Infrared detectors: an overview Antoni Rogalski * Institute of Applied Physics, Military University of Technology, 2 Kaliskiego St., 00-908 Warsaw 49, Poland Abstract The paper presents progress in infrared (IR) detector technologies during 200 history of their development. Clas- sification of two types of IR detectors (photon detectors and thermal detectors) is done on the basis of their principle of operation. The overview of IR systems and detectors is presented. Also recent progress in different IR technologies is described. Discussion is focused mainly on current and the most rapidly developing detectors: HgCdTe heterostructure photodiodes, quantum well AlGaAs/GaAs photoresistors, and thermal detectors. The outlook for near-future trends in IR technologies is also presented. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Photon detectors; Thermal detectors; Focal plane arrays; Two-colour detectors 1. Introduction Looking back over the past 1000 years we no- tice that infrared (IR) radiation itself was un- known until 200 years ago when Herschel’s experiment with thermometer was first reported. He built a crude monochromator that used a thermometer as a detector so that he could mea- sure the distribution of energy in sunlight. In April 1800 he wrote [1]: Thermometer No. 1 rose 7 de- grees in 10 minutes by an exposure to the full red coloured rays. I drew back the stand... thermometer No. 1 rose, in 16 minutes, 8 3 8 degrees when its centre was 1=2 inch out of the visible rays. The early history of IR was reviewed about 40 years ago in two well-known monographs [2,3]. The most important steps in development of IR detectors are the following: • in 1921 Seebeck discovered the thermoelectric effect and soon thereafter demonstrated the first thermocouple, • in 1829 Nobili constructed the first thermopile by connecting a number of thermocouples in se- ries [4], • in 1933 Melloni modified thermocouple design and used bismuth and antimony for it [5]. Langley’s bolometer appeared in 1880 [6]. Langley used two thin ribbons of platinum foil, connected so as to form two arms of a Wheatstone bridge. Langley continued to develop his bolome- ter for the next 20 years (400 times more sensitive than his first efforts). His latest bolometer could detect the heat from a cow at a distance of a quarter of mile. Thus, at the beginning the devel- opment of IR detectors was connected with ther- mal detectors. The photon detectors were developed in XX century. The first IR photoconductor was devel- oped by Case in 1917 [7]. In 1933 Kutzscher at Infrared Physics & Technology 43 (2002) 187–210 www.elsevier.com/locate/infrared * Fax: +48-22-685-9109. E-mail address: [email protected](A. Rogalski). 1350-4495/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S1350-4495(02)00140-8
Transcript
Infrared detectors: an overview
Antoni Rogalski *
Institute of Applied Physics, Military University of Technology, 2 Kaliskiego St., 00-908 Warsaw 49, Poland
Abstract
The paper presents progress in infrared (IR) detector technologies during 200 history of their development. Clas-
sification of two types of IR detectors (photon detectors and thermal detectors) is done on the basis of their principle of
operation. The overview of IR systems and detectors is presented. Also recent progress in different IR technologies is
described. Discussion is focused mainly on current and the most rapidly developing detectors: HgCdTe heterostructure
photodiodes, quantum well AlGaAs/GaAs photoresistors, and thermal detectors. The outlook for near-future trends in
IR technologies is also presented. � 2002 Elsevier Science B.V. All rights reserved.
Looking back over the past 1000 years we no-tice that infrared (IR) radiation itself was un-known until 200 years ago when Herschel’sexperiment with thermometer was first reported.He built a crude monochromator that used athermometer as a detector so that he could mea-sure the distribution of energy in sunlight. In April1800 he wrote [1]: Thermometer No. 1 rose 7 de-grees in 10 minutes by an exposure to the full redcoloured rays. I drew back the stand. . . thermometerNo. 1 rose, in 16 minutes, 83
8degrees when its centre
was 1=2 inch out of the visible rays.The early history of IR was reviewed about 40
years ago in two well-known monographs [2,3].The most important steps in development of IRdetectors are the following:
• in 1921 Seebeck discovered the thermoelectriceffect and soon thereafter demonstrated the firstthermocouple,
• in 1829 Nobili constructed the first thermopileby connecting a number of thermocouples in se-ries [4],
• in 1933 Melloni modified thermocouple designand used bismuth and antimony for it [5].
Langley’s bolometer appeared in 1880 [6].Langley used two thin ribbons of platinum foil,connected so as to form two arms of a Wheatstonebridge. Langley continued to develop his bolome-ter for the next 20 years (400 times more sensitivethan his first efforts). His latest bolometer coulddetect the heat from a cow at a distance of aquarter of mile. Thus, at the beginning the devel-opment of IR detectors was connected with ther-mal detectors.
The photon detectors were developed in XXcentury. The first IR photoconductor was devel-oped by Case in 1917 [7]. In 1933 Kutzscher at
University of Berlin, discovered that lead sulphide(from natural galena found in Sardinia) wasphotoconductive and had response to about 3 lm[8].
Many materials have been investigated in theIR field. Observing a history of the development ofthe IR detector technology, a simple theorem, afterNorton [9], can be stated: All physical phenomenain the range of about 0.1–1 eV can be proposed forIR detectors. Among these effects are: thermo-electric power (thermocouples), change in elec-trical conductivity (bolometers), gas expansion(Golay cell), pyroelectricity (pyroelectric detec-tors), photon drag, Josephson effect (Joseph-son junctions, SQUIDs), internal emission (PtSiSchottky barriers), fundamental absorption (in-trinsic photodetectors), impurity absorption (ex-trinsic photodetectors), low-dimensional solids(superlattice (SL) and quantum well (QW) detec-tors), different type of phase transitions, etc.
Fig. 1 gives approximate dates of significantdevelopment efforts for the materials mentioned.The years during World War II saw the origins ofmodern IR detector technology. Interest has cen-tred mainly on the wavelengths of the two atmo-spheric windows 3–5 and 8–14 lm, though inrecent years there has been increasing interest inlonger wavelengths stimulated by space applica-tions [10].
2. Classification of IR detectors
Progress in IR detector technology is connectedmainly to semiconductor IR detectors, which areincluded in the class of photon detectors. In this
class of detectors the radiation is absorbed withinthe material by interaction with electrons. Theobserved electrical output signal results from thechanged electronic energy distribution. The pho-ton detectors show a selective wavelength depen-dence of the response per unit incident radiationpower. They exhibit both perfect signal-to-noiseperformance and a very fast response. But toachieve this, the photon detectors require cryo-genic cooling. Cooling requirements are the mainobstacle to the more widespread use of IR systemsbased on semiconductor photodetectors makingthem bulky, heavy, expensive and inconvenient touse. Depending on the nature of interaction, theclass of photon detectors is further sub-dividedinto different types. The most important are: in-trinsic detectors, extrinsic detectors, photoemissive(metal silicide Schottky barriers) detectors, andquantum well detectors. Table 1 shows the com-parison of various IR detectors.
