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Narrow gap semiconductor photodiodes


									                                                          Invited Paper

                                Narrow gap semiconductor photodiodes
                                           Antoni Rogaiskia and Manijeh Razeghi"
             ajnstitute of Applied Physics, Military University of Technology, 0 1 -489 Warsaw, Poland
            bCenter for Quantum Devices, Department ofElectrical Engineering and Computer Science
                                 Northwestern University, Evanston, Illinois 60 201


    At present efforts in infrared (IR) detector research are directed towards improving the performance of single element
    devices and large electronically scanned arrays, and to obtain higher operating temperature of detectors. Another
    important aim is to make JR detectors cheaper and more convenient to use. Investigations of the performance of narrow
    gap semiconductor photodiodes are presented. Recent progress in different JR photodiode technologies is discussed:
    HgCdTe photodiodes, InSb photodiodes, alternative to HgCdTe 111—V and Il—VI ternary ahoy photodiodes, and monolithic
    lead chalcogemde photodiodes. Investigations of the performance of photodiodes operated at short wavelength IR (SWIR),
    1—3 tm; medium wavelength IR (MWIR), 3—S pm; and long wavelength IR (LWIR), 8—14 jtm; are presented.
       The operating temperature for HgCdTe detectors is higher than for other types of photon detectors. HgCdTe detectors
    with background limited performance operate with thermoelectric coolers in the medium wavelength range, instead the
    long wavelength detectors operate at 100 K. HgCdTe is characterized by high absorption coefficient and quantum
    efficiency and relatively low thermal generation rate compared to other detectors.

    Keywords: infrared detectors, HgCdTe photodiodes, InSb photodiodes, InGaAs photocliodes, lead salt photodiodes,

                                                      1. INTRODUCTION
       The years during World War II saw the origins of modem infrared (IR) detector technology. Recent success in applying
    JR technology to remote sensing problems has been made possible by the successful development of high-performance JR
    detectors over the last five decades. Many materials have been investigated in the JR field. Spectral detectivity curves for a
    number of commercially available JR detectors are shown in Fig. 1. Interest has centered mainly on the wavelengths of the
    two atmospheric windows 3—5 .tm and 8—14 tim, though in recent years there has been increasing interest in longer
    wavelengths stimulated by space applications.
       During the 1950s JR detectors were built using single-element-cooled lead salt detectors, primarily for anti-air-missile
    seekers. Usually lead salt detectors were polycrystalline and were produced by vacuum evaporation and chemical
    deposition from a solution, followed by a post-growth sensitization process.' The first extrinsic photoconductive detectors
    were reported in the early 1950s. Since the techniques for controlled impurity introduction became available for
    germanium at an earlier date, the first high performance extrinsic detectors were based on the use of germanium. Extrinsic
    photoconductive response from copper, zinc, and gold impurity levels in germanium made devices possible in the 8- to 14-
    .tm long wavelength spectral window (LWJR) and beyond to the 14- to 3O-im very long wavelength region (VLWIR). The
    extrinsic photoconductors were widely used at wavelengths beyond 10 pm, prior to the development of the intrinsic
    detectors. They must be operated at lower temperatures to achieve perfonnance similar to that of intrinsic detectors, and a
    sacrifice in quantum efficiency is required to avoid impracticably thick detectors.
       At the same time, rapid advances were being made in narrow bandgap semiconductors that would later prove useful in
    extending wavelength capabilities and improving sensitivity. The first such material was InSb, a member of the newly
    discovered Ill—V compound semiconductor family. The end of the 1950s saw the introduction of semiconductor alloys in
    Ill—V. IV—VI, and Il—VI material systems. These alloys allowed the bandgap of the semiconductor and hence the spectral
    response of the detector to be custom tailored for specific applications. In 1959, research by Lawson and co-workers
    triggered development of variable bandgap Hg,CdTe (HgCdTe) alloys, providing an unprecedented degree of freedom on
    infrared detector design.2

