“Our theoretical work showed that light-induced conductivity in nanowires can be increased by more than 10 times over similar bulk structures under the same illumination level. The work from Deli Wang’s lab has confirmed some of our calculations and provides further support for the idea that nanowires will be increasingly incorporated into photodetection and photovoltaic applications,” said Asbeck.
In the new work, short pulses of ultraviolet light (hundreds of femtoseconds wide) were detected on time scales in the nanosecond range. Moreover, using electronic measurement of photocurrent, the engineers reported internal photoconductive gain (G) as high as 10^8 – one of the highest ever reported.
"Although nanowire detectors offer both high speed and high gain, the most important figure of merit for the device is the signal-to-noise ratio or the sensitivity,” explained Yu-Hwa Lo, an author on the Nano Letters paper and the director of NANO3, the clean nanofabrication facility at Calit2's UCSD campus.
“Because of the unique geometry of nanowires, the active volume that produces dark current, a source of noise, is only one thousandth that of a normal size photodetector. This enables nanowire detectors to achieve very high sensitivity, provided that light can be efficiently coupled into the nanowires. Several methods have been proposed to achieve light coupling efficiency, such as placing the nanowires in an optical resonant cavity. In theory, a nanowire detector can achieve single photon sensitivity, which is the ultimate sensitivity for any photodetector,” said Lo.
The engineers also show that molecular oxygen absorbed at the surface of zinc oxide (ZnO) nanowires capture free electrons present in n-type ZnO nanowires and make them especially good at keeping holes and electrons apart. The oxygen mechanism the authors outline explains much of the enhanced sensitivity reported in ZnO nanowire photodetectors.
The engineers fabricated and characterized UV photodetectors made from ZnO nanowires with diameters of 150 to 300 nanometers and lengths ranging from 10 to 15 micrometers. The researchers studied the photoconductivity of zinc oxide nanowires over a broad time range and under both air and vacuum.
Analytical studies performed by Peter Asbeck and ECE graduate student Lingquan Wang and published in the proceedings of IEEE-NANO 2006 support the mechanism outlined in the Nano Letters paper.
According to Wang, this work also highlights how moving to the nanoscale can sometimes throw intuitions out the window.
“The surface trap states that help to make nanowires such sensitive light detectors are the very same surface features that engineers desperately avoid when manufacturing semiconductors for computer transistors, where they hamper performance,” Wang said. ###
ZnO Nanowire UV Photodetectors with High Internal Gain
C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo, and D. Wang
Department of Electrical and Computer Engineering, Jacobs School of Engineering, UniVersity of California, San Diego, 9500 Gilman DriVe, La Jolla, California 92093-0407, Nano Letters 7(4), 1003-1009 (2007), DOI: 10.1021/nl070111x
Funders: Office of Naval Research (ONR-Nanoelectronics), the National Science Foundation (NSF), Sharp Labs of America.
Notes: Thanks to a collaboration with Alan Heeger’s lab at UC Santa Barbara where Cesare Soci and Deli Wang did some of their PhD work, the engineers performed electronic measurement of the photocurrent during very short times using a technique pioneered for the study of conjugated polymers. The authors are also grateful for their collaboration with Ed Yu’s lab at UCSD’s Jacobs School and the staff of Calit2’s Nano3 Facility for maintenance of the nanofabrication environment.
Contact: Daniel Kane firstname.lastname@example.org 858-534-3262 University of California - San Diego, April 23, 2007
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