The second class of IR detectors is composed ofthermal detectors. In a thermal detector the inci-dent radiation is absorbed to change temperatureof the material, and the resultant change in somephysical properties is used to generate an electricaloutput. The detector element is suspended on lags,which are connected to the heat sink. Thermaleffects are generally wavelength independent; thesignal depends upon the radiant power (or its rateof change) but not upon its spectral content. Inpyroelectric detectors a change in the internalspontaneous polarisation is measured, whereas inthe case of bolometers a change in the electricalresistance is measured. In contrast to photon de-tectors, the thermal detectors typically operate atroom temperature. They are usually characterized
Fig. 1. History of the development of IR detectors.
by modest sensitivity and slow response but theyare cheap and easy to use. The greatest utility inIR technology has found bolometers, pyroelectricdetectors and thermopiles.
Up till the nineties, thermal detectors have beenconsiderably less exploited in commercial andmilitary systems in comparison with photon de-tectors. The reason for this disparity is that ther-mal detectors are popularly believed to be ratherslow and insensitive in comparison with photondetectors. As a result, the worldwide effort to de-velop thermal detectors was extremely small rela-tive to that of photon detector. In the last decadehowever, it has been shown that extremely goodimagery can be obtained from large thermal de-tector arrays operating uncooled at TV framerates. The speed of thermal detectors is quite ad-equate for non-scanned imagers with two-dimen-sional (2D) detectors. The moderate sensitivity ofthermal detectors can be compensated by a largenumber of elements in 2D electronically scannedarrays. With large arrays of thermal detectors the
best values of NEDT, below 0.1 K, could bereached because effective noise bandwidths < 100Hz can be achieved.
Uncooled, monolithic focal plane arrays (FPAs)fabricated from thermal detectors may revolutio-nise development of thermal imagers. Recently,very encouraging results have been obtained withmicromachined silicon bolometer [10,12] and py-roelectric detector arrays [10,13].
3. Focal plane arrays
There are a number of architectures used in thedevelopment of IR FPAs. In general, they may beclassified as hybrid and monolithic. The centraldesign questions involve performance advantagesversus ultimate producibility. Each applicationmay favour a different approach depending onthe technical requirements, projected costs, andschedule.
Table 1
Comparison of IR detectors (after Ref. [11])
Detector type Advantages Disadvantages
Thermal Light, rugged, reliable, and low cost Low detectivity at high frequency
Room temperature operation Slow response (ms order)
Photon
Intrinsic
IV–VI Available low-gap materials Poor mechanical
Well studied Large permittivity
II–VI Easy band-gap tailoring Non-uniformity over large area
Well-developed theory and exp. High cost in growth and processing
Multicolour detectors
III–V Good material and dopants Heteroepitaxy with large lattice mismatch
Advanced technology
Possible monolithic integration
Extrinsic Very long wavelength operation Extremely low temperature operation
Relatively simple technology
Free carriers Low-cost, high yields Low quantum efficiency
Large and close packed 2D arrays Low temperature operation
Quantum wells
Type I Matured material growth Low quantum efficiency
Good uniformity over large area Complicated design and growth
Multicolour detectors
Type II Low Auger recombination rate Complicated design and growth
Easy wavelength control Sensitive to the interfaces
In the monolithic approach, some of the mul-tiplexing is done in the detector material itself thanin an external readout circuit. The basic element ofa monolithic array is a metal–insulator–semicon-ductor (MIS) structure as shown in Fig. 2(c). AMIS capacitor detects and integrates the IR-gen-erated photocurrent. Although efforts have beenmade to develop monolithic FPAs using narrow-gap semiconductors, silicon-based FPA technol-
ogy with Schottky-barrier detectors is the onlytechnology, which has matured to a level ofpractical use.
Hybrid FPAs detectors and multiplexers arefabricated on different substrates and mated witheach other by the flip-chip bonding (Fig. 3) orloophole interconnection. In this case we can op-timise the detector material and multiplexer inde-pendently. Other advantages of the hybrid FPAs
are near 100% fill factor and increased signal-processing area on the multiplexer chip. In theflip-chip bonding, the detector array is typicallyconnected by pressure contacts via indium bumpsto the silicon multiplex pads. The detector arraycan be illuminated from either the frontside orbackside (with photons passing through the trans-parent detector array substrate). In general, thelatter approach is most advantageous. In HgCdTehybrid FPAs, photovoltaic detectors are formedon thin HgCdTe epitaxial layer on transparentCdTe or CdZnTe substrates. For HgCdTe flip-chip hybrid technology, the maximum chip size isof the order of 10 mm square. In order to over-come this problem, the technology is being devel-oped with sapphire or silicon as the substrate ofHgCdTe detectors. When using opaque materials,substrates must be thinned to 10–20 lm in order toobtain sufficient quantum efficiencies and reducethe crosstalk.
There is a large research activity directed to-wards 2D staring arrays detectors consisting ofmore than 106 elements. IR FPAs have nominallythe same growth rate as dynamic random accessmemory (RAM) integrated circuits (ICs) (it isconsequence of Moore’s Law, which predicts theability to double transistor integration on each ICabout every 18 months) but lag behind in size byabout 5–10 years. ROIC’s are somewhat analo-gous to dynamic RAM––only readouts require a
minimum of three transistors per pixel comparedto one per memory cell. Consequently, whereasvarious 64 � 64 FPAs were available in the early1980s, several vendors are now producing mono-lithic FPAs in the TV-compatible 1040 � 1040formats. Fig. 4 illustrates the trend of array sizeover the past 25 years and some projections ofwhat will evolve in the coming decade. Rockwellhas developed the world’s largest HgCdTe shortwavelength IR (SWIR) FPA for astronomy andlow-background applications. The format of thedevice is a hybrid 2048 � 2048 with a unit cell sizeof 18 lm � 18 lm. Table 2 contains descriptionof representative IR FPAs that are commerciallyavailable as standard products and/or catalogueitems from the major manufactures. Ten yearsago, high quality single element detectors oftenwere priced over $2000, but now, some current IRFPA production costs are less than $1 per detectorand even greater reductions are expected in thenear future.
Two types of silicon addressing circuits have beendeveloped: CCDs and complementary metal-oxide-semiconductor (CMOS) switches. The photogener-ated carriers are first integrated in the well formedby a photogate and subsequently transferred toslow (vertical) and fast (horizontal) CCD shiftregisters.
An attractive alternative to the CCD readout iscoordinative addressing with CMOS switches. The
Fig. 4. Increase in array format size over the past 25 years and projections for the coming decade. PtSi, InSb, and HgCdTe have been
following the pace of dynamic RAM, offset by about a decade. QWIP detectors have been recently reported in sizes as large as
advantages of CMOS are that existing foundries.Design rules of 0.25 lm are in production withpre-production runs of the 0.18 lm design rules.At present, CMOS with minimum feature 6 0.5lm is also enabling monolithic visible CMOSimagers.
4. Viewpoint on IR detector technologies
During the past four decades mercury cadmiumtelluride (HgCdTe) has became the most im-portant semiconductor for the middle and longwavelength (k ¼ 3–30 lm) IR photodetectors. Theshort wavelength region has been dominated byIII–V compounds (InGaAs, InAsSb, InGaSb).