                                                                                           SPIE Vol. 3287 • 0277-786X198/$1O.OO
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                             1            1.5      2       3        4        5 678910                         15         20            30             40
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Fig. 1. Comparison of the D* of various commercially available infrared detectors when operated at the indicated
     temperature. Chopping frequency is 1000 Hz for all detectors except the thermopile (10 Hz), thermocouple (10 Hz),
     thermistor bolometer (10 Hz), Golay cell (10 Hz) and pyroelectric detector (10 Hz). Each detector is assumed to view
     a hemispherical surround at a temperature of 300 K. Theoretical curves for the background-limited D* for ideal
     photovoltaic and photoconductive detectors and thermal detectors are also shown.

The performance of JR detectors is determined by fundamental physical properties of materials used for their fabrication.
HgCdTe as the most important intrinsic semiconductor alloy systems for IR detectors is well established. The specific
advantages of HgCdTe are the direct energy gap, ability to obtain both low and high carrier concentrations, high mobility
of electrons and low dielectric constant. The extremely small change of lattice constant with composition makes it possible
to growhigh quality layered and graded gap structures. HgCdTe can be used for detectors operated at various modes, and
can be optimized for operation at the extremely wide range of the IR spectrum (1—30 sun) and at temperatures ranging
from that of liquid helium to room temperature. HgCdTe has, however, the most serious technological problems of any
semiconductor material in mass production. The difficulties with this material have made it desirable to examine other
material systems to see whether performance can be improved. Table 1 listed important parameters of the materials used in
JR detectors fabrication.

                                     Table 1 . Some physical properties of narrow gap semiconductors

           Material                      Eg (eV)             n, (cm3)                                   (1O cm2/Vs)                (lO cm2Ns)
                                   77K          300K     77K       300K                                 77K        300K           77K 300K
        InAs                     0.414          0.359   6.5x103 9.3x10'4                   14.5          8           3            0.07                0.02
        InSb                     0.228          0.18    2.6x109 1.9x10'6                   17.9         100          8                1               0.08
        In53Ga47As               0.66           0.75                     5.4x10'           14.6          7         1.38                               0.05
        PbS                      0.31           0.42       3x107         1.0x1015          172          1.5        0.05            1.5                0.06
        PbSe                     0.17           0.28     6x10"           2.0x1016          227           3         0.10             3                 0.10
        PbTe                     0.22           0.31    l.5x10'°         1.5x10'6          428           3         0.17             2                 0.08
        Pb1_SnTe                  0.1           0.1     3.0x1013         2.0x1016          400           3         0.12             2                 0.08
        Hg1CdTe                   0.1           0.1     3.2x10'3         2.3x1016          18.0         20           1            0.044               0.01
        Hg1CdTe                   0.25          0.25    7.2x108          2.3x10'5          16.7          8          0.6           0.044               0.01

       Several structures are possible to observe the photovoltaic effect. These include p—n junctions, heterojunctions,
    Schottky barriers and MIS photo-capacitors. Each of these different types of devices has certain advantages for JR
    detection, depending on the particular application.3 Recently, more interest has been focused on the first two structures, so
    further considerations are restricted to p—n junctions and heterostructures. Photodiodes with their very low power
    dissipation and easy multiplexing on focal plane silicon chip, can be assembled in two-dimensional (2-D) arrays
    containing a very large ( 106) number ofelements, limited only by existing technologies. Figure 2 shows the physical and
    electrical mating of a 2-D array of detectors to a silicon multiplexer, with individual interconnections between each
    detector and the corresponding input to the multiplexer. Systems based upon such focal plane arrays (FPAs) can be
    smaller, lighter with lower power consumption, and can result in much higher performance than systems based on first
    generation detectors. Photodiodes can also have less low frequency noise, faster response time, and the potential for a more
    uniform spatial response across each element. However, the more complex processes needed for photovoltaic detectors
    have influenced on slower development and industrialization of the second generation systems, particularly for large
                                                                                 HgCdTe     Metakzation
                                                                         \ / _Loophole interconnect
                                                                     ___ 1 -HgCdTe
                                                                                          Metal pad

                                                 (a)                               (b)

    Fig. 2. Hybrid IP FPA interconnect techniques between a detector array and silicon multiplexer: (a) indium bump
         technique, (b) loophole technique (after Ref 4).

