Wednesday, 31 July 2013

Researchers discover universal law for light absorption in 2D semiconductors

Researchers discover universal law
for light absorption in 2D
semiconductors

(From left) Eli Yablonovitch, Ali Javey
and Hui Fang discovered a simple law
of light absorption for 2D
semiconductors that should open
doors to exotic new optoelectronic
and photonic technologies. Credit:
Roy Kaltschmidt, Berkeley Lab
From solar cells to optoelectronic
sensors to lasers and imaging
devices, many of today's
semiconductor technologies hinge
upon the absorption of light.
Absorption is especially critical for
nano-sized structures at the interface
between two energy barriers called
quantum wells, in which the
movement of charge carriers is
confined to two-dimensions. Now,
for the first time, a simple law of
light absorption for 2D
semiconductors has been
demonstrated.
Working with ultrathin membranes of
the semiconductor indium arsenide ,
a team of researchers with the U.S.
Department of Energy (DOE)'s
Lawrence Berkeley National
Laboratory (Berkeley Lab) has
discovered a quantum unit of photon
absorption , which they have dubbed
"A Q ," that should be general to all
2D semiconductors , including
compound semiconductors of the III-
V family that are favored for solar
films and optoelectronic devices. This
discovery not only provides new
insight into the optical properties of
2D semiconductors and quantum
wells , it should also open doors to
exotic new optoelectronic and
photonic technologies .
"We used free-standing indium
arsenide membranes down to three
nanometers in thickness as a model
material system to accurately probe
the absorption properties of 2D
semiconductors as a function of
membrane thickness and electron
band structure," says Ali Javey, a
faculty scientist in Berkeley Lab's
Materials Sciences Division and a
professor of electrical engineering
and computer science at the
University of California (UC) Berkeley.
"We discovered that the magnitude
of step-wise absorptance in these
materials is independent of thickness
and band structure details."
In this FTIR microspectroscopy study,
light absorption spectra are obtained
from measured transmission and
reflection spectra in which the
incident light angle is perpendicular
to the membrane. Credit: Javey group
Javey is one of two corresponding
authors of a paper describing this
research in the Proceedings of the
National Academy of Sciences ( PNAS).
The paper is titled "Quantum of
optical absorption in two-dimensional
semiconductors." Eli Yablonovitch, an
electrical engineer who also holds
joint appointments with Berkeley Lab
and UC Berkeley, is the other
corresponding author.
Previous work has shown that
graphene, a two-dimensional sheet
of carbon, has a universal value of
light absorption . Javey, Yablonovitch
and their colleagues have now found
that a similar generalized law applies
to all 2D semiconductors. This
discovery was made possible by a
unique process that Javey and his
research group developed in which
thin films of indium arsenide are
transferred onto an optically
transparent substrate, in this case
calcium fluoride.
"This provided us with ultrathin
membranes of indium arsenide, only
a few unit cells in thickness, that
absorb light on a substrate that
absorbed no light," Javey says. "We
were then able to investigate the
optical absorption properties of
membranes that ranged in thickness
from three to 19 nanometers as a
function of band structure and
thickness."
Indium arsenide is a III–V
semiconductor with electron mobility
and velocity that make it an
outstanding candidate for future
high-speed, low-power opto-
electronic devices.
Using the Fourier transform infrared
spectroscopy (FTIR) capabilities of
Beamline 1.4.3 at Berkeley Lab's
Advanced Light Source, a DOE
national user facility, Javey,
Yablonovitch and their co-authors
measured the magnitude of light
absorptance in the transition from
one electronic band to the next at
room temperature. They observed a
discrete stepwise increase at each
transition from indium arsenide
membranes with an AQ value of
approximately 1.7-percent per step.
"This absorption law appears to be
universal for all 2D semiconductor
systems," says Yablonovitch. "Our
results add to the basic
understanding of electron–photon
interactions under strong quantum
confinement and provide a unique
insight toward the use of 2D
semiconductors for novel photonic
and optoelectronic applications."
More information: http://
www.pnas.org/
content/110/29/11688.short
Provided by Lawrence Berkeley
National Laboratory

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