Infrared detectors: an overview - Antoni Rogalski

24 Pages·2002·1.04 MB·English

InfraredPhysics&Technology43(2002)187–210
www.elsevier.com/locate/infrared
Infrared detectors: an overview
Antoni Rogalski *
InstituteofAppliedPhysics,MilitaryUniversityofTechnology,2KaliskiegoSt.,00-908Warsaw49,Poland
Abstract
The paper presents progress in infrared (IR) detector technologies during 200 history of their development. Clas-
sificationoftwotypesofIRdetectors(photondetectorsandthermaldetectors)isdoneonthebasisoftheirprincipleof
operation.TheoverviewofIRsystemsanddetectorsispresented.AlsorecentprogressindifferentIRtechnologiesis
described.Discussionisfocusedmainlyoncurrentandthemostrapidlydevelopingdetectors:HgCdTeheterostructure
photodiodes,quantumwellAlGaAs/GaAsphotoresistors,andthermaldetectors.Theoutlookfornear-futuretrendsin
IR technologiesis also presented. (cid:1) 2002Elsevier Science B.V.Allrights reserved.
Keywords:Photondetectors;Thermaldetectors;Focalplanearrays;Two-colourdetectors
1. Introduction • in 1921 Seebeck discovered the thermoelectric
effectandsoonthereafterdemonstratedthefirst
Looking back over the past 1000 years we no- thermocouple,
tice that infrared (IR) radiation itself was un- • in 1829 Nobili constructed the first thermopile
known until 200 years ago when Herschel’s byconnectinganumberofthermocouplesinse-
experiment with thermometer was first reported. ries [4],
He built a crude monochromator that used a • in 1933 Melloni modified thermocouple design
thermometer as a detector so that he could mea- and used bismuth and antimony for it [5].
surethedistributionofenergyinsunlight.InApril
1800 he wrote [1]: Thermometer No. 1 rose 7 de- Langley’s bolometer appeared in 1880 [6].
grees in 10 minutes by an exposure to the full red Langley used two thin ribbons of platinum foil,
colouredrays.Idrewbackthestand...thermometer connectedsoastoformtwoarmsofaWheatstone
No.1rose,in16minutes,83degreeswhenitscentre bridge. Langley continued to develop his bolome-
8
was 1=2 inch out of the visible rays. ter for the next 20 years (400 times more sensitive
The early history of IR was reviewed about 40 than his first efforts). His latest bolometer could
years ago in two well-known monographs [2,3]. detect the heat from a cow at a distance of a
The most important steps in development of IR quarter of mile. Thus, at the beginning the devel-
detectors are the following: opment of IR detectors was connected with ther-
mal detectors.
The photon detectors were developed in XX
*Fax:[phone]. century. The first IR photoconductor was devel-
E-mailaddress:[email protected](A.Rogalski). oped by Case in 1917 [7]. In 1933 Kutzscher at
1350-4495/02/$-seefrontmatter (cid:1) 2002ElsevierScienceB.V.Allrightsreserved.
PII:S1350-4495(02)00140-8
188 A.Rogalski/InfraredPhysics&Technology43(2002)187–210
Fig. 1.HistoryofthedevelopmentofIRdetectors.
