REACTIVE COEVAPORATION OF DYBACUO SUPERCONDUCTING FILMS - THE SEGREGATION OF BULK IMPURITIES ON ANNEALED MGO(100) SUBSTRATES

This article reports results from a study on the effect bulk impurities in MgO substrates may have when substrates are annealed to improve film microstructure and transport properties. DyBaCuO thin films are prepared by molecular beam reactive coevaporation on both annealed and unannealed MgO(100). While predeposition annealing has led to improved film quality for some groups, under our conditions just the opposite occurs. We point out the existence of Ca segregation to the MgO surface as a factor that needs investigation because Ca is a common contaminant in commercially supplied MgO. Atomic force microscope images show decoration of annealed substrates with submicron size particles that may be composed of the segregated Ca. These particles seem to underlie the distinct differences seen in the surface microstructure of films on annealed and unannealed substrates. In the case of annealed substrates a more random microstructure is promoted. Under identical growth conditions, outgrowths present in films on unannealed (mechanically polished) substrates are oriented in an ordered pattern. These findings suggest that the expected benefits from predeposition annealing can be negated when significant surface segregation of bulk contaminants occurs.