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Nano Res
DOI 10.1007/s12274-015-0731-0
Evidencing of germanium nanopyramid arrays for near100% absorption in visible regime
Qi Han1, Yongqi Fu1(), Lei Jin1, Jingjing Zhao3, Zongwei Xu2, Fengzhou Fang2, Jingsong Gao3,
Weixing Yu4
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0731-0
http://www.thenanoresearch.com on January 28, 2015
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1
Evidencing of germanium nanopyramid arrays for
near-100% absorption in visible regime
Qi Han1, Yongqi Fu1* , Lei Jin1 ,Jingjing Zhao3 , Zongwei
Xu2,
1
Fengzhou Fang2, Jingsong Gao3,
Weixing Yu4
University of Electronic Science and Technology of
China, China
2 Tianjin University, China
3 Chinese Academy of Sciences, China
4 Shenzhen University, China
In the visible regime, the experimentally measured absorption of
germanium nanopyramid arrays can even reach to the level of as high
as 100%.
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Evidencing of germanium nanopyramid arrays for near100% absorption in visible regime
Qi Han1,
Yongqi Fu1(), Lei Jin1, Jingjing Zhao3, Zongwei Xu2, Fengzhou Fang2, Jingsong Gao3,
Weixing Yu4
Received: day month year
ABSTRACT
Revised: day month year
Solar energy is regarded as one of the most plentiful harvest sources of
Accepted: day month year
renewable energy. An extraordinary light harvesting property of a germanium
(automatically inserted by
the publisher)
© Tsinghua University Press
periodic nanopyramid array is reported in this letter. Both of our theoretical
and experimental results demonstrate that the nanopyramid array can gain the
perfect broadband absorption in the wavelength from 500 nm to 800 nm.
and Springer-Verlag Berlin
Especially in the visible regime, the experimentally measured absorption can
Heidelberg 2014
even reach to the level of as high as 100%. Further analyses reveal that intrinsic
antireflection effect and slowlight waveguide mode play an important role in
KEYWORDS
the ultra-high absorption. It is helpful for research and development of
nanopyramid array,
absorption coefficient,
antireflection coatings,
slowlight mode
photovoltaic devices.
1
Introduction
Sustainable energy is a challenging problem for
Address correspondence to Yongqi Fu, yqfu@uestc.edu.cn
human bein gs in current ce ntury. Rec ently,
tremendous progress has been made for the purpose
of developing photovoltaics that can be potentially
2
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mass deployed since photovoltaic cells can serve as
harness a large fraction of the incident solar
a virtually unlimited clean source of energy by
photons. In this regard, simulation studies have
converting sunlight into electrical power[1-4]. In
previously shown that three-dimensional (3D) cell
these progresses, optical absorption has been paid
structures
attention as an effective and powerful way to boost
nanopillar arrays [11-13],nanocone arrays [14-16],
energy conversion efficiencies. The ability to
nanohole
deposit sunlight is of profound interest for
[21,22] ,and other perfect absorbers[23-26] have
high-performance
proven to possess unique optical characteristics for
solar
cell
applications.
such
as
arrays
nanowire
[17-20],
arrays
nanodisk
[7-10],
arrays
Conventional silicon thin film-based photovoltaic
light absorbing and harvesting.
cells are inefficient even though nanostructures in
nanopyramid arrays
silicon photovoltaic cells is of particular importance
coefficient which is caused from both an intrinsic
as these currently make up over 80% of the
antireflection effect [15] and stimulating slowlight
photovoltaic market in the world[5]. Advantage of
waveguide mode of weakly coupled resonances.
the silicon-based photovoltaic cells with short
[10,27] In this letter, evidences from both theoretical
collection length for excited carriers results in
experimental
significant
collection
germanium nanopyramid arrays surely retain the
efficiency. Silicon, however absorbs poorly over the
ability to reach perfect absorption in visible regime.
improvement
in
carrier
peak of the solar spectrum, requiring the use of over
100 μm thick Si wafers for sufficient absorption.
2
study
can
are
Germanium
improve absorption
provided
that
the
Experimental
Germanium belongs to indirect-gap materials as
A silica glass substrate supported germanium
same as silicon. In contrast, germanium has lower
thin film of 1 m in thickness was coated using
band gap. The gap results in a phenomenon that the
electron beam evaporation technique. Figure 1 (a)
photons with lower energy can excite electrons
shows the schematic
migration from valence band to conduction band.
nanopyramid
And electron-hole pairs produce accordingly. It
experiment was carried out using our focused ion
means the germanium thin film-based structures
beam (FIB) machine (FEI NOVA200 Nanolab) which
can increase optical absorption coefficient in
is equipped with a liquid gallium ion source. This
broader band than that of silicon material.