Universityof Berlin,discovered that leadsulphide class of detectors the radiation is absorbed within
(from natural galena found in Sardinia) was the material by interaction with electrons. The
photoconductive and had response to about 3 lm observed electrical output signal results from the
[8]. changed electronic energy distribution. The pho-
Many materials have been investigated in the ton detectors show a selective wavelength depen-
IRfield.Observingahistoryofthedevelopmentof dence of the response per unit incident radiation
theIRdetectortechnology,asimpletheorem,after power. They exhibit both perfect signal-to-noise
Norton [9], can be stated: All physical phenomena performance and a very fast response. But to
in the range of about 0.1–1 eV can be proposed for achieve this, the photon detectors require cryo-
IR detectors. Among these effects are: thermo- genic cooling. Cooling requirements are the main
electric power (thermocouples), change in elec- obstacletothemorewidespreaduseofIRsystems
trical conductivity (bolometers), gas expansion based on semiconductor photodetectors making
(Golay cell), pyroelectricity (pyroelectric detec- them bulky, heavy, expensive and inconvenient to
tors), photon drag, Josephson effect (Joseph- use. Depending on the nature of interaction, the
son junctions, SQUIDs), internal emission (PtSi class of photon detectors is further sub-divided
Schottky barriers), fundamental absorption (in- into different types. The most important are: in-
trinsic photodetectors), impurity absorption (ex- trinsicdetectors,extrinsicdetectors,photoemissive
trinsic photodetectors), low-dimensional solids (metal silicide Schottky barriers) detectors, and
(superlattice (SL) and quantum well (QW) detec- quantum well detectors. Table 1 shows the com-
tors), different type of phase transitions, etc. parison of various IR detectors.
Fig. 1 gives approximate dates of significant ThesecondclassofIRdetectorsiscomposedof
development efforts for the materials mentioned. thermal detectors. In a thermal detector the inci-
The years during World War II saw the origins of dent radiation is absorbed to change temperature
modern IR detector technology. Interest has cen- of the material, and the resultant change in some
tred mainly on the wavelengths of the two atmo- physicalpropertiesisusedtogenerateanelectrical
spheric windows 3–5 and 8–14 lm, though in output.Thedetectorelementissuspendedonlags,
recent years there has been increasing interest in which are connected to the heat sink. Thermal
longer wavelengths stimulated by space applica- effects are generally wavelength independent; the
tions [10]. signal depends upon the radiant power (or its rate
of change) but not upon its spectral content. In
pyroelectric detectors a change in the internal
2. Classification of IR detectors spontaneous polarisation is measured, whereas in
the case of bolometers a change in the electrical
ProgressinIRdetectortechnologyisconnected resistance is measured. In contrast to photon de-
mainly to semiconductor IR detectors, which are tectors, the thermal detectors typically operate at
included in the class of photon detectors. In this room temperature. They are usually characterized
A.Rogalski/InfraredPhysics&Technology43(2002)187–210 189
Table 1
ComparisonofIRdetectors(afterRef.[11])
Detectortype Advantages Disadvantages
Thermal Light,rugged,reliable,andlowcost Lowdetectivityathighfrequency
Roomtemperatureoperation Slowresponse(msorder)
Photon
Intrinsic
IV–VI Availablelow-gapmaterials Poormechanical
Wellstudied Largepermittivity
II–VI Easyband-gaptailoring Non-uniformityoverlargearea
Well-developedtheoryandexp. Highcostingrowthandprocessing
Multicolourdetectors
III–V Goodmaterialanddopants Heteroepitaxywithlargelatticemismatch
Advancedtechnology
Possiblemonolithicintegration
Extrinsic Verylongwavelengthoperation Extremelylowtemperatureoperation
Relativelysimpletechnology
Freecarriers Low-cost,highyields Lowquantumefficiency
Largeandclosepacked2Darrays Lowtemperatureoperation
Quantumwells
TypeI Maturedmaterialgrowth Lowquantumefficiency
Gooduniformityoverlargearea Complicateddesignandgrowth
Multicolourdetectors
TypeII LowAugerrecombinationrate Complicateddesignandgrowth
Easywavelengthcontrol Sensitivetotheinterfaces
by modest sensitivity and slow response but they best values of NEDT, below 0.1 K, could be
are cheap and easy to use. The greatest utility in reached because effective noise bandwidths<100
IR technology has found bolometers, pyroelectric Hz can be achieved.
