A Single Dot Marks the Spot for Nanowire LEDs
From wires nanometers in size, an industry hopes that mighty lights will grow. That goal — as well as the ability to perform quantum optics experiments — has gotten a boost, thanks to researchers from the Kavli Institute of Nanoscience at Delft University of Technology and from Philips Research Labs in Eindhoven, both in the Netherlands. The group has fabricated nanowire LEDs and demonstrated the devices’ electroluminescence.
The glowing dot is a nanoscale LED, appearing with zero bias (left) and with 2-V forward bias (right) applied. Scale bar = 2 μm. Reprinted with permission of the American Chemical Society.
As have scientists before them, the group used an axial geometry so that the LEDs were the result of doping changes along the length of the nanowire. This arrangement enabled them to inject electrons and holes into a precisely defined active region about the size of a quantum dot. The resulting light extraction efficiency was high because the optically active region was not buried in a material with a high refractive index.
The “quantum dot” region self-aligns to the N- and P-type ends of the nanowire, making the devices promising for electrically driven quantum optics experiments. As for lighting, hopes arise from the characteristics of nanowire LEDs, according to the institute’s Valery Zwiller. “The very high light extraction efficiency, in addition to the fact that high-quality nanowires can be grown on cheap substrates such as silicon, makes nanowire LEDs very attractive for the lighting/display industry.”
In this scanning electron microscope image of a P-N InP nanowire LED, the P side (upper right, or thick, side) is covered with a highly P-doped InGaAs shell (entering from the right). Researchers are investigating whether such shells improve P-type contacts to the nanowire LEDs. Courtesy of Valery Zwiller and Maarten van Kouwen, Delft University of Technology.
The researchers chose InP nanowires and InAsP active regions because the emission from InAsP can be tuned to infrared telecommunications wavelengths. There is a strong interest in electrically driven single-photon devices for that spectral range.
The Philips part of the team, headed by Erik P.A.M. Bakkers, grew the nanowires, whereas the institute’s researchers performed the subsequent processing. The fabrication of the devices involved making good electrical contact between the nanowire and a conducting strip. Zwiller noted that this step and the doping process were not easy and that getting the process to work took months of effort.
In the end, team members achieved a device yield of 50 percent. They also demonstrated both electro- and photoluminescence from the quantum dot-size regions with an emission wavelength width of ∼1 nm. They measured this using a spectrometer from Acton Research Corp. of Acton, Mass., and a Roper Scientific Inc. liquid nitrogen-cooled CCD camera.
Although the results are promising, Zwiller noted some problems. One, the emission was about 100 times wider than comparable non-nanowire quantum dots; two, the electroluminescence was weaker than expected. To help the former, the researchers are working to surround the quantum dot region with a barrier or shell of undoped InP. For the second, they are considering ways to produce better nanowire contacts.
Their next goal is to demonstrate single-photon generation, which will enable the measuring of a single electron spin on very fast time scales and open up experiments in other areas. “Nanowires could merge single-electron transport experiments with single-photon optics experiments,” Zwiller said.
Nano Letters, February 2007, pp. 367-371.
- The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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