**6. MWIR/LWIR Detector Array Technologies and Applications (InSb, HgCdTe, HOT, SLS and Bolometers)**

Most objects in earth's environment emit radiation in the MWIR/LWIR wavelength range, commonly referred to as the thermal band. For example, the human body, by virtue of being at a temperature of ~300K, emits radiation that peaks around 10 microns. Also, most chemi‐ cal species have spectral signatures in this infrared regime due to fundamental absorption processes associated with vibrational states of the molecules. Thus, in many applications that require the observation and identification of chemical species using point detection or standoff detection, such as pollution monitoring, gas leak detection, gas sensing and spec‐ troscopy, one needs to work in the MWIR/LWIR bands. Another distinct advantage is that the atmosphere has clear transmission windows in the MWIR and LWIR bands, making it very attractive for terrestrial applications.

Figure 13 presents the transmission of the integrated InSb sensor with cold filters for the two MWIR bands that are commonly referred to as the blue band (shorter wavelength) and the red band with the longer spectral response. The optical coatings on the detector arrays and the optics are designed for maximum transmission and to minimize the spectral crosstalk

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**Figure 14.** Dual-color Integrated Detector Cooler Assembly (IDCA) with two InSb FPAs and two cold radiation shields

Figure 14 presents the schematics of various key building blocks for a Dual-color Integrated Detector Cooler Assembly (IDCA) with two InSb FPA's connected to their circuit card as‐ semblies. Each of the InSb focal plane arrays has been optimized for the blue and red bands of interest in the broad MWIR region. As a dual band system, both FPAs can operate simul‐

MCT is the material of choice for a variety of high performance IRFPA systems for a variety of defense and commercial applications. Many of these applications use state-of-the-art HgCdTe growth using a bulk Cadmium ZincTelluride (CdZnTe) substrate. However, as the push for larger array sizes continues, it is recognized that an alternative substrate technolo‐

A significant effort has been under way in developing CdTe/Si or GaAs as a desired substrate. This substrate technology has been successful for short-wavelength (SWIR) and mid wave‐ length (MWIR) focal plane arrays; current HgCdTe/Si material quality is being further devel‐ oped for long-wavelength (LWIR) arrays, due to the high density of dislocations present in the material [59]. To remedy the high dislocation counts, researchers are focusing on both compo‐ site substrate development and improvement, and on HgCdTe/Si post-growth processes [60]. The impact of ex-situ annealing on the quality of the epitaxial surface is shown in the Figure 15 for HgCdTe/Si substrates. Several groups have demonstrated HgCdTe/Simaterial with dislo‐

cm-2. This is a five times reduction in the baseline material dis‐

**6.2. Multicolor HgCdTe (MCT) Detectors Arrays for MWIR/LWIR Applications**

gy for large area needs to be developed for HgCdTe IRFPAs.

location density that is currently used in the fabrication of devices.

between the bands, typically less than 0.1%, as can be seen in Figure 13.

with integrated optics [58].

taneously at high frame rates.

cation density measuring 1 x 106

#### **6.1. InSb Detector Array**

InSb detector arrays have found many applications in MWIR due to their spatial uniformity, low dark current and image quality. This technology has evolved over the years in response to the stringent requirements for applications in missile seekers and missile warning sys‐ tems (MWS) [57-58]. For these applications, the IR imagers need to exhibit high dynamic range, fast frame rates, high resolution, very wide fields of view (FOV), and high sensitivity. The wide FOV optical design must consider the large incident angle of incoming photons, which if not included, can cause the appearance of ghost images and imaging of strong illu‐ mination sources outside the FOV. High spatial resolutions are achieved by using large ar‐ rays ( 640x512 and 1280x1024 ) with small pixels (unit cell size of 15 μm). The bandgap of this binary alloy is relatively constant and cannot be varied much as is the case with HgCdTe or SLS devices. Hence, to use InSb for high sensitivity multicolor applications re‐ quires the incorporation of filters to select bands of interest.

**Figure 13.** Normalized transmission of the cold filter in MWIR bands that are commonly referred to Blue (MWIR -1) and Red (MWIR-2) [58].