detectors and thermopiles. Uncooled,monolithicfocalplanearrays(FPAs)
Uptillthenineties,thermaldetectorshavebeen fabricated from thermal detectors may revolutio-
considerably less exploited in commercial and nise development of thermal imagers. Recently,
military systems in comparison with photon de- very encouraging results have been obtained with
tectors. The reason for this disparity is that ther- micromachined silicon bolometer [10,12] and py-
mal detectors are popularly believed to be rather roelectricdetector arrays [10,13].
slow and insensitive in comparison with photon
detectors. As a result, the worldwide effort to de-
velop thermal detectors was extremely small rela-
tive to that of photon detector. In the last decade 3. Focal plane arrays
however, it has been shown that extremely good
imagery can be obtained from large thermal de- Thereareanumberofarchitecturesusedinthe
tector arrays operating uncooled at TV frame development of IR FPAs. In general, they may be
rates. The speed of thermal detectors is quite ad- classified as hybrid and monolithic. The central
equate for non-scanned imagers with two-dimen- design questions involve performance advantages
sional (2D) detectors. The moderate sensitivity of versus ultimate producibility. Each application
thermal detectors can be compensated by a large may favour a different approach depending on
number of elements in 2D electronically scanned the technical requirements, projected costs, and
arrays. With large arrays of thermal detectors the schedule.
190 A.Rogalski/InfraredPhysics&Technology43(2002)187–210
In the monolithic approach, some of the mul- ogy with Schottky-barrier detectors is the only
tiplexingisdoneinthedetectormaterialitselfthan technology, which has matured to a level of
inanexternalreadoutcircuit.Thebasicelementof practical use.
a monolithic array is a metal–insulator–semicon- Hybrid FPAs detectors and multiplexers are
ductor (MIS) structure as shown in Fig. 2(c). A fabricated on different substrates and mated with
MIS capacitor detects and integrates the IR-gen- each other by the flip-chip bonding (Fig. 3) or
erated photocurrent. Although efforts have been loophole interconnection. In this case we can op-
made to develop monolithic FPAs using narrow- timise the detector material and multiplexer inde-
gap semiconductors, silicon-based FPA technol- pendently. Other advantages of the hybrid FPAs
Fig. 2.MonolithicIRFPAs:(a)all-silicon;(b)heteroepitaxy-on-silicon;(c)non-silicon(e.g.,HgCdTeCCD)(afterRef.[14]).
Fig. 3.HybridIRFPAwithindependentlyoptimisedsignaldetectionandreadout:(a)indiumbumptechnique,(b)loopholetech-
nique.
A.Rogalski/InfraredPhysics&Technology43(2002)187–210 191
are near 100% fill factor and increased signal- minimum of three transistors per pixel compared
processing area on the multiplexer chip. In the to one per memory cell. Consequently, whereas
flip-chip bonding, the detector array is typically various 64(cid:1)64 FPAs were available in the early
connected by pressure contacts via indium bumps 1980s, several vendors are now producing mono-
to the silicon multiplex pads. The detector array lithic FPAs in the TV-compatible 1040(cid:1)1040
can be illuminated from either the frontside or formats. Fig. 4 illustrates the trend of array size
backside (with photons passing through thetrans- over the past 25 years and some projections of
parent detector array substrate). In general, the what will evolve in the coming decade. Rockwell
latterapproachismostadvantageous.InHgCdTe has developed the world’s largest HgCdTe short
hybrid FPAs, photovoltaic detectors are formed wavelength IR (SWIR) FPA for astronomy and
on thin HgCdTe epitaxial layer on transparent low-background applications. The format of the
CdTe or CdZnTe substrates. For HgCdTe flip- device isa hybrid2048(cid:1)2048 witha unit cell size
chip hybrid technology, the maximum chip size is of 18 lm(cid:1)18 lm. Table 2 contains description
of the order of 10 mm square. In order to over- of representative IR FPAs that are commercially
come this problem, the technology is being devel- available as standard products and/or catalogue
oped with sapphire or silicon as the substrate of items from the major manufactures. Ten years
HgCdTe detectors. When using opaque materials, ago, high quality single element detectors often
substratesmustbethinnedto10–20lminorderto werepriced over$2000,butnow,some currentIR
obtain sufficient quantum efficiencies and reduce FPAproductioncostsarelessthan$1perdetector
the crosstalk. and even greater reductions are expected in the
There is a large research activity directed to- near future.