                                       2. THEORY OF INFRARED PHOTODIODES

       The photodetector is a slab of homogeneous semiconductor with the actual "electrical" area Ae that is coupled to a
    beam of infrared radiation by its optical area A0. Usually, the optical and electrical areas of the device are the same or
    close. The use of optical concentrators can increase the AO/AC ratio.
        Detectivity D* as the main parameter characterizing normalized signal to noise performance of detectors is equal5'6

                                                  D* =_1.h/2i[2(G +R)tJ2                                                      (1)
       For a given wavelength and operating temperature, the highest performance can be obtained by maximizing
    i/[t(G+R)1112. This means that high quantum efficiency must be obtained with a thin device.
       Assuming a single pass of the radiation and negligible frontside and backside reflection coefficients, the highest
    detectivity can be obtained for t = 1.26/a.7 In this optimum case 1 = 0.7 16 and detectivity is equal

                                                                    / a     \l/2
                                                       D*=O.4521                                                              (2)

        To achieve a high performance the thermal generation must be suppressed to possible the lowest level. This is usually
    done with cryogenic cooling of the detector. For practical purposes, the ideal situation occurs when the thermal generation
    is reduced below the optical generation.
        At equilibrium the generation and recombination rates are equal, and we have

                                                                       I \112
                                                       D*=O.3111                                                        (3)
                                                                   hc G)
    The ratio of absorption coefficient to the thermal generation rate a/G is the fundamental figure of merit of any material
for infrared photodetectors which directly determines the detectivity limits of the devices.
    An optimized photodetector of any type should consist of the following regions (Fig. 3):8
. Lightly doped narrow gap semiconductor region, which acts as an absorber of IR radiation. Its band gap, doping and
      geometiy should be selected.
.    Electric contacts to the narrow gap region which sense optically generated charge carriers. Contacts should not
     contribute to the dark current of the device.
.    Passivation of the narrow gap region. The surfaces of the absorber regions must be insulated from the ambient by a
     material, which also does not contribute to the generation of carriers. In addition, the carriers, which are optically
     generated in absorber, are kept away from surfaces, where recombination can reduce the quantum efficiency. For the
     best sensitivity the frontside face should perfectly transmit IR radiation.
.    Refractive, reflective or diffractive concentrator of JR radiation.
S    Backside minor for double pass of IR radiation.

                                                                            Heterojunction contacts




                           Fig.   3.   Schematic of 3-D heterostructure photodetector (after Ref. 8).

   In practice, such a device can be obtained using 3-dimensional gap and doping engineering, with the narrow gap
absorber buried in a wide gap semiconductor. The undoped wide gap material can be used as a window, and/or a
concentrator of incoming radiation. Doped n— and p—type semiconductors are used for contacts. The device must be also
supplied with contact metallization and electrical leads. Thermal generation in the contact regions is virtually eliminated
by making them wide gap.
   The generation-recombination processes are associated with the predominant recombination mechanisms. There are
three important generation and recombination mechanisms: Shockley-Read, radiative, and Auger mechanisms. The
Shockley-Read mechanism occurs via lattice defects and impurity energy levels within the forbidden energy gap. These
mechanism sets can be controlled by the procedure used to grow the material; consequently, the Shockley-Read process is
not a fundamental limit to the performance of the detector. The radiative generation-recombination and Auger
mechanisms are fundamental band-to-band processes, which are determined by the electronic band structure of the
semiconductor. It appears that Auger mechanisms dominate generation and recombination processes in high-quality InSb-
like narrow gap semiconductors. The generation rate due to the Auger 1 and Auger 7 processes can be described as6'9

