NEW BUSINESS FIELDS

Two-Dimensional Near Infrared Sensor with Low Noiseand Wide Wavelength Range
Yasuhiro IGUCHI*, Hiroki MORI, Masaki MIGITA, Youichi NAGAI, Hiroshi INADA, Kei FUJII,Takashi ISHIZUKA and Katsushi AKITA

A two-dimensional near infrared image sensor with the cut-off wavelength of 2.35 µm has been successfully developedby using InGaAs/GaAsSb type-II quantum well structures as its absorption layer. The 250-pair InGaAs (5nm)/GaAsSb(5nm) quantum well structures lattice-matched to InP substrates were grown by metal organic vapor phase epitaxy. Thep-n junctions were formed in the absorption layer of each pixel by the selective diffusion of zinc. The sensor chip with320 × 256 pixels at 30 µm pitch, in which each pixel has an 18 µm × 18 µm light receiving area, was hybridized to aread-out IC with indium bumps. More than 99% pixels operated properly. A Peltier cooler was used as the coolingsystem, enabling the downsizing of sensing systems. The dark current of each pixel was 3 pA at -60°C, which is lower
than that of the conventional sensor at the same temperature. The responsivity showed good linearity with respect to theinput power. Therefore, this sensor is expected to be used for the detection of weak optical signals and the quantitative,
real-time image analyses of various materials in the food and phermaceutical industries.
Keywords: near infrared, sensor, type-II quantum well, low noise, imaging

1. Introduction
Many molecules have overtones and combination tonesof fundamental vibrations in the near infrared region between 1.0 µm and 2.5 µm. As the absorptions of these tones
are weaker than that of fundamental vibrations in the mid
infrared region over 3 µm, the near infrared light arrives at
the inside of matarials and therefore is expected for thenon-destractive analyses. Recently, the near infrared spectroscopy has been attracting notice from the view point of
the safety and the quality control in many production fieldsuch as pharmaceutial and food industries, or from the view
point of diagnoses in lifescience fields. Sumitomo ElectricIndustries, Ltd. has produced the near infrared diagnostic
equipment “Compovision” which makes it possible to image
the distribution of composition and concentration of materials in real time non-destructively, which is difficult by
the conventional imaging inspection method.
In the near infrared region, the identification of material is generally difficult because the absorptions of many
materials overlap and the forms of spectra are complicated.
However, this problem has been solved by the statistical
data processing technology such as chemometrics. On theother hand, five items as follows are needed for the nearinfrared sensor: (1) High responsivity and low noise for detecting weak absorption such as overtones and combination tones; (2) Operation not with a large-scale coolingsystem such as liquid nitrogen but with a small cooling system such as Peltier device; (3) High speed operation forreal-time inspection; (4) Good linearity of responsivity withrespect to the strength of light for quantative analyses; and(5) Array sensor for imaging (in particular, a two-dimentional array sensor is preferable to do both spectral measurement and spatial position measurement at the sametime). In this paper, a two-dimentional near infrared sensor which is a key device for the compositional imaging system “Compovision” will be introduced.

2. Near Infrared Sensor with Type-II Quantum
Well Structure
The near infrared sensors are classified into two types.
One is the photoconductive type such as PbS and PbSe,
and the other is the photovoltaic type such as Ge, InAs,InSb, HgCdTe and InGaAs. In general, the photovoltaic
type is superior to the photoconductive one from the view
point of response time and linearity of responsivity to thestrength of incident light. Among the photovoltaic type,Ge, InAs, InSb and HgCdTe sensors have high dark currents and need large-scale cooling systems such as one
using liquid nitrogen. On the other hand, the InGaAs sensor lattice matched to InP substrate shows low dark current
and high responsivity. Also, mass production technologyof the InGaAs sensor has progressed because it is used forthe photodiode of optical fiber communications, and thetwo-dimensional array sensor is already on the market.
However, because of its cut-off wavelength of 1.7 µm, thesensor can only analyze restricted materials. The cut-off
wavelength can be made longer up to about 2.6 µm by increasing the indium composition of InGaAs. But the darkcurrent due to crystal defects becomes large because of lattice-mismatch to InP. In addition, it is difficult to fabricate
two-dimensional array sensors with large chip size becauseof the difficulty of homogeneous epitaxial growth. Currently, the sensor which has a two-dimensional array andcan operate in the near infrared region up to 2.5 µm is
made of HgCdTe. However, it is difficult to apply this sensor to the wide-use analytical equipment because not only
it is very expensive due to its large-sale cooling system but
also its constituent elements have big impact against theenvironment.
Recently, type-II quantum well structures attract attention as materials to realize both longer cut-off wavelengthand lower dark current, which is difficult for the semiconductor bulk materials. The InGaAs/GaAsSb type II quan-

98 · Two-Dimensional Near Infrared Sensor with Low Noise and Wide Wavelength Rangetum well structure is a candidate for 2 µm-range infraredsensor material(1),(2). As shown in Fig. 1, the absorption
which occurs at the overlapped part between the wave-function of electrons in the conduction band of InGaAs andthe wave-function of holes in the valence band of GaAsSbcorresponds to the wavelength of 2 µm range. In this material system, both InGaAs and GaAsSb are lattice-matchedto InP substrate and the crystal defects due to lattice mismatch is not generated and therefore low dark current isexpected. Also, because the effective small bandgap is realized by combining large bandgap materials, noise current
due to thermal excitation or Auger recombination is expected to be suppressed. Moreover, the cut-off wavelength
can be easily adjusted by changing the layer thickness ofquantum wells.