wards 2D staring arrays detectors consisting of Twotypesofsiliconaddressingcircuitshavebeen
more than 106 elements. IR FPAs have nominally developed:CCDsandcomplementarymetal-oxide-
the same growth rate as dynamic random access semiconductor(CMOS)switches.Thephotogener-
memory (RAM) integrated circuits (ICs) (it is ated carriers are first integrated in the well formed
consequence of Moore’s Law, which predicts the by a photogate and subsequently transferred to
ability to double transistor integration on each IC slow (vertical) and fast (horizontal) CCD shift
about every 18 months) but lag behind in size by registers.
about 5–10 years. ROIC’s are somewhat analo- AnattractivealternativetotheCCDreadoutis
gous to dynamic RAM––only readouts require a coordinativeaddressingwithCMOSswitches.The
Fig. 4.Increaseinarrayformatsizeoverthepast25yearsandprojectionsforthecomingdecade.PtSi,InSb,andHgCdTehavebeen
following the pace of dynamic RAM, offset by about a decade. QWIP detectors have been recently reported in sizes as large as
640(cid:1)480pixels(afterRef.[15]).
192 A.Rogalski/InfraredPhysics&Technology43(2002)187–210
Table 2
RepresentativeIRFPAsofferedbysomemajormanufacturers
Manufacturer/website Size/architec- Pixelsize Detectormaterial Spectral Operatingtem- D(cid:2)ðk Þ
p
ture (lm) range(lm) perature(K) (cmHz1=2W(cid:5)1)/
NETD(mK)
Raytheon/ 256(cid:1)256/H 30(cid:1)30 InSb 1–5.5 10–77
www.raytheon.com 1024(cid:1)1024/H 27(cid:1)27 InSb 0.6–5.0 35
320(cid:1)240/H 50(cid:1)50 Si:AsBIB 2–28 4–10
128(cid:1)128/H 40(cid:1)40 HgCdTe 9–11 80
256(cid:1)256/H 30(cid:1)30 HgCdTe 8.5–11 77–100
320(cid:1)240/M 48(cid:1)48 VO (bolometer) 8–14 300 50
x
328(cid:1)245/H 35(cid:1)35 Pyro(BST) 8–14 300 >50
Rockwell/Boeing/ 256(cid:1)256/H 40(cid:1)40 HgCdTe >15 77
www.boeing.com 640(cid:1)480/H 27(cid:1)27 HgCdTe >10 77
www.rsc.rockwell.com 225(cid:1)256/H 40(cid:1)40 HgCdTe 1–4.6 120 >10
640(cid:1)480/H 27(cid:1)27 HgCdTe 1–4.6 120 >25
2048(cid:1)2048/H 18(cid:1)18 HgCdTe 1–2.5 95–120 >1(cid:1)1014
320(cid:1)240/M 48(cid:1)48 VO (bolometer) 8–14 300 50
x
Mitsubishi/www.mits- 256(cid:1)256/M 52(cid:1)40 PtSi 3–5 77 0.036
ubishi-imaging.com 512(cid:1)512/M 26(cid:1)20 PtSi 3–5 77 0.033
1024(cid:1)1024/M 17(cid:1)17 PtSi 3–5 77 0.10
BAESystems 256(cid:1)256/H 30(cid:1)30 HgCdTe 1–10 80
432(cid:1)432/H HgCdTe 1–5 80
640(cid:1)480/M 28(cid:1)28 VO (bolometer) 8–14 (cid:6)300 60
x
Sofradir/ 128(cid:1)128/H 50(cid:1)50 HgCdTe 7.7–10 80 1:1(cid:1)1011/10
Infrared.sofradir.com 128(cid:1)128/H 50(cid:1)50 HgCdTe 3.7–4.8 90 4:3(cid:1)1011/7
