We have successfully fabricated extended SWIR photodetectors with the cutoff wavelength of 2.5 μm by using InGaAs (-0.3 %)/GaAsSb(+0.3 %) strain compensated type-II quantum wells as an absorption layer. The 250-pair InGaAs/GaAsSb quantum wells were grown on an InP substrate by metal organic vapor phase epitaxy. The p-n junction was formed in the absorption layer by selective zinc diffusion. Dark current was low and showed diffusion current limited mode. Quantum efficiency in the wavelength region between 2.0 μm and 2.5 μm which corresponds to the type-II absorption became twice as high as that of the normal lattice-matched InGaAs/GaAsSb type-II quantum wells.
Short wavelength infrared (SWIR) focal plane array (FPA), has an attractive application such as night vision, chemical sensing, remote monitoring of infrastructure and so on. In spite of the many trials on alternative material, FPA with HgCdTe (MCT) keep predominant position in SWIR region, especially over wavelength of 1.7μm. However, MCT is not suitable for commercial application due to its containing environmentally hazardous substances. For a commercial use, so far, Sumitomo Electric has developed FPA with InGaAs/GaAsSb type-II quantum well structures, which are based on maturity of III-V compound semiconductor epitaxial and device fabrication technology. Recently, we have successfully extended cutoff-wavelength up to 2.5μm, which showed comparable spectral range to MCT. By adopting asymmetrically the thicker layer of InGaAs in quantum wells, we modified spectral response related to the type-II transition in the quantum well. The 250-pair InGaAs/GaAsSb quantum wells structure lattice-matched InP substrates were grown by metal organic vaper phase epitaxy. The p-n junction of each pixel was formed by selective zinc diffusion. Dark current density was less than 1μA/cm2 at 213K, which means comparably to low dark current of MCT. Temperature dependence of dark current density showed diffusion current limited mode. These results means InGaAs/GaAsSb type-II FPA is a promising candidate for commercial applications. In the presentation, we will report the characteristics of InGaAs/GaAsSb type-II quantum well and the operational results of SWIR FPA.
One of JAXA’s future missions, using an imaging Fourier Transform Spectrometer (FTS), requires the focal plane array (FPA) that has high sensitivity up to the very long-wavelength infrared (VLWIR) region. Since a Type-II superlattice (T2SL) is the only known infrared material to exhibit performance that is theoretically predicted to be higher than that of HgCdTe additionally the cutoff wavelength can be tailored in the wavelength region of 3-30 μm, we started the research and development of the T2SL detector in 2009. In order to confirm our final goal, which is to realize the FPA with a cutoff wavelength of 15 μm, we first fabricated the 320 × 256 (QVGA format) InAs/GaInSb T2SL FPA with a cutoff wavelength of 15 μm, and the large-format 640 × 512 (VGA format) T2SL FPA is followed because the other missions, using an infrared imager, require the large-format FPA. The noise-equivalent delta temperature measured with F1.4 optics was 0.15 K for QVGA format T2SL FPA at 77 K. It was 0.35 K for VGA format T2SL FPA at 77 K, but there is non-uniformity, and further improvements are necessary to achieve high performance FPAs.
The authors estimate signal-to-noise ratios (SNRs) and contrasts for both InGaAs SWIR camera (cut-off wavelength λco~1.7 μm) and type II superlattice (T2SL) SWIR camera (λco~2.3 μm), under such situations as human skin as an object and vegetation as surroundings which are illuminated only by OH night airglow. In estimating the number of signal electrons, the measured spectral properties of quantum efficiencies for both InGaAs and T2SL detectors are used along with reflectance spectra of human skin and materials, while atmospheric transmission spectra are calculated with MODTRAN. As to noise electrons, shot noise resulting from dark current of InGaAs or T2SL detector is added to photon noise and ROIC (Read-Out Integrated Circuit) noise. The SNR values for the T2SL camera are found larger than those for the InGaAs camera. The contrasts of human skin vs surroundings are positive for the T2SL camera, while those for the InGaAs camera are negative.
In the short wavelength infrared (SWIR) region, InGaAs/GaAsSb type-II quantum well absorption structures are proposed as an attractive material for realizing low dark current. Recently QVGA format (array size 320×256) focal plane array (FPA) with cutoff-wavelength of 2.35 μm was demonstrated for commercial use by our group. We succeeded in extending cut-off wavelength of FPA consisting of InGaAs/GaAsSb type-II quantum well up to 2.5 μm. The 250-pairs InGaAs/GaAsSb quantum well structure lattice matched to InP substrate was grown by metal organic vapor phase epitaxy (MOVPE). The p-n junction of each pixel was formed by selective zinc diffusion method. Dark current of pixel showed the diffusion current limited mode and slightly better than that of HgCdTe with a same cutoff-wavelength. We present the electrical and optical characteristics of InGaAs/GaAsSb type-II quantum well FPA with cutoff-wavelength of 2.5 μm.