                                                GA =                   = —L-1 +                                         (4)
                                                       2t1 2t7 2'r1                  7)

where i and 'r are the intrinsic Auger 1 and Auger 7 recombination times. Then Auger dominated detectivity is equal
[see Eq. (1)]
                                                                  LI 'EM
                                                       —                         I
                                                           2"2hc t"2   n+p/7)

where y = tA7 A1 • Since the resulting Auger generation rate achieves its minimum for p y112n, it leads to important
conclusion about optimum doping. The optimum performance of Auger limited detectors can be achieved with a lightly p—
type doping with hole concentration p = y"2n1.
     Assuming that the saturation dark current I of a photodiode is only due to thermal generation in the base layer and that
its thickness is low compared to the diffusion length, we have6'9

                                                           Is = qGAt                                                           (6)

where G is the generation rate in the base layer. Because the zero bias resistance-area product is



      Taking into account the Auger 7 mechanism in extrinsic p—type region of nt—on—p photodiode, we receive

                                                          R0A=2kTtA7                                                           (9)
and the same equation for p—on—n photodiode

                                                          R0A=2kTt1                                                        (10)
where Na and Nd are the acceptor and donor concentrations in the base region, respectively.
  Hitherto, photodiodes were analyzed in which the dark current was limited by diffusion. However, several additional
excess mechanisms are involved in detennining the dark current-voltage characteristics of the photodiode. The dark
current is the superposition of current contributions from three diode regions: bulk, depletion region and surface.
   Below, we will be concerned with the current contribution of high-quality photocliodes with high R,A products limited
.     generation-recombination within the depletion region,
.     tunneling through the depletion region,
.     surface effects.
      The first two mechanisms are schematically illustrated in Fig. 4.
      The generation-recombination current of the depletion region can be described as

                                                          GR"to—   qwn1
                                                                                                                           (     )

if a trap is near the intrinsic level of the band gap energy. In this equation t0 is the carrier lifetime in depletion region. The
surface leakage current can be described in the same way.
    The space-charge region generation-recombination current varies with temperature as n1, i.e., less rapidly than
diffusion current which varies as n.
      The zero bias resistance can be found by differentiating Eq. (11) and setting V = 0:

                                              (R A) =1-)
                                               °     GR
                                                        ivo                 qn1w

where Vb = kT1n(NaNd/n). In evaluating Eq. (12), the term of greatest uncertainty is t0.
                                                                       .—Depletion region —.j
                             Electron diffusion

                                                     •.          .

                         Minority carriers are generated
                         thermally by intrinsic processes                                      Direct    and to nd tunneling
                         or via traps

                                                                       Generation via                    Trap assied tunneling
                                                                       traps in the                          ———
                                                                       depletion region


                                                                                                                               Hole diffusion

Fig. 4. Schematic representation of some of the mechanisms by which dark current is generated in a reverse biased p—n
       junction (after Ref. 10).

    The third type of dark current component that can exist is a tunneling current caused by electrons directly tunneling
across the junction from the valence band to the conduction band (direct tunneling) or by electrons indirectly tunneling
across the junction by way of intermediate trap sites in the junction region (indirect tunneling or trap-assisted tunneling —
see Fig. 4).
    The tunneling current (and R<A product) is critically dependent on doping concentration. Fig. 5 shows the dependence
of the R2,A product components on the dopant concentrations for one-sided abrupt HgCdTe, PbSnTe and PbSnSe
photodiodes at 77 K (Eg 0. 1 eV). To produce high RA products for HgCdTe and lead salt photodiodes, the doping
concentration of 1016 cm3 and 1017 cm3 (or less) are required, respectively. The maximum available doping levels due to
onset of tunneling are more than an order of magnitude higher with IV—V1 than with HgCdTe photodiodes. This is due
to their high permittivities t because tunneling contribution of the R0A product contains exp[const(m5c5 / N)2 Eg]


                                                  10     _
                                                             -i)A1            (RQ )
                                                         _                           (ROA)T

                                                                  n-pP junction \ _ . . Pb033Sn,,76Se
                                                                                                      Pb0 78Sn0Te

                                                             ——— n-pjunction I

                                                  I02,4          II
                                                                      't;l&         1016
                                                                                           '      '
                                                                                                        '. 017
                                                                                                                 ' '

                                                                              Na,   Nd (cm)

Fig. 5. The dependence of the R0A product on doping concentration for one-sided abrupt HgCdTe, PbSnTe and PbSnSe
       photodiodes at 77 K Eg = 0.1 eV) (after Ref 11).

