1-2-3 contact: a new superconducting film.

1-2-3 contact: A new superconducting film

Using high-temperature superconductors for magnetically levitated trains and loss-free transmission of electricity remain dreamy prospects. But researchers have had success in depositing working thin films of the poorly understood superconductors onto silicon and other materials. New microwave communications technologies and faster computers made with denser, speedier chips may emerge as the first practical payoffs of the new superconductors, they say.

In new studies that complement and build on earlier ones, materials scientist Jagdish Narayan and his graduate students at North Carolina State University in Raleigh have developed a relatively low-temperature technique for depositing thin films of the now famous "1-2-3" superconductor -- composed of one part yttrium (as counted by atoms), two parts barium, three parts copper and roughly seven parts oxygen -- onto several materials including silicon, the basic substrate for most electronic devices.

The Raleigh researchers first vaporize small portions of a 1-2-3 pellet with laser pulses. An electrically charged ring between the pellet and the heated substrate, such as silicon or strontium titanate, and homogenizes their speeds to ensure a smooth layering, Narayan says. A nozzle near the substrate adds oxygen, resulting in a 1-2-3 superconducting film thoroughly aligned with the underlying substrate's crystal structure. At the highest processing temperature his group studied, 650[deg.]C, "these films are defect free," Narayan told SCIENCE NEWS. At the lowest temperature, 500[deg.]C, the films and alignments are less pristine, he notes.

In a paper scheduled to appear in the June 5 APPLIED PHYSICS LETTERS, the scientists claim their crystalline films are near-perfect, stable and capable of carrying record-high currents, although other researchers point out that no standards have been established for measuring the currents and temperatures. Narayan's group also reports depositing its films at temperatures roughly 100[deg.]C cooler than previously reported by his group and others. Lower processing temperatures minimize chemical reactions and physical movements that can disrupt the substrate/film interface. But since lower temperatures also reduce the films' crystalline quality, choosing processing conditions is a delicate balancing act.

Getting the 1-2-3 films onto silicon has proved difficult because the two materials' crystal structures do not exactly match and they expand and contract at different rates as the temperature changes. After such hybrid structures are cycled through temperature ranges, "the films begin to look like the bottom of a riverbed," remarks physicist Venky Venkatesan of Bell Communications Research (Bellcore) in Red Bank, N.J. Moreover, silicon cannot withstand the higher processing temperatures typically used for making bulk 1-2-3 superconductors.

To overcome these incompatibilities, researchers can use expensive strontium titanate as a sort of material diplomat. Even at lower processing temperatures, strontium titanate's crystal structure can accommodate silicon on one side and the 1-2-3 superconducting films on the other to form a less stressed and more stable three-layer structure. Still, for commercial applications, Narayan and others continue to seek cheaper processes that do not require the extra layer.

In the past months, researchers at Bellcore, Hitachi and elsewhere have reported progress. High-quality 1-2-3 superconducting films are nearing practical applications in hybrid superconductor-semiconductor chips and in military and consumer communications devices that operate at extremely high frequencies in the microwave region of the electromagnetic spectrum, Venkatesan says.