Resistivity Under Pressure: Copper-Doped Germanium

September 4, 1998

By Paul Preuss, [email protected]

Germanium, doped with copper and cooled to the temperature of liquid helium, is ordinarily a good insulator. But it becomes an astonishingly good conductor of electricity when squeezed along one axis of its crystalline structure. The surprising discovery was made by Oscar Dubon, Eugene Haller and Wladyslaw Walukiewicz of the Material Sciences Division.

Eugene Haller and Wladyslaw Walukiewicz found that by compressing copper-doped germanium at low temperature, resistance dropped a trillion- fold,  permitting new studies of semiconductor transitions.   Photo by Roy Kaltschmidt

"A little pressure induces a change of over a dozen orders of magnitude--one trillion fold!--in conductivity," says Haller, pointing to a plot that displays the resistivity of a copper-doped germanium sample abruptly dropping as pressure increases.

The increase in conductivity is so rapid and so large that the effect may be useful in designing instruments to detect extremely small changes of temperature or pressure. Even more important, the phenomenon offers condensed-matter researchers a new way to perform detailed studies of the insulator-metal transition in semiconductors.

What makes semiconductors so useful in the electronics industry and elsewhere is that their conductivity can be changed from insulating to conducting behavior by adding minute quantities of certain impurities, a process called "doping." In the well-ordered crystalline structure of an undoped semiconductor, the overlapping orbitals of the electrons form distinct energy bands. The lower-energy valence bands are full of electrons--so full that the electrons cannot move--while the higher-energy conduction bands, where electrons can move, are empty. If there are no extra charge carriers in these bands, the crystal is an insulator.

In a semiconductor like germanium, the bands lie relatively close together on the energy scale, and even a little heat can give valence-band electrons enough energy to leap to the conduction band, which makes the crystal a conductor. The effect can be enhanced or modified by doping. If some of the pure crystal's atoms are replaced by others, such as phosphorus atoms, electrons are added to the conducting band. Doping with boron atoms leaves positively charged "holes" in the valence band, which also allows charge to flow.

At sufficiently high temperatures, doping germanium with copper can contribute holes to germanium's valence band. But at low concentrations, the average separation of copper atoms is much larger than the size of the outermost copper orbitals, and the charge can move only by "hopping" between adjacent copper sites. Since this is an unlikely process, the germanium crystal still acts as an insulator.

"We decided to take germanium that was doped with copper, but was still a good insulator, and apply uniaxial stress," Walukiewicz explains. It was a simple matter to apply over four kilobars of pressure [over 4,000 times atmospheric pressure at sea level] to a one-by-one-by-five millimeter chip of copper-doped germanium by squeezing it in a vise.

What Dubon, Walukiewicz and Haller found was that pressure applied along one direction changed the electron orbitals around the copper atoms, effectively enlarging these orbitals enough so that even at low concentrations they overlapped and permitted conduction.

The overlapping orbitals of the dopant atoms themselves form their own separate band, with properties that depend on charge and electron spin. Solid-state researchers have been intrigued by these doping-induced bands since they were first proposed by theorist John Hubbard almost 40 years ago.

In most semiconductors, however, the higher-energy Hubbard component merges with the crystal's valence band near the insulator-metal transition point, making Hubbard bands exceedingly difficult to study. Uniaxially stressed copper-doped germanium is free of these problems and represents a pristine case of isolated Hubbard bands.

"You can say we have a new, smaller-gap semiconductor within the germanium semiconductor," says Walukiewicz. "The placement of the copper is random, but its electronic structure is a completely delocalized extended state."

Walukiewicz sees opportunities for many areas of research, such as studying random systems "using germanium as a matrix to keep copper in place: random but well defined. Random systems are much more difficult to study than crystals, but also much more common."

Haller agrees that the possibilities are numerous. "We have seen this huge physical effect, an increase in conductivity of many orders of magnitude achieved simply by applying pressure. And it is intrinsically interesting to be able to study the Hubbard band in a new state, clearly separated from the valence band of germanium. We can do that for the first time."

Photo: Eugene Haller and Wladyslaw Walukiewicz found that by compressing copper-doped germanium at low temperature, resistance dropped a trillion-fold, permitting new studies of semiconductor transitions. Photo by Roy Kaltschmidt (XBD9807-01754-01.tif)


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