                                                  3. InGaAs PHOTODIODES

        In053Ga47As alloy lattice matched to the InP substrate has already been shown to be a suitable detector material for
    near-JR (1.O—1.7-m) spectral range. InGaAs ternary alloy having lower dark current and noise than indirect-bandgap
    germanium, the competing near-W material, the material is addressing both entrenched applications including low light
    level night vision (in the region 1—3 m) and new applications such as remote sensing, eye-safe range finding and process
        InGaAs-detector processing technology is similar to that used with silicon, but the detector fabrication is different. The
    InGaAs detector's active material is deposited onto a substrate using chloride VPE or MOCVD techniques adjusted for
    thickness, background doping, and other requirements. Planar technology evolved from the older mesa technology and at
    present is widely used due to its simple structure and processing as well as the high reliability and low cost.'2'13
        Figure 6 shows that the highest quality InGaAs photodiodes have been grown by MOCVD.'4 Their performance agrees
    with the radiative limit and is comparable with HgCdTe photodiodes.


                                                          Wavelength (pm)

            Fig. 6. Near JR and short wavelength JR detector R0A vs. wavelength at 295 K and 250 K (after Ref. 14).

       Linear array formats of 256, 5 12 and 1024 elements have been fabricated for environmental sensing from 0.8 p.m to 2.6
    tim. The sizes ofpixels are different; from 30x30 j.tm2 (with spacing of 50 .tm), 25x500 .tm2 to 13x500 im2 (with spacing
    of 25 .tm). Sensors Unlimited offers lOx 10x6-cm line-scan cameras incorporating linear InGaAs FPAs of up to 512
    elements on a 50-tim pitch.'5 Two versions of the units are available, optimized for wavelength bands of either 0.8 to 1.7
    mor1.0to2.2. pm.
       The first two-dimensional 128x 128 In53Ga47As hybrid EPA for the 1 .0—1.7 pm spectral range was demonstrated by
    Olsen et al. in 1990.16 Room-temperature measurements of a camera based on a 128x 128 InGaAs FPA (with Rockwell's
    readout CMOS multiplexer) indicated that the mean value of D' was 1.03x i0' cmHz2W with the standard deviation
    45% of the mean and 94.8% of the pixels with a D' greater than 50% of the mean value.'7 At 230 K the mean D*was
    3.5x10'4 cmHz"2W'.
                                                   4. InSb PHOTODIODES
       InSb photodiodes have been available since the late fifties and are generally fabricated by impurity diffusion, ion
    implantation, LPE, or MBE.3 Wiimners et al.'8"9 have presented the status of InSb photodiode technology for a wide
    variety of linear and FPAs. Fabrication techniques for InSb photocliodes use gaseous diffusion, and a subsequent etch
    results in a p-type mesa on n-type substrate with donor concentration about 10' cm3.
       Typical InSb photodiode RA product at 77 K is 2x 106 1cm2 at zero bias and 5x 106 cm2 at slight reverse biases of
    approximately 100 mV. This characteristic is beneficial when the detector is used in the capacitive discharge mode. As
    element size decreases below 10 cm, some slight degradation in resistance due to surface leakage occurs.

