ARTICLE IN PRESS ProgressinQuantumElectronics32(2008)89–120 www.elsevier.com/locate/pquantelec Review Quantum-dot infrared photodetectors: Status and outlook (cid:2) P. Martyniuk, A. Rogalski InstituteofAppliedPhysics,MilitaryUniversityofTechnology,2KaliskiegoStr.,00-908Warsaw,Poland Abstract This paper reviews the present status and possible future developments of quantum-dot infrared photodetectors (QDIPs). At the beginning the paper summarizes the fundamental properties of QDIPs. Next, an emphasis is put on their potential developments. Investigations of the performance of QDIPs as compared to other types of infrared photodetectors are presented. A model is based on fundamental performance limitations enabling a direct comparison between different infrared material technologies. It is assumed that the performance is due to thermal generation in the active detector’s region. In comparative studies, the HgCdTe photodiodes, quantum well infrared photodetectors (QWIPs), type-II superlattice photodiodes, Schottky barrier photoemissivedetectors,dopedsilicondetectors,andhigh-temperaturesuperconductordetectorsare considered. Theoreticalpredictionsindicatethatonlytype-IIsuperlatticephotodiodesandQDIPsareexpected tocompetewithHgCdTephotodiodes.QDIPstheoreticallyhaveseveraladvantagescomparedwith QWIPsincludingthenormalincidenceresponse,lowerdarkcurrent,higheroperatingtemperature, higherresponsivityanddetectivity.TheoperatingtemperatureforHgCdTedetectorsishigherthan for other types of photon detectors. It is also shown, that BLIP temperature of QDIP strongly dependson nonuniformityin the QD size. ComparisonofQDIPperformancewithHgCdTedetectorsgivesclearevidencethattheQDIPis suitable for high operation temperature. It can be expected that improvement in technology and design of QDIP detectors will make it possible to achieve both high sensitivity and fast response useful for practicalapplication atroom temperatureFPAs. Comparison of theoretically predicted and experimental data indicates that, as so far, the QDIP devices have not fully demonstrated their potential advantages and are expected to posses the (cid:2) Correspondingauthor.Tel./fax:+48226839109. E-mailaddress:[email protected](A.Rogalski). 0079-6727/$-seefrontmatterr2008ElsevierLtd.Allrightsreserved. doi:10.1016/j.pquantelec.2008.07.001 ARTICLE IN PRESS 90 P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 fundamental ability to achieve higher detector performance. Poor QDIP performance is generally linked to nonoptimal band structure and controlling the QDs size and density (nonuniformity in QDsize). r2008Elsevier Ltd. Allrightsreserved. Keywords:Quantum-dotinfraredphotodetectors;Quantumwellinfraredphotodetectors;HgCdTephotodiodes; Type-IIsuperlattices;RAproduct;Detectivity Contents 1. Introduction . .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. 90 2. Anticipated advantagesof QDIPs. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. 92 3. Performance limits of infrared photodetectors . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. 95 3.1. QDIPmodel. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. . 96 3.2. Normalized darkcurrent.. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. . 98 3.3. Detectivity .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. . 99 3.4. BLIPtemperature. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. 100 4. QDIPsvs.HgCdTe photodiodes.. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . . 101 4.1. Fundamentalfigure of merit .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. 101 4.1.1. Photodiode . .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. 101 4.1.2. QDIP . .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. 105 4.2. Experimental verification.. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. 108 4.2.1. Performance at lowtemperature . .. .. .. . .. .. .. .. .. .. .. . .. .. .. 108 4.2.2. Performance at highertemperature.. .. .. . .. .. .. .. .. .. .. . .. .. .. 110 5. Conclusions . .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . . 114 Appendix A . .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . . 114 A.1. HgCdTe. . .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. 