Epitaxial growth（MOVPE)

Wafer process（Zn diffusion）（electrode formation）（passivation）

Micro bump bonding
IndiumbumpSensor chip

read-out IC
Sapphire lidPackaging
Ceramic PKG

Peltier device

Fig. 2. Fabrication flow of sensor
In0.53Ga0.47As (Eg=0.75eV)
Conduction band

0.49 eV ~ 2.5 µm
Valence band

GaAs0.51Sb0.49 (Eg=0.74eV)

Fig. 1. Band structure of InGaAs/GaAsSb type-II quantum wells (layerthickness is 5nm/5nm)

3. Fabrication of Two-Dimensional Array Sensor
A two-dimensional array sensor with 320 × 256 pixels
was fabricated. Each pixel has an 18 µm × 18 µm light receiving area and is arranged by 30 µm pitch. The fabrication process is illustrated in Fig. 2.
An InGaAs buffer layer, an InGaAs/GaAsSb quantum
well absorption layer (layer thickness: 5nm/5nm, pairnumber: 250), an InGaAs layer and an InP cap layer weregrown on a S-doped (100) InP substrate by metal organic
vapor phase epitaxy (MOVPE).
The p-n junctions were formed in the absorption layer
by the selective thermal diffusion of zinc in the 18 µm × 18
µm region arranged by 30 µm-pitch using SiN mask. The
p-electrodes made of Au-Zn were formed on the InP caplayer of each pixel and the n-electrode made of Au-Ge-Ni
were formed on the rear surface of the InP substrate as acommon electrode. As this sensor is the rear-illumination
type, SiON as an anti-reflection film was deposited on therear surface of the InP substrate.
The sensor chip was bonded to a complementary
metal oxide semiconductor (CMOS) read-out integrated
circuit (read-out IC) by indium bumps. After that, the hybridized chip was assembled in a ceramic package with a
Peltier device as shown in Photo 1. The sapphire lid with

Photo 1. Two-dimensional array sensor

anti-reflection film was used for a window. The photo-current generated in each pixel of the sensor chip was output
as voltage signals through the capacitive transimpedanceamplifier (CTIA) of the read-out IC. The signals werechanged to digital signals by the field-programmable gatearray (FPGA).

4. Characteristics of the Sensor
A portrait obtained by the developed two-dimensional
array sensor is shown in Photo 2. It was photographed
under the illumination of a conventional halogen lampwith the frame rate of 100 Hz and the integration time of1 msec. The signal was output by 14 bit. The clear gradation that reflects the quantity of moisture was obtained at
the skin and hair. The pixels with defect were less than 1%
in the sensor.
4-1 Dark current
The temperature dependence of dark current at
VR=60 mV of a senor with an 18 µm × 18 µm light receiving
area is shown in Fig. 3. The dark current (Id) at the temperature (T) is generally described by the following equation:
Id ∝ exp(-Eg/nkT),
where Eg is a bandgap energy of the absorption layer, k isBoltzmann constant, and n is a constant. The dark current
at the lower bias voltage consists of diffusion current gen-SEI TECHNICAL REVIEW · NUMBER 76 · APRIL 2013 · 99

Photo 2. Portrait photographed by the developed two-dimensional sensor

300K

250K200K

VR = 60 mV

10-9
10-10

HgCdTe10-11

InGaAs/GaAsSb
type-II

10-1210-13

VR = 0V

0.8
3.0

3.5

4.0

4.5

5.0

5.5

1000/T (K -1)

Fig. 3. Temperature dependence of dark currents of InGaAs/GaAsSb
type-II quantum well sensor and HgCdTe sensor

Responsivity (A/W)

Dark current (A)10-8

HgCdTe sensor were compared at the same light receiving
area and bias voltage(3). In Fig. 3, dark currents of theHgCdTe sensor are also shown as the dotted line. Our developed sensor has lower dark current than the HgCdTe
sensor by one order when compared at the same temperature. This shows that the InGaAs/GaAsSb type-II quantum
well structure is promising as the sensor material with lownoise. The histogram of dark currents in 320 × 256 pixels
at -60°C is shown in Fig. 4. The dark current of each pixel
is between 2 pA and 4 pA. This result shows the dark currents are homogeneous.
4-2 Responsivity
The responsivity characteristics in the wavelength region over 2 µm were measured using a spectrophotometer.
As shown in Fig. 5, the sensor can detect the light up to
2.35 µm, which corresponds to the effective bandgap energy of the type-II quantum well structure.
Furthermore, responsivities at the wavelength of 1.29µm, 1.53 µm, 1.96 µm and 2.20 µm were measured using
band pass filters. As shown in Fig. 6, the linearity of responsivity with respect to the strength of incident light was goodfor every wavelength. Almost all of the operable pixels(>99%) showed the same responsivity characteristics.