    InSb photovoltaic detectors are widely used for ground-based infrared astronomy and for applications aboard the Space
Infrared Telescope Facility (SIRTF).
   Recently, impressive progress has been made in the performance of InSb hybrid FPAs. The U.S. Naval Observatory
and the National Optical Astronomy Observatories in collaboration with the Santa Barbara Research Center developed a
1024x 1024 InSb FPA.2° It is the largest single chip IR array in use today. The architecture of this device consists of four
independent 512x512 quadrants with eight outputs per quadrant. An array of this size is only possible because the InSb
detector material is thinned to less than 10 pm which allows it to accommodate the InSb/silicon thermal mismatch.
    InSb photodiodes can also be operated in the temperature range above 77 K. Of course, the RA products degrade in this
region. At 120 K, RA products of i04 cm2 are still achieved with slight reverse bias, making BLIP operation possible.
The quantum efficiency in InSb photodiodes optimized for this temperature range remains unaffected up to 160 K.'9

                                                 5. HgCdTe PHOTODIODES

   Hitherto, the realization of HgCdTe photodiodes has usually based on the most common nt—p and —n structure
(symbol "+" denotes strong doping, underlined "" wider gap). In such diodes, the lightly doped narrow gap absorbing
region ("base" of the photodiode), determines the dark current and photocurrent. In these photodiodes the base p-type
layers (or n-type layers) are sandwiched between CdZnTe substrate and high-doped (in n—p structures) or wider-gap (in
p+—n structure) regions. Due to backside illumination (through CdZnTe substrate) and internal electric fields (which are
"blocking" for minority carriers), influence of surface recombinations on the photodiodes performance is eliminated. The
influence of surface recoinbination can be also prevented by the use of suitable passivation. Both optical and thermal
generations are suppressed in the nt-region due to the Burstein-Moss effect and in the p-region due to wide gap. Thus
R,A product of double-layer heterojunction (DLHJ) structure is higher than that of homostructure.
    The thickness of the base region should be optimized for near unity quantum efficiency and low dark current. This is
achieved with a base thickness slightly higher than the inverse absorption coefficient for single pass devices: t 1/a
(which is 1O jim) or half of the 1/a for double pass devices (devices supplied with a retroreflector). Low doping is
beneficial for low thermal generation and high quantum efficiency.
    A schematic of mesa DLHJ structure used in fabrication of pt—n HgCdTe photocliodes together with its band diagram
are illustrated in Fig. 7. The n—type base, which is the absorbing region, is deliberately doped with indium at a level of
about (1—3)x iO cm3. The composition of the base material is chosen for the wavelength of interest. P—n junction is
formed using arsenic as the dopand at a level of about 1018 cm3. P-type capping layers with composition y > x have a
thickness 1—2 m The electrical junction is positioned near the metallurgical interface and it is wise to place the junction
in the small band gap layer to avoid deleterious effects on the quantum efficiency and dark currents. Passivation of
HgCdTe has been done by several techniques which comprehensive review was given by Nemirovsky and Bahir.2'
Recently, however, most laboratories have been using CdTe or CdZnTe (deposited by MBE, MOCVD, spuuering and e-
beam evaporation) for photodiode passivation.22
                        Au contact

                                     CdTe passivation

                 tArseriic-9ped           n-type cap y

                      n-type basex         Y>X

                      CdZnTe substrate

                         (a)                                                           (b)

           Fig. 7. DLHJ mesa ptn HgCdTe photodiode: (a) schematic cross-sectional view; (b) band diagram.

         Rogaiski and Ciupa have compared the performance of n—p and           LWIR HgCdTe photodiodes.9 It appears, that for
     the lowest doping levels achievable in controllable manner in the base regions of photodiodes (Na 5< 1015 cm3 for nt—p
     structure, and Nd 5 x 10' cm3 for p—n structure) the performance of both types of photodiodes is comparable for a given
     cutoff wavelength and temperature.
         The temperature dependence of R0A product of six hybrid np photodiode arrays with different cut-off wavelength is
     shown in Fig. 8. The intrinsic carrier concentration n dependence is also shown. Note that the SWIR array is generation-
     recombination limited from room temperature to 120 K. The MWJR devices at T <              150   K are also dominated by
     generation-recombination current. The transition from diffusion to generation-recombination limited case is clearly seen in
     the LWIR arrays.