114 A.2. QWIP .. . .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. 115 A.3. Photoemissive detectors. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. 116 A.4. Extrinsic detectors. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. .. .. 117 A.5. High-temperature superconductor (HTSC) .. .. . .. .. .. .. .. .. . .. .. .. .. .. 117 References .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. .. .. .. . . 118 1. Introduction SincetheinitialproposalbyEsakiandTsuin1970[1]andtheadventofmolecularbeam epitaxy (MBE), the interest in semiconductor low-dimensional solids has increased continuouslyovertheyears,drivenbytechnologicalchallenges,newphysicalconceptsand phenomena as well as promising applications. A new class of materials with unique optoelectronic properties has been developed. Zero-dimensional quantum confined semiconductor heterostructures have been investigated theoretically and experimentally for some time [2–4]. At present, nearly defect-free quantum-dot devices can be fabricated reliably and reproducibly. Also new types of infrared photodetectors taking advantage of the quantum confinement obtained in semiconductor heterostructures have been emerged. ARTICLE IN PRESS P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 91 As it was indicated by Kinch [5], photon detectors can be divided into two broad classes, namely majority and minority carrier devices. We can distinguish six infrared (IR) material systems: 1. Direct bandgap semiconductors—minority carriers (cid:2) binary alloys: InSb, InAs (cid:2) ternary alloys: HgCdTe, InGaAs (cid:2) type-II, -III superlattices: InAs/GaInSb, HgTe/CdTe 2. Extrinsic semiconductors—majority carriers (cid:2) Si:As, Si:Ga, Si:Sb (cid:2) Ge:Hg, Ge:Ga 3. Type-I superlattices—majority carriers (cid:2) GaAs/AlGaAs QWIPs 4. Silicon Schottky barriers—majority carriers (cid:2) PtSi, IrSi 5. Quantum dots—majority carriers (cid:2) InAs/GaAs QDIPs 6. High-temperature superconductors (HTSC)—minority carriers AllofthesematerialsystemshavebeenseriouslyplayersintheIRmarketplacewiththe exception of the HTSC and quantum-dot infrared photodetectors (QDIPs). The dates given in Fig. 1 show the chronology of significant development efforts on the materials mentioned.FirstobservationsofintersubleveltransitionsinthefarIRwerereportedinthe early1990s,eitherinInSb-basedelectrostaticallydefinedquantumdots[6]orinstructured MWIR LWIR contact contact LWIR Video MWIR Substrate λ λ 2 1 rd 3 Gen nd st 2 Gen FPA+ROIC 1 Gen, Scan to image FPA+ROIC As P mal detectors TlS2 PbSPbSe Ge:XInSb HgCdTe PbSnTeSi:X Si:X/CCDPtSi/CCD HgCdTe/CCD HgCdTe SPRITE InGaAs QWIP Two-colour FPAsBolometer FPAsPyroelectric FPAs QDIPVery large FPAs MEMS FPAs Four-colour FPAs Two-colour QDIP F Ther 1940 1950 1960 1970 1980 1990 2000 2010 Detector roadmap Fig.1. Historyofthedevelopmentofinfrareddetectorsandsystems.Threegenerationsystemscanbeconsidered for principal military and civilian applications: first generation (scanning systems), second generation (staring systems—electronicallyscanned)andthirdgeneration(multicolourfunctionalityandotheron-chipfunctions). ARTICLE IN PRESS 92 P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 two-dimensional (2-D) electron gas [7]. The first QDIP was demonstrated in 1998 [8]. Ever since great progress has been made in their development and performance characteristics [9,10] and in their applications to thermal imaging focal plane arrays (FPA) [11]. Thebeginningoftheinterestinquantum-dotresearchcanbetracedbacktoasuggestion byArakawaandSakakiin1982[2]thattheperformanceofsemiconductorlaserscouldbe improved by reducing the dimensionality of the active regions of these devices. Initial efforts at reducing the dimensionality of the active regions focused on using ultrafine lithography coupled withwet ordrychemicaletchingtoform 3-D structures.Itwas soon realized, however, that this approach introduced defects (high density of surface states) that greatly limited the performance of such quantum dots. Initial efforts were mainly focused on the growth of InGaAs nanometer-sized islands on GaAs substrates. In 1993, thefirstepitaxialgrowthofdefect-freequantum-dotnanostructureswasachievedbyusing