One of JAXA’s future missions, using an imaging Fourier Transform Spectrometer (FTS), require the focal plane array (FPA) that has high sensitivity and a very long-wavelength infrared (VLWIR) cutoff wavelength. Since a Type-II superlattice (T2SL) is the only known infrared material to have a theoretically predicted performance superior to that of HgCdTe and the cutoff wavelength can be tailored in the wavelength region of 3-30 μm, we started the research and development of the T2SL detector in 2009. In order to confirm our final goal which is to realize an FPA with a cutoff wavelength of 15 μm, we fabricated InAs/GaInSb T2SL infrared detectors with a cutoff wavelength of 15 μm. We show the results of the dark current and responsivity measurement of single pixel detectors and the development status of FPAs including the image taken by a 320 × 256 InAs/GaInSb T2SL FPA with a cutoff wavelength of 15 μm.
HgCdTe (MCT) is predominantly used for infrared imaging applications even in SWIR region. However, MCT is
expensive and contains environmentally hazardous substances. Therefore, its application has been restricted mainly
military and scientific use and was not spread to commercial use. InGaAs/GaAsSb type-II quantum well structures are
considered as an attractive material for realizing low dark current PDs owing to lattice-matching to InP substrate. Moreover,
III-V compound material systems are suitable for commercial use. In this report, we describe successful operation of focal
plane array (FPA) with InGaAs/GaAsSb quantum wells and mention improvement of optical characteristics. Planar type
pin-PDs with 250-pairs InGaAs(5nm)/GaAsSb(5nm) quantum well absorption layer were fabricated. The p-n junction was
formed in the absorption layer by the selective diffusion of zinc. Electrical and optical characteristics of FPA or pin-PDs
were investigated. Dark current of 1μA/cm2 at 210K, which showed good uniformity and led to good S/N ratio in SWIR
region, was obtained. Further, we could successfully reduce of stray light in the cavity of FPA with epoxy resin. As a
result, the clear image was taken with 320x256 format and 7% contrast improvement was achieved. Reliability test of
10,000 heat cycles was carried out. No degradations were found in FPA characteristics of the epoxy coated sample. This
result means FPA using InGaAs/GaAsSb type-II quantum wells is a promising candidate for commercial applications.
The cutoff wavelength of 6μm is preferable for the full usage of the atmospheric window in the mid-wavelength region. An InAs/GaSb type-II superlattice (T2SL) is the only known infrared material that has a theoretically predicted high performance and also the cutoff wavelength can be easily controlled by changing the thickness of InAs and GaSb. In this study, we used a p-i-n structure with InAs/GaSb T2SL absorber and also barrier layers which was grown on a Tedoped GaSb substrate by molecular beam epitaxy. A mesa-type focal plane array (FPA) with 320×256 pixels and 30μm pixel pitch was fabricated. Mesa structures were formed by inductively coupled plasma reactive ion etching with halogen gas mixture. Prior to the deposition of the SiO2 passivation film, N2 plasma treatment was applied for reducing the dark currents. Measured dark current of the sensor was 4x10-7A/cm2 at temperature of 77K and reverse bias of -20mV. The quantum efficiency was 0.35 and the detectivity was 4.1x1012cm/Hz1/2W. The sensor array was hybridized with the commercially available readout integrated circuit using indium bumps. The noise equivalent differential temperature measured with F/2.3 optics was 31mK at 77K. The operability was over 99%. This FPA is suitable for full usage of the atmospheric window in the mid-wavelength region.
Stray light in focal plane array (FPA) deteriorates the accuracy of hyper spectral imaging. Multiple reflections between FPA window and peripheral region of a sensor chip are considered to be the major sources of stray light. One idea for suppressing the stray light is to shield the incident light on the peripheral region of the sensor chip by narrowing the FPA window. However, it is limited by the tolerance of assembly. In this study we have examined an epoxy coating on the peripheral region such as ROIC contact pad area, AlN substrate and bonding wire. Sensor chip with InGaAs/GaAsSb type-II quantum well structures, which has the cut-off wavelength of 2.35 μm, 320×256 pixels were bonded to ROIC through indium bumps, assembled to AlN substrate and to a four stage TEC. To avoid the degradation by the stress to the chip and bonding wire, low elastic modulus epoxy was selected. Stray light suppression was confirmed by the sensor signal output of epoxy coated samples, 3% contrast improvement was achieved. Further, reliability test of 10,000 heat cycles between -75°C and 25°C was carried out. No degradations were found in sensor characteristics of the epoxy coated sample. These results suggest that the epoxy coating in SWIR FPA is effective in suppressing the stray light and suitable for hyper spectral imaging.