There have been numerous attempts to replaceHgCdTe with alternative materials. At present,several other variable gap alloy systems are knownincluding closely related mercury alloys HgZnTe,HgMnTe, lead tin tellurides and selenides, InAsSb,III–VI compounds with thallium and bismuth,free-carrier detectors and low-dimensional solids.The main motivations, behind the numerousattempts to replace HgCdTe, are technologicalproblems of this material. One of them is weakHg–Te bond, which results in bulk and surface andinterface instabilities. Uniformity and yield are stillissues. Nevertheless, HgCdTe remains the leadingsemiconductor for IR detectors. The most impor-tant reasons for this are:
• No one of the new materials offers fundamentaladvantages over HgCdTe. While the figure ofmerit, (a=G) (where a is the absorption coeffi-cient and G is the thermal generation rate) [16],of various narrow-gap semiconductors seems tobe very close to that of HgCdTe, the free-carrierdetectors and GaAs/AlGaAs superlattice de-vices have several order of magnitude smallera=G.
• HgCdTe exhibits extreme flexibility, it can betailored for optimised detection at any regionof IR spectrum, dual and multicolour devicescan be easily constructed.
• The present development of IR photodetec-tors has been dominated by complex band-gapheterostructures. Among various variable band-
gap semiconductor alloys, HgCdTe is the onlyone material covering the whole IR spectralrange having nearly the same lattice parameter.The difference of lattice parameter betweenCdTe (Eg ¼ 1:5 eV) and Hg0:8Cd0:2Te (Eg ¼ 0:1eV) is �0.2%. Replacing small fraction of Cdwith Zn or Te with Se can compensate the re-sidual lattice mismatch. The independence oflattice parameter on composition is a major ad-vantage of HgCdTe over any other materials.
Heterojunctions are helpful in achieving highperformance in practice. For example, the narrow-gap HgCdTe that absorbs IR radiation can beburied encapsulated in wider gap HgCdTe pre-venting instabilities due to the weak Hg–Te bonds.
When background-photon noise is the domi-nant noise mechanism, the detector is operating inan ideal mode, and is said to exhibit backgroundlimited performance (BLIP). BLIP temperature isdefined that the device is operating at a tempera-ture at which the dark current equals the back-ground photocurrent, given a field of view (FOV),and a background temperature. In Fig. 5, plots ofthe calculated temperature required for BLIP op-eration in 30� FOV, are shown as a function ofcutoff wavelength. We can see that the operat-ing temperature of ‘‘bulk’’ intrinsic IR detectors(HgCdTe and PbSnTe) is higher than for othertypes of photon detectors. HgCdTe detectors with
Fig. 5. Estimation of the temperature required for background
limited operation of different types of photon detectors. For the
calculations FOV ¼ 30� and TB ¼ 300 K are assumed (after
BLIP operate with thermoelectric coolers in theMWIR range, but the LWIR detectors (86 kc 6 12lm) operate at �100 K. HgCdTe photodiodesexhibit higher operating temperature compared toextrinsic detectors, silicide Schottky barriers andQWIPs. However, the cooling requirements forQWIPs with cutoff wavelengths below 10 lm areless stringent in comparison with extrinsic detec-tors and Schottky-barrier devices.
Recently, more interest has been focused on p–njunction heterostructure photodiodes. In compar-ison with heterostructures, homojunctions havea lower breakdown voltage and a large reverseleakage current. Another aspect of photodiodedesign is the required bandwidth for high-speedoperation.
Photodiodes with their very low power dissi-pation, easy multiplexing on focal plane siliconchip and less stringent noise requirements for thereadout devices and circuits, can be assembled in2D arrays containing a very large number of ele-ments, limited only by existing technologies.
Epitaxy is the preferable technique to obtaindevice-quality materials. Among the various epit-axial techniques, liquid phase epitaxy (LPE) is themost mature method. LPE growth must be carriedout at relatively high growth temperature withadherent interdiffusion and resulting graded in-terfaces. Recent efforts are aimed mostly at
low growth temperature techniques: metalorganicchemical vapour deposition (MOCVD) and mo-lecular beam epitaxy (MBE).
The baseline detector architecture for SWIRInGaAs and HgCdTe is Pþ-on-n device (symbol Pdenotes wider gap). Fig. 6 shows the double layerheterostructure (DLPH) device cross-sections forboth types of photodiodes. Apart from the abovedescribed benefits, incorporation of a buried nar-row-band-gap active layer in the DLPH reducestunnelling currents and increases the total doseradiation hardness, both of which are essentialdetector attributes for remote sensing applications.The thickness of the base region should be opti-mised for near unity quantum efficiency and a lowdark current. This is achieved with a base thick-ness (typically about 5–7 lm) slightly higher thanthe inverse absorption coefficient for single passdevices. Low doping is beneficial for a low thermalgeneration and high quantum efficiency. Since thediffusion length in absorbing region is typicallylonger than its thickness, any carriers generated inthe base region can be collected giving rise to thephotocurrent.
4.1. InGaAs photodiodes
The need for high-speed, low-noise InxGa1�xAs(InGaAs) photodetectors for use in lightwave
Fig. 6. Double layer planar heterostructure cross-section schematics for SWIR InGaAs (a) and HgCdTe (b) photodiodes.
communication systems operating in the 1–1.7 lmwavelength region (x ¼ 0:53) is well established.In0:53Ga0:47As alloy is lattice matched to the InPsubstrate. Having lower dark current and noisethan indirect-band-gap germanium, the competingnear-IR material, the material is addressing bothentrenched applications including low light levelnight vision and new applications such as remotesensing, eye-safe range finding and process con-trol [18]. By changing the alloy composition ofthe InGaAs absorption layer, the photodetectorresponsivity can be maximized at the desiredwavelength of the end user to enhance the signal-to-noise ratio.
InGaAs-detector processing technology is sim-ilar to that used with silicon, but the detectorfabrication is different. The InGaAs detector’sactive material is deposited onto a substrate usingchloride VPE or MOCVD techniques adjusted forthickness, background doping, and other require-ments. Planar technology evolved from the oldermesa technology and at present is widely used dueto its simple structure and processing as well as thehigh reliability and low cost (see Fig. 6(a)).
Standard In0:53Ga0:47As photodiodes have ra-diative-limited room temperature detectivity of1013 cm Hz1=2 W�1. With increasing cutoff wave-length, detectivity decreases, what is shown in Fig.7. The highest quality InGaAs photodiodes havebeen grown by MOCVD [19]. Their performanceis comparable with HgCdTe photodiodes.
Linear array formats of 256, 512 and 1024 ele-ments have been fabricated for environmentalsensing from 0.8 to 2.6 lm. The size of pixels aredifferent; from 30 � 30 lm2 (with spacing of 50lm), 25 � 500 lm2 to 13 � 500 lm2 (with spacingof 25 lm). Sensors Unlimited offers 10 � 10 � 6cm line-scan cameras incorporating linear InGaAsFPAs of up to 512 elements on a 50 lm pitch. Aroom temperature staring cameras are based on128 � 128 and 320 � 240 InGaAs FPAs.
4.2. InSb photodiodes
InSb material is far more mature than HgCdTeand good quality more than 7 cm diameter bulksubstrates are commercially available [20]. InSb
photodiodes have been available since the late fif-ties and they are generally fabricated by impuritydiffusion and ion implantation. Epitaxy is notused; instead, the standard manufacturing tech-nique begins with bulk n-type single crystal waferswith donor concentration about 1015 cm�3. Wim-mers et al. have presented the status of InSbphotodiode technology for a wide variety of linearand FPAs [21,22].