MBE [12]. Most of the practical quantum-dot structures today are synthesized both by MBE and MOCVD. Under certain growth conditions, when the thickness of the film with the larger lattice constant exceeds a certain critical thickness, the compressive strain within the film is relieved by the formation of coherent island. These islands may be quantum dots. Coherent quantum-dot islands are generally formed only when the growth proceeds in whatisknownasStranski–Krastanowgrowthmodel[13].Theonsetofthetransformation ofthegrowthprocessfroma2-Dlayer-by-layergrowthmodetoa3-Dislandgrowthmode resultsinaspottyRHEEDpattern.Thisis,incontrasttotheconventionalstreakypattern, generally observed for the layer-by-layer growth mode. The transition typically occurs afterthedepositionofacertainnumberofmonolayers.ForInAsonGaAs,thistransition occurs after about 1.7 monolayers of InAs have been grown; this is the onset of islanding and, hence, quantum-dot formation. The most advanced III–V IR detectors, which utilize intersubband or subband to continuum transitions in quantum wells, are GaAs/AlGaAs quantum well infrared photodetectors (QWIPs). The imaging performance of FPA fabricated with this material system is comparable to the state of art of HgCdTe [14,15]. This paper summarizes the fundamental properties of QDIPs. The intent is to concentrate on device approaches and present stage of development. A secondary aim is to compare the potential QDIP performance with different material systems used in IR detector technology. Our intention is to concentrate on fundamental phenomena and minimize any confusion that might exist within the minds of scientists. The paper completes two previously published papers by Kinch [5] and Phillips [16]. 2. Anticipated advantages of QDIPs ThesuccessofquantumwellstructuresforIRdetectionapplicationshasstimulatedthe development of QDIPs. In general, QDIPs are similar to QWIPs but with the quantum wells replaced by quantum dots, which have size confinement in all spatial directions. Fig. 2 shows the schematic layers of a QWIP and a QDIP. In both cases, the detection mechanism is based on the intraband photoexcitation of electrons from confined states in theconductionbandwellsordotsintothecontinuum.Theemittedelectronsdrifttowards thecollectorintheelectricfieldprovidedbytheappliedbias,andphotocurrentiscreated. It is assumed, that the potential profile at the conduction band edge along the growth ARTICLE IN PRESS P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 93 QWIP QDIP Injection from contact Top contact Growth Trapping direction Emitter Emission xN xN hν Barrier Barrier Photocurrent Well Dots Barrier Barrier Bottom contact Under bias Collector Fig. 2. Schematic layers of QWIP and QDIP (a) and potential profile for both structures under bias (b). For QDIP,influenceofwettinglayerisneglected(afterRef.[17]). Direction of IR radiation photoelectron current flow Ohmic contact Quantum dot layer Wetting layer Active region Contact layer (Multi periods) Barrier layer Ohmic contact Spacer layer Contact layer Buffer layer Substrate Fig.3. Schematicdiagramofconventionalquantum-dotdetectorstructure. direction for both structure have a similar shape as shown in Fig. 2(b). In practice, since the dots are spontaneously self-assembled during growth, they are not correlated between multilayers in active region. TwotypesofQDIPstructureshavebeenproposed:conventionalstructure(vertical)and lateral structure. In a vertical QDIP, the photocurrent is collected through the vertical transport of carriers between top and bottom contacts (see Fig. 3). The device heterostructure comprises repeated InAs QD layers buried between GaAs barriers with topandbottomcontactlayersatactiveregionboundaries.Themesaheightcanvaryfrom 1 to 4mm depending on the device heterostructure. The quantum dots are directly doped (usually with silicon) in order to provide free carriers during photoexcitation, and an AlGaAs barrier can be included in the vertical device heterostructure in order to block dark current created by thermionic emission [18,19]. ARTICLE IN PRESS 94 P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 1.2 GaAs barrier 1.0 InGaAwsell GaAs sivity barrier on 0.8 p s e d r 0.6 e z IR ali 0.4 m or N 0.2 InAs QD 0 3 5 7 9 11 13 Wavelength (μm) Fig.4. DWELLinfrareddetector:(a)theoperationmechanism,(b)experimentallymeasuredspectraltunability byvaryingwellwidthfrom55to100A˚ (afterRef.