Many of the missile applications in the MWIR band require the use of two color InSb detec‐ tors, in order to discriminate the missile signature from the clutter background and reduce the false alarm rate. One band detects the target while the other band subtracts the back‐ ground for noise suppression. The actual wavelength bands in 3-5 micron range vary from application to application [58].

Figure 13 presents the transmission of the integrated InSb sensor with cold filters for the two MWIR bands that are commonly referred to as the blue band (shorter wavelength) and the red band with the longer spectral response. The optical coatings on the detector arrays and the optics are designed for maximum transmission and to minimize the spectral crosstalk between the bands, typically less than 0.1%, as can be seen in Figure 13.

troscopy, one needs to work in the MWIR/LWIR bands. Another distinct advantage is that the atmosphere has clear transmission windows in the MWIR and LWIR bands, making it

InSb detector arrays have found many applications in MWIR due to their spatial uniformity, low dark current and image quality. This technology has evolved over the years in response to the stringent requirements for applications in missile seekers and missile warning sys‐ tems (MWS) [57-58]. For these applications, the IR imagers need to exhibit high dynamic range, fast frame rates, high resolution, very wide fields of view (FOV), and high sensitivity. The wide FOV optical design must consider the large incident angle of incoming photons, which if not included, can cause the appearance of ghost images and imaging of strong illu‐ mination sources outside the FOV. High spatial resolutions are achieved by using large ar‐ rays ( 640x512 and 1280x1024 ) with small pixels (unit cell size of 15 μm). The bandgap of this binary alloy is relatively constant and cannot be varied much as is the case with HgCdTe or SLS devices. Hence, to use InSb for high sensitivity multicolor applications re‐

**Figure 13.** Normalized transmission of the cold filter in MWIR bands that are commonly referred to Blue (MWIR -1)

Many of the missile applications in the MWIR band require the use of two color InSb detec‐ tors, in order to discriminate the missile signature from the clutter background and reduce the false alarm rate. One band detects the target while the other band subtracts the back‐ ground for noise suppression. The actual wavelength bands in 3-5 micron range vary from

very attractive for terrestrial applications.

170 Optoelectronics - Advanced Materials and Devices

quires the incorporation of filters to select bands of interest.

**6.1. InSb Detector Array**

and Red (MWIR-2) [58].

application to application [58].

**Figure 14.** Dual-color Integrated Detector Cooler Assembly (IDCA) with two InSb FPAs and two cold radiation shields with integrated optics [58].

Figure 14 presents the schematics of various key building blocks for a Dual-color Integrated Detector Cooler Assembly (IDCA) with two InSb FPA's connected to their circuit card as‐ semblies. Each of the InSb focal plane arrays has been optimized for the blue and red bands of interest in the broad MWIR region. As a dual band system, both FPAs can operate simul‐ taneously at high frame rates.

#### **6.2. Multicolor HgCdTe (MCT) Detectors Arrays for MWIR/LWIR Applications**

MCT is the material of choice for a variety of high performance IRFPA systems for a variety of defense and commercial applications. Many of these applications use state-of-the-art HgCdTe growth using a bulk Cadmium ZincTelluride (CdZnTe) substrate. However, as the push for larger array sizes continues, it is recognized that an alternative substrate technolo‐ gy for large area needs to be developed for HgCdTe IRFPAs.

A significant effort has been under way in developing CdTe/Si or GaAs as a desired substrate. This substrate technology has been successful for short-wavelength (SWIR) and mid wave‐ length (MWIR) focal plane arrays; current HgCdTe/Si material quality is being further devel‐ oped for long-wavelength (LWIR) arrays, due to the high density of dislocations present in the material [59]. To remedy the high dislocation counts, researchers are focusing on both compo‐ site substrate development and improvement, and on HgCdTe/Si post-growth processes [60]. The impact of ex-situ annealing on the quality of the epitaxial surface is shown in the Figure 15 for HgCdTe/Si substrates. Several groups have demonstrated HgCdTe/Simaterial with dislo‐ cation density measuring 1 x 106 cm-2. This is a five times reduction in the baseline material dis‐ location density that is currently used in the fabrication of devices.

