This paper reports on the unipolar medium wavelength infrared (MWIR) InAs/GaSb/B-Al 0.2 Ga 0.8 Sb type-II superlattice (T2SL) nBn detector's photoelectrical performance. In our model, the heterojunction barrier-active region (absorber) was assumed to be decisive as the contributing dark current mechanism limiting nBn's detector performance. The voltage drop analysis on the nBn structure was introduced to estimate the bias drop on the heterojunction barrier-active region. It was assumed that the contact n + -barrier heterojunction's layer has an insignificant influence on the electrical properties of the detector. In addition, a bulk-based model with an effective band gap of T2SL material has been assumed in the device modeling. Both current-voltage (I-V) and differential resistance-area product RA(V,T), characteristics of nBn's detector were found to be dominated by diffusion and generation-recombination currents in the zero-bias and the low-bias regions. At medium values of reverse voltages, the dark current was mostly affected by trap-assisted tunneling, whereas the band-to-band tunneling revealed its contribution at high values of reverse bias (V > 0.7 V). The RA(V,T) characteristics' fitting procedure allowed estimation of both diffusion and generation-recombination lifetimes as well as the trap energy level temperature dependence within T2SL energy gap. It was predicted that at T = 77 K, the RA product and detectivity reached values of 1000 cm 2 and 4 × 10 11 cm Hz 1/2 W -1 , respectively. The corresponding values at room temperature were 0.01 cm 2 and D * = 5 × 10 8 cmHz 1/2 W −1 , respectively. Finally, InAs/GaSb/B-Al 0.2 Ga 0.8 Sb T2SLs nBn's state of the art was compared to the performance of InAs/GaSb T2SLs PIN photodiodes and the HgCdTe bulk photodiodes operated at near-room temperature. It was shown that the RA product of the MWIR T2SLs nBn detector has reached a comparable level with the state of the art of the HgCdTe bulk photodiodes.
This paper provides update on development of the Peltier cooled detectors optimized for wavelengths above 13 µm. Initially, the devices made by Vigo were mostly used for uncooled detection of CO2 laser radiation. Over the years the performance and speed of response has been steadily improved. At present the uncooled or Peltier cooled photodetectors can be used for sensitive and fast response detection in the mid-wavelength and long-wavelength infrared spectral range. The devices have found important applications in IR spectrometry, quantum cascade laser based gas analyzers, laser radiation alerters and many other IR systems. Recent efforts were concentrated on the extension of useful spectral range to > 13 µm, as required for its application in Fourier transform IR spectrometers. This was achieved with improved design of the active elements, use of monolithic optical immersion technology, enhanced absorption of radiation, dedicated electronics, series connection of small cells in series, and last but not least, applying more efficient Peltier coolers. Practical devices are based on the complex HgCdTe heterostructures grown on GaAs substrates with metal-organic chemical vapor deposition technique with immersion lens formed by micromachining in the GaAs substrates. The results are very encouraging. The devices cooled with miniature 4 stage Peltier coolers mounted in TO-8 style housings show significant response at wavelength exceeding 16 µm.
The history, status, and recent progress in the middle and long wavelength Hg 1-x HgCdTe photodetectors in Poland The Institute of Physics Polish Academy of SciencesThe HgCdTe research began in 1960 at the Institutes of Physics of Warsaw University and Polish Academy of Scien− ces (IFPAN), Warsaw. Bridgman technique with sealed thick wall quartz ampoules was used to grow material sui− table for research and experimental devices. The explosions caused by the high mercury pressure were quite frequent that time, resulting in a few series accidents. The grown ma− terial was used for optical and galvanomagnetic studies. The studies were aimed at determination of energy band struc− ture of the material. The Hg 1−x Cd x Te studies were for a long time the main topics of numerous meetings on solid state physics and electronics in Poland. The studies were impor− tant contribution to the understanding of fundamental pro− perties of the material.Simple photoconductive, photoelectromagnetic, and photovoltaic devices based on the bulk MCT crystals were demonstrated already in 1963 [5]. Interesting, the first pho− tovoltaic devices were based on n−on−p junctions that were formed at perimeter of MCT ingots during cool down pe− riod. Later, IFPAN scientists concentrated their efforts on diluted magnetic semiconductors such as Hg 1−x Mn x Te, less useful for infrared photodetectors. The Military University of TechnologyIn 1968, device−oriented research started at the Military Uni− versity of Technology (MUT), Warsaw (see Ref. 6 and related papers therein). Initially, modification of the isothermal vapour phase epitaxy (ISOVPE) discovered by Cohen−Solal et al. [7] in France, was applied. The original process relies on near− −equilibrium isothermal evaporation of HgTe upon a thick CdTe substrate [ Fig. 1(a)]. Interdiffusion of the grown layer with the substrate material yields variable gap structures.In the Polish modification [8] [ Fig. 1(b)], the hybryde substrates with thin CdTe layers were used rather than the bulk CdTe. The layers had been deposited by vacuum evapo− ration on low−cost and readily available substrates such as mica, silicon, sapphire and other. Limited amount of HgTe was then deposited by ISOVPE and the process was carried out up to complete homogenization. As the result, extremely uniform epitaxial layers were grown.
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