We present the development status of Type II superlattice (T2SL) infrared detector in JAXA. Since 2009, we have
started a basic research on InAs/GaSb T2SL infrared detectors. Our final goal is to realize the T2SL array detector
having a cutoff wave length of λc=15 μm.
In order to confirm a technical feasibility of 15 μm cutoff T2SL detector, we fabricated T2SL samples having a different
thickness of InAs/ 7 monolayers (ML) GaSb. These crystals are designed for the cutoff wavelength from 6 μm to 15
μm. The X-ray Diffraction measurement shows a mismatch between the substrate and superlattice layers is below
0.006%. The surface morphology of the samples with an atomic force microscope is 1.5-3.3 Å RMS for 5×5 μm square
regions. We also fabricated single pixel detectors with these crystals. We show the results of the spectral response
measurement using a FTIR system. We also show the development status of an array detector. The array detector having
the cutoff wavelength of 6 μm is successfully demonstrated. However, further improvements are required for a future 15
μm cutoff array detector.
The performance of space-borne infrared detectors is required higher sensitivity, higher resolution, or larger format in
comparison with that of ground-based infrared detectors. In order to realize higher mission requirements, JAXA decided
to position the infrared detector technology as one of the strategic technologies of JAXA and to promote the
development of the infrared detectors.
InAs/GaSb Type II superlattice (T2SL) is the only known infrared material that has a theoretically predicted higher
performance than HgCdTe. If the T2SL detector is realized, it can be applied for high sensitivity infrared sensors, which
are required for many advanced instruments such as an imaging Fourier Transform Spectrometer. The final goal of the
T2SL detector development is to realize an array detector having a cutoff wavelength of λc=15μm.
We have started a basic research on the T2SL detector. In this paper, we report on the first results of the development of
T2SL detectors of mid-wave infrared regime. The detector structure is a pin photodiode with SL of 9 InAs monolayers
(MLs) and 7 GaSb MLs. We present results of optical evaluation of the detector. The cutoff wavelength is 5.5μm at 30K.
The responsivity is 0.33±0.05A/W at 4.5 μm.
We designed InAs/Ga<sub>0.6</sub>In<sub>0.4</sub>Sb superlattice (SL) material for terahertz-range photodetectors. Depending on the
thicknesses of the InAs and Ga<sub>0.6</sub>In<sub>0.4</sub>Sb layers, the SL energy gap Eg can be adjusted to be between 8-25 meV, which corresponds to a cut-off frequency from 2 to 6 THz. Different designs were numerically evaluated by using the eightband
k•p model. The calculations show that the SL energy gap is sensitive to monolayer (ML) scale variations in layer
thickness, and that realization of the design parameters requires better than 1ML accuracy of epitaxial growth.
A 40-period strained Ga<sub>0.6</sub>In<sub>0.4</sub>Sb SL with alternating InSb (1ML) and GaAs (1ML) interfaces was grown by a
molecular beam epitaxy on a GaSb substrate; the target energy gap Eg was 9 meV. The SL samples were characterized
by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence and absorption spectroscopy
measurements. Despite the large lattice mismatch between InAs and Ga<sub>0.6</sub>In<sub>0.4</sub>Sb, the XRD and AFM measurements
showed that the SL had good structural and surface quality and an accurate layer structure. The surface roughness was
We have demonstrated the operation of a far-infrared
frontside-illuminated GaAs/AlGaAs quantum well photodetector
based on intersubband absorption in a quantum well (QW) with a targeted peak frequency of 3 THz (wavelength: ~100
μm). A multiple quantum well structure consists of 20 periods of 18 nm QWs interleaved by 80 nm barriers with an Al
alloy content of 2%. We measured the following performance characteristics: dark current, responsivity, and spectral
response. A responsivity of 13 mA/W at an electric bias of 40 mV and an operating temperature of 3 K was obtained
with a peak response close to the designed detection frequency. The dark current density was a few μA/cm<sup>2</sup> and was
limited by thermally assisted tunneling through the barriers. We looked also at possible designs to optimize the device's
Terahertz imaging and spectroscopy have attracted a lot of attention in recent years, because monocycle
terahertz radiation can be generated using an ultra-short pulse laser and semiconductor device technologies. The
availability of monocycle terahertz radiation sources has encouraged innovative research and development
activities worldwide in an extremely wide range of applications, from security to medical systems. However, the
fundamental device technology, namely the semiconductor emitter, amplifier, modulator, focal plane array
detector, and optical thin film among others, in the terahertz frequencies has not yet been fully established.
Therefore, a measurement system in the terahertz range remains a costly alternative. We report in this paper our
recent developments of a terahertz quantum cascade laser (THz-QCL) and a terahertz quantum well
photo-detector (THz-QWIP). We believe that the combination of a semiconductor emitter (THz-QCL) and a
semiconductor detector array (THz-QWIP) is a good choice for developing a cost-effective measurement system
for a given terahertz range (from 1.5 THz to 5.0 THz), because both of these items are based on mass-production
semiconductor fabrication techniques.
We fabricated the THz-QCLs using a resonant longitudinal-optical phonon depopulation (RPD) scheme, which is
made up of both a GaAs/AlGaAs material system and a GaSb/AlGaSb material system. The GaAs/AlGaAs
THz-QCL has already successfully demonstrated a high peak power (about 30 milliwatts in pulsed operation)
operation at 3.1 THz and a high operating temperature (123K). On the other hand, we have fabricated a
THz-QWIP structure consisting of 20 periods of GaAs/Al<sub>0.02</sub>Ga<sub>0.98</sub>As quantum wells with a grating coupler on the
top of detector devices, and successfully operated it at 3 THz with a responsivity of 13mA/W. We now believe
we are ready to make a cost-effective measurement system, although both of the devices still require cryogenic
Gallium-doped germanium (Ge:Ga) extrinsic photoconductor is one of a excellent quantum detector in the terahertz range. Design of a novel wave-guide Ge:Ga photoconductor integrated with silicon solid immersion lens and fabrication technology for linear arrays is presented. The possibilities to extend this technology for realizing large format Ge:Ga waveguide 2D-array detector are discussed.
We have designed and fabricated GaAs/AlGaAs QWIP photodetector for THz range of spectrum (3THz, 100 μm). To evaluate suitability of this type of detector for real-time THz imaging, a prototype of a small array have been built by integrating detector elements with cryogenic readout electronics. Up to 32 individual channels can be measured with
this system at temperatures down to 4K. In this paper we present the design and expected performance of GaAs/AlGaAs THz QWIP integrated with cryogenic readout electronics (CRE), and discuss key development issues related to the design.