                                                            Temperature (K)

                                                             iooorr (K)

         Fig. 8. RA dependence on reciprocal temperature of six detector hybrids covering the 1— infrared spectrum
                                                         (after Ref. 23).

         Usually, p-type base material is characterized by relatively high trap concentration, which dominates the excess carrier
     lifetime by the Shockley-Read-Hall recombination mechanism. Its influence depends on technological limits. However, the
     quality of p-type material has considerably improved in the last several years. Destefanis and Chamonal have developed a
     modified process for planar nt—p HgCdTe homojunctions with a large improvement in detector performance.24 The n—p
     homojunctions were made by ion implantation in 8—12 m thick HgCdTe liquid phase epilayers on CdZnTe substrates. The
     RA improvement of one order of magnitude (in the range between 400 to 650 cm2 at 77 K for a 10 p.m cutoff
     wavelength detector) has been observed. This effect was obtained as a result of an increase of the minority carrier lifetime
     in the base p-type region (close to the Auger 7 limited lifetime) and a slightly thinner epitaxial p—type layer.
         Up to the present, photovoltaic HgCdTe FPAs have been mainly based on p—type material. Linear (240, 288, 480, and
     960 elements), 2—D scanning arrays with time delay and integration (ThI), and 2—D staring formats from 32x32 up to
     1024x 1024 have been made.'4'25'26 Pixel sizes ranging from 18-tim square to over 1 mm have been demonstrated. The best
     results have been obtained using hybrid architecture, which permits independent optimization of the materials parameters
     and device fabrication processes for detectors and the signal-processing electronics exploiting advances in a
     complementaly metal-oxide-semiconductor (CMOS) fabrication processes.

                                       6. LEAD CHALCOGENIDE PHOTODIODES

         For a period of decade from the late 1960s to the mid 1970s, because of production and storage problems, HgCdTe
     alloy detectors were in serious competition with IV—VI alloy devices (mainly PbSnTe) for developing photodiodes.27'28
     PbSnTe alloy seemed easier to prepare and appeared more stable. Development of PbSnTe photodiodes was discontinu.d
     because the chalcogemdes suffered two significant drawbacks. The first one was a high dielectric constant that resulted in

high diode capacitance and therefore limited frequency response. For scanning systems under development at that time,
this was a serious limitation. However, for stating imaging systems under development today using 2-D arrays, this would
not be as significant of an issue. The second drawback to IV—VI compounds is their very high thermal coefficients of
expansion. This limited their applicability in hybrid configurations with silicon multiplexers. Today, with the ability to
grow these materials on alternative substrates such as silicon, this too would not be a fundamental limitation. Moreover, as
regards ease of manufacture, homogeneity and costs, photovoltaic IV—Vl arrays on Si substrates offer substantial
advantages compared to HgCdTe. The maximum available doping levels due to onset of tunneling are more than an order
of magnitude higher with IV-VIs than with HgCdTe photodiodes (see Section 2).
    To overcome thermal mismatch problems between silicon readout substrate and lead chalcogemde detector for large
arrays, Zogg et al.29'3° have used an epitaxial stacked CaF2—BaF2 buffer layer of 200 mu thickness. The entire MBE
growth procedure of 3 pm thick lead chalcogenide epitaxial layer on Si(1 1 1) chips containing integrated circuits with
standard Al metallizations has been performed.29 This has been done by lowering the growth temperature to 450°C and
using a room-temperature substrate surface cleaning procedure. The detectors were delineated with a vacuum-deposited
blocking Pb contacts which inverts the surface region of p—type material, thereby creating an induced p—n junctions. A
common ohmic contacts are formed with evaporated platinum.
    Figure 9 shows R,A products at 77 K as a function of cutoff wavelength for different lead salt photodiodes on Si with
stacked BaF2/CaF2 and CaP2 buffer layers.3° Although these values are considerably above the BLIP limit (for 300 K, 2it
FOV and 11 = 50%), they are still significantly below theoretical limit given by Auger recombination. The performance of
Iv—vI photodiodes is inferior to HgCdTe photodiodes; their RA products are two orders of magnitude below values for
p+—n HgCdTe photodiodes. R,A products of PbSnSe photodiodes with cutoff wavelength 10.5 m, were about 1 cm2 at
77 K. The temperature dependence of RA is diffusion limited down to 100 K, while depletion-limited noise dominates
below this temperature.


