On the surface, nanowires would seem to make poor light detectors, but it is a nanowire’s surface that produces just the opposite outcome. Those are the findings of a team of scientists from the University of California, San Diego, that investigated the mechanisms responsible for high internal gain in ultraviolet photodetectors made with zinc oxide nanowires.When struck by photons above a certain energy, a nanowire generates holes and electrons. Because the holes are readily trapped at the surface, the electrons flow more freely, leading to increased photoconduction. Reprinted with permission of the American Chemical Society.The result of the research could be new imaging devices, according to team member Deli Wang, an assistant professor of electrical and computer engineering. “The nanowire photodetectors offer great promise for very low light detection.”Nanowires are a few hundred nanometers in diameter and 10 or so microns long. Somewhat surprisingly — given their size and limited light absorption — nanowires, particularly ones composed of ZnO, make great photodetectors. Wang’s group, for example, fabricated ZnO nanowires with internal photoconductive gains as high as 100 million, one of the highest ever reported.To discover what is behind this performance, the researchers constructed ZnO nanowires with diameters of 150 to 300 nm and lengths of 10 to 15 μm. They brought these devices into contact with thin metal film electrode microstrips and measured the photocurrent under various conditions.In doing this, they used a variety of light sources and detectors. For the steady-state current voltage response, they used a mercury arc lamp with a 340- to 440-nm filter and varied the light intensity. To achieve the frequency and spectral response of the photocurrent, they used a xenon arc discharge lamp modulated by a mechanical chopper in conjunction with a monochromator from PerkinElmer Inc. of Waltham, Mass., and an amplifier from Stanford Research Systems Inc. of Sunnyvale, Calif. They determined the intensity with a silicon photodiode from Newport Corp. of Irvine, Calif.A scanning electron micrograph shows a typical zinc oxide nanowire photodetector. The nanowire runs below the contacting electrodes, which are spaced 2 μm apart. Courtesy of Deli Wang, University of California, San Diego.For fast transient photocurrent measurements, they borrowed a setup at the University of California, Santa Barbara, which had a Ti:sapphire laser from Spectra-Physics of Mountain View, Calif. They used the third harmonic, a 267-nm-wavelength beam with a pulse duration of less than 150 fs. With this system, they achieved a measurement temporal resolution of less than 1 ns.The scientists measured the photoconduction at ambient pressure and at vacuum over periods that spanned from nanoseconds to hundreds of seconds. Based on their results, they concluded that oxygen played a critical role, even on the fastest time scale. Its presence led to hole-trap states at the nanowire surface. In an electronic semiconductor device, surface trap states impair performance. Here they boosted photoconduction gain because they kept the holes and electrons produced by a photon from recombining, allowing current to flow more freely. That effect, together with the large surface-to-volume ratio and size of the nanowires, explained the photoconduction gain.With the mechanism understood, the researchers are now studying different semiconductor materials and nanowire structures, looking to extend the spectral response to the visible and near-infrared. They also are trying to commercialize the technology. “Most likely, the first commercial applications will be for imaging, such as the night-vision, surveillance or automotive sectors, where sensitivity is highly valuable,” Wang said.Nano Letters, April 2007, pp. 1003-1009.