Growth Of YBCO Thin/Thick Films By Photo-assisted Metel-organic Chemical Vapor Deposition (MOCVD) And Characteristic Studies Of These YBCO Films

Many HTS superconductors are being engineered for use in high-current applications. In these HTS superconductors, because of its excellent electrical property high temperature superconductor (high-Tc) YBa2Cu3O7-x (YBCO) thin films have significant potential for applications in microwave devices, so more and more people attach importance to it. For these reason we plan to applied high quality YBCO thin films on larger area LAO(100) substrates by photo-assisted MOCVD. We have designed and assembled two photo-assisted MOCVD systems, one of them has a quartz reactor, the other one has a reactor made of stainless steel. In this study a systemic study of the characteristic of YBCO films prepared by the first photo-assisted MOCVD systems whose reactor was made of quartz was described.1. Design of the photo-assisted MOCVD systemsTwo photo-assisted MOCVD systems were designed and assembled. The precursors(Y(tmhd),Ba(tmhd) and Cu(tmhd) ) used in our MOCVD system are solid state under room temperature, moreover they can not boil away easily under more higher temperature. So in our MOCVD systems the pipelines should be more shorter and can not have many turnings. The system is made up of eight parts, they are gas purifying plants, gas transport plants, precursor -supply system, gas mixing plants, lamp house, the reactor, 3 2 2 temperature control device and tail gas treatment. As yet these two photo-assisted MOCVD systems are running well. After the MOCVD system was fit in we optimized the MOCVD system with a quartz reactor.2. Enactment of the gas flows, substrate temperature and the chamber pressureWe find the optimal parameters for YBCO thin films prepared by the photo-assisted MOCVD system: The substrate temperature is between 825℃and 855℃, the carrier gas flows of the Ba(tmhd)2,Y(tmhd)3,Cu(tmhd)2 are 300-450 sccm,100-150 sccm and 100-150 sccm, respectively;the O2 and N2O flow are 300-450 sccm and 200-300 sccm, respectively;the operating pressure is between 6 torr and 8 torr.3. Hot wall and cold wallWe compared the characteristic of the YBCO films that prepared by hot wall and cold wall MOCVD, and find that the purely c-axis YBCO can prepared by cold wall MOCVD more easily.4. The predominance of processing electronic materials by photoWe analyzed the predominance of processing electronic materials by photo relative to that by heat. We know that chemical or physical reaction thermal dissociation is fundamentally limited by Boltzmann distribution or thermal activation, but light in the UV, visible, as well as IR region can supply the necessary thermal energy to the sample. Various film preparation techniques, such as laser ablation, magnetron sputtering, and metal-organic chemical vapor deposition (MOCVD), have been utilized for epitaxial growth of high quality YBCO films. Among them, photo-assisted MOCVD technique has its two obvious advantages, the capability to grow both thin and thick high-quality YBCO films, one is YBCO films can be prepared with very high growth rate(above 200nm/min) another is YBCO films grown by photo-assisted MOCVD have single-crystal-like qualities, no SEM visualizable grain boundaries. So this technique has the capability to grow both thin and thick high-quality YBCO films with high growth rate. All of these advantages are all because of the photo assisting. In the photo-assisted MOCVD system, if the temperature of the substrate is settled, when the height of the substrate is changed the power of the lamps will change. And the color temperature of the filament will change according to the power. We know that in black-body radiation when the temperature of the blackbody is increased the energy eradiated will increase, so the wavelengh of the light that eradiated from blackbody is shorter. So in the experimental we set the power of the lemps beyound 85% when YBCO growing, here the color temperature of the filament is higher than 3000K the light that eradiated from filament include UV, visible, as well as IR region.5. Chamber pressure and characteristic of the YBCO filmsWe analyzed how the chamber pressure effect on the characteristic of the YBCO films. The crystal structures of YBCO films grown under different chamber pressure were analyzed by the assist of SEM and XRD-2θscan. In our system if growth temperature is fixed, as the chamber pressure increasing the growth of pure c-axis YBCO films will change to growth of a-axis YBCO films, and the growth rate will increase at the begining and then decrease. From these experiences we find the optimization of the chamber pressure for pure c-axis YBCO grown.6. Simulation for gas flow patterns and temperatureA three-dimensional simulation for gas flow patterns was conducted with commercial software FLUENT 6.0 for the first time. In the simulation, growth temperature, chamber pressure and growth time were all fixed; the only process variable was the susceptor inclination angle. The temperature of the substrate change tinily when this angle is changed. But as the angle changes, the gas flow pattern in the reactor chamber is changed accordingly. This change of gas flow pattern affect the growth mechanism of the growing YBCO films. And the experimental results were found to be well correlated with the simulations. We find that the simulation is very useful for the experimental. The experimental and the simulation show all show that when the angle between the axis of gas inlet and the susceptor surface is 22.5o the YBCO films have best characteristic.7. The barium precursor's sublimation/evaporation can not be well controlled and the solutionFrom the experimental we find that the barium precursor's sublimation/evaporation can not be well controlled. We solved this problem by mending the structure of the precursors'ovens.8. The thickness of the YBCO films can be controledBy evaporating the barium precursor employed with its heating temperature being appropriately programmed during the time interval of film growth the steady time of the barium precursor's evaporating can last 15 minutes. By this method the thickness of the YBCO films can be well controlled under 4.5μm. And the characteristic of the YBCO thick films is as well as that of the thin films. The XRD-ωscan of two YBCO films (0.6μm and 4.5μm) with different thickness were analyzed, the FWHM are 0.29o and 0.30o.