Nucleation Study of ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta }$ (YBCO) Films by Modified TFA-MOD Approach

In this paper we studied the nucleation process of YBCO films on lattice matched substrates through modified TFA-MOD approach. An experiment designed to investigate gas flow speed on different locations of the same sample shows that nucleation does not occur on regions with HF built up. Extending conversion time shows little or no effect in these regions in terms of nucleation. Thus, a criterion has been established for the continuous nucleation and growth with respect to full conversion and gas flow speed. On the other hand, interfacial nucleation morphology study shows that temperature has huge impact on the nucleation mode. A transition from dendrite (at low temperature) to cell (medium temperature) and to super-small cell/layered (high temperature) has been observed for YBCO films derived from modified TFA-MOD approach. By controlling the nucleation mode, high Jcs above 5 MA/cm² can be routinely grown for 0.3 mum to 0.65 mum YBCO films. Experimental results reveal that the method of thinning YBCO film down to interfacial level by chemical solution developed in this research is a very useful tool for nucleation and growth study.