FIB machine is integrated with a scanning electron
Meanwhile,
photons
microscope (SEM), and a gas-assistant etching (GAE)
excitation, more heat can be produced, and higher
functions. This machine employs a focused Ga + ion
temperature also improves efficiency of generation
beam milling under energy of 30 keV, a probe
and separation of electron-hole pairs as well as
current of 4 pA~19.7 nA, and beam limiting
photoelectric conversion efficiency in germanium
aperture size of 25 m~350 m. For the smallest
structures in comparison to the silicon materials.
beam currents, the beam was focused down to 5 nm
In
material-based
in diameter at full width and half maximum
structures can lift the internal quantum efficiency
(FWHM). Assisting by a computer program, the
and short circuit current to increase the efficiency of
milling process is carried out by means of varying
photovoltaic cells.
the ion dose for different relief depths. The defined
other
with
words,
the
same
germanium
energy
illustration
structure.
The
of
periodic
nanofabrication
Whereas thin film photovoltaic cells offer a viable
area for the FIB milling using bitmap function[28,
pathway to reduce fabrication costs and material
29] is 9  9 m2. During the milling process, the
usage[6], their energy conversion efficiencies can
beam current and ion dose of 20 pA and 0.25
still be improved significantly by enabling them to
nC/m2
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were
adopted
respectively.
Working
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Nano Res.
distance from sample surface to facet of ion column
and just joining each other, and then the 3D
is 20 mm. Electron mode of the FIB milling was
pyramid array is formed finally.
selected for the germanium thin film. Charging
effect originating from the germanium is slightly
3
Results and discussion
during scanning and can be ignored here. Unlike
Figures 1 (c), (d) and (e) exhibit scanning electron
conventional two-dimensional (2D) structures, one
microscope (SEM) images of the nanopyramid
step FIB milling of the three-dimensional (3D)
arrays. For convenient characterization, total eight
patterns
nanopyramid arrays
is
quite
challenge.
Firstly,
precise
thin
were fabricated on
film,
and
the side
the
calibration is required to ensure the accurate depth
germanium
length
milling. Then a designed 2D pattern (see Fig. 1b)
of each square nanopyramid array is determined
was called into the defined milling area. Feature
to be 20 μm. These germanium nanopyramid arrays
size of the pattern was defined on the basis of the
had a uniform height of about 500 nm and bottom
rate of self-extension effect which is generated by
length size of about 200 nm. All germanium
redeposition effect of the ion beam bombardment
nanopyramid arrays are closely packed and display
while designing the pattern. Initially, a concave
in short-range order.
V-shape of the reverse pyramid structure with a few
For design of the nanopyramid structures, we
nanometer depth is produced. After 15 min.
need to take into account the wave effects by means
continuous
are
of solving Maxwell’s equations for the full wave
over-milled so as to enable the reverse pyramid
vector. A finite-difference and time-domain (FDTD)
milling,
the
squared
areas
structure originated from the square area extending
(a)
(b)
(c)
(d)
(e)
Figure 1 (a) Schematic diagram of the nanopyramid arrays absorber; (b) binary bitmap pattern designed for fabrication of the
nanopyramide array; (c) overall SEM images of total eight nanopyramid arrays; (d) SEM image of a germanium nanopyramide array;
(e) zoom-in SEM images of the germanium nanopyraim arrays with measured sizes in vertical and horizontal.
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Nano Res.
algorithm has been proven to be effective for
numerically calculating such periodic structures. In
our analyses, we use FDTD solution 8.5 (from
Lumerical Solution Inc.) and set a fine grid
dimension of 5 nm5 nm5 nm, which is less than
1/100 of the shortest wavelength in the whole
computation domain so as to ensure convergence of
the calculation process. The wavelength is ranging
from 500 nm to 800 nm. Figure 2 (a) shows the
experimental setup for spectroscopy.
(b)
The absorption spectrum of the nanopyramid
arrays and thin film are shown in Fig. 2 (b) and
(c).In order to simulate reality of situation, we put
eight nanopyramid arrays in a field of view. Th e
abs orption s pectrum is the average of eight
nanopyramid arrays. We observed a substantial
absorption enhancement over the entire usable solar
spectrum in nanopyramid arrays structure. In
comparison
to
the
experimental
results,
corresponding numerical calculation results show
(c)
that the absorption coefficient declines slightly from
1 to 0.983 in the nanopyramid arrays, but the
absorption coefficient still maintains at the level of
Figure 2 (a) Experimental setup of the spectrometry system, an
perfect absorption which is higher than the
optical fiber-based spectrometer from Ocean Optics is
previous reports.[10, 15, 24, 27] The absorption
employed; (b) measured and simulated results of absorption
coefficient is larger than the other reported
coefficient on samples with nanopyramid arrays; (c) measured
nanostructures in the wavelength from 400 nm to
and simulated results of absorption coefficient on samples with
800 nm. Absorption coefficient in calculation is
flat thin film.
smaller than that of experiments. This can be
attributed to SiO2 impurities on the sample play a
dominant role in increasing absorption coefficient
as
an
antireflection
coating.
