Nanowires Form Electrically Driven Laser
A team of scientists at Harvard University in Cambridge, Mass., has developed an electrically driven laser based on freestanding semiconductor nanowires. The laser is relatively inexpensive to produce and has the flexibility to be integrated with conventional electronics.
In the nanowire injection laser, light is emitted from the end of the nanowire. The metal electrode (the rectangular structure) conformally coats the nanowire. Courtesy of Charles M. Lieber.
Electrically driven semiconductor lasers have proved successful in applications such as communications, information storage, and medical diagnostics and therapeutics because of well-developed planar semiconductor growth and processing that enable the reproducible fabrication and integration of the devices with other technologies. Because these processes still carry high costs, researchers have explored the use of organic molecules, polymers and inorganic nanostructures, which can be molded into devices through various chemical processes. Nevertheless, electrically driven lasing in organic systems has been problematic and, until recently, had not been attempted in nanostructures.
"The research evolved naturally from my overall research program, which is focused on using different semiconductor nanowires as functional building blocks for assembling a broad range of nanoscale electronic and optoelectronic/photonic devices," said Charles M. Lieber of the university's department of chemistry and chemical biology. Nanowires recommend themselves for use in electrically driven lasers because they have defect-free structures that offer electrical transport comparable to that of high-quality planar inorganic devices and because a single nanowire can serve as an optical cavity and as a gain medium.
The researchers concentrated on single-crystal, 80- to 200-nm-diameter CdS nanowires with a wurtzite structure with a  growth axis. They grew the nanowires using their nanocluster-mediated technique, which, Lieber noted, yields single-crystal nanowires of precisely controlled chemical composition, physical dimensions, and electronic and optical properties. In contrast, processes such as template-assisted growth typically produce polycrystalline structures that limit the application of nanowires.
To assess the potential for use in injection lasers, the researchers constructed a hybrid device, integrating N-type CdS nanowire laser cavities with P-Si electrodes defined in heavily P-doped planar substrates. Their findings indicate that single semiconductor wires should offer a means to produce integrated electrically driven photonic devices.
Such lasers promise to increase flexibility and to reduce costs in current applications. They also may enable new uses, including integrated single- and multicolor emitters for lab-on-a-chip systems; nanosensors for biochemical defense, medicine and general chemical and biological detection; near-field optical nanolithography; and integrated optoelectronics for inter/intrachip optical communication.
To commercialize the devices, however, the scientists must significantly improve the stability, which will require them to develop more efficient cavities and injection schemes. "We believe that both issues could be addressed at the nanowire growth stage prior to device assembly, by preparing Bragg gratings at the nanowire ends through axial composition modulation and using core-shell nanowire structure to enable uniform injection into the active medium/cavity," Lieber said.
If the lasers are to be used in integrated optoelectronics for inter/intrachip communication, he added, techniques that offer the precise assembly of nanowires at specific locations will be necessary.
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