Diamond electronics on deck

By Kimberly Patch, Technology Research News

Diamonds naturally form in the thickest part of the earth's crust, 75 or more miles deep, where more than half a million pounds of pressure per square inch and temperatures topping 900 degrees Celsius push carbon atoms into a compact crystalline structure. Diamonds ride to the surface in streams of molten lava when volcanoes erupt.

Only a material with unusual properties could survive such an intense process.

Researchers have known for a long time that pure diamond is potentially superior to silicon in several ways that would make for better electronic components. In theory, diamond components would be able to withstand extreme conditions like heat, radiation and high voltage, and could boost the bandwidth of wireless communications channels.

But it has proved difficult to make sufficiently pure diamond. Like natural diamond, the manufactured variety tends to contain impurities and defects that hinder their ability to conduct electricity.

Semiconductor materials used to make components that precisely guide currents of electrons must be extremely pure. The silicon wafers used for most of today's electronics are the purest material manufactured in large quantities, with fewer than one impurity or defect per trillion atoms.

Researchers from ASEA Brown Boveri (ABB) in Sweden and DeBeers Industrial Diamonds in England have brought diamond-based electronics closer to realization by showing that it is possible to manufacture diamond pure enough to reliably conduct electricity.

Two distinct processes are used to manufacture diamond for industrial use. High-pressure, high-temperature (HPHT) techniques have been used for more than 30 years to produce tiny diamonds for applications like coating the tips and edges of cutting tools. For the past decade researchers have been working to make more pure diamond using chemical vapor deposition (CVD), which induces the carbon in a carbon-containing gas to accumulate on a surface.

The ABB-DeBeers researchers' achieved success in chemical vapor deposition "by very precise control of synthesis conditions, and ensuring that deposition occurs under conditions of high purity," said Steven Coe, research manager at DeBeers. "This is a case where a whole series of incremental improvements have collectively enabled" a step forward, he said.

The manufactured diamond's charge carrier mobility, a measure of how easily negatively charged electrons or positively-charged holes vacated by electrons can move through a material, is actually higher than theoretical predictions, said Jan Isberg, an ABB researcher who is now a researcher at Uppsala University in Sweden. "We have... shown that the electronic quality of the diamond can be improved to reach higher carrier mobilities than in any previously-made diamond," said Isberg.

The researchers' diamond showed room temperature electron carrier mobility as high as 4,500 square centimeters per volt second, Isberg said. This is about three times higher than silicon. This means electric current can travel through the diamond as fast as 45,000 centimeters per second in the presence of a one volt electric field applied over a one millimeter-thick area of diamond, he said.

The experiments also showed carrier lifetimes "orders of magnitude better" than previously shown in diamond, said Isberg.

In a semiconductor substance like diamond, an electric field jolts electrons into a higher energy state to induce current to flow; the carrier lifetime is the average time an electron stays in this high-energy state. In natural diamonds, carrier lifetimes are shorter than one billionth of a second. The researchers clocked carrier lifetimes as high as two millionths of a second in their diamond material. "The lifetime is limited by defects and impurities in the material, so long lifetimes is an indication of pure material," said Isberg.

Electrically viable diamond is especially valuable because of diamond's other unusual properties. It is the hardest known substance and best known heat conductor, and it withstands the damaging effects of electric current better than any other known material. It is also chemically inert, offers low friction, and is transparent to lightwaves from the ultraviolet range to the far-infrared range.

In general, the material could enable electronic devices with "superior performance regarding power efficiency, power density, high frequency properties, power loss, and cooling," Isberg said.

Its ability to withstand electrical current makes it a good candidate for higher power, higher frequency wireless devices. "You can make smaller transistors for the same voltage levels, which improves the switching speed and thereby [increases the] frequency," said Isberg. Transmitting information at higher frequencies enables wireless devices with more bandwidth, because more information can be transmitted in the same amount of time over radio waves that are shorter and thus faster.

Diamond transistors could in theory deliver one watt of power at 100 gigahertz, or billion cycles per second, said Isberg. This is five times faster has been achieved using the semiconductor Gallium Arsenide.

Diamond-based electronics would also be better than existing semiconductor materials for high-temperature applications, said Isberg. Diamond conducts heat 15 times more efficiently than silicon, and therefore cools faster.

The ability to disburse heat quickly is a plus for electronics materials because some of the energy that powers the transistors in devices like computer chips is inevitably given off as waste heat. The faster the chip and the more transistors it contains, the more heat is given off. The fans in many of today's PCs are required to prevent the main processor chips, which contain as many as 40 million transistors each, from overheating.

Diamond would be especially appropriate for electronics required to withstand high-temperature environments like automobile engines and space, said Isberg.

The strong chemical bond structure of diamond means that the atoms are not easily displaced by radiation. This would make for more stable components for radiation-detection and high-voltage devices, and for high-radiation environments in space, said Isberg.

The researchers' next step is to construct electronic components like transistors and diodes from the manufactured diamond material.

Simple diamond radiation and UV detectors for medical applications and space could be ready for practical use in as few as two years, said Isberg. Diamond diodes and transistors for use in telecommunications, high-voltage and high-temperature applications will take four to six years to develop, he said.

The work is an important breakthrough in this type of diamond synthesis, said Yury Gogotsi, a professor of materials engineering at Drexel University. "Many years of research activities and huge money invested in CVD diamond in the past decade did not produce the films or crystals of the quality necessary for the electronic application of diamond," he said. The initial disappointments have in recent years slowed funding for and development of the process, he said.

This finding may reverse that trend, said Gogotsi. "Demonstration of electronic-quality diamond produced by CVD may result in a new wave of interest... and financial support for the research on CVD diamond and diamond electronics."

Isberg's research colleagues were Johan Hammersberg, Erik Johansson and Tobias Wikström of ABB, and Daniel J. Twitchen, Andrew J. Whitehead and Geoffrey A. Scarsbrook of De Beers Industrial Diamonds in England. They published the research in the September 6, 2002 issue of Science. The research was funded by ABB in Sweden and De Beers.

Timeline:   2 years, 4-6 years
Funding:   Corporate
TRN Categories:  Chemistry; Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "High Carrier Mobility in Single-Crystal Plasma-Deposited Diamond," Science, September 6, 2002.


September 18/25, 2002

Page One

Molecule chip demoed

Diamond electronics on deck

Huge lasers could spark fusion

Diamonds improve quantum crypto

Software agents ask for help


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