0.6
0.4
0.2
02.0erated near the depletion region and generation-recombination current generated via traps in the depletion region.
The n-value is unity when the diffusion current is dominant, and becomes two when the generation-recombination current is dominant. Considering Eg=0.49eV in thecase of the developed sensor, the n-value is calculated tobe 0.92 from the slope on the graph of Fig. 3, which shows
the diffusion current is dominant. The dark currents of the2.1

2.2

2.3

2.42.5Wavelength (µm)

Fig. 5. Spectral responsivity

1.0Output (arb. unit)

Frequency (pixel)

50,00040,00030,00020,0001.29µm0.6

1.96µm2.20µm0.4

0.2

10,0000λ=1.53µm

0.870,00060,0002.60

1234560Dark current (pA)

Fig. 4. Histogram of dark currents in 320×256 pixels.
Measured at VR=1.2V and T=213K

100 · Two-Dimensional Near Infrared Sensor with Low Noise and Wide Wavelength Range

020406080

Incident light strength (arb. unit)

Fig. 6. Linearity of responsivities

100These results show that our sensor is suitable for analyzing
the distribution of composition and concentration of materials by imaging.

(1)

(2)

5. Conclusion(3)

With growing expectations for the non-destructive andnon-invasive analyzing equipment using near infrared spectroscopy, the two-dimensional infrared sensor that can detect light in the near infrared region up to the 2 µm bandhas become more important. In this development, the InGaAs/GaAsSb type-II quantum well structure was adopted
to obtain longer cut-off wavelength and lower dark current,
both of which are difficult to obtain with semiconductorbulk materials. As a result, a two-dimensional array sensor
with 320 × 256 pixels was realized. This sensor can detect
light in the near infrared region up to 2.35 µm and is operated with a Peltier cooling device. Furthermore, this sensor has a good linearity of responsivity and high frame rateof 100 Hz.
Photo 3 compares sample images of the mixture ofsugar and salt photographed by the developed two-dimensional array sensor and a charge coupled device (CCD)
camera. Although sugar cannot be distinguished from salt
in the visible image photographed by a CCD camera, it canbe distinguished in the near infrared image as sugar isdark-colored. The developed sensor can be used not only
for material distinction but also for quantitative analysis because of its good linearity of responsivity.
The production costs of this sensor will be reduced byusing MOVPE technology, which is suitable for the mass
production of epitaxial wafers, and mass production techniques that have been cultivated through the developmentof photodiodes for optical fiber communication. With theabove described features, this sensor is expected to be usedin new fields as an efficient general-purpose 2 µm-range infrared sensor.

References
R. Sidhu, N. Duan, J. Campbell and A. Holmes: “A Long-Wavelength
Photodiode on InP Using Lattice-Matched GaInAs-GaAsSb Type-II
Quantum Wells,” IEEE Photon. Technol. Lett., 17, (2005) 2715
H. Inada, H. Mori, Y. Nagai, Y. Iguchi, T. Saitoh, K. Fujii, T. Ishizuka
and K. Akita: “MOVPE grown InGaAs/GaAsSb Type-II Quantum
Well Photodiode for SWIR Focal Plane Array,” Proc. of SPIE, 8012,(2011) 801220
P. Chorier, P. Tribolet: “High performance HgCdTe SWIR detectors
for hyperspectral instruments,” Proc. of SPIE, 4540 (2001) 328

Contributors (The lead author is indicated by an asterisk (*).)

Y. IGUCHI*• Ph.D.
Group Manager, Transmission Devices
R&D Laboratories

H. MORI• Assistant Manager, Transmission Devices R&D Laboratories

M. MIGITA• Assistant Manager, Transmission Devices R&D Laboratories

Y. NAGAI• Assistant General Manager, Transmission Devices R&DLaboratories

H. INADA• Senior Assistant General Manager, Transmission Devices
R&D Laboratories

K. FUJII• Semiconductor Technologies R&D Laboratories

T. ISHIZUKA• Ph.D.
Assistant Manager, Semiconductor Technologies R&DLaboratories

K. AKITA(a) Visible image(b) Near infrared image• Ph.D.
Group Manager, Semiconductor Technologies R&DLaboratories

Photo 3. Comparison of visible image and near infrared image of sugarsalt mixture.
As both materials are white, they cannot be distinguished fromeach other in (a). In (b), however, they can be distinguished assugar absorbs near infrared light and appears dark.

· Compovision is a trademark or registerd trademark of Sumitomo ElectricIndustries, Ltd.

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