Typical InSb photodiode RA product at 77 K is2 � 106 X cm2 at zero bias and 5 � 106 X cm2 atslight reverse biases of approximately 100 mV.This characteristics is beneficial when the detectoris used in the capacitive discharge mode. As ele-ment size decreases below 10�4 cm�2, some slightdegradation in resistance due to surface leakageoccurs.
InSb photodiodes can also be operated in thetemperature range above 77 K. Of course, the RAproducts degrade in this region. At 120 K, RAproducts of 104 X cm2 are still achieved with slightreverse bias, making BLIP operation possible. Thequantum efficiency in InSb photodiodes optimisedfor this temperature range remains unaffected upto 160 K. Detectivity increases with reducedbackground flux (narrow FOV and/or cold filter-ing) as illustrated in Fig. 8.
Fig. 7. Room temperature detectivity of InGaAs photodiodes
with cutoff wavelength at 1.6, 1.9, 2.2, and 2.6 lm, respectively.
InSb photovoltaic detectors are widely used forground-based IR astronomy and for applicationsaboard the Space Infrared Telescope Facility.Recently, impressive progress has been made in theperformance of InSb hybrid FPAs. An array sizeof 1024 � 1024 is possible because the InSb de-tector material is thinned to <10 lm (after surfacepassivation and hybridisation to a readout chip)which allows it to accommodate the InSb/siliconthermal mismatch [23]. Linear array formats of64, 128 and 256 elements are also produced withfrontside-illuminated detectors for both high-background and astronomy applications. Elementsizes depend on device format and range from20 � 20 to 200 � 200 lm.
The cryogenically cooled InSb and HgCdTearrays have comparable array size and pixel yieldat MWIR spectral band. However, wavelengthtunability and high quantum efficiency have madeHgCdTe the preferred material.
4.3. HgCdTe photodiodes
HgCdTe photodiodes are available to cover thespectral range from 1 to 20 lm. Fig. 9 illustratesrepresentative spectral response from photodiodes.Spectral cutoff can be tailored by adjusting theHgCdTe alloy composition.
Epitaxial techniques are preferred technique toobtain device-quality HgCdTe material for IRdevices. Epitaxial growth of the HgCdTe detector
array on a Si substrate, rather than CdZnTe, hasemerged as a particularly promising approachto scale up wafer dimensions and achieve a cost-effective production [24]. MBE offers unique ca-pabilities in material and device engineeringincluding the lowest growth temperature, super-lattice growth and potential for the most sophis-ticated composition and doping profiles. Thegrowth temperature is less than 200 �C for MBEbut around 350 �C for MOCVD, making it moredifficult to control the p-type doping in theMOCVD due to the formation of Hg vacancies.
Different HgCdTe photodiode architectureshave been fabricated that are compatible withbackside and frontside-illuminated hybrid FPAtechnology [18,25]. The fabrication of HgCdTephotodiodes was usually based on the most com-mon nþ–p and Pþ–n DLHJ structure. In thesephotodiodes the base p-type layers (or n-typelayers) are sandwiched between CdZnTe substrateand high-doped (in nþ–p structures) or wider-gap(in Pþ–n structure) regions. Due to backside illu-mination (through CdZnTe substrate) and internalelectric fields (which are ‘‘blocking’’ for minoritycarriers), influence of surface recombinations onthe photodiodes performance is eliminated. Bothoptical and thermal generations are suppressed inthe nþ-region due to the Burstein–Moss effect andin the Pþ-region due to wide gap. The influence ofsurface recombination is also prevented by the useof suitable passivation. Passivation of HgCdTehas been done by several techniques which com-prehensive review was given by Nemirovsky and
Fig. 8. Detectivity as a function of wavelength for an InSb
photodiode operating at 77 K (after Judson, Infrared Detectors
Catalog, 1999).
Fig. 9. Representative spectral response data for HgCdTe
Bahir [26]. Recently, however, most laboratorieshave been using CdTe or CdZnTe (deposited byMBE, MOCVD, sputtering and e-beam evapora-tion) for photodiode passivation [27]. For exam-ple; the important element of the DLHJ structureare an In-doped (Nd � 1015 cm�3) HgCdTe ab-sorber layer with a wider band-gap cap layergrown by LPE (MBE or MOCVD), arsenic diffu-sion (or implantation; Na � 1018 cm�3), a passiva-tion layer, metal contacts to the diode, and indiumbumps for mating to the readout IC multiplexer.
It appears, that for the lowest doping levels,achievable in controllable manner in the base re-gions of photodiodes (Na ¼ 5 � 1015 cm�3 for nþ–pstructure, and Nd ¼ 5 � 1014 cm�3 for pþ–n struc-ture), the performance of both types of photodi-odes is comparable for a given cutoff wavelengthand temperature [28].
The dependence of the base region diffusionlimited RoA product on the long wavelength cutofffor P-on-n LWIR HgCdTe photodiodes at differ-ent temperatures is shown in Fig. 10. This figurealso includes the experimental data reported bymany authors for DLHJ p-on-n structures. Theupper experimental data are situated about a halfof an order below ultimate theoretical predictions.With a lowering of the operation temperature ofphotodiodes, the discrepancy between the theo-retical curves and experimental data increases,which is due to additional currents in the junctions(such as tunnelling current or surface leakagecurrent) that are not considered. Photodiodes withlower performance usually contain metallurgicaldefects such as dislocation clusters and loops, pinholes, striations, Te inclusions, and heavy terrac-ing. It should be noticed that the upper experi-mental data in very long wavelength range (above14 lm) at lower temperature (40 K) coincides verywell with theoretical predictions.
Fig. 11 presents a comprehensive comparison ofthe performance of MWIR P-on-n HgCdTe pho-todiodes on CdZnTe and Si substrates for cutoffwavelengths ranging from 3.5 to 5 lm. The deviceswith highest performance are processed fromMBE-grown epilayers on bulk CdZnTe substrates.The shorter cutoff devices (with kc � 3 lm) arediffusion-limited down to at least 125 K. The de-vices with longer cutoff wavelength (with kc � 5
lm) appear to be diffusion-limited down to �110K. Below this temperature the experimental dataobscure the probable onset of generation-recom-bination and/or tunnelling current limitations.
Up to the present, photovoltaic HgCdTe FPAshave been mainly based on p-type material. Linear(240, 288, 480, and 960 elements), 2D scanningarrays with time delay and integration (TDI), and2D staring formats from 32 � 32 up to 2048 �2048 have been made [18]. Pixel sizes ranging from18 lm square to over 1 mm have been demon-strated. The best results have been obtained usinghybrid architecture.
detection in the 3–5 lm spectral range [30,31].Radiation is transmitted through the p-type siliconand is absorbed in the metal PtSi (not in thesemiconductor), producing hot holes which arethen emitted over the potential barrier into thesilicon, leaving the silicide charged negatively.Negative charge of silicide is transferred to a CCDby the direct charge injection method.
The effective quantum efficiency in the 3–5 lmatmospheric window is very low, of the order of1%, but useful sensitivity is obtained by means ofnear full frame integration in area arrays. Thequantum efficiency has been improved by thin-ning PtSi film and implementation of an ‘‘opti-cal cavity’’. Due to very low quantum efficiency,the operating temperature of Schottky-barrierphotoemissive detectors is lower than anothertypes of IR photon detectors (see Fig. 5).
