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.
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.
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.
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.
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