Both composite substrate development, whether it is using Si, GaAs or some other alterna‐ tive substrate system, and HgCdTe material improvement are active areas of study within the Infrared community. In fact, it might be a combination of techniques currently being de‐ veloped that ultimately lead to HgCdTe grown on scalable alternative substrates supplant‐

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Figure 16 shows a HgCdTe/Si FPA architecture hybridized via indium interconnects to the silicon Readout Integrated Circuit (ROIC). The FPA in Figure 16 consists of MBE grown HgCdTe single band detector arrays with in-situ doped, P-on-N architecture fabricated on a

**Figure 16.** Architecture of an IRFPAs made with HgCdTe/Si hybridized to a Read Out Integrated Circuit (ROIC) [61].

Using MBE system (see Figure 17), researchers produced epitaxial HgCdTe layers on (211) Si substrates with very low macro defect density and uniform Cd composition across the ep‐ itaxial wafers. These HgCdTe/Si composite wafers have shown growth defect densities less

strates, due to the better crystalline quality of the starting substrate [61].

, approximately 100 times better than can be achieved on CdZnTe sub‐

ing HgCdTe grown on bulk CdZnTe substrates for large area array applications [60].

6-inch silicon substrate.

**Figure 17.** VG V100 Molecular Beam Epitaxial System

than 10 defects /cm2

**Figure 15.** Optical microscopy images of the HgCdTe/Si before and after ex-situ cycle annealing. Note the landmark defect used to identify the same area of material for both pre and post anneal images [59].


**Table 1.** Advantages of Si-based composite substrate technology for HgCdTe material development [59].

Table 1 highlights the advantages for transitioning away from a bulk CdZnTe substrate tech‐ nology for large area HgCdTe IR detectors and focal plane arrays. Past effort has focused on us‐ ing a Si-based composite substrate technology, specificallyCdTe/Si, for HgCdTe material development. As shown in Table 1, Si has better attributes with respect to bulk CdZnTe in ev‐ ery category except for lattice and thermal matching to HgCdTe. The lattice mismatch between Si and HgCdTe is 19.3% and has proven to be a significant challenge to overcome. To address this issue, a great effort within the HgCdTe community has been expended on developing MBE grown CdTe/Si as a composite substrate for subsequent HgCdTe growth. Much research and investigation has gone into understanding and improving the surface passivation, nuclea‐ tion, buffer layer growth, and material characterization of CdTe/Si material itself. Currently, CdTe (112)/Si (112) is of extreme high quality with x-ray rocking curve full width at half maxi‐ mum (FWHM) values measuring less than 60 arcsec for an 8 μm thick epilayer [59]. Significant efforts are also being expended in developing HgCdTe on GaAs substrates.

Both composite substrate development, whether it is using Si, GaAs or some other alterna‐ tive substrate system, and HgCdTe material improvement are active areas of study within the Infrared community. In fact, it might be a combination of techniques currently being de‐ veloped that ultimately lead to HgCdTe grown on scalable alternative substrates supplant‐ ing HgCdTe grown on bulk CdZnTe substrates for large area array applications [60].

Figure 16 shows a HgCdTe/Si FPA architecture hybridized via indium interconnects to the silicon Readout Integrated Circuit (ROIC). The FPA in Figure 16 consists of MBE grown HgCdTe single band detector arrays with in-situ doped, P-on-N architecture fabricated on a 6-inch silicon substrate.

**Figure 16.** Architecture of an IRFPAs made with HgCdTe/Si hybridized to a Read Out Integrated Circuit (ROIC) [61].

**Figure 17.** VG V100 Molecular Beam Epitaxial System

**Figure 15.** Optical microscopy images of the HgCdTe/Si before and after ex-situ cycle annealing. Note the landmark

Maximum size 7 x 7 cm2 6 inch diameter Si Maximum area ∼50 cm<sup>2</sup> ∼180 cm<sup>2</sup> Si Scalability No Yes Si Cost \$220/cm<sup>2</sup> ∼\$1/cm<sup>2</sup> Si Thermal match to Si ROIC No Yes Si Robustness Brittle Hard Si Lattice match to MCT Yes No CZT Surface Smooth Smooth None Orientation available (112) (112) None Vendors 1 (foreign) Numerous (domestic) Si Substrate quality (dislocations) < 10000 cm<sup>2</sup> < 100 cm<sup>2</sup> Si Impurities Low Extremely low Si

**Table 1.** Advantages of Si-based composite substrate technology for HgCdTe material development [59].

efforts are also being expended in developing HgCdTe on GaAs substrates.