Schottky photoemission is independent of suchfactors as semiconductor doping, minority carrierlifetime, and alloy composition, and, as a result ofthis, has spatial uniformity characteristics that arefar superior to those of other detector technolo-gies. Uniformity is only limited by the geometricdefinition of the detectors.
Most of the reported Schottky-barrier FPAshave the interline transfer CCD architecture.Using a 1.2 lm charge sweep device technology, alarge fill factor of 71% was achieved with a26 � 20 lm2 pixel in the 512 � 512 monolithicstructure [31]. The noise equivalent temperaturedifference (NETD) was estimated as 0.033 K withf =1:2 optics at 300 K. The 1040 � 1040 elementCSD FPA has the smallest pixel size (17 � 17 lm2)among 2D IR FPAs. Current PtSi Schottky-bar-rier FPAs are mainly manufactured in 150 mmwafer process lines with around 1 lm lithographytechnologies; the most advanced Si technologyoffers 200 mm wafers process with 0.25 mm designrules. However, the performance of monolithicPtSi Schottky-barrier FPAs has reached a plateau,and a slow progress from now on is expected.
Various semiconductor photoemissive struc-tures for far IR detection have been discussed byPerera [32].
4.5. Extrinsic photoconductors
Extrinsic photoresistors are used in a widerange of the IR spectrum extending from a few lmto �300 lm. They are the principal detectors op-erating in the range k > 20 lm [33]. Detectorsbased on silicon and germanium have found thewidest application as compared with extrinsicphotodetectors on other materials. Si has severaladvantages over Ge; for example, three orders ofmagnitude higher impurity solubilities are attain-able, hence thinner detectors with better spatialresolution can be fabricated from silicon. Si haslower dielectric constant than Ge, and the relateddevice technology of Si has now been more thor-oughly developed, including contacting methods,surface passivation and mature MOS and CCDtechnologies. Moreover, Si detectors are charac-terized by superior hardness in nuclear radiationenvironments. Fig. 12 illustrates the spectral re-sponse for several extrinsic detectors.
The availability of a highly developed siliconMOS technology facilities the integration of largedetector arrays with charge-transfer devices forreadout and signal processing. The well-estab-lished technology also helps in the manufacturingof uniform detector arrays and the formation of
Fig. 11. Comparison of 125 K detector performance for
MWIR HgCdTe photodiodes grown on Si and CdZnTe by
MBE and photodiodes grown on CdZnTe by LPE. Each data
point represents an array-median RoA product measured at 125
low-noise contacts. Although the potential of largeextrinsic silicon FPAs for terrestrial applicationshas been examined, interest has declined in favourof HgCdTe and InSb with their more convenientoperating temperatures. Strong interest in dopedsilicon continues for space applications, particu-larly in low-background flux and for wavelengthsfrom 13 to 20 lm, where compositional control isdifficult for HgCdTe. The shallower impurity en-ergies in germanium allow detectors with spectralresponse up to beyond 100 lm wavelength andmajor interest still exists in extrinsic germaniumfor wavelengths beyond about 20 lm.
To maximize the quantum efficiency and de-tectivity of extrinsic photoconductors, the dopinglevel should be as high as possible. This idea isrealized in blocked impurity band (BIB) devices.The longer spectral response of the BIB Si:As de-vice compared with the bulk Si:As device (see Fig.12) is due to the higher doping level in the formerthat reduces the binding energy of an electron. Fora detailed analysis of the BIB detector see Szmu-lowicz and Madarsz [35].
BIB devices made from either doped silicon ordoped germanium are sensitive in the IR wave-length range of 2 and 220 lm. BIB devices in largestaring array formats are now becoming com-mercially available. The best results have beenachieved to date for Si:As BIB hybrid FPAs pro-duced by Hughes Technology Center in Carlsbad[36,37] and Rockwell International Science Center
in Anaheim [38]. Hybrid FPAs with Si:As BIBdetectors operating in 4–10 K temperature rangehave been optimised for low, moderate, and highIR backgrounds. The 256 � 256 format with 30lm pixels and 240 � 320 format with 50 lm pixelsare available for low- and high-background ap-plications, respectively. Antimony-doped silicon(Si:Sb) arrays and 128 � 128 pixel Si:Sb hybridFPAs having response to wavelengths >40 lmhave been also demonstrated, primarily for use atlow and moderate backgrounds. Germanium BIBdevices have been developed on an experimentalbasis, but they have not been reported in large 2Darray formats yet.
4.6. GaAs/AlGaAs QWIPs
Among the different types of quantum well IRphotodetectors (QWIPs), technology of the GaAs/AlGaAs multiple quantum well detectors is themost mature [39,40].
QWIP technology is based on the well-devel-oped A3B5 material system, which has a largeindustrial base with a number of military andcommercial applications. QWIP cannot competewith HgCdTe photodiode as the single device es-pecially at higher temperature operation (>70 K)due to fundamental limitations associated withintersubband transitions [17]. However, the ad-vantage of HgCdTe is less distinct in tempera-ture range below 50 K due to problems involvedin a HgCdTe material (p-type doping, Shockley–Read recombination, trap-assisted tunnelling,surface and interface instabilities). Even thoughthat QWIP is a photoconductor, several its prop-erties such as high impedance, fast responsetime, long integration time, and low power con-sumption, well comply requirements of large FPAsfabrication. Due to the high material quality atlow temperature, QWIP has potential advantagesover HgCdTe for VLWIR FPA applications interms of the array size, uniformity, yield and costof the systems.
Fig. 13 shows two detector configurations usedin fabrication of QWIP FPAs. In the bound-to-continuum QWIP the photoelectron can escapefrom the quantum well to the continuum transportstates without being required to tunnel through the
Fig. 12. Examples of extrinsic silicon detector spectral re-
sponse. Shown are Si:In, Si:Ga, and Si:As bulk detectors and a
barrier. As a result, the voltage bias required toefficiently collect the photoelectrons can be re-duced dramatically, thereby lowering the darkcurrent. It appears that the dark current decreasessignificantly when the first excited state is droppedfrom the continuum to the well top without sac-rificing the responsivity.
In a miniband transport QWIP (see Fig. 13(b)),IR radiation is absorbed in the doped quantumwells, exciting an electron into the miniband andtransporting it in the miniband until it is collectedor recaptured into another quantum well. Theminiband QWIPs show lower photoconductivegain than bound-to-continuum QWIPs because thephotoexcited electron transport occurs in the mini-band where electrons have to transport throughmany thin heterobarriers resulting in a lower mo-bility.
Rogalski [41] has used simple analytical ex-pressions for detector parameters described byAndersson [42]. Fig. 14 shows the dependence ofdetectivity on the long wavelength cutoff for n-typeGaAs/AlGaAs QWIPs at different temperatures.The satisfactory agreement with experimental data
in wide range of cutoff wavelength 86 kc 6 19 lmand temperature 356 T 6 77 K has been obtained,considering the samples having different doping,different methods of crystal growth, different spec-tral widths, different excited states, and even in onecase a different materials system (InGaAs).