[21]). In addition to the standard InAs/GaAs QDIP, several other heterostructure designs have been investigated for use as IR photodetectors [9,10]. An example is InAs QDs embedded in a strain-relieving InGaAs quantum well which are known as dot-in-a-well (DWELL) heterostructures (see Fig. 4) [11,20]. This device offers two advantages: challenges in wavelength tuning through dot-size control can be compensated in part by engineering the quantum well sizes, which can be controlled preciselyandquantumwellscantrapelectronsandaidincarriercapturebyQDs,thereby facilitatinggroundstaterefilling.Fig.4(b)showsDWELLspectraltuningbyvaryingwell geometry. The lateral QDIP collects photocurrent through transport of carriers across a high- mobilitychannelbetweentwotopcontacts,operatingmuchlikeafield-effecttransistor.As previously, again AlGaAs barriers are present, but instead of blocking the dark current, these barriers are used to both modulation-dope the quantum dots and to provide the high-mobility channel. Lateral QDIPs have demonstrated lower dark currents and higher operating temperatures than vertical QDIPs since the major components of the dark currentarisefrominterdottunnellingandhoppingconduction[22].However,thesedevices willbedifficulttoincorporateintoaFPAhybrid-bumpbondedtoasiliconreadoutcircuit. Because of this, more efforts is directed to improve the performance of vertical QDIPs, which are more compatible with commercially available readout circuits. Thequantum-mechanicalnatureofQDIPsleadstoseveraladvantagesoverQWIPsand other types of IR detectors that are available. As in the HgCdTe, QWIP and type-II superlattice technologies, QDIPS provide multi-wavelength detection. However, QDs provide many additional parameters for tuning the energy spacing between energy levels, such as QD size and shape, strain, and material composition. The potential advantages in using QDIPs over quantum wells are as follows: (cid:2) Intersubband absorption may be allowed at normal incidence (for n-type material). In QWIPs,onlytransitions polarized perpendicularlytothegrowthdirection areallowed, ARTICLE IN PRESS P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 95 duetoabsorption selection rules.The selection rules inQDIPsareinherentlydifferent, and normal incidence absorption is observed. (cid:2) Thermal generation of electrons is significantly reduced due to the energy quantization in all three dimensions. As a result, the electron relaxation time from excited states increasesduetophononbottleneck.GenerationbyLOphononsisprohibitedunlessthe gap between the discrete energy levels equals exactly to that of the phonon. This prohibition does not apply to quantum wells, since the levels are quantized only in the growthdirection and a continuum existsin the other two directions (hence generation- recombination by LO phonons with capture time of few picoseconds). Thus, it is expected that S/N ratio in QDIPs will be significantly larger than that of QWIPs. (cid:2) LowerdarkcurrentofQDIPsisexpectedthanofHgCdTedetectorsandQWIPsdueto 3-D quantum confinement of the electron wavefunction. Both the increased electron lifetime and the reduced dark current indicate that QDIPs should be able to provide high-temperature operation. In practice, however, it has been a challenge to meet all of above expectations. Carrier relaxation times in QDs are longer than the typical 1–10ps measured for quantum wells. It is predicted that the carrier relaxation time in QDs is limited by electron–hole scattering [23], rather than phonon scattering. For QDIPs, the lifetime is expected to be even larger, greater than 1ns, since the QDIPs are majority carrier devices due to absence of holes. The main disadvantage of the QDIP is the large inhomogeneous linewidth of the quantum-dot ensemble variation of dot size in the Stranski–Krastanow growth mode [16,24]. As a result, the absorption coefficient is reduced, since it is inversely