128(cid:1)128/H 50(cid:1)50 HgCdTe 2.5–4.2 195 7:5(cid:1)1011/36
320(cid:1)240/H 30(cid:1)30 HgCdTe 3.7–4.8 120 1:0(cid:1)1012/8
320(cid:1)240/M 45(cid:1)45 AmorphousSi 8–14 (cid:5)20to60(cid:3)C 80
(bolometer)
Sarnoff/ 320(cid:1)244/M 23(cid:1)32 PtSi 1–5 77
www.sarnoff.com 640(cid:1)480/M 24(cid:1)24 PtSi 1–5 77
Eastman-Kodak/ 486(cid:1)640/M 25(cid:1)25 PtSi 1–5 77
www.kodak.com
Marconi/www. 128(cid:1)128/H 50(cid:1)50 HgCdTe 8–12 77
gec-marconi.com/ 384(cid:1)288/H 30(cid:1)30 HgCdTe 3–5 80 15
infra-red/ 256(cid:1)128/H 56(cid:1)56 Pyro(PST) 8–14 (cid:6)300 90
384(cid:1)288/H 40(cid:1)40 Pyro(PST) 8–14 (cid:6)300 130
AEG/www.aeg.com/ 256(cid:1)256/M 24(cid:1)24 PtSi 3–5 77 75
e_produkte.htm 486(cid:1)640/M 24(cid:1)24 PtSi 3–5 77 70
256(cid:1)256/H 40(cid:1)40 HgCdTe 8–10 80 20
640(cid:1)512/H 24(cid:1)24 HgCdTe 3–5 80 20
256(cid:1)256/H 40(cid:1)40 QWIP 8–10 60 10
640(cid:1)512/H 24(cid:1)24 QWIP 8–10 60 20
JPL/www.jpl.nasa.gow 128(cid:1)128/H 50(cid:1)50 QWP 15(k) 45 30
c
256(cid:1)256/H 38(cid:1)38 QWIP 9(k) 70 40
c
640(cid:1)486/H 18(cid:1)18 QWIP 9(k) 70 36
c
SensorsUnlimited/ 128(cid:1)128/H 60(cid:1)60 InGaAs 0.9–1.7 300 >1013
www.sensorsinc.com 320(cid:1)240/H 40(cid:1)40 InGaAs 0.9–1.7 300 >1012
H––hybrid,M––monolithic.
A.Rogalski/InfraredPhysics&Technology43(2002)187–210 193
advantages of CMOS are that existing foundries. gap semiconductor alloys, HgCdTe is the only
Design rules of 0.25 lm are in production with one material covering the whole IR spectral
pre-production runs of the 0.18 lm design rules. range having nearly the same lattice parameter.
At present, CMOS with minimum feature 60.5 The difference of lattice parameter between
lm is also enabling monolithic visible CMOS CdTe (E ¼1:5 eV) and Hg Cd Te (E ¼0:1
g 0:8 0:2 g
imagers. eV) is (cid:6)0.2%. Replacing small fraction of Cd
with Zn or Te with Se can compensate the re-
sidual lattice mismatch. The independence of
4. Viewpoint on IR detector technologies lattice parameter on composition is a major ad-
vantage of HgCdTe over any other materials.
Duringthepastfourdecadesmercurycadmium
telluride (HgCdTe) has became the most im- Heterojunctions are helpful in achieving high
portant semiconductor for the middle and long performanceinpractice.Forexample,thenarrow-
wavelength(k¼3–30lm)IRphotodetectors.The gap HgCdTe that absorbs IR radiation can be
short wavelength region has been dominated by buried encapsulated in wider gap HgCdTe pre-
III–V compounds (InGaAs, InAsSb, InGaSb). ventinginstabilitiesduetotheweakHg–Tebonds.
