Zinc oxide nanowires make ultrasensitive photodetectors
30 April 2007
The geometry of semiconducting nanowires makes them uniquely suited for
light detection, according to a new University of California study that
highlights the possibility of nanowire light detectors with single-photon
|A single zinc
oxide (ZnO) nanowire held down by metal contacts. Nanowire segments
between the contacts can serve as photodetectors.
Nanowires are crystalline fibres about one thousandth the width of a
human hair, and their inherent properties are expected to enable new
photo-detector architectures for sensing, imaging, memory storage, intrachip
optical communications and other nanoscale applications, according to a new
study in an upcoming issue of the journal
The University of California San Diego (UCSD) engineers illustrate why
the large surface areas, small volumes and short lengths of nanowires make
them extremely sensitive photodetectors — much more sensitive than larger
photodetectors made from the same materials.
"These results are encouraging and suggest a bright future for nanowire
photodetectors, including single-photon detectors, built from nanowire
structures,” said Deli Wang, an electrical and computer engineering (ECE)
professor from the UCSD Jacobs School of Engineering and corresponding
author on the Nano Letters paper.
For a nanowire to serve as a photodetector, photons of light with
sufficient energy must hit the nanowire in such a way that electrons are
split from their positively charged holes. Electrons must remain free from
their holes long enough to zip along the nanowire and generate electric
current under an applied electric field — a sure sign that light has been
|Drawing of a
single nanowire photodetector. When light strikes the nanowire in
such a way that an electron and hole in the semiconducting material
split apart, the electron runs along the wire and increases the
wire's current and light is detected.
||Schematic of the
trapping and photoconduction mechanism in ZnO nanowires. At the top
of each box are "energy band diagrams" ("b" represents the situation
in darkness and "c" under UV illumination). In ZnO nanowires (as
compared to some other semiconducting nanowires), the lifetime of
the unpaired electrons is further inreased by oxygen molecules
desorption from the surface when holes neutralize the oxygen ions.
The new research demonstrates that the geometry of nanowires — with so
much surface area compared to volume — makes them inherently good at
trapping holes. Dangling bonds on vast nanowire surfaces trap holes, and
when holes are trapped, the time it takes electrons and holes to recombine
increases. Delaying the reunion of an electron and its hole increases the
number of times that electron travels down the nanowire, which in turn
triggers an increase in current and results in “internal photoconductive
“Different kinds of nanowires detect different wavelengths of light. You
could make a red-green-blue photodetector on the nanoscale by combining the
right three kinds of nanowires,” said Cesare Soci, one of two primary
authors on the Nano Letters paper and a postdoctoral researcher in the Deli
Wang lab at the Jacobs School. The other primary author is Arthur Zhang, a
graduate student in the lab of Yu-Hwa Lo, an electrical engineering
professor at the Jacobs School.
This work supports recent theoretical work
from Peter Asbeck’s High Speed Device Group, also at the Jacobs School.
“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 108 — one of the highest ever reported.
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.
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