，需视对所生长的膜的需要而定。
在光照退火影响YBCO外延膜脱氧及吸氧特性的研究中。讨论了光照退火及热退火方法对YBCO外延膜脱氧及吸氧速度的影响，以及平衡态氧含量的影响。
首先对YBCO外延膜在460oC和550oC的退火温度下，分别使用光照退火和热退火方法进行吸氧及脱氧退火处理，研究了两种退火方法对YBCO外延膜吸氧及脱氧速度的影响。发现光照对YBCO外延膜的吸氧起到促进的作用，即在光照条件下YBCO外延膜的吸氧速度比无光照条件下的吸氧速度快很多；但是对YBCO外延膜的脱氧起到抑制的作用，即在光照条件下YBCO外延膜的脱氧速度比无光照条件下的脱氧速度慢很多。本论文通过YBCO外延膜吸氧及脱氧过程中光照对氧分子O_2分解成氧原子O的化学反应的影响合理地解释了这两种现象，认为在光照退火时卤钨灯辐射出来的高能光子加速了氧分子O_2分解成氧原子O的化学反应，从而解释了以上实验结果。另外通过比较不同温度及氧分压下退火处理后YBCO外延膜达到平衡状态的氧含量，发现YBCO外延膜在热力学平衡状态时的氧含量只与退火温度T及氧分压P_(O2)等热力学状态函数有关，与是否使用光照退火和吸氧及脱氧速度无关。这也说明光处理电子材料时，光照的作用只会影响反应的速度，而对反应的热力学稳定态不会产生影响。
同时在该部分还讨论了使用XRD测试的方法确定YBCO外延膜氧含量的可行性。这种确定YBCO外延膜氧含量相对变化的方法主要依据是由YBCO粉末材料的XRD-2θ测试和碘滴定法以确定的c轴晶格长度和O含量的线性关系。但是相对于粉末材料，在薄膜材料中，会存在衬底与外延膜的晶格失配、热缺陷、相纯度及界面态等因素，很明显这些因素会影响YBCO外延膜的晶格常数。因此相同氧含量下YBCO外延膜的晶格常数会与YBCO粉末材料的有所差别，所以YBCO外延膜的氧含量与c轴晶格长度的关系与粉末材料所确定的关系会有所差别。在本文的研究中为了排除掉外延膜中晶格失配、热缺陷、相纯度及界面态等因素对c轴晶格长度带来的影响，使用相同生长条件下制备的YBCO外延膜样品进行氧含量的比较。并且由氧含量为6.0及6.9的YBCO外延膜样品所对应的c轴晶格长度确定了实验中所使用的外延膜样品的c轴晶格长度和O含量的线性关系，利用该线性关系计算同类YBCO外延膜样品中氧含量的相对变化。认为该方法虽然不能直接确定YBCO外延膜的绝对氧含量，但是可以较精确地确定YBCO外延膜氧含量的相对变化。
最后在围绕使用光辅助MOCVD法在双轴织构Ni带衬底上生长YBCO外延膜初步研究中，讨论了卤钨灯光照加热方法对Ni带衬底温度的影响。通过比较Ni带衬底与LAO衬底上生长的YBCO外延膜讨论了衬底的热导率对YBCO外延膜生长速度的影响。同时还从热力学角度分析了YBCO外延膜a轴取向及c轴取向生长的原因。
通过对c轴取向YBCO外延膜的生长温度区间的实验发现在Ni带衬底上生长c轴取向YBCO外延膜的最佳设定温度约为770oC，而在LAO衬底上生长c轴取向YBCO外延膜的最佳设定温度在800oC左右。其主要原因有两点，其一是在卤钨灯光照加热衬底时，由于Ni带衬底与LAO衬底的透光特性不同，而造成了衬底与托盘之间的导热特性不同。对于LAO衬底而言，其加热方式为衬底托盘对LAO导热进行加热；而对于Ni衬底而言，则是被卤钨灯直接加热，并且还需衬底托盘与其有良好的接触来进行均匀散热。这一区别造成了相同设定温度下Ni衬底的实际温度远高于LAO衬底的实际温度。其二是因为Ni是一种良好的导体，而LAO是一种介电材料，Ni衬底的导热性能比LAO要高很多。因此分别以两种材料为衬底生长YBCO外延膜时，在相同的设定生长温度下，Ni衬底表面的反应物原子具有更高的活性，因而更容易形成c轴取向生长。
同时发现在Ni带衬底上c轴取向YBCO外延膜的生长速度远大于LAO衬底上的生长速度。通过两种衬底的热导率不同分析了生长速度不同的原因，认为就Ni衬底及LAO衬底而言，由于其热导率不同，因而表面吸附分子的迁移及反应速度也会不同。Ni相对于LAO具有更强的导热能力，因此更容易给表面吸附分子传递能量。所以在Ni衬底表面，被吸附源分子的迁移及反应速度也更快，从而导致在Ni衬底上的YBCO外延膜生长速度更快。
另外本论文从热力学角度分析了YBCO外延膜a轴取向及c轴取向生长的原因，认为YBCO外延膜的c轴取向和a轴取向生长为两种不同的热力学稳定态。a轴取向YBCO外延膜生长所需的生长温度低于c轴生长温度这一现象表示形成a轴取向YBCO外延膜所需要克服的激活能低于c轴取向生长所需要克服的激活能。然而在a轴取向YBCO外延膜的高温退火实验中也发现，长时间的高温退火可以导致a轴取向外延膜向c轴取向转变，但是c轴取向外延膜不会向a轴取向转变，这一现象说明YBCO的c轴取向比a轴取向具有更低的自由能。
High temperature superconductor (HTS) YBCO possesses high critical current density(J_c), high critical transition temperature (Tc), low surface resistance (Rs), and goodsuperconducting properties under magnetic fields. So it is very popular in superconductingapplication. For low current applications, YBCO films are usually grown on apropriatecrystaline wafers. These YBCO epitaxial films can be ultilipd for microwave filters, SQUID,MRI, etc.. For high current applications, YBCO films are usually coated on opportune metalsubstrate. These coated conductors can be used in power transmission, superconductingcurrent limiter, superconducting magnetic energy storage (SMES), etc. Various filmpreparation techniques, such as laser ablation, magnetron sputtering, co-evaporation, andmetalorganic chemical vapor deposition (MOCVD), have been utilized for epitaxial growthof high quality YBCO films for coated conductors. Among them, MOCVD has certainunique advantages over others, such as uniform growth of films over large areas, thecapability to grow high-quality YBCO films