There have been numerous attempts to replace When background-photon noise is the domi-
HgCdTe with alternative materials. At present, nantnoisemechanism,thedetectorisoperatingin
severalothervariablegapalloysystemsareknown an ideal mode, and is said to exhibit background
including closely related mercury alloys HgZnTe, limited performance (BLIP). BLIP temperature is
HgMnTe,leadtintelluridesandselenides,InAsSb, defined that the device is operating at a tempera-
III–VI compounds with thallium and bismuth, ture at which the dark current equals the back-
free-carrier detectors and low-dimensional solids. ground photocurrent,given a fieldof view (FOV),
The main motivations, behind the numerous and a background temperature. In Fig. 5, plots of
attempts to replace HgCdTe, are technological the calculated temperature required for BLIP op-
problems of this material. One of them is weak eration in 30(cid:3) FOV, are shown as a function of
Hg–Tebond,whichresultsinbulkandsurfaceand cutoff wavelength. We can see that the operat-
interfaceinstabilities.Uniformityandyieldarestill ing temperature of ‘‘bulk’’ intrinsic IR detectors
issues. Nevertheless, HgCdTe remains the leading (HgCdTe and PbSnTe) is higher than for other
semiconductor for IR detectors. The most impor- types of photon detectors. HgCdTe detectors with
tant reasons for this are:
• No one of the new materials offers fundamental
advantages over HgCdTe. While the figure of
merit, (a=G) (where a is the absorption coeffi-
cient and G is the thermal generation rate) [16],
of various narrow-gap semiconductors seems to
beveryclosetothatofHgCdTe,thefree-carrier
detectors and GaAs/AlGaAs superlattice de-
vices have several order of magnitude smaller
a=G.
• HgCdTe exhibits extreme flexibility, it can be
tailored for optimised detection at any region
of IR spectrum, dual and multicolour devices
can be easily constructed.
Fig. 5.Estimationofthetemperaturerequiredforbackground
• The present development of IR photodetec-
limitedoperationofdifferenttypesofphotondetectors.Forthe
tors has been dominated by complex band-gap
calculations FOV¼30(cid:3) and T ¼300 K are assumed (after
B
heterostructures. Amongvariousvariableband- Ref.[17]).
194 A.Rogalski/InfraredPhysics&Technology43(2002)187–210
Fig. 6.Doublelayerplanarheterostructurecross-sectionschematicsforSWIRInGaAs(a)andHgCdTe(b)photodiodes.
BLIP operate with thermoelectric coolers in the low growth temperature techniques: metalorganic
MWIRrange,buttheLWIRdetectors(86k 612 chemical vapour deposition (MOCVD) and mo-
c
lm) operate at (cid:6)100 K. HgCdTe photodiodes lecular beam epitaxy (MBE).
exhibit higher operating temperature compared to The baseline detector architecture for SWIR
extrinsic detectors, silicide Schottky barriers and InGaAsandHgCdTeisPþ-on-ndevice (symbol P
QWIPs. However, the cooling requirements for denotes wider gap). Fig. 6 shows the double layer
QWIPs with cutoff wavelengths below 10 lm are heterostructure (DLPH) device cross-sections for
less stringent in comparison with extrinsic detec- both types of photodiodes. Apart from the above
tors and Schottky-barrier devices. described benefits, incorporation of a buried nar-
Recently,moreinteresthasbeenfocusedonp–n row-band-gap active layer in the DLPH reduces
junction heterostructure photodiodes. In compar- tunnelling currents and increases the total dose
ison with heterostructures, homojunctions have radiation hardness, both of which are essential
a lower breakdown voltage and a large reverse detectorattributesforremotesensingapplications.
