Technology of infrared (IR) avalanche photodiodes (APDs) gradually moves from simple single element APD to 2D focal plane arrays (FPA). Spectral covering of APDs is expanded continuously from classic 1.3 μm to longer wavelengths due to using of narrow-gap semiconductor materials like Hg1-xCdxTe. APDs are of great interest to developers and manufacturers of different optical communication, measuring and 3D reconstruction thermal imaging systems. Major IR detector materials for manufacturing of high-performance APDs became heteroepitaxial structures InxGa1-xAsyP1-y and Hg1-xCdxTe. Progress in IR APD technology was achieved through serious improvement in material growing techniques enabling forming of multilayer heterostuctures with separate absorption and multiplication regions (SAM). Today SAM-APD design can be implemented both on InxGa1-xAsyP1-y and Hg1-xCdxTe multilayer heteroepitaxial structures. To create the best performance optimal design avalanche heterophotodiode (AHPD) it is necessary to carry out a detailed theoretical analysis of basic features of generation, avalanche breakdown and multiplication of charge carriers in proper heterostructure. Optimization of AHPD properties requires comprehensive estimation of AHPD’s pixel performance depending on pixel’s multi-layer structure design, layers doping, distribution of electric field in the structure and operating temperature. Objective of the present article is to compare some features of 1.55 μm SAM-AHPDs based on InxGa1-xAsyP1-y and Hg1-xCdxTe.
Technology of infrared (IR) photovoltaic (PV) focal plane arrays (FPA) covering spectral range from 1.6 to 14 μm gradually moves from simple quasi-matrix (linear) arrays like as 4×288 pixels to large format high definition arrays 1280×1024 pixels and more. Major infrared detector materials for PV technology are InSb and its alloys and ternary alloys Hg1-xCdxTe. Progress in IR PV technology was provided in last decade by serious improvement in material growing techniques. Increasing of PV array format is related always to decreasing of pixel size and spacing between neighbor pixels to minimal size reasonable from point of view of infrared physics. So pitch is small (15-25 μm) in
large format arrays. Ambipolar diffusion length of photogenerated charge carriers can exceed pitch many times in high quality absorption layers of PV arrays. It means that each pixel can collect excess charge carriers generated far from n+-p junction border. Optimization of resolution, filling factor and cross-talking level of small-pitched PV FPA requires comprehensive estimation of photodiode's (PD) pixel performance depending on pixel and array design, material properties and operating conditions. Objective of the present work was to develop general approach to estimate collection of photogenerated charge carriers in small-pitched arrays.
Multilayer heterostructures of Hg1-xCdxTe alloy grown by Molecular Beam Epitaxy (MBE) on large size alternative
substrates Si, GaAs and Ge are considered as one of productive alternative materials for issue of large format
photovoltaic (PV) infrared (IR) focal plane arrays. However reaching of ultimate performance of small-pitched
photodiode's (PD) covering spectral range from 8 to 12 μm depends on electronic properties of both individual layers
and heterostructure interfaces. Due to small thickness of heterostructure layers, interfaces are located close to active
regions of p-n junction and hence generation-recombination processes at interfaces will contribute to value of current
flowing through junction. As usual measured dark current value of small-sized PD is higher than estimated from
calculation and cannot be explained by discrepancy between real and estimated charge carriers concentration in
absorption layers where p-n junction is formed. Objective of the present work was to calculate the contribution of
recombination of charge carriers via electronic states on nearby inner interface to dark current of Hg1-xCdxTe LWIR PD (λco equals to 9.5-10.3 μm at Top=77 K) and its variation with absorption layer parameters and compare it to
measured data on small-pitched arrays. We have concluded previously that at high recombination rate dark current can
grow in orders of value.
