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/cm<sup>2</sup> 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.
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.
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/cm<sup>2</sup> 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 SiO<sub>2</sub> passivation film, N<sub>2</sub> plasma treatment was applied for reducing the dark currents. Measured dark current of the sensor was 4x10<sup>-7</sup>A/cm<sup>2</sup> at temperature of 77K and reverse bias of -20mV. The quantum efficiency was 0.35 and the detectivity was 4.1x10<sup>12</sup>cm/Hz<sup>1/2</sup>W. 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.