leakage current. Another aspect of photodiode The thickness of the base region should be opti-
design is the required bandwidth for high-speed misedfornearunityquantumefficiencyandalow
operation. dark current. This is achieved with a base thick-
Photodiodes with their very low power dissi- ness (typically about 5–7 lm) slightly higher than
pation, easy multiplexing on focal plane silicon the inverse absorption coefficient for single pass
chip and less stringent noise requirements for the devices.Lowdopingisbeneficialforalowthermal
readout devices and circuits, can be assembled in generation and high quantum efficiency. Since the
2D arrays containing a very large number of ele- diffusion length in absorbing region is typically
ments, limited only by existing technologies. longer thanitsthickness,anycarriersgeneratedin
Epitaxy is the preferable technique to obtain the base region can be collected giving rise to the
device-quality materials. Among the various epit- photocurrent.
axial techniques, liquid phase epitaxy (LPE) is the
mostmaturemethod.LPEgrowthmustbecarried 4.1. InGaAs photodiodes
out at relatively high growth temperature with
adherent interdiffusion and resulting graded in- The need for high-speed, low-noise In Ga As
x 1(cid:5)x
terfaces. Recent efforts are aimed mostly at (InGaAs) photodetectors for use in lightwave
A.Rogalski/InfraredPhysics&Technology43(2002)187–210 195
communicationsystemsoperating inthe1–1.7 lm
wavelength region (x¼0:53) is well established.
In Ga As alloy is lattice matched to the InP
0:53 0:47
substrate. Having lower dark current and noise
thanindirect-band-gapgermanium,thecompeting
near-IR material, the material is addressing both
entrenched applications including low light level
night vision and new applications such as remote
sensing, eye-safe range finding and process con-
trol [18]. By changing the alloy composition of
the InGaAs absorption layer, the photodetector
responsivity can be maximized at the desired
wavelength 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 detector
fabrication is different. The InGaAs detector’s
active material is deposited onto a substrate using Fig. 7.RoomtemperaturedetectivityofInGaAsphotodiodes
chlorideVPE or MOCVDtechniques adjusted for withcutoffwavelengthat1.6,1.9,2.2,and2.6lm,respectively.
thickness, background doping, and other require-
ments. Planar technology evolved from the older
mesatechnologyandatpresentiswidelyuseddue photodiodes have been available since the late fif-
toitssimplestructureandprocessingaswellasthe ties and they are generally fabricated by impurity
high reliability and low cost (see Fig. 6(a)). diffusion and ion implantation. Epitaxy is not
Standard In Ga As photodiodes have ra- used; instead, the standard manufacturing tech-
0:53 0:47
diative-limited room temperature detectivity of niquebeginswithbulkn-typesinglecrystalwafers
(cid:9)1013 cmHz1=2W(cid:5)1. With increasing cutoff wave- with donor concentration about 1015 cm(cid:5)3. Wim-
length,detectivitydecreases,whatisshowninFig. mers et al. have presented the status of InSb
7. The highest quality InGaAs photodiodes have photodiodetechnologyforawidevarietyoflinear
been grown by MOCVD [19]. Their performance and FPAs [21,22].
is comparable with HgCdTe photodiodes. TypicalInSbphotodiodeRAproductat77Kis
Linear array formats of 256, 512 and 1024 ele- 2(cid:1)106 Xcm2 at zero bias and 5(cid:1)106 Xcm2 at
ments have been fabricated for environmental slight reverse biases of approximately 100 mV.
sensing from 0.8 to 2.6 lm. The size of pixels are This characteristics is beneficial when the detector
different; from 30(cid:1)30 lm2 (with spacing of 50 is used in the capacitive discharge mode. As ele-
lm), 25(cid:1)500 lm2 to 13(cid:1)500 lm2 (with spacing ment size decreases below 10(cid:5)4 cm(cid:5)2, some slight
of 25 lm). Sensors Unlimited offers 10(cid:1)10(cid:1)6 degradation in resistance due to surface leakage
cmline-scancamerasincorporatinglinearInGaAs occurs.