with high growth rate and beingsingle-crystal-like. So it is attractive for industrial applications. For preparation of a c-axisYBa2Cu3O6.9film by photo-assisted MOCVD, there are two characteristics because of photoactivation. Firstly, photo-assisted MOCVD often can have much higher growth rate (>500nm/min) than traditional MOCVD technique (<200nm/min); secondly, the c-axis YBCOfilms grown by photo-assisted MOCVD often can have very high crystalline quality. Mythesis work is done with three core issues that in the preparation of YBCO films byphoto-assisted MOCVD. Firstly, morphology and superconducting properties of photo-assisted MOCVD processed YBCO films as variation of precursors’ proportion wasstudied. Then we discussed the effects of photo-assisted and thermal annealing on oxidationand de-oxidation of YBCO films. Finally, the preparation of YBCO film on biaxiallytextured Ni substrates and the effects of thermal conductivity of different substrates, onwhich YBCO films grown by photo-assisted MOCVD were studied.
Firstly, crystalline quality and superconducting properties of photo-assisted MOCVDprocessed YBCO films as variation of sublimation of the Cu-based precursor was studied. Itwas found that by decreasing appropriately the sublimated gas ratio of the Cu-based precursor,as specified by the_(mol)e ratio of (Cu/Y)_(mol), and practically performed by gradually loweringthe sublimation temperature Tsubof the Cu-based precursor, surface growth particles on c-axisoriented YBCO epi-films prepared by photo-assisted MOCVD could then be effectivelysubdued. But the holes and other defects would be happened in YBCO film if the (Cu/Y)_(mol)value was too small. Finally, it is found that with the condition that Ba precursor in excessamount, YBCO film sample A03has it (Cu/Y)_(mol)as2.3, with essentially no outgrowths on itssurface.
From the J_c analysis, it also found that as the (Cu/Y)_(mol)value decreasing, a trend of J_creduce can be found. The main causes of the jc lower trend is possibility of formation ofholes in quantity and size with the film becomes higher with the (Cu/Y)_(mol)value decreasing.The holes will cause lower J_cto the YBCO film obviously. Overall, the sample, preparedunder condition that (Cu/Y)_(mol)=2.3, with some bits of small size Cu-O outgrowths onsurface (sample A03), has a J_c value as2.6MA/cm~2（0T，77K）. Moreover for sampleA04and A05prepared under much lower (Cu/Y)_(mol), there is no outgrowths on top but many holes,have the J_c value of1.8MA/cm~2. For the5YBCO film samples used in this study, sampleA01has the highest J_c, which value is3.4MA/cm~2. However it also has the outgrowths withmost types, maximum quantity, and larger size. Thus, how to optimize the YBCO filmsepitaxial parameters should be guisted by the need of applications.
Then effects of photo-assisted and thermal annealing on oxidation and de-oxidation ofYBCO films were studied. In this study, the effects of photo-assisted annealing and thermal annealing on oxidation and de-oxidation rates and the equilibrium oxygen content of YBCOfilm were discussed.
