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Millimeter and terahertz wave photodetectors have a wide range of applications. However, the state-of-the-art techniques lag far behind the urgent demand due to the structure and performance limitations. Here, we report sensitive and direct millimeter and terahertz wave photodetection in compact InGaAs-based subwavelength ohmic metal–semiconductor–metal structures. The photoresponse originates from unidirectional transportation of nonequilibrium electrons induced by surface plasmon polaritons un
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Antenna-assisted subwavelength metal–
InGaAs–metal structure for sensitive
and direct photodetection of millimeter
and terahertz waves
JINCHAO TONG,
1,2,†
YUE QU,
2,3,†
FEI SUO,
1,†
WEI ZHOU,
2
ZHIMING HUANG,
2,4,5
AND DAO HUA ZHANG
1,6
1
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
2
State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
3
University of Chinese Academy of Sciences, Beijing 100049, China
4
Key Laboratory of Space Active Opto-electronics Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences,
Shanghai 200083, China
5
e-mail: zmhuang@mail.sitp.ac.cn
6
e-mail: EDHZHANG@ntu.edu.sg
Received 21 September 2018; revised 4 November 2018; accepted 11 November 2018; posted 13 November 2018 (Doc. ID 346503);
published 21 December 2018
Millimeter and terahertz wave photodetectors have a wide range of applications. However, the state-of-the-art
techniques lag far behind the urgent demand due to the structure and performance limitations. Here, we report
sensitive and direct millimeter and terahertz wave photodetection in compact InGaAs-based subwavelength
ohmic metal–semiconductor–metal structures. The photoresponse originates from unidirectional transpor-
tation of nonequilibrium electrons induced by surface plasmon polaritons under irradiation. The detected
quantum energies of electromagnetic waves are far below the bandgap of InGaAs, offering, to the best of our
knowledge, a novel direct photoelectric conversion pathway for InGaAs beyond its bandgap limit. The achieved
room temperature rise time and noise equivalent power of the detector are 45 μs and 20 pW · Hz
−1∕2
, respectively,
at the 0.0375 THz (8 mm) wave. The detected wavelength is tunable by mounting different coupling antennas.
Room temperature terahertz imaging of macroscopic samples at around 0.166 THz is also demonstrated. This
work opens an avenue for sensitive and large-area uncooled millimeter and terahertz focal planar arrays.
© 2018
Chinese Laser Press
https://doi.org/10.1364/PRJ.7.000089
1. INTRODUCTION
Millimeter and terahertz (THz) wave technologies have at-
tracted unprecedented attention owing to their significance
in almost all scientific and technological fields, including re-
mote sensing, spectroscopy, imaging, and communications
[1–5]. However, they still face challenges in high-performance
building blocks such as detectors. Conventional photodetec-
tion based on bandgap or intersubband transitions in optoelec-
tronic semiconductors does not perform well in millimeter and
terahertz wave ranges due to their relatively small photon en-
ergy (1 THz–4.14 meV) and strong background thermal noise.
Commercially available Golay cells, pyroelectric elements, and
bolometers, despite their widespread use, either suffer from
slow response (typically, ms level for Golay cell and pyroelectric
elements) or require cryogenic cooling (typically, liquid-
helium-cooled temperature for Si bolometers) for normal op-
eration due to the intrinsic thermal response mechanism [6].
Schottky barrier diodes have high speed but require very
advanced growth and fabrication techniques [7].
In the past decades, the development of sensitive millimeter
and terahertz wave detectors has been highly active. Terahertz
field effect transistors (FETs), with a fast response speed and good
responsivity, are promising candidates. They are usually built with
a nanoscale transistor channel where a plasma wave is excited to
enable the photodetection [8–10]. Terahertz photoconductive
antennas (THz-PCs) have been very successfully and maturely
used in terahertz time domain systems (THz-TDSs). To allow
indirect detection of terahertz radiation, a local femtosecond laser
is required to pump the semiconductor (usually requires high re-
sistivity and ultra-short carrier lifetime) located in the gap of the
antenna [11,12]. Terahertz quantum-well infrared photodetec-
tors are based on intersubband transitions and require the oper-
ation in extremely low-temperature conditions and high-quality
quantum wells [13]. In addition, two-dimensional materials,
Research Article
Vol. 7, No. 1 / January 2019 / Photonics Research 89
2327-9125/19/010089-09 Journal © 2019 Chinese Laser Press
as well as topological insulators, have also been investigated for
their capability of detecting terahertz radiation [14–18]. Among
them, graphene-based terahertz detectors, including FETs [13],
hot electron bolometers [14], and photothermoelectric devices
[15], have been demonstrated with a noise equivalent power
(NEP) less than 10 nW · Hz
−1∕2
for room temperature opera-
tion, sensitivity comparable to that of a Si bolometer, and pulse
response as fast as 110 ps, respectively. Terahertz photodetection,
using bismuth selenide (Bi
2
Se
3
)[19], has also been reported re-
cently, exhibiting the ability of topological insulators for terahertz
photoelectric conversion. Besides, terahertz single-photon detec-
tors have also been realized at temperatures of T<1K,dem-
onstrating an extremely low NEP of 10
−22
W · Hz
−1∕2
[20].