A key factor in QWIP FPA performance isthe light-coupling scheme. A distinct feature ofQWIPs is that the optical absorption strength isproportional to an incident photon’s electric-fieldpolarisation component normal to the quantumwells. For imaging, it is necessary to be able tocouple light uniformly to 2D arrays of these de-tectors, so a diffraction grating or other similarstructure is typically fabricated on one side of thedetectors to redirect a normally incident photoninto propagation angles more favourable for ab-sorption. The pixels of 2D arrays are thinned toabout 5 lm in thickness. The thinning traps dif-
Fig. 13. Band diagram of demonstrated QWIP structures: (a)
bound-to-extended and (b) bound-to-miniband. Three mecha-
nisms creating dark current are also shown in (a): ground-state
fracted light inside the illuminated pixels, increas-ing responsivity and eliminating crosstalk. Thethinning also allows the detector array to stretchand accommodate the thermal expansion mis-match with the Si ROIC.
Gunapala and co-workers at Jet PropulsionLaboratory (JPL) demonstrated a 256 � 256QWIP FPA in an Amber hand-held camera. Thecurrent state of the art for QWIP FPA size hasbeen 640 � 480 recently demonstrated by JPL[43,44] and Lockheed Martin [45]. The measuredmean NEDT of the QWIP camera was 36 mK atan operating temperature of T ¼ 70 K at 300 Kbackground [44].
4.7. Thermal detectors
IR semiconductor imagers use cryogenic orthermoelectric coolers, complex IR optics, andexpensive sensor materials. Typical costs of cryo-genically cooled imagers of around $50 000 restricttheir installation to critical military applicationsallowing conducting of operations in completedarkness. Very encouraging results have beenobtained with micromachined silicon bolometerarrays and pyroelectric detector arrays. Severalcountries have demonstrated imagers with NEDTof 100 mK or better, and the cost of simple systemsis sometimes below $10 000. It is expected thathigh-performance imager system costs will be re-duced to less than $1000 [46] and above IR cam-eras will become widely available in the nearfuture. Although developed for military applica-tions, low-cost IR imagers are used in non-militaryapplications such as: drivers aid, aircraft aid, in-dustrial process monitoring, community services,firefighting, portable mine detection, night vision,border surveillance, law enforcement, search andrescue, etc.
4.7.1. Micromachined silicon bolometersThe most popular thermistor material used in
fabrication of the micromachined silicon bolome-ters is vanadium dioxide, VO2. From the point ofview of IR imaging application, probably themost important property of VO2 is its high nega-tive temperature coefficient of resistance (TCR) atambient temperature, which exceeds 4% per degree
for single element bolometer and about 2% forFPA.
The final microbolometer pixel structure isshown in Fig. 15. The microbolometer consists ofa 0.5 lm thick bridge of Si3N4 suspended about 2lm above the underlying silicon substrate. The useof a vacuum gap of �2.5 lm, together with aquarter wave resonant cavity between the bolo-meter and the underlying substrate, can produce areflector for wavelengths near 10 lm. The bridge issupported by two narrow legs of Si3N4. The Si3N4
legs provide the thermal isolation between themicrobolometer and the heat-sink readout sub-strate and support conductive films for electricalconnection. A bipolar input amplifier is normallyrequired, and this can be obtained with biCMOSprocessing technology. Encapsulated in the centreof the Si3N4 bridge is a thin layer (500 �AA) ofpolycrystalline VOx.
Honeywell has licensed this technology to sev-eral companies for the development and produc-tion of uncooled FPAs for commercial andmilitary systems. At present, the compact 320 �240 microbolometer cameras are produced byRaytheon, Boeing, and Lockheed Martin in theUnited States. The US government allowed thesemanufactures to sell their devices to foreigncountries, but not to divulge manufacturing tech-nologies. In recent years, several countries, in-cluding the United Kingdom, Japan, Korea, andFrance have picked up the ball, determined todevelop their own uncooled imaging systems. As aresult, although the US has a significant lead, someof the most exciting and promising developmentsfor low-cost uncooled IR systems may come fromnon-US companies, e.g., microbolometr FPAswith series p–n junction elaborated by Mitsubishi
Fig. 15. Bridge structure of Honeywell microbolometer (after
Electric [48]. This approach is unique, based on anall-silicon version of microbolometer.
The 240 � 320 arrays of 50 lm microbolome-ters are fabricated on industry-standard wafer (4in. diameter) complete with monolithic readoutcircuits integrated into underlying silicon. Radfordet al. [49] have reported a 240 � 320 pixel arraywith 50 lm square vanadium oxide pixels, forwhich the average NETD (f =1 optics) was 8.6 mK.Larger arrays size was described by Altman andcolleagues at Lockheed Martin [50]; they reporteda 640 � 480 FPA with 28 � 28 m2 pixels withNETD (f =1 optics) of about 60 mK.
At present, several research programmes arefocused towards enhancement of performancelevel in excess of 109 cm Hz1=2 W�1. It is antici-pated that new materials will form the basis of thenext generation of semiconductor film bolome-ters. The most promising material appears to beamorphous silicon [51].
4.7.2. Pyroelectric detectorsThe imaging systems based on pyroelectric ar-
rays, usually need to be operated with opticalmodulators which chop or defocus the incomingradiation. This may be an important limitation formany applications in which chopperless operationis highly desirable (e.g., guided munitions). Hith-erto, most of the ferroelectric detectors have beenoperated well-below Curie temperature TC, wherethe polarisation is not affected by changes in am-bient temperature. It is, however, possible to op-erate ferroelectrics at or above TC, with an appliedbias field, in the mode of a ‘‘dielectric bolometer’’.
Several materials have been examined in di-electric bolometer mode. Barium strontium titan-ate (BST) ceramic is a relatively well-behavedmaterial with a very high permittivity. Texas in-struments (TI) has improved the performance ofpyroelectric FPAs using a bias voltage applied tomaintain and optimise the pyroelectric effect nearthe phase transition [52]. Fig. 16 shows details ofthe completed pyroelectric detector device struc-ture. For the United Kingdom array programmelead scandium tantalate (PST) material has beenchosen [53].
Although many applications for this hybridarray technology have been identified, and imagers
employing these arrays are in mass production, nohybrid technology advances are foreseen. Thereason is that the thermal conductance of thebump bonds is so high that the array NETD (f =1optics) is limited to about 50 mK. Pyroelectricarray technology therefore is moving towardmonolithic silicon microstructure technology. Themonolithic process should have fewer steps andshorter cycle time. Most ferroelectrics tend to losetheir interesting properties as the thickness is re-duced. However, some ferroelectric materials seemto maintain their properties better than others.This seems particularly true for lead titanate(PbTiO3) and related materials, whereas BST, thematerial used in hybrid detectors, does not hold itsproperties well in thin-film form. Various tech-niques for the deposition of thin ferroelectric filmshave been investigated, including radio frequencymagnetron sputtering, dual ion beam sputtering,sol–gel processing, and laser ablation.
5. Dual-band IR focal plane arrays
Multicolour capabilities are highly desirable foradvance IR systems. Systems that gather data inseparate IR spectral bands can discriminate bothabsolute temperature and unique signatures ofobjects in the scene. By providing this new di-mension of contrast, multiband detection also en-ables advanced colour processing algorithms tofurther improve sensitivity above that of single-colour devices. Currently, multispectral systemsrely on cumbersome imaging techniques that eitherdisperse the optical signal across multiple IR FPAs
or use a filter wheel to spectrally discriminate theimage focused on single FPA. Consequently, theseapproaches are expensive in terms of size, com-plexity, and cooling requirements. Both HgCdTephotodiodes and QWIPs offer the multicolour ca-pability in the MWIR and LWIR range.
