Advance May Help
the Semiconductor Industry
micrograph shows a mock field-effect transistor with a layer of
crystalline strontium titanate instead of silicon dioxide as the
gate electrode. The layer was grown in registry with the silicon
template making up the transistor's base. ORNL tests show that the
A barrier to future increases in computing power is a
restriction set by a compound of silicon itself. As transistors are
downsized, the use of silicon dioxide to control
electron flow will limit transistor performance.
After 10 years of research, Rodney McKee of ORNL's Metals
and Ceramics (M&C) Division, Fred Walker of the University of Tennessee,
and Matt Chisholm of the Solid State Division, have found a solution.
They demonstrated that amorphous silicon dioxide conventionally used
on silicon chips can be replaced with a crystalline oxide whose superior
electrical properties will allow reduction in transistor size without
loss of performance. In March 1999 the team built a field-effect transistor
(FET) using crystalline strontium titanate and demonstrated that it
performs as well as conventional transistors.
A FET is a common switching device used in modern electronic
equipment. This tiny semiconducting device consists of three metal electrodes
and a silicon base. When a conventional FET is turned on, electrons
injected by a source electrode flow as a current through the silicon
base for collection at a drain electrode. To turn the transistor off,
a gate electrode between the other electrodes applies an electrical
voltage to a dielectric film, causing it to "pinch off" the
current by raising the silicon base's resistance. In this way, a transistor
can function as an on-and-off switch. It can also store bits of information
(a "1" if switched on, a "0" if switched off).
As transistor size is reduced, the silicon dioxide layer
needed for the dielectric film will eventually become so thin (<3 nm)
that it will be useless. The reason: it will leak electrons through
quantum mechanical tunneling.
For decades, crystalline oxides have been considered
a promising solution to this problem for the semiconductor industry.
These materials have the physical thickness to support an electric field
yet are able to store electrical charges more effectively. But no one
had been able to produce the high-quality layer needed to support shrinking
semiconductors. Using molecular beam epitaxy, a precisely controlled
process for growing thin films under ultrahigh vacuum, and a $400 video
camera to tape the deposition of oxide films on silicon, McKee and Walker
learned which conditions allow film crystals to grow in registry with
the silicon crystal template beneath, producing a perfect film.
Because strontium titanate exerts a stronger influence
on the transistor's conductivity than silicon dioxide, the gate electrode
can take up less space, compressing the area between the source and
drain electrodes and shortening the distance the electrons would travel.
The benefits? Transistors with strontium titanate are likely to be smaller