Ways to improve performance of scanning and starring types photovoltaic (PV) infrared (IR) Hg1-xCdxTe focal plane arrays (FPA) including Hg1-xCdxTe Long-Wave (LWIR) PV FPA covering spectral range from 8 to 12 μm requires comprehensive estimation of photodiode's (PD) performance depending on Hg1-xCdxTe material properties and operating conditions and comparing it to real data published for commercial products. Advanced Infrared Focal Plane Arrays including extended Long-Wave (LWIR) 8-14 μm operating at temperatures Top=80-100 K can be developed with due accounting of correlation between measured and correctly calculated performance. Optimized PD with n+-p junction is characterized by lower dark current value than previously reported in same kind n+-p junction. Lowest dark current is preferable for proper multiplexing of PD arrays to Silicon Read-out Integrated Circuits (ROICs) and hence to get highest possible performance as well. Comparative analysis of LWIR PD performance at 80 K is needed also to understand ways to improve FPA useful quality. Objective of the present work was to calculate Hg1-xCdxTe LWIR PV FPA (λco equals to 9.5-10.3 μm at Top=80 K) performance variation with doping level, absorber thickness, surface recombination rate and compare it to measured data on arrays with time-and-delay integration (TDI) mode of operation. Commercial LWIR PV FPAs are fabricated as usual on Hg1-xCdxTe layers grown by Liquid Phase Epitaxy (LPE).
Realization of affordable large format high performance photovoltaic (PV) infrared (IR) Hg1-xCdxTe based focal plane
arrays (FPA) covering spectral ranges Mid-Wave (MWIR) from 3 to 5.5 μm and extended Long-Wave (LWIR) from 8
to 14 μm requires comprehensive estimation of photodiodes performance depending on Hg1-xCdxTe material properties
and operating conditions. Advanced Infrared Focal Plane Arrays include Mid-Wave (MWIR) 3-5.5 μm operating at
temperatures Top=80-100 K and at higher temperatures (HOT) Top=200-240 K, extended Long-Wave (LWIR) 8-14 μm
operating at temperatures Top=80-100 K and multi-color arrays. Perhaps novel FPA will be based on photodiodes (PD)
with p-n junction opposite to usually used n+-p junction. PD with optimal p-n junction could have lower dark current
value than same size n+-p junction. It is desirable for proper multiplexing of PD arrays to Silicon Read-out Integrated
Circuits (ROICs). Comparative analysis of LWIR PD performance at 80 K and 100 K is needed also due to strong
tendency to lowering weight and power consumption of perspective megapixel FPA. Objective of the present work was
to calculate Hg1-xCdxTe MWIR and LWIR PV FPA (λp equals to 4.5-4.8 μm at Top=225 K responding 2-3 stages thermal
electric cooler temperature and 8.0-9.0 and 10.0-10.5 μm at Top=80-100 K) performance variation with doping level,
absorber thickness, surface recombination rate and operating temperature.
Full replacement of scanning type thermal imaging (TI) equipment by starring type TI is on agenda of XXI century.
Realization requires development and launching production of affordable large format high performance photovoltaic
(PV) infrared (IR) focal plane arrays (FPA) covering spectral range from 1.3 to 14 μm. Advanced Infrared Focal Plane
Arrays include Short-Wave (SWIR) 1.3-2.5 μm operating at near room temperature Top=300 K, Mid-Wave (MWIR) 3-
5.5 μm operating at higher temperatures (HOT) Top=200-240 K, extended Long-Wave (LWIR) 8-14 μm operating at
temperatures Top=80-100 K and multi-color arrays. Challenge is serious and adequate research and development works
have to be done. One of major approach is fabrication of megapixel IRFPA based on Hg1-xCdxTe epitaxial multi-layer
structures grown by molecular beam epitaxy (MBE). To be successful with approach it is necessary to optimize design
of pixel and develop reproducible MBE growth technology issuing epitaxial structures with perfect layers and
interfaces. Perhaps novel FPA will be based on photodiodes (PD) with p-n junction opposite to usually used n+-p
junction. PD with optimal p-n junction could have lower dark current value than same size n+-p junction. It is desirable
for proper multiplexing of PD arrays to Silicon Read-out Integrated Circuits (ROICs). Objective of the present work was
to examine Hg1-xCdxTe LWIR PV FPA (λp ranged from 10 to 11 μm at Top=80-100 K) performance variation with
doping level, absorber thickness, interface shunting and operating temperature.
