Smooth step to superconducting devices

PHILIP BALL

Thin films of nobium alloys like this could be replaced by slivers of magnesium diboride. © Applied Superconductivity Center

A new superconductor might begin to make good on its promise thanks to a technique developed in the United States for fashioning it into smooth, ultra-thin films.¹

Magnesium diboride (MgB₂) astonished researchers last year when it was found to conduct with no electrical resistance at - 234 oC, several degrees higher than similar superconductors.² But turning this discovery to practical use has been hampered by the difficulty of making the thin films of MgB₂ needed in superconducting electronic circuits.

Now Xiaoxing Xi of Pennsylvania State University and co-workers have found a cheap and simple method for making high-quality films of MgB₂.

Improvements to the method will be needed, says superconductor technology specialist John Rowell of Northwestern University in Illinois, but it should ultimately make MgB₂ electronics feasible.

In principle, integrated circuits made from superconductors could operate faster than those based on semiconductors such as silicon, leading to quicker information technologies. Already, superconducting quantum interference devices (SQUIDs) are used to detect very small magnetic fields, for example, in magnetic resonance imaging (MRI).

Unfortunately, these circuits and devices can only operate at extremely low temperatures. Typically, SQUIDs made from niobium and its alloys must be cooled with liquid helium to within about 4 degrees of absolute zero (- 269 oC). Superconducting properties are lost at higher temperatures.

So researchers are always anxious to find materials that can sustain superconductivity at higher temperatures. Ceramic materials called high-Tc copper oxides superconduct at temperatures several dozen degrees higher than MgB₂, but MgB2 is cheaper.

Film studies

Two methods for making thin films of MgB₂ have already been explored. One, heating films of boron in the presence of magnesium vapour, gives films with good superconducting properties but rough surfaces. Devices such as SQUIDs generally require several films laid on top of one another, so this roughness is a severe handicap.

The other method - depositing magnesium and boron vapours simultaneously - gives smooth but rather imperfect films with lowered superconducting temperature limits.

Xi's team uses a heating coil to evaporate lumps of magnesium at around 700 oC. The magnesium vapour then combines with diborane, a gaseous compound of boron and hydrogen, in a high-pressure atmosphere of hydrogen gas. Thin films of MgB₂ grow on plates of a hard material such as sapphire or silicon carbide.

The hydrogen is the key: it stops magnesium oxide contaminating the films and hindering their superconductivity.

1. Zeng, X. et al. In situ epitaxial MgB₂ thin films for superconducting electronics. Nature Materials, 1, 35 - 38, (2002). |Article|
2. Nagamatsu, J. et al. Superconductivity at 39 K in magnesium diboride. Nature, 410, 63 - 64 , (2001). |Article|

© Nature News Service / Macmillan Magazines Ltd 2002