Considerable progress has been recently dem-onstrated in multispectral HgCdTe detectors em-ploying MBE and MOCVD for the growth ofvariety devices [54–61]. Also QWIP’s technologydemonstrates considerably progress in fabricationof multicolour FPAs [40,44,62–66]. Devices for thesequential and simultaneous detection of two clo-sely spaced sub-bands in the MWIR and LWIRradiation have been demonstrated.
5.1. Dual-band HgCdTe FPAs
The two-colour detector arrays are usuallybased upon an n–P–N HgCdTe triple layer het-erojunction (TLHJ) design. The TLHJ detectors
consist of back-to-back photovoltaic p–n junc-tions. This device architecture is realised by plac-ing a longer wavelength HgCdTe photodiodesimply behind shorter wavelength photodiode.Also quaternary device structure is used [60].
Both sequential-mode and simultaneous modedetectors are fabricated from the multilayer ma-terials. The mode of detection is determined by thefabrication process. Figs. 17 and 18 show the ele-ments of arrays of two-colour photovoltaic unitcells in both modes. The sequential-mode detectorhas a single indium bump per unit cell that permitssequential bias selectivity of the spectral bandsassociated with operating tandem photodiodes.The problems with the bias-selectable device arethe following: its construction does not allow in-dependent selection of the optimum bias voltagefor each photodiode, and there can be substantialmedium wavelength crosstalk in the long wave-length detector. To overcome the problems of thebias-selectable device, the independently accessed
Fig. 17. Cross-section of integrated two-colour detectors in an n–P–N layer structure for sequential operating mode.
Fig. 18. Cross-section of integrated two-colour detectors in an n–P–N layer structure for simultaneous operating mode.
back-to-back photodiode dual-band detectors havebeen proposed. The simultaneous mode detectoremploys an additional electrical contact to theshared-type centre layer so that each junction canbe accessed independently with both signal chan-nels integrated simultaneously. Longwave band fillfactor is reduced from that of the midwave, sincesome junction area is sacrificed to provide contactto the buried cap layer, and spatial coincidence isaltered.
Critical step in device formation is connectedwith in situ doped p-type As-doped layer withgood structural and electrical properties to preventinternal gain from generating spectral crosstalk.The band-gap engineering effort consists of in-creasing the CdTe mole fraction and the effectivethickness of the p-type layer to suppress out-off-band carriers from being collected at the terminal.
Fig. 19 shows examples of spectral responsefrom MWIR/MWIR, MWIR/LWIR, and LWIR/LWIR two-colour devices.
Fill factors of 128 � 128 MWIR/MWIR FPAsas high as 80% were achieved by using a singlemesa structure to accommodate the two indiumbump contacts required for each unit cell with 50lm size [9]. The NEDT for both bands was below25 mK and imagery was acquired at temperaturesas high as 180 K with no visible degradation inimage quality. The camera used for these mea-surements had a 50 mm, f =2:3 lens.
Recently, Rockwell and Boeing have extended asingle-colour DLHJ planar technology to two-colour architecture [61]. A cross-section of a typi-cal backside illuminated pixel is shown in Fig. 20.The band 1 absorber (shorter wavelength) is grownfirst, with the band 2 absorber (longer wavelength)
grown on top. To prevent diffusion of carriersbetween two bands, a wide-band-gap 1 lm thicklayer separates these absorbing layers. The diodesare formed by implanting arsenic as a p-type do-pant and activating with an anneal giving unipolaroperation for both bands. The band 2 implantedarea is a concentric ring around the band 1 dimple.Because the lateral carrier diffusion length is largerthan the pixel pitch in the MWIR material and theband 1 junction is small, the pixel is isolated by dryetching a trench around each pixel to reduce car-rier crosstalk. The entire structure is capped with aslightly wider band-gap layer to reduce surfacerecombination and simplify passivation. Two-col-our 128 � 128 FPAs with low-1013 cm�2 s�1 back-ground limited detectivity performance have beenobtained for MWIR (3–5 lm) devices at T < 130K and for LWIR (8–10 lm) devices at T � 80 K.
5.2. Dual-band QWIP FPAs
Device capable of simultaneously detecting twoseparate wavelengths can be fabricated by verticalstacking of the different QWIP layers during epit-axial growth (see Fig. 21). Separate bias voltages
Fig. 19. Spectral response curves for two-colour HgCdTe detectors in various dual-band combinations of spectral bands (after Ref.
can be applied to each QWIP simultaneously viathe doped contact layers that separate the multiplequantum well detector heterostructures.
Typical operating temperature for QWIP de-tectors is in the region of 40–100 K. The biasacross each QWIP can be adjusted separately, al-though it is desirable to apply the same bias toboth colours. As shown in Fig. 22, the responsivityof both QWIPs is around 300–350 mA/W at atemperature 40 K and at an operating bias of þ1.5lm applied to common contact. It appears that thecomplex two-colour processing has not compro-mised the electrical and optical quality of eithercolour in the two-colour device.
A key factor in QWIP FPA performance is thelight-coupling scheme. Different light-couplingmechanisms are used in QWIPs. Most QWIP ar-
rays use 2D grating, which is very wavelengthdependent, and efficiency gets lower when the pixelsize gets smaller. Lockheed Martin has used rect-angular and rotated rectangular 2D gratings fortheir two-colour LW/LW FPAs. Although ran-dom reflectors have achieved relatively high quan-tum efficiencies with large test device structure, it isnot possible to achieve the similar high quantumefficiencies with random reflectors on small FPApixels due to the reduced width-to-height aspectrations. In addition, it is difficult to fabricaterandom reflectors for shorter wavelength detectorsrelative to long wavelength detectors due to thefact that feature sizes of random reflectors arelinearly proportional to the peak wavelength of thedetectors. As a result, the quantum efficiency be-comes a more difficult issue for QWIP multicolourFPA than for single colour.
Two-colour detectors that cover both MWIRand LWIR atmospheric windows are especiallyimportant in many applications. To cover MWIRrange a strained layer InGaAs/AlGaAs materialsystem is used. InGaAs in MWIR stack produceshigh in-plane compressive stain which enhancesthe responsivity [67]. The MWIR/LWIR FPAsfabricated by Sanders consist of an 8.6 lm GaAs/AlGaAs QWIP on top of 4.7 lm strained InGaAs/GaAs/AlGaAs heterostructure. The fabricationprocess allowed fill factors of 85% and 80% for theMW and LW detectors. The first FPAs with thisconfiguration had an operability in excess of 97%,and NETD value better 35 mK.
Recently, Gunapala et al. [64] have demon-strated the first 8–9 and 14–15 lm two-colourimaging camera based on a 640 � 486 dual-bandQWIP FPA, which can be processed with dual ortriple contacts to access the CMOS readout mul-tiplexer. Single indium bump per pixel is usableonly in the case of interlace readout scheme (i.e.,odd rows for one colour and the even rows for theother colour) which uses an existing single-colourCMOS readout multiplexer. However, the disad-vantage is that it does not provide a full fill factorfor both wavelength bands.