Ultimate performance small-pitched infrared focal plane arrays (FPA) are of the great interest for development and production of state-of-the-art special and common use thermal imagers. Novel MBE-grown Hg1-xCdxTe/CdZnTe/GaAs heterostructures (MCT/CZT on GaAs HS) are considered perspective for implementation of sophisticated FPA concepts including multi-color and HOT (high operating temperature). Performance of MWIR and LWIR photoconductive (PC) and photovoltaic (PV) infrared detectors fabricated on the base of a. m. heterostructures are presented. The main feature of developed technology is formation of multi-layer device structure in single MBE growth run with precise control of thickness and alloy composition "x" across individual layers and hence throughout heterostructure. Flexible HS design results in half-finished products of PC and PV detectors with optimized parameters of absorber and perfect interfaces between absorber and blocking layers providing effective suppression of surface recombination and surface leakage currents. Giant peak responsivity RV (λco=10.5 μm, 500 K, 1200 Hz) over 6,0×105 V/W was reached on LWIR PC. Average values D* = 1.8×1011 cmHz1/2W-1 and NEDT = 9 mκ were measured on 4×288 PV FPA with λco = 11.2 μm at T=78 K just as 23 mK and 19 mκ on 320×256(240) PV FPA with λco = 5.5 and λco = 10.2 μm at T=78 K.
Narrow-gap low-doped n-Hg1-xCdxTe (x = 0.18-0.35) material with electron concentration at liquid nitrogen temperature (77 K) n77≈(1-10)×1014 cm-3 is preferably used by all manufacturers of infrared (IR) radiation detectors as absorbing layer (n-absorber) of high performance Mid-Wave (MWIR), Long-Wave (LWIR) and Very Long-Wave (VLWIR) photoconductors (PC) covering spectral range from 3 to 25 μm. Low-doped n-Hg1-xCdxTe (x = 0.21-0.65) material with n77≈(1-5)×1015 cm-3 is perspective for development and production of novel small- pitched photovoltaic (PV) Short-Wave (SWIR), MWIR, LWIR and multi- color infrared focal plane arrays (FPA) covering spectral range from 1.3 to 12 μm. Novel FPA is based on photodiodes (PD) with p-n junction opposite to usually used n+-p junction. PD with optimal p-n junction could have lower dark current value than same size n+-p junction. It is very desirable for adequate multiplexing of PD arrays by Silicon Read-out Integrated Circuits (ROIC). Low-doped n-Hg1-xCdxTe could be grown by different techniques: bulk crystallization; liquid phase epitaxy (LPE); molecular beam epitaxy (MBE) having seriously different referring (melting/crystallization/annealing) temperatures. Objective of the present work was to examine and compare some crystalline, chemical and physical parameters and impact of PC device performance on their variation.
Spectral density of "generation-recombination" noise voltage <δV2gr> ("g-r noise") in photoconductive Hg1-xCdxTe infrared radiation detectors with absorber n-Hg1-xCdxTe layer was calculated. Variations of <δV2gr> with doping level (n ≈ Nd), ambient background flux density (Qbgr, Tbgr ≈ 300 K), electrical bias (Vb/Ib) and pixel active area were analyzed. Spectral density of low-frequency noise as superposition of Flicker-noise "1/f", g-r noise resulting from fluctuations in generation-recombination rates of equilibrium (thermal) charge carriers <δV2gr,th> and excess charge carriers exited by background photons <δV2gr,bgr> and Johnson-Nyquist noise <δV2JN> were examined in small active area (30 μm x 30 μm and 50 μm x 50 μm) Hg1-xCdxTe photoconductors based on MBE-grown multi-layer structures. Noise measurements were performed on Long-Wave (LWIR) PC MCT detectors with responsivity peak wavelength 10 ≤ λp ≤ 12 μm at operating temperature Top ≈ 78 K. Measured dependencies of g-r noise voltage spectral density have confirmed BLIP mode of photoconductors up to FOV=10 degrees where D*(λp) exceed 2x1011 Jones.
Compact small-pitched infrared focal plane arrays (FPA) having ultimate performance are of the great interest for development and production of state-of-the-art special and common use thermal imaging systems lighter in weight and with lower energy consumption. Novel MBE-grown Hg1-xCdxTe epitaxial multi-layer structures are considered perspective for manufacturing of such FPA. Objective of the present work was to examine the impact of small-pitched Long-Wave Infrared (LWIR) Hg1-xCdxTe photoconductor device performance on variation of background flux density. Peak wavelength λp was ranged from 10.5 to 11.5 μm at 78-100 K. High performance small active area photoconductors based on MBE-grown multi-layer structures consisting of homogeneous narrow-gap n-Hg1-xCdxTe absorbing layer (n-absorber) both side blocked by thin graded-gap Hg1-xCdxTe layers have been fabricated and examined. Availability of innovative Hg1-xCdxTe epitaxial material (half-finished products of photoconductors - three-layer sensitive structures grown by MBE in single run) gives opportunity to manufacture and offer versatile detectors with flexible tuning of electro-optical parameters. Multi-element (2 x 32=64 elements) Hg1-xCdxTe photoconductors with pixel's active area size 30 μm x 30 μm and pitch 45 µm were tested. Electro-optical measurements have shown improved value of peak responsivity and detectivity close to theoretically predicted for model photoconductor.