Table 1 highlights the advantages for transitioning away from a bulk CdZnTe substrate tech‐ nology for large area HgCdTe IR detectors and focal plane arrays. Past effort has focused on us‐ ing a Si-based composite substrate technology, specificallyCdTe/Si, for HgCdTe material development. As shown in Table 1, Si has better attributes with respect to bulk CdZnTe in ev‐ ery category except for lattice and thermal matching to HgCdTe. The lattice mismatch between Si and HgCdTe is 19.3% and has proven to be a significant challenge to overcome. To address this issue, a great effort within the HgCdTe community has been expended on developing MBE grown CdTe/Si as a composite substrate for subsequent HgCdTe growth. Much research and investigation has gone into understanding and improving the surface passivation, nuclea‐ tion, buffer layer growth, and material characterization of CdTe/Si material itself. Currently, CdTe (112)/Si (112) is of extreme high quality with x-ray rocking curve full width at half maxi‐ mum (FWHM) values measuring less than 60 arcsec for an 8 μm thick epilayer [59]. Significant

**Substrate Technology**

**Bulk CdZnTe Si Advantage**

defect used to identify the same area of material for both pre and post anneal images [59].

172 Optoelectronics - Advanced Materials and Devices

Using MBE system (see Figure 17), researchers produced epitaxial HgCdTe layers on (211) Si substrates with very low macro defect density and uniform Cd composition across the ep‐ itaxial wafers. These HgCdTe/Si composite wafers have shown growth defect densities less than 10 defects /cm2 , approximately 100 times better than can be achieved on CdZnTe sub‐ strates, due to the better crystalline quality of the starting substrate [61].

The HgCdTe/Si epitaxial substrates with a P-on-N configuration can be fabricated into mesa delineated detectors using the same etch, passivation, and metallization schemes as detec‐ tors processed on HgCdTe/CdZnTe substrates. Detector fabrication processes across the full area of 6-inch HgCdTe/Si wafers have routinely produced high performing detector pixels from edge to edge of the photolithographic limits across the wafer, offering 5 times the printable area as compared with 6×6 cm CdZnTe substrates. Large-format (2Kx2K) MWIR FPAs fabricated using large area HgCdTe layers grown on 6-inch diameter (211) silicon sub‐ strates demonstrated NEDT operabilities better than 99.9% (see Figure 18). SWIR and MWIR detector performance characteristic son HgCdTe/Si substratesare comparable to those on the established HgCdTe/CdZnTe wafers. HgCdTe devices fabricated on both types of substrates have demonstrated very low dark current, high quantum efficiency and full spectral band fill factor characteristic of HgCdTe [61].

each band, with noise equivalent differential temperature (NEDT) operabilities of 99.98% for

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**Figure 19.** Cross-section of single-mesa dual-band detector architecture applied to HgCdTe on Si (Left). Shown on the

High performance MWIRand LWIR FPAs are normally cooled to cryogenic temperatures at about 80 K, in order to suppress the dark current noise from overwhelming the photo‐ generated signals. If the operating temperature of the FPA is increased without degrading image quality, then smaller coolers can be used and the SWaP and cost of the system could be reduced. Of course, a more significant advantage can result if the operating tem‐ perature is increased to > 200 K where a low cost thermoelectric cooler can be implement‐ ed. There is a growing effort to increase the operating temperature of MWIR and LWIR infrared detectors by: reducing leakage currents; reducing thermal generation rates in the active region and minimizing the active volume of the detector without reducing quan‐ tum efficiency. While a number of strategies can be used to achieve high operating tem‐ perature (HOT) detectors, arecent DARPA program (AWARE-Broadband) focused on reducing detector material volume via a photon trap/photonic crystal approach to reduce