The
Considering the high refractive index contrast
ultra-high
between germanium and air, a large portion of
absorption may be explained in detail in physical
incident light is reflected back from the surface of
point of view as follows.
the germanium thin-film and thus cannot be used to
generate current in future solar cell. Currently,
quarter-wavelength transparent thin films become
the industrial standard for antireflection coatings
for the thin film solar cells. However, this
quarter-wavelength
antireflection
coatings
is
typically designed to suppress reflection at a
specific wavelength.[30, 31] In the nanopyramid
(a)
arrays, the electromagnetic field can be coupled
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Nano Res.
efficiently into the pyramids cavity at resonances,
and resulting in a significant light-trapping ability
boost. For the nanopyramid arrays with small
filling factor, most of the incident light cannot be
coupled into the nanopyramid arrays, but is
absorbed in a single path through the array. In
contrast, the nanopyramid arrays with large filling
factor provide efficient supported modes. Hence the
(a)
(b)
(c)
(d)
absorption is greatly increased when these modes
are well coupled and concentrated in the array.[32]
The
contribution
for
light
absorption
enhancement from the structured area as shown in
Fig. 2(b), in comparison to the absorption of the flat
thin film without any structures shown in Fig. 2(c),
originates from lighting trapping and antireflection
effect. The nanopyramid arrays suppress reflection
because the pyramid geometry provides an
Figure 3 Electromagnetic field distribution at y=0 plane for
averaged, graded index from air to germanium as
different wavelengths. (a) λ=500 nm; (b) λ=600 nm; (c) λ=700
the side length increases from zero to maximum
nm; and (d) λ=800 nm.
value at the planar film surface. The reflection
suppression is broadband since the index-matching
which
are
close
to
the
interface
between
is largely independent to wavelength. On the other
nanopyramid and air regions. The vortexes locate
hand, the nanostructures can be regarded as a
exactly at the places where the electromagnetic field
special type of metamaterials. To achieve effective
highly concentrates. These are typical features in
antireflection, the periodicity of the array has size
slowlight waveguides with the slowlight defined as
ranging in the subwavelength regime based on the
the propagation of light at a quite low group
incoming light to see an effective averaged
velocity in comparison to the light speed in
index.[15] In addition, a high aspect ratio of the
vacuum.[33-35] For example, in Ref. 33 the authors
arrays is preferred so as to provide a smooth index
demonstrated that the designed air/dielectric/metal
transition from air to germanium.
waveguide could support such kind of vortex mode
Meanwhile, such nanopyramid arrays -bas ed
propagating very slowly along the slab and even
ultra-short vertical waveguides support slowlight
being trapped at a critical core thickness. Later, a
mode at different wavelengths so that the incident
nanopyramid/air/nanopyramid
light at different wavelengths is captured at the
waveguide has also been presented to slow down
position of different tooth widths (lateral size of the
the electromagnetic
single pyramid). As can be seen in Fig. 3, light is
complete standstill at certain regions of the air core
m a i n l y abs o r b e d o n t h e t o p o r e dge of
waveguide. Light with different wavelengths as
nanopyramid. Most of the incident energy firstly
different photons energy has different penetration
propagates downwardly along z-direction in the air
depths. In short wavelength, most of light is
ga p r e gi on wi t h o ut p e n et rat i n g in t o t h e
absorbed on the top of nanopyramid arrays. With
nanopyramid arrays and then whirls into the
increasing of the wavelength, focusing regime of the
nanopyramid region. The whirling forms vortexes
electromagnetic field is shifting to the bottom of the
slowlight
propagating waves to a
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Nano Res.
nanopyramid. This means, light with different
the working mode of confocal imaging in which the
wavelengths is absorbed in the nanopyramid
sample is stationary and only stage scans in
arrays.
horizontal plane. This is a finely focused far-field
In order to intuitively observe the absorption
imaging with resolution within diffraction limit.
effect occurred on the areas of structured and
Fiber tip is not necessary in this imaging mode.
non-structured, we utilized an apertured near-field
Intensity distributions can be derived from the
scanning optical microscope (a-NSOM, modeled
confocal images, as shown in Fig. 4. As can be seen
Multiview 2000TS from Nanonics Inc.) to gain the
from Figs. 4 (a) and (b), optical intensity in both
optical intensity distributions. Firstly, we selected
(a)
(b)
(c)
(d)
Figure 4 Intensity distributions for both structured and non-structured areas, (a) nanopyramid arrays scan area is 5 m5 m; (b) flat
thin film area without structure, imaging area is 5 m5 m; (c) hybrid section (bright part indicates flat thin film only), imaging area
is 20 m20 m. (d) NSOM image of the hybrid section. The color bars indicate relative light intensity. The value approaching 0
means near-100% absorption from the area with the pyramid arrays.
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nanopyramid arrays and flat thin film regime are
energy nowadays, it is reasonable to believe that the
apparently distinct nevertheless imaging resolution
proposed
is low compared to feature size of the structures. It
applications in those light harvesting related areas.
can be seen from the color bar that intensity in the
nanopyramid arrays is about zero, and about 0.06 in
flat thin film while the maximum intensity is 0.16.
This result is well matched with absorption
coefficient above. Figure 4 (c) exhibits intensity
distribution in a section of the sample which
absorber
will
find
its
potential
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