For the next-generation 8 to 14μm long-wavelength infrared (LWIR) sensing, type-II superlattices (T2SLs) detectors have enormous potential to appeal to various applications including space, medical imaging, and defense. In typical absorber design, the sufficiently thick active layer (AL) is required to achieve high quantum efficiency (QE), whereas it can cause a high dark current and increase the cost. Moreover, a simple increase in AL thickness does not provide an increase in QE due to the limited carrier lifetime in T2SLs. A possible solution to the weak absorption in the AL of the T2SL-based detectors involves incorporating the AL into an optically resonant cavity. In this work, optically enhanced QE for the broadband T2SL nBn detectors will be presented through the guided-mode-resonance (GMR) structures on the top surface of the T2SL. We fabricated T2SL nBn detectors with periodic gratings on the top surface. The devices showed much-enhanced QE due to multiple resonances, as well as Fabry-Perot resonance in the thin AL with a lower dark current characteristic than the reference T2SL detector. Furthermore, we also found the broadening of the cutoff wavelength, which is typically limited by the material property, by scaling the dimension of the diffraction grating for a strong resonance beyond the cut-off region. In conclusion, the GMR-based LWIR T2SL detectors can show a significant performance enhancement in QE and extend the detection range beyond the cut-off wavelength while maintaining a low dark current.
Each MWIR and LWIR detectror, which is widely used in various civil and military including chemical identification, atmospheric monitoring, guided weapon and surveillance reconnaissance systems, is advantageous for detecting hot and cool targets, respectively. Dual-band or multi-band detector that is able to detect more than two bands with only single detector has excellent recognition and identification capablities. Therefore, various groups have studied dual-band or multi-band detector since 1998. In this work, a 20 μm 640×512 dual-band midwave and longwave infrared detector with nBn structure was studied. A nBn detector is not only effective in reducing dark current, but it is relatively simple to implement a dual-band detector by growing MWIR and LWIR absorber layers on both sides of a barrier layer. The dual-band detector acquires each MWIR and LWIR bands by selecting the applied bias direction. Consequently, the 20 μm 640×512 dual-band MWIR/LWIR FPA hybridized to read-out integrated circuit (ROIC) exhibited that an average noise equivalent temperature difference (NETD) and operability of both MWIR and LWIR modes were less than 25 mK and more than 99.5 %, respectively.
In modern infrared systems, barrier infrared detectors (BIRDs) have been widely used because a barrier is effective in reducing dark current by Shockley-Reed-Hall (SRH) process. Many researches have been studied on design of the barrier that prevents majority carrier flow and permits minority carrier flow. In this paper, we have studied on type-II superlattice (T2SL) nBn detectors having an unipolar barrier, where design and epi. growth are relatively simple for MWIR high operating temperature (HOT) and cooled LWIR detectors. InAs/InAsSb nBn for MWIR detection and InAs/GaSb nBn for LWIR detection were designed and fabricated. The fabricated MWIR and LWIR devices showed a dark current density of ≤ 2×10-6 A/cm2 at 150 K and ≤ 5×10-6 A/cm2 at 80 K, respectively. Also, 15 μm VGA MWIR and LWIR FPAs showed excellent performance with an average noise equivalent temperature difference (NETD) of ≤ 20 mK and operability of 99.5 % at 150 K and 80 K, respectively. MWIR HOT detector exhibited measured NETD similar to theoretical NETD considering dark current. And 10 μm SXGA HOT MWIR detector for high resolution imaging showed perfornance with an average NETD of ≤ 25 mK and operability of ≥99.5 % up to 130 K.
Quantum efficiency (QE) is a important parameter of infrared detector. InAs/GaSb T2SL LWIR detector has a low QE due to a small absorption coefficient compared to MCT detector. The QE can be generally increased by thickening the absorber layer but a thick absorber layer increases dark current. In this work, the QE of the InAs/GaSb T2SL LWIR photodetector was improved by Fabry-Perot resonance phenomenon. Resonance cavity is formed between front side metal and backside semiconductor-air interface mirror. At a specific wavelength, the QE is periodically increased by the resonance cavity. To broaden the resonant wavelength band, a grid pattern was formed on the backside of the detector. Consequently, the average QE of T2SL LWIR detector was improved up to ~33 % in the entire 7-9 m wavelength.