proportional totheensemblelinewidth.Large,inhomogeneouslybroadenedlinewidthhasadeleterious effectonQDIPperformance.Subsequently,thequantumefficiencyQDdevicestendtobe lower than what is predicted theoretically. Vertical coupling of quantum-dot layers also reduces theinhomogeneous linewidthof thequantum-dotensemble;however,it may also increase the dark current of the device, since carriers can tunnel through adjacent dot layers more easily. As in other type of detectors, also nonuniform dopant incorporation adversely affects the performance of the QDIP. Therefore, improving QD uniformity is a key issue in the increasing absorption coefficient and improving the performance. Thus, the growth and design of unique QD heterostructure is one of the most important issues related to achievement of state-of-the art QDIP performance. 3. Performance limits of infrared photodetectors The total generation rate of IR detector is a sum of the optical and thermal generation G ¼G þG . (1) th op The optical generation may be due to the signal or background radiation. For IR detectors, usually background radiation is higher compared to the signal radiation. If the thermal generation is reduced much below the background level, the performance of the device is determined by the background radiation (BLIP conditions for background limited IR photodetector). This condition can be described as [5] ZF t B 4n , (2) t th ARTICLE IN PRESS 96 P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 wheren isthedensityofthermalcarriersatthetemperatureT,tisthecarrierlifetime,F th B is the total background photon flux density (unitcm(cid:3)2s(cid:3)1) reaching the detector, and t is the detector’s thickness. Re-arranging, we have for the BLIP requirements ZF n B4 th, (3) t t i.e., the photon generation rate per unit volume needs to be greater than the thermal generationrateperunitvolume.Thecarrierscanbeeithermajorityorminorityinnature. Using Z¼at, where a is the absorption coefficient in the material, we obtain n F 4 th ¼G . (4) B th at Thenormalizedthermalgeneration,G ¼n /(at),predictstheultimateperformanceof th th anyIRmaterialandcanbeusedtocomparetherelativeperformanceofdifferentmaterials as a function of temperature and energy gap (cutoff wavelength). Itshouldbenotedthattheimportanceofthethermalgenerationrateasamaterialfigure of merit was recognized for the first time by Long [25]. It was used in many papers by English workers [26,27] related to high operating temperature (HOT) detectors. Eq. (4) wasintroducedbyKinch[5],whichisthethermalgenerationratewithin1/adepthperunit of area, as the figure of merit. This formula is actually the inverse a/G figure of merit th previously proposed by Piotrowski and Gawron [28]. In further considerations we use a simple set of fundamental detector parameters described in excellent Kinch’s paper [5] to compare the performance of different material systems used in IR detector technology. In the case of QDIPs, a model developed by Phillips is adapted [16]. 3.1. QDIP model Fig. 5 shows a schematic view of the QDIP structure under considerations. Simple estimationindicatesthatthequantum-dotdensityd¼1/s2,wheresistheinterdotspacing. We will consider a planar array of quantum dots with conduction band structure containing two confined energy levels (E and E ) and the excited state transition 1 2 coinciding with the barrier conduction band minimum. Fig.5. Schematicviewofthequantum-dotarray(a)andconductionbandstructureofthedot(b)(afterRef.[16]). ARTICLE IN PRESS P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 97 DuetodiscretenatureofQDs,thefillfactorFshouldbeincludedforopticalabsorption in quantum dots. This factor can be estimated in a simple way as p ffiffiffiffi 3 V F ¼ , (5) s where V is the quantum-dot volume. For self-assembled QDs, a Gaussian distribution has been observed for the electronic andopticalspectra.PhillipsmodelledtheabsorptionspectraforanensembleofQDsusing a Gaussian line shape in the shape " # n s ðE(cid:3)E Þ2 aðEÞ¼a 1 QDexp (cid:3) g , (6) o d s s2 ens ens where a is the maximum absorption coefficient, n is the areal density of electrons in the o 1 quantum-dotgroundstate,disthequantum-dotdensity,andE ¼E (cid:3)E istheenergyof g 2 1 the optical transition between ground and excited states in the QDs. The expressions s QD ands arethestandarddeviationsintheGaussianlineshapeforintrabandabsorptionin ens a single quantum dot and for the distribution in energies for the QD ensemble, respectively. It should be noticed that Eq. (6) estimates the absorption coefficient for the necessary presents of electrons in the QD ground state. The terms n /d and s /s 1 QD ens describe a decrease in absorption due to absence of available electrons in the QD ground state and inhomogeneous broadening, respectively. Tocalculatethermaldistributionofcarrierdensity,theFermidistributionisused.Then, the electron densities in the QD sheet for the energy level n is given by Z gd (cid:3) ðE(cid:3)E Þ2(cid:4) n ¼ p exp (cid:3) n fðE ÞdE, (7) n ffipffiffis s2 n where g is the degeneracy factor for the energy level, E is the mean energy, s is the n standard deviation in energy for the Gaussian line shape (to describe the spread of QD energy levels, again a Gaussian distribution with standard deviation s is used [29]). Because, however, soE ¼E (cid:3)E , the Gaussian line shape function describing carrier g 2 1 densities in QD ground and excited states has little effect, and then Eq. (7) can be simplified to n ¼gdFðE Þ. (8) n n Next,takingintoaccountthechargeneutralitycondition,the2-Dcarrierdensitiesmay be given by Z 1 N ¼n þn þn ¼2dfðE Þþ8dfðE Þþ g2DðEÞfðE ÞdE, (9) d 1 2 b 1 2 c 0 where N is the sheet density dopant level. We assume a degeneracy of g¼2 (two-spin d states) for the QD ground state, and g¼8 (fourfold degeneracy and two-spin state each [30]) for the quantum-dot excited state. As it was mentioned previously, the excited state coincides with the conduction band minimum of the barrier material; E ¼E . Then, the c 2 thermal carrier density is n +n , where n is the carrier density in the conduction band. 2 b b ARTICLE IN PRESS 98 P.Martyniuk,A.Rogalski/ProgressinQuantumElectronics32(2008)89–120 3.2. Normalized dark current The normalized dark current density, given by G q, is presented as [see Eq. (4)] th qsdt J ¼G q¼ ðn þn Þ. (10) dark th a n Ft 2 b o 1 In the calculation we assume the material parameters chosen by Phillips [16]: a ¼5(cid:4)104cm(cid:3)2, V¼5.3(cid:4)10(cid:3)19cm(cid:3)3, d¼5(cid:4)1010cm(cid:3)2, t¼1ns, N ¼1(cid:4)1011cm(cid:3)2 o d andthedetectorthicknesst¼1/a .Theseparametersarerepresentativeforself-assembled o InAs/GaAs quantum dots reported in the literature. The dopand concentration corresponds to two electrons per QD. In further analysis, for clarity, we assume that inhomogeneous broadening of the dot ensemble is neglected (s /s ¼1). QD ens The normalized dark current densities for the various materials used in IR detector technologies in LWIR spectralregion (E ¼0.124eV, l ¼10mm) are shownin Fig. 6. In g c addition, the f/2 background flux current density is also shown. The extrinsic silicon, the HTSCandthephotoemissive(siliconSchottkybarrier)detectorsarehypothetical,butare includedforcomparison.Inthecalculations,carriedoutfordifferentmaterialsystemswe have followed the procedures used in Kinch’s paper [5] (see Appendix), except QDIPs where the Phillips’ model is used [16]. In the MWIR and LWIR regions, the dominant position have HgCdTe photodiodes. QWIPsaremainlyusedinLWIRtacticalsystemsoperatingatlowertemperature,typically 65–70K, where cooling is not an issue. Large detector arrays with more than one million detector elements are fabricated by several manufacturers using these material systems. Beyond 15mm, good performance is achieved using extrinsic silicon detectors. These detectors are termed impurity band conduction (IBC) detectors and found niche market for the astronomy and civil space communities because HgCdTe has not yet realized its potential at low temperatures and reduced background. 10-1 2m) QWIP λc = 10 μm c A/ y ( 10-3 f/2 FOV,T = 300 K sit en HTSC Photoemissive d nt Extrinsic urre 10-5 c QDIP k ar d ed 10-7 z ali HgCdTe m or N 10-9 20 40 60 80 100 120 Temperature (K) Fig.6. TemperaturedependenceofthenormalizeddarkcurrentofvariousLWIRmaterialtechnologies.Thef/2 backgroundfluxcurrentdensityisalsoshown.