                                                     Cut-off wavelength (

Fig. 9. R0A products at 77 K versus cutoff wavelength for different lead salt photodiodes on Si with stacked BaF2/CaF2 or
       CaF2 buffer layers. The BLIP limit for 300 K, 2it FOV and 1 = 50% is included (after Ref. 30). The solid line
       represents calculated and experimental data for   HgCdTe photodiodes according to Ref. 9.

         The research group at the Swiss Federal Institute of Technology fabricated linear lead chalcogenide photodiode sensor
     monolithic arrays with 2x 128 pixels and with cutoff wavelengths ranging from 3 to 12 .tm, which have been used in one-
     direction mechanically scanned thermal imaging camera.

                                          7. BACKGROUND LIMITED OPERATION

         In Fig. 10 plots of the calculated temperature required for background limited (BLIP) operation in 300 FOV are shown
     as a function of cutoff wavelength. We can see that the operating temperature of "bulk" intrinsic IR detectors (HgCdTe and
     PbSnTe) is higher than for other types of photon detectors. HgCdTe detectors with background limited performance
     operate with thermoelectric coolers in the MWIR range, instead the LWIR detectors (8 X 12 pm) operate at 100 K.
     HgCdTe is characterized by high optical absorption coefficient and quantum efficiency and relatively low thermal
     generation rate compared to extrinsic detectors, silicide Schottky barriers and quantum well IR photodetectors (QWIPs).

                                                      FOV = 30°, Scene temperature =
                                                         p-on-n HgCdTe Auger limited photodiodes
                                                         Nd = 5x10'4 cm3, t = 10
                                                             n-on-p HgCdTe Auger limited photodiodes
                                                             N=5x1O5 cm4, t= 1Ojm
                                                                   HgCdTe Auger limited photoconductors
                                                                   N4 = 3x10'4 cm4, t = 10 m
                                                                         n'-on-p PbSnTe Auger limited
                                                                         N, = iO'7 cm4, t = 10 tm


                                                     5                 10                   15            20
                                                              Cutoff wavelength (tim)

     Fig. 10. Estimation of the temperature required for background limited operation of different types of photon detectors. In
            the calculations FOV = 30° and TB = 300 K are assumed (after Ref. 31).

                                                         8. CONCLUSIONS

         To summarize, despite serious competition from alternative technologies and slower progress than expected, HgCdTe
     is unlikely to be seriously challenged for high-performance applications, applications requiring multispectral capability
     and fast response. The recent successes of competing cryogenically cooled detectors are due to technological, not
     fundamental issues. There are good reasons to think that the steady progress in epitaxial technology would make HgCdTe
     devices much more affordable in the near future. The much higher operation temperature of HgCdTe compared to
     Schottky barrier devices and low-dimensional solid devices may become a decisive argument in this case. In applications
     for short-range thermal imaging systems a serious challenge comes from solid state arrays of thermal detectors
     (pyroelectric and bolometers) which are expected to take over and increase the market for uncooled short-range imaging

     Acknowledgment. This work was partially supported by the KEN (Poland) under grant number PBZ 28.11.

     1.  R. J. Cushman, "Film-type infrared photoconductors," Proc. IRE 47, 1471—1475 (1959).
     2. W. D. Lawson, S. Nielson, E. H. Putley and A. S. Young, "Preparation and properties of HgTe and mixed crystals of
          HgTe-CdTe," J. Phys. Chem. Solids 9, 325—329 (1959).
     3.   A. Rogalski (Editor), Infrared Photon Detectors, SPIE Optical Engineering Press, Bellingham (1995).

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