The deep mesa process for pixel isolation with ICP-RIE (Inductively Coupled Plasma – Reactive Ion Etching) was studied to develop InAs/GaSb type-Ⅱ superlattice (T2SL) LWIR photodetector with nBn structure. To reduce the lateral diffusion current component of the dark current components, it is essential to accomplish a proper deep dry etching process that can completely isolate absorption layer. In this work, ICP-RIE dry etching was studied to implement the smooth, vertical and isolated pixels. By increasing substrate temperature and adjusting the ratio of Ar in BCl3/Ar gas, it was found that the etch rate was largely increased and mesa shpae has become perpendicular and smooth. It was also found that dark current density was increased as the surface roughness increased. For the best sufrgace roughness, the dark current density of 15 μm pitch device fabricated exhibited 4.92x10-6 A/cm2 at and applied bias of -0.1 V and a temperature of 80 K.
We report our recent work on the fabrication of type-II superlattice (T2SL) LWIR nBn photodetectors. It is well known that the dangling bonds or the oxidized element on the etched mesa sidewall increase a dark current. Therefore, the passivation and treatment process for the mesa surface is the key for detector performance. In this work, we present an in-situ surface plasma treatment after the dry-etch process for the pixel isolation. To investigate the effects of the plasma treatment for the various gases (CHF3, H2, and H2/Ar), the optical and electrical analysis were performed. The results show that H2/Ar plasma treatment was effective for removing Sb-oxides at dry-etched surface. The fabricated devices which was measured at -0.1 V and 80 K shows the dark current density of -3.9 x 10-6 A/cm-2 .
High operating temperature(HOT) is the key for low size, weight and power(SWaP) detector development and SWaP detector is the key for modern weapon system such as unmanned aerial vehicle(UAV) and man portable system. The low dark current that determines the operating temperature can be achieved by adopting InAs/InAsSb type-II superlattice(T2SL) absorber and nBn structure. In this work, HOT mid-wavelength infrared(MWIR) detector with InAs/InAsSb T2SL absorber and AlAsSb barrier was developed. The AlAsSb barrier shows excellent lattice match with GaSb substrate. Only the dry etch for pixel reticulation was applied to fabricate the device. At 80 K, dark current density is 2e-9 A/cm2 at the bias -0.2 V and, at 130 K, 2e-7 A/cm2 at the bias -0.1 V. The quantum efficiency was measured for both front side illumination and back side illumination. The back side illumination offers higher quantum efficiency than the front side illumination. The average quantum efficiency is about 50 % for front side illumination with 3 μm absorber. The 640 x 512 VGA format focal plane array(FPA) with 15 μm pitch was fabricated to study the temperature dependency of electro-optical characteristics. It was found that mean noise equivalent temperature difference(NETD) below 150 K is 15 mK, which is limited by the well capacitance. As the temperature increases NETD increases proportional to the dark current.
Large format high resolution FPAs are the key elements for medium to high performance applications including enhanced vision, thermal sights, and industrial applications. In this work, the characteristics of recently developed 10 μm pitch SXGA InSb detector are presented. To develop the 10 μm pitch SXGA InSb detector, three important technical issues were resolved. At first, physically isolating pixels to reduce the crosstalk was adopted to enhance the Modulation Transfer Function (MTF). It was found that the MTF of fabricated detector tested with slanted edge method was improved largely. The MTF of 10 μm pitch FPAs at the Nyquist frequency showed the same MTF of 15 μm pitch device at the Nyquist frequency. Therefore the zoomed image of 10 μm pitch device will have the same image quality as the 15 μm device. Another important issue is the indium bump fabrication process. To fabricate fine bump with uniform height, electrodeposition technology was developed. With this method, uniform indium bump over the 8' ROIC could be achieved. Finally, to achieve large capacitance, 0.18 μm CMOS technology was adopted. To use 0.18 μm CMOS technology, the ROIC should have to be designed all again. The designed and fabricated ROIC has 2.4 Me- with 3.3 V bias voltage and has 8 output channels with 20 MHz output rate. The developed 10 μm pitch InSb SXGA detector showed median NETD (Noise Equivalent Temperature Difference) of 22.6 mK. To measure the stability of developed 10 _m pitch InSb SXGA detector, system NETD(SNETD) was measured after thermal cycling. The SNETD of 30 mK was measured for more than 200 thermal cycling, which shows that the output of developed FPA is very stable.
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