NOVEL SOLID-PHASE EPITAXIAL-GROWTH OF YBA2CU3O7-DELTA FILMS FROM PRECURSOR OXIDES

The pulsed laser deposition technique has been used to study the process of layering and intermixing of precursor oxide phases of YBa2Cu3O7-delta (YBCO) during deposition from separate component targets. The films are grown on (100)-oriented SrTiO3 substrates by ablation from the component targets YBa2Ox/CuO or Y2Cu2O5/BaCuO2 under process conditions similar to those needed for the growth of YBCO films from a composite target. For precursor oxide layers below a critical thickness, which can be as large as 1000 angstrom, facile formation of high-quality epitaxial YBCO films is observed resulting from interdiffusion and reaction of the constituent phases. On the other hand, while some degree of intermixing occurs above the critical thickness, there is no evidence of formation of the YBCO phase. The novel solid-phase epitaxy observed below the critical thickness is believed to result from diffusion of species through grain boundaries of the initial precursor oxide layer, followed by reaction at the substrate interface to epitaxially nucleate the YBCO layers. The process continues until all of the initial oxide layer is consumed and can be repeated with deposition of subsequent alternate layers. The dynamics of the diffusion process has been investigated using in situ resistance measurements to monitor the growth of the YBCO phase. The results show that the time scale for interdiffusion and reaction is very rapid (approximately 10-100 ms), and occurs soon after arrival of the ablated species from the second component target on the surface of the initial precursor oxide layer. These results have important implications for the choice of component oxide blocks for layer-by-layer growth of artificially structured films.