The YBCO film samples were annealed at460oC and550oC photo-assisted andthermally for oxidation and de-oxidation. Then effects of these two annealing processes onoxidation/de-oxidation rate YBCO were analyzed. It is found that the photo-activation isclearly more effective for oxidation of YBCO films than thermal activation. The oxidationrate of the YBCO film is obviously higher by photo-assisted annealing than thermalannealing. However, the photo-assisted annealing is less effective for de-oxidation of YBCOfilms than thermal annealing. It indicates that the de-oxidation rate of YBCO film isobviously lower by photo-assisted annealing than thermal annealing. These two results canbe explained by the fact that in oxidation and de-oxidation processing of YBCO films, O_2molecules adsorbed on surface can be dissociated into oxygen atoms more easily byphoto-assisted process than thermally. We think that the reaction rate of O_2moleculesdissociating into oxygen atoms can be speed up by high energy photons provided by thehigh power halogen tungsten lamps. From this the experimental results cited above can beexplained. On the other hands, from the comparison study of equilibrium oxygen content ofYBCO film processed at different annealing temperature and oxygen partial pressure, it isfound that the stabilized on equilibrium state oxygen-content x of a c-axis oriented YBCOfilm can be specified by thermodynamic variables such as annealing temperature T, oxygenpartial pressure PO_2, irrespective of whether annealed by photo-assisted or thermalactivation process. This means that in case of photo-assisted processing the photo activationcan have effect on reaction rates, but no effect on thermodynamic equilibrium conditions forthe material.
At the same time the feasibility of the technique to evaluate the oxygen content x in aYBCO film indirectly from values of the c-axis lattice parameter determined by XRD2θ-scan patterns was discussed in this part. Adoption of this indirect method is based on a“linear relationship” between c-axis parameter determined by XRD2θ-scan data andoxygen content of powder samples of YBCO obtained by the standard iodometric titration. However, comparing with the powder samples of YBCO, effects such as stresses andgradients as well as interfacial layers happened in films of YBCO must be concerned.Certainly, these effects will cause differences in c-axis values resulted between powder andfilm samples of YBCO. Thus the c-axis parameter values will be different for the YBCOfilms and powder that have the same oxygen content. So the linear relationship betweenc-axis parameter and oxygen content will also be different for the film sample and powdersample of YBCO. In this part the all small pieces of YBCO film samples used for thiscomparison study (about2×5mm2) were cut from a2" c-axis oriented epitaxial YBCO film.So the effects from stresses and gradients as well as interfacial layers are similar for all ofthose YBCO film samples. Then a similar line was drawn based on the c-axis parameters ofYBCO film samples that with oxygen content of6.0and6.9, just as the “linear relationship”found between c-axis value and oxygen content x of powder YBCO samples as cited earlier.Then based on this line, all the relative changes of oxygen content values at various c-axisparameters of YBCO film samples used in this study can be estimated by the relativechanges of c-axis values. Thus, we think that the relative changes of oxygen content x of afilm sample can be estimated reliably from the changes of the respective c-axis parametersof these YBCO film samples.
Finally, a preliminary research on preparation of YBCO film on biaxially textured Nisubstrates by photo-assisted MOCVD was presented. In this issue, the effects ofphoto-assisted on substrate temperature of biaxially textured Ni were discussed. The effect ofthermal conductivity of substrates on YBCO film growth rate was analyzed from this study.At the same time, the reason of c-axis and a-axis orientation growth of YBCO film wasanalyzed from the point of thermodynamics.
From the experiments, it is found that the growth temperature is about770oC for c-axisYBCO film on biaxially textured Ni substrates, and about800oC for that grown on LAOsubstrates. There are two reasons for this difference between different substrates. The firstreason is that, because the light transmittance is different for Ni substrate and LAO substrate,so the heat transfer behavior between the susceptor and these two substrates is different. For substrates as LAO, the substrate is heated by susceptor as heat conducted from susceptor tosubstrate. However, for substrate as Ni tape, the substrate is heated by tungsten halogen lamp.More over it should keep well contact between substrate and susceptor to dissipate heatenergy equally from the substrate. For this case, the actual temperature of Ni substrate will bemuch higher than that of LAO substrate at the same set temperature. The second reason isthat, because Ni is a good conductor, but LAO is a dielectric material, so the thermalconductivity of Ni substrate is higher than that of LAO substrate. Thus the heat can beconducted more easily from Ni to reactant atoms than from LAO to reactant atoms.
At the same time, it is found the growth rate of YBCO films on Ni substrates is higherthan that on LAO. The reason of this different of growth rate can be got form the differenceof thermal conductivity between these tow substrates. Because the thermal conductivity isdifferent from Ni to LAO, so the mobility and reaction rate of the molecule on surfaces ofsubstrates are different. The heat can be conducted more easily from Ni to reactant atomsthan from LAO to reactant atoms. Thus the mobility and reaction rate of the molecule onsurfaces of Ni substrate are higher than that on LAO substrate. And then the growth rate ofYBCO film on Ni substrate is higher than that on LAO substrate.
On the other hand, the reason of c-axis and a-axis orientation growth of YBCO film wasanalyzed from the point of thermodynamics. We think that, they are two differentthermodynamical equilibrium states for c-axis and a-axis orientated YBCO films. It isknown, the growth temperature of a-axis YBCO film is lower than that of c-axis YBCO film.This means the activation energy for growth of a-axis YBCO film is lower than c-axisYBCO film. We also find from the experiment of annealing a-axis YBCO at highertemperature, the a-axis YBCO film can change to be c-axis at a high anneal temperature forlong time, but the c-axis YBCO film can not change to be a-axis. This means the free energyof the c-axis YBCO film is lower than that of a-axis YBCO film.
引文
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