High performance large active area photoconductors based on MBE-grown multi-layer structures consist of homogeneous narrow-gap n-Hg1-xCdxTe absorbing layer (n-absorber) blocked by thin adjacent graded-gap Hg1-xCdxTe layers have been fabricated and examined. Large active area (from 0.25 mm x 0.25 mm to 2.25 mm x 2.25 mm) Hg1-xCdxTe photoconductors with improved responsivity in Mid-Wave 3.0-5.5 μm (MWIR); Long-Wave 8-14 μm (LWIR) and Very Long-Wave 14-20 μm (VLWIR) infrared spectral ranges are very attractive for use in state-of-the art IR imaging, analytical and spectroscopic equipment. Synergy of advanced Hg1-xCdxTe detectors with IR-fiber optics, especially based on polycrystalline infrared (PIR-) fiber (4-18 μm) cables and bundles, provides above mentioned equipment with qualitatively new possibility like as remote probing of the objects which are difficult to access or beyond direct optical access. Availability of innovative Hg1-xCdxTe epitaxial material (half-finished products of photoconductors - three-layer sensitive structures grown by MBE in single run) open perspective to manufacture and offer improved detectors for much number of applications. Low temperature MBE growth technique provides better tuning of detectors' spectral responsivity curves to the ordered spectral ranges. Measurements performed on fabricated photoconductors showed significantly increased value of peak responsivity and high level of detectivity.
Photoconductors based on multi-layer structure consists of homogeneous narrow-gap n-Hg1-xCdxTe absorbing layer (n-absorber) blocked by thin adjacent graded-gap Hg1-xCdxTe layers have been fabricated and examined. A possible giant increase in responsivity of Long-Wave Infrared (LWIR) photoconductor (spectral range from 8 to 14 μm) and Very Long-Wave Infrared (VLWIR) photoconductor (spectral range longer than 14 μm) at 78-100K operating temperature was predicted. Prediction is based on suggestion that interfaces in three-layer sensitive structure grown by MBE in single run and consists of n-absorber and adjacent graded-gap layers of native material and same type of conductivity will be free of both recombination centers and charge states. Theoretical analysis has shown that formation of diffusion barrier within graded-gap layers is occurred during illumination of photoconductor. That diffusion barrier prevents excess holes excited in homogenous absorber layer from moving to surfaces. Therefore excess holes will recombine preferably in active region of photoconductor, thus giving high quantum efficiency and good responsivity. Measurements performed on fabricated photoconductors showed near ideal background limited performance (BLIP) with significantly increased value of peak responsivity. Wide shape of spectral responsivity curves is evidence that surface recombination at interfaces was eliminated.
Innovative Polycrystalline IR-fibers (PIR-) from Silver Halide open a variety of promising fiber applications in 4-18 μm range. PIR-fibers coupled with IR-detectors enable unique flexible 2D & 3D IR-imaging, including IR-endoscopy with PIR-fiber bundles. FTIR- and other IR-spectrometers with PIR-fiber probes may be used for remote process molecular control, for in-vivo diagnostics of tissue and for pollution monitoring in liquid and gaseous phase in "finger-print" part of spectrum.