In recent years, surface plasmon polaritons (SPPs) in sub-
wavelength structures have been attracting more and more atten-
tion, as they have a wide variety of applications, depending on
the dimensions of the plasmonic structures and operational fre-
quency range [21–25]. Millimeter and terahertz wave photode-
tection can be realized by localized-SPP-induced nonequilibrium
electrons in antenna-assisted subwavelength ohmic metal–
semiconductor–metal (OMSM) structure [20]. In the previous
work, the detection was verified by simulation and experiments
from the devices made of gold and InSb. However, as the current
epitaxial growth technique for InSb on an insulating substrate
has a lattice mismatch issue, the InSb slices were fabricated
by polishing bulk wafers into thin films, which was difficult
for fabricating large planar arrays with multiple pixels.
Here, we report direct millimeter and terahertz wave photo-
detectors based on In
0.53
Ga
0.47
As epitaxially grown on lattice-
matched InP substrates. In this device, a subwavelength
OMSM structure is used to convert the absorbed photons into
localized SPPs, which then induce nonequilibrium electrons near
the metal–semiconductor interfaces, while the antenna increases
the number of photons coupled into the OMSM structure. The
nonequilibrium electrons induced by SPPs will form unidirec-
tional photocurrent under an external voltage bias. Normal tera-
hertz antennas (designed for different frequency ranges) are used
to improve the coupling efficiency of incident radiation into the
subwavelength detecting structure. The compact devices omit
local laser pumping in THz-PCs and complicated treble elec-
trodes in THz-FETs, exhibiting excellent performance with re-
spect to the state of the art for room temperature operation. At
0.0375 THz (8 mm), the typical rise time and noise equivalent
power of the detector are 45 μsand20 pW · Hz
−1∕2
,respec-
tively. Beyond the proof of concept, millimeter and terahertz im-
aging at 0.166 THz (1.81 mm) has been demonstrated for
macroscopic samples in a realistic setting. The detected photons
have a far smaller quantum energy than the bandgap of
In
0.53
Ga
0.47
As, allowing a novel direct optoelectrical property
beyond the bandgap limit. Moreover, the sensitive terahertz de-
tection is based on In
0.53
Ga
0.47
As∕InP,whichisamaturetech-
nology, offering a promising avenue for sensitive and large-area
uncooled millimeter and terahertz focal planar arrays.
2. EXPERIMENTAL SETUP AND METHODS
A. Device Design
In the device architecture, the length L of InGaAs is much less
than the wavelength of the incident radiation λ (Fig. 1). Under
transverse magnetic (TM) polarized illumination, the planar
dipole-like antenna (or other type antennas), which is designed
for a spe cific wavelength, will efficiently couple photons into
the structure. Owing to the negative permittivity of InGaAs
[26] in the frequency range of interest, localized SPPs will
be excited by the coupled photons within InGaAs, especially
near the InGaAs–Au interfaces on the top facet [27]. The
SPPs then induce nonequilibrium electrons by transferring en-
ergy to electrons in InGaAs. With zero bias, the SPP-induced
nonequilibrium electrons have a symmetric distribution due to
the symmetry of the device structure. However, when an ex-
ternal bias is applied, the SPP-induced electrons will flow
through InGaAs, leading to the generati on of photocurrent.
InGaAs is a well-known III–V semiconductor that is com-
monly used as an interband transition-based photodetection
material for the near-infrared (NIR) range, with both photocon-
ductive and photovoltaic architectures [28]. It has been maturely
used as focal planar arrays in the NIR range. It is lattice matched
to the InP substrate when the component of indium is approx-
imately 0.53, which offers a good material quality and capability
of large-scale wafer growth. The metal organic chemical vapor
deposition grown In
0.53
Ga
0.47
As in our work has a bandgap
of ∼752 meV, an electron concentration of ∼5.9 × 10
16
cm
−3
,
and an electron mobility of 7000–8000 cm
2
· V
−1
· s
−1
at room
temperature. The substrate is 350 μm SI InP (100) doped with
Fe. The resistivity is 5 × 10
6
Ω · cm. The plasma frequency of
In
0.53
Ga
0.47
As is near 3.2 THz, which is in the terahertz wave
range and allows negative permittivity below it [25,29]. The pla-
nar antenna depicted in Fig. 1 is configured as a half-wave dipole
designed for a specific wavelength (∼0.0375 THz∕8mm)[30].
It is worth noting that this detection strategy based on InGaAs is
quite different from the InGaAs-based [31](GaAs[10 ,11]or
Ge [32]) indirect terahertz photoconductive antennas, where
t
x
y
z
W
L
s
InGaAs
InP
Antenna
Wire bonding
(a) (b)
(c)
Fig. 1. Schematic of the antenna-assisted subwavelength ohmic
Au–InGaAs–Au photodetector (not drawn to scale). (a) Full view
of the schematic of the photodetector. (b) Scanning electron micro-
scope image of the fabricated device with wire bonding. The scale
bar represents 1 mm. (c) The zoom in tridimensional view for the
central part of the structure. L is the length of the semiconductor,
and s is the spacing between the edges of the two ohmic contacts.
W and t are the width and thickness of the InGaAs layer, respectively.
The left-bottom inset is the cross section of the In
0.53
Ga
0.47
As∕InP
detector.
90 Vol. 7, No. 1 / January 2019 / Photonics Research
Research Article
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