The device structure, shown in Fig. 23, consistsof a 30 period stack (500 �AA AlGaAs barrier and a60 �AA GaAs well) of very LWIR (VLWIR) struc-ture and a second 18 period stack (500 �AA AlGaAs
Fig. 22. Typical responsivity spectra at 40 K and a common
bias of 1.5 V, recorded simultaneously for two QWIPs in the
same pixel (after Ref. [63]).
Fig. 21. Structure of two-colour stacked QWIP (after Ref.
barrier and a 40 �AA GaAs well) of LWIR structureseparated by a heavily doped 0.5 lm thick inter-mediate GaAs contact layer. The VLWIR QWIPstructure has been designed to have a bound-to-quasibound intersubband absorption peak at14.5 lm, whereas the LWIR QWIP structure hasbeen designed to have a bound-to-continuum in-tersubband absorption peak at 8.5 lm, sincephotocurrent and dark current of the LWIR de-vice structure is relatively small compared to theVLWIR portion of the device structure.
Fig. 24 shows schematic side view of the inter-lace dual-band GaAs/AlGaAs FPA. Two different2D periodic grating structures were designed to
independently couple the 8–9 and 14–15 lm radi-ation into detector pixels in even and odd rows ofthe FPAs. The top 0.7 lm thick GaAs cap layerwas used to fabricate the light-coupling 2D peri-odic gratings for 8–9 lm detector pixels, whereasthe light-coupling 2D periodic gratings of the 14–15 lm detector pixels were fabricated throughLWIR MQW layers. In such a way, this gratingscheme short circuited all 8–9 lm sensitive detec-tors in all odd rows of the FPAs. The FPA wasback-illuminated through the flat thinned sub-strate membrane (�1000 �AA). Very thin substrateadapts the thermal expansion and contraction co-efficient of the silicon readout multiplexer, com-pletely eliminates the thermal mismatch problembetween the silicon readout and the GaAs baseddetector array, completely eliminates pixel-to-pixel crosstalk, and finally, significantly en-hances an optical coupling of IR radiation intoQWIP pixels.
The performance of dual-band FPAs were tes-ted at a background temperature of 300 K, withf =2 cold stop, and at 30 Hz frame rate. The meanvalue of quantum efficiency at operating temper-ature T ¼ 40 K and bias VB ¼ �2 V is 12.9% and8.9% in LW and VLW spectral range, respectively.The estimated NEDT of LWIR and VLWIR de-tectors at 40 K are 36 and 44 mK, respectively.
Fig. 24. Structure cross-section of the interlace dual-band FPA (after Ref. [64]).
Fig. 23. Conduction band diagram of the LWIR and VLWIR
6. Anticipated evolution of IR technology in the near
future
The future applications of IR detector systemsrequire:
• higher pixel sensitivity,• further increase in pixel density to above 106
pixels,• cost reduction in IR imaging array systems
through the use of less cooling sensor technol-ogy combined with integration of detectors andsignal-processing functions (with much moreon-chip signal processing),
• improvement in the functionality of IR imagingarrays through development of multispectralsensors.
To reduce the real cost of the IR image systems,one must take action on all the elements, whichmake up the cost to the user. The cost can bebroken down into three parts: the chip (detector þROIC), the dewar, integration and tests. The usermust add the cryogenic machine cost that is notnegligible compared to the component one’s. Thisexplains why the cost of PtSi or QWIPs is notmarkedly less than that of photon detectors of thesame complexity, even though the raw materials(silicon and GaAs) is much less than for HgCdTe.A possible reduction in the purchase price iscounterbalanced by a significant increase in oper-ating costs.
Detector maturity is a function of the accumu-lated experience and development effort, thecomplexity of the device required, and the inherentdifficulty presented by the material technology. Atpresent, HgCdTe photodiodes and BIB extrinsicsilicon detectors are not fully mature. PtSi tech-nology is mature and has received a plateau. Othertwo detector technologies such as InSb and siliconbolometers are still evolving significantly as ap-plications for larger array configurations andsmaller pixel sizes continue to push the technology.
Thermal detector arrays will increase in size andimprove in thermal sensitivity to a level satisfyinghigh performance applications at ambient tem-perature. It is supposed that the silicon microbo-lometers arrays and the monolithic pyroelectric
arrays will capture the low-cost markets. Currentuncooled bolometer FPAs have achieved NEDTless than 10 mK with f =1 optics, what open thedoor to the use of less expensive slower opticalsystems.
It is supposed that sales of IR thermal imagingequipment to the automobile market will begin torapidly change the relative ratio between military/government and commercial IR markets. Todayonly about 10% of the market is commercial.After a decade the commercial market can growto over 70% in volume and 40% in value, largelyconnected with volume production of uncooledimagers for automobile driving [9]. In large volumeproduction for automobiles drivers the cost ofuncooled imaging systems will decrease to below$1000. Of course, these systems will cover othersegments of the transportation industry: trucks,trains, ships, barges, buses, and airplanes.
For same applications requiring uncooleddetectors, the slow response speed is unaccept-able. Recently, a number of concepts (e.g., non-equilibrium device [68], multijunction HgCdTephotodiodes [69], optical immersion) and newmaterials (InAsSb, InAs/GaSb-based type II su-perlattices) [11] have been proposed to improveperformance of photon detectors operating at nearroom temperature. The measurements show thepossibility to achieve detectivity of � 1 � 109
cm Hz1=2 W�1 at the 8–9 lm range and potentially,the devices can be assembled in large FPAs.
Despite serious competition from alternativetechnologies and slower progress than expected,HgCdTe is unlikely to be seriously challenged forhigh-performance applications, applications re-quiring multispectral capability and fast response.The recent successes of competing cryogenicallycooled detectors are due to technological, notfundamental issues. The steady progress in epit-axial technology would make HgCdTe devicesmuch more affordable in the near future. Themuch higher operation temperature of HgCdTe,compared to Schottky-barrier devices and low-dimensional solid devices, may become a decisiveargument in this case.
The fundamental performance limits ofHgCdTe photodiodes have not been reached yet.Continued development of the in situ vapour
phase epitaxy methods (MBE and MOCVD) willallow band-gap engineering heterojunction devicesof increasing quality and complexity. Also con-tinued development of epitaxial growth on alter-native substrates such as silicon will reduce thecost of 2D arrays. Development of dual-band ar-ray will continue and three-band detectors willsoon be demonstrated. To provide high resolutionspectroscopic imaging larger HgCdTe FPAs willbe used in Fourier-transform (FT) interferometers.Photodiodes will replace photoresistors for detec-tion out to 15 lm since they characterised by morelinear response.
The situation concerning quantum well struc-tures and superlattices is not clear; however, un-ique detection capabilities may arise from thelow-dimensional solids. Situation of LWIR QWIPsis clear. The initial results show promise for thegrowth of QWIPs on silicon wafers and appli-cations for integration with silicon-based elec-tronics. It is expected that the QWIP hand-held,cost-effective camera will find imaging and spec-troscopy applications in LWIR spectral band.Powerful possibilities of QWIP technology areconnected with VLWIR FPA applications andwith multicolour detection. Three-band and four-band FPAs will soon be demonstrated in near thefuture.
A new IR detector concept is microelectrome-chanical structures (MEMSs). This technologyis a marriage of photolithography and mechanics.FPAs based on MEMS technology and a visibleoptical readout system may offer lower-cost LWIRimaging systems [70].
Finally, considerable development of signal-processing function into FPAs can be anticipated.
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