FPAs of up to 512 elements on a 50 lm pitch. A InSb photodiodes can also be operated in the
room temperature staring cameras are based on temperaturerangeabove77K. Of course, theRA
128(cid:1)128 and 320(cid:1)240 InGaAs FPAs. products degrade in this region. At 120 K, RA
productsof104 Xcm2 arestillachievedwithslight
reversebias,makingBLIPoperationpossible.The
4.2. InSb photodiodes quantum efficiency inInSbphotodiodes optimised
for this temperature range remains unaffected up
InSbmaterialisfar morematurethanHgCdTe to 160 K. Detectivity increases with reduced
and good quality more than 7 cm diameter bulk background flux (narrow FOV and/or cold filter-
substrates are commercially available [20]. InSb ing) as illustrated in Fig. 8.
196 A.Rogalski/InfraredPhysics&Technology43(2002)187–210
Fig. 9. Representative spectral response data for HgCdTe
Fig. 8. Detectivity as a function of wavelength for an InSb photodiodes.
photodiodeoperatingat77K(afterJudson,InfraredDetectors
Catalog,1999).
array on a Si substrate, rather than CdZnTe, has
emerged as a particularly promising approach
InSb photovoltaicdetectorsarewidelyused for to scale up wafer dimensions and achieve a cost-
ground-based IR astronomy and for applications effective production [24]. MBE offers unique ca-
aboard the Space Infrared Telescope Facility. pabilities in material and device engineering
Recently,impressiveprogresshasbeenmadeinthe including the lowest growth temperature, super-
performance of InSb hybrid FPAs. An array size lattice growth and potential for the most sophis-
of 1024(cid:1)1024 is possible because the InSb de- ticated composition and doping profiles. The
tectormaterialisthinnedto<10lm(aftersurface growth temperature is less than 200 (cid:3)C for MBE
passivation and hybridisation to a readout chip) but around 350 (cid:3)C for MOCVD, making it more
which allows it to accommodate the InSb/silicon difficult to control the p-type doping in the
thermal mismatch [23]. Linear array formats of MOCVD due to the formation of Hg vacancies.
64, 128 and 256 elements are also produced with Different HgCdTe photodiode architectures
frontside-illuminated detectors for both high- have been fabricated that are compatible with
background and astronomy applications. Element backside and frontside-illuminated hybrid FPA
sizes depend on device format and range from technology [18,25]. The fabrication of HgCdTe
20(cid:1)20 to 200(cid:1)200 lm. photodiodes was usually based on the most com-
The cryogenically cooled InSb and HgCdTe mon nþ–p and Pþ–n DLHJ structure. In these
arrays have comparable array size and pixel yield photodiodes the base p-type layers (or n-type
at MWIR spectral band. However, wavelength layers) are sandwiched between CdZnTe substrate
tunability and high quantum efficiency have made and high-doped (in nþ–p structures) or wider-gap
HgCdTe the preferred material. (in Pþ–n structure) regions. Due to backside illu-
mination(throughCdZnTesubstrate)andinternal
4.3. HgCdTe photodiodes electric fields (which are ‘‘blocking’’ for minority
carriers), influence of surface recombinations on
HgCdTephotodiodesareavailabletocoverthe the photodiodes performance is eliminated. Both
spectral range from 1 to 20 lm. Fig. 9 illustrates optical and thermal generations are suppressed in
representativespectralresponsefromphotodiodes. the nþ-region due to the Burstein–Moss effect and
Spectral cutoff can be tailored by adjusting the inthePþ-regiondue towide gap.Theinfluenceof
HgCdTe alloy composition. surfacerecombinationisalsopreventedbytheuse
Epitaxial techniques are preferred technique to of suitable passivation. Passivation of HgCdTe
obtain device-quality HgCdTe material for IR has been done by several techniques which com-
devices. Epitaxial growth of the HgCdTe detector prehensive review was given by Nemirovsky and