Spectral density of "generation-recombination" noise voltage <δV2gr> ("g-r noise") in Photoconductive Mercury-Cadmium Telluride / Hg1-xCdxTe (PC MCT) infrared radiation detectors with absorber n-Hg1-xCdxTe layer was calculated. Variations of <δV2gr> with doping level (n ≈ Nd), ambient background flux density (Qbgr, Tbgr ≈ 300 K), electrical bias (Vb/Ib) and design of sensitive pixel were analyzed. Spectral density of low-frequency noise as superposition of Flicker-noise "1/f", g-r noise resulting from fluctuations in generation-recombination rates of equilibrium (thermal) charge carriers <δV2gr, th> and excess charge carriers exited by background photons <δV2gr,bgr> and Johnson-Nyquist noise <δV2JN> were examined in Hg1-xCdxTe photoconductors based on MBE-grown multi-layer structures. Noise measurements were performed on Long-Wave (LWIR) PC MCT detectors with responsivity peak wavelength 10 ≤ λp ≤ 12 μm at operating temperature Top ≈ 290-300 K and 78 K. Registration and recording of noise voltage spectral density graphs were performed in frequency range from 6 Hz to 12.5 kHz with resolution equals to 6 Hz. Tested PC MCT detectors show extremely low spectral density of excess Flicker-noise with cut-off frequency (Fco) ranging from 10 to 300 Hz. Measured dependencies of g-r noise voltage spectral density are correlated with calculations.
New generation of Mercury-Cadmium-Telluride (MCT) high performance infrared radiation (IR) detectors with IR-fiber input has been developed and fabricated. This new product is originated from 25 years experience in MCT detectors and IR fiber optics technologies. Range of products includes single- and multi-element detectors designed for registration of optical signals in spectral range from 2 to 18 μm. Detectors design is integrated or modular and includes package, sensitive element, cooling system, operating temperature sensor, optical components such as narrow band-pass filter and/or lens and/or different kind of optical window, optical connection unit and fiber pig-tail or fiber cable. Cooling system options include thermoelectric cooler, long-holding time dewar filled with liquid nitrogen, Joule-Thomson micro-liquidizer and Stirling-cycle cooler. Registered infrared radiation is delivered to sensitive area of detector through either Polycrystalline InfraRed (PIR-) Fiber (4 - 18 μm) or Chalcogenide IR-glass (CIR-) Fiber (2 - 6 μm). Unique feature intrinsic to Hg1-xCdxTe (MCT) alloys to form continuous series of alloy compositions "x" with proportionally changed energy gap Eg(x,T) allows to tune spectral responsivity of detector sensitive element with ordered spectral range and hence to use every time the highest sensitive detector.
Low-frequency flicker-noise “1/f” spectra were examined on Photoconductive Mercury-Cadmium Telluride / Hg1-xCdxTe (PC MCT) infrared radiation detectors. Noise measurements were performed on PC MCT detectors with responsivity peak wavelength λp from 3,0 to 5,5 μm and from 10,5 to 12,0 μm at operating temperature Top ≈ 205 - 210 K and ≈ 78 K respectively. Tested high quality PC MCT infrared radiation detectors show extremely low spectral density of excess “1/f” noise and low values of noise-knee frequency from 10 to 150 Hz and demonstrate high-level performance.
x1÷11 μm optical code sensor with Gray Code system based on miniaturized Photoconductive Mercury-Cadmium Telluride / Hg1-xCdxTe (PC MCT) detector with three-stage thermoelectric cooler (TEC) has been developed and fabricated. Performance of PC MCT detector is presented evidencing good detectivity D* and responsivity RV values in spectral range from, 1,0 to 10,6μm. Sensitive area structure allows producing of four digits Gray Code. Sensor can be used in electro-optical systems for spatial recognition of powerful sources of coherent infrared radiation.
Variations of peak detectivity D*(λp) and peak responsivity RV(λp) values with background flux density have been examined on Photoconductive Mercury-Cadmium Telluride / Hg1-xCdxTe (PC MCT) infrared radiation detectors. Measurements were performed at operating temperature Top ≈ 78 K. Performance was measured on multi-element PC MCT infrared radiation detectors (N = 3x32 and 128 elements) with responsivity peak wavelength λp from 10,5 to 12,0 μm at the integral background flux density varied from (8-10)E+17 to (6-8)E+15 photon•cm-2•sec-1. Tested PC MCT infrared radiation detectors demonstrate background limited performance (BLIP).
Wide nomenclature of Photoconductive Mercury-Cadmium Telluride / Hg1-xCdxTe (PC MCT) infrared radiation detectors with three-stage thermoelectric cooler (TEC) has been developed and fabricated. Performance of PC MCT infrared detectors are presented evidencing excellent peak detectivity D*(λp) and peak responsivity RV(λp) values in spectral ranges from 3,0 to 5,5 μm and from 7,5 to 10,6 μm.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.