Two Colors, One Dopant

Hank Hogan

A group of researchers from the Institute of Ion Beam Physics and Materials Research at Forschungszentrum Dresden-Rossendorf in Germany has discovered that the rare-earth element europium can do something rare. When used in a silicon wafer as a dopant, it makes a metal-oxide semiconductor electroluminescent device that changes color from red to blue when the drive current changes from low to high.

Although the group had worked with rare-earth dopants before, institute director Manfred Helm said that the results surprised its members, noting that some changes of intensity within the various emission lines had been observed with other dopants, but not so strikingly and, in particular, not so well seen by the naked eye.

Europium-doped CMOS devices shift color from red to blue (left) with a change in current from 20 μA (a) to 1 mA (b) to 2.5 mA (c). The change is related to the two oxidation states of the dopant (right). Reprinted with permission of the American Institute of Physics.

Unlike other rare-earth elements, europium has not been investigated extensively as a dopant in metal-oxide semiconductor devices. The researchers expected that its presence would result in a red emission.

To study this, they created CMOS structures and implanted them with europium, leading to a dopant concentration that ranged from 0.5 to 3 percent. After annealing the implant damage, the scientists coated the 1- to 500-μm-diameter devices with indium tin oxide and patterned them to make a transparent gate electrode atop each one. They then injected current, using a Jobin Yvon monochromator and a Hamamatsu photomultiplier tube to record the resulting electroluminescence from 350 to 750 nm.

At low currents, the devices were visibly red, as expected, whereas at high currents, they were blue, which the investigators had not foreseen. Measurements showed that the emission at 616 nm was orders of magnitude greater than that at 400 nm when injection currents were microamps or less for a 200-μm-diameter device. As the current climbed into the milliamp range, the ratio between the colors switched.

As for why this happens, the group postulated that the phenomenon results from the two oxidation states of europium. It is hoped that further experiments, which are planned but not yet completed, will pin down the exact microscopic mechanism.

Other goals include reducing the device’s operating voltage by using thinner oxide. Possible uses of the technique include microdisplays. “There may be other applications for ‘cheap’ biosensing — directly at the point of care, for example — because no separate excitation laser would be needed,” Helm said.

Applied Physics Letters, April 30, 2007, Vol. 90, 181121.