Focal plane array based on InAs/GaSb type-II superlattice (T2SL) is expected as an alternative to HgCdTe. To get more competitive performance of T2SL detector, we need building up more reliable fabrication process. Especially, mesa formation and passivation with understanding of surface leakage mechanism is critical issue. Generally, the existence of dangling bonds at crystal surface or damaged layer and native oxides on etched mesa sidewall leads to surface leakage currents, which mostly degrade the detector performance. Many researchers adopted SiO2 film as an effective passivation layer, which was deposited by plasma enhanced chemical vapor deposition at low temperature. However, good passivation requires not only stable film, but also an effective surface treatment before passivation. There are few reports, which discuss the relation between treatment before passivation and device characteristics in T2SL photodetectors. In this work, we present dry etching mesa formation and the effect of pretreatment of passivation on T2SL p-i-n photodetector fabrication. We investigate R0A-Perimeter/Area relation and R0A temperature dependence with in-situ plasma treatment prior to the passivation. From results of electrical characterization and interface analysis using STEM, it is recognized that in-situ N2 plasma treatment was effective to surface leakage reduction.
Infrared sensors with type II quantum well structure have gained great attention and have shown advanced progress.
InGaAs/GaAsSb type II quantum well structures are considered as an attractive material system for realizing low dark
current PDs owing to lattice-matching to InP substrate. In this report, we describe successful operation of PIN-PDs with
InGaAs/GaAsSb quantum wells grown by metal-organic vapor phase epitaxy (MOVPE). MOVPE method is well-known
to have good uniformity which leads to mass-production of focal plane array. Planer type pin-PDs were adopted. The p-n
junction was formed in the absorption layer by the selective diffusion of zinc. Electrical and optical characteristics of
pin-PDs such as well number dependence of responsivity, were investigated. Dark current was 9.0 μA/cm2 at 233 K,
which has better uniformity compared to those of MBE sample, and responsivity of 0.8 A/W in SWIR region were
obtained. This result indicates that planer photodiode using MOVPE grown InGaAs/GaAsSb type II quantum wells is a
promising candidate for consumer applications.
Low dark current photodiodes (PDs) in the short wavelength infrared (SWIR) upto 2.5μm region, are expected for
many applications. HgCdTe (MCT) is predominantly used for infrared imaging applications. However, because of high
dark current, MCT device requires a refrigerator such as stirling cooler, which increases power consumption, size and
cost of the sensing system. Recently, InGaAs/GaAsSb type II quantum well structures were considered as attractive
material system for realizing low dark current PDs owing to lattice-matching to InP substrate. Planar type PIN-PDs were
successfully fabricated. The absorption layer with 250 pair-InGaAs(5nm)/GaAsSb(5nm) quantum well structures was
grown on S-doped (100) InP substrates by solid source molecular beam epitaxy method. InP and InGaAs were used for
cap layer and buffer layer, respectively. The p-n junctions were formed in the absorption layer by the selective diffusion
of zinc. Diameter of light-receiving region was 140μm. Low dark current was obtained by improving GaAsSb crystalline
quality. Dark current density was 0.92mA/cm2 which was smaller than that of a conventional MCT. Based on the same
process as the discrete device, a 320x256 planar type focal plane array was also fabricated. Each PD has 15μm diameter
and 30μm pitch and it was bonded to read-out IC by using indium bump flip chip process. Finally, we have successfully
demonstrated the 320 x256 SWIR image at room temperature. This result means that planer type PD array with the type
II InGaAs/GaAsSb quantum well structure is a promising candidate for uncooled applications.
Enhanced performance has been observed for molecular organic light emitting diodes (MOLEDs) consisting of two to four organic layers sequentially vacuum vapor deposited onto patterned indium-tin oxide (ITO) on polyester (PET) films. For the device structures studied, the performance of diodes fabricated on polyester is comparable to or better than their analogs on glass substrates. For example, at 100 A/m2, a luminous power efficiency of 4.4 lm/W and external quantum yield of 2.7 percent is observed for a device structure consisting of two hole transport layers, a doped emitting layer and an electron transport layer on a polyester substrate. The same device made on a glass substrate has a luminous power efficiency of 3.5 lm/W and external quantum yield of 2.3 percent. The enhanced performance of the plastic MOLEDs is attributed to increased optical output coupling. Electrical and optical performance for comparative device structures has been characterized by current-voltage-luminance measurements and electroluminescence spectra, and ITO surface morphology has been studied by Atomic Force Microscopy.