The principle of volume reduction is demonstrated in Figure 20 which illustrates the effect of reducing the fill factor on device performance for a baseline shrinking mesa and an ideal‐ ized photon trap detector. The fill factor is defined as the volume of material remaining div‐ ided by the volume of the unit cell. The mesa reduction in volume initially reduces the NEDT as noise generating volume is removed, until the volume removed causes the signal to be reduced relative to the noise. Two types of IR photon trapping structures have been investigated: In AsSb pyramidal arrays and HgCdTe pillars and holes. Photon trap detectors on MBE HgCdTe/Si epitaxial wafers (see Figure 21) exhibit improved performance com‐

Right is Scanning electron micrograph of 20-micron-unit-cell dual-band detectors array [62].

**6.3. High Operating Temperature (HOT) Detectors**

dark current without degrading quantum efficiency [63-64].

the MWIR band and 99.6% for the LWIR band at 84 K [62].

**Figure 18.** NEDT measured from 2K×2K HgCdTe/Si MWIR HgCdTe/Si FPA with 15 micron unit cell demonstrating bet‐ ter than 99.9% operability [61].

#### *6.2.1. HgCdTe on Silicon Two-Color IRFPAs*

As noted above, the motivation for HgCdTe growth on large-area Si substrates is to enable larger array formats and potentially reduced FPA cost compared to smaller, more expensive CdZnTe substrates. In addition to the successful demonstration of single color IRFPA on composite HgCdTe/Si substrates, researchers produced MWIR/LWIR dual band FPAs on large area Si substrates. The device structure is based on a triple-layer N-P-N heterojunction (TLHJ) architecture grown by molecular-beam epitaxy (MBE) on 100 mm (211) Si wafers with ZnTe and CdTe buffer layers [62]. The MWIR/LWIR dual band epitaxial wafers have low macro defect densities (<300 cm-2). Inductively coupled plasma etched detector arrays with 640x480 dual band pixels (20 μm) are mated to dual-band readout integrated circuits (ROICs) to produce FPAs (see Figure 19). The measured 80 K cutoff wavelengths are 5.5 μm for MWIR and 9.4 μm for LWIR, respectively. The FPAs exhibit high pixel operabilities in each band, with noise equivalent differential temperature (NEDT) operabilities of 99.98% for the MWIR band and 99.6% for the LWIR band at 84 K [62].

**Figure 19.** Cross-section of single-mesa dual-band detector architecture applied to HgCdTe on Si (Left). Shown on the Right is Scanning electron micrograph of 20-micron-unit-cell dual-band detectors array [62].

#### **6.3. High Operating Temperature (HOT) Detectors**

The HgCdTe/Si epitaxial substrates with a P-on-N configuration can be fabricated into mesa delineated detectors using the same etch, passivation, and metallization schemes as detec‐ tors processed on HgCdTe/CdZnTe substrates. Detector fabrication processes across the full area of 6-inch HgCdTe/Si wafers have routinely produced high performing detector pixels from edge to edge of the photolithographic limits across the wafer, offering 5 times the printable area as compared with 6×6 cm CdZnTe substrates. Large-format (2Kx2K) MWIR FPAs fabricated using large area HgCdTe layers grown on 6-inch diameter (211) silicon sub‐ strates demonstrated NEDT operabilities better than 99.9% (see Figure 18). SWIR and MWIR detector performance characteristic son HgCdTe/Si substratesare comparable to those on the established HgCdTe/CdZnTe wafers. HgCdTe devices fabricated on both types of substrates have demonstrated very low dark current, high quantum efficiency and full spectral band

**Figure 18.** NEDT measured from 2K×2K HgCdTe/Si MWIR HgCdTe/Si FPA with 15 micron unit cell demonstrating bet‐

As noted above, the motivation for HgCdTe growth on large-area Si substrates is to enable larger array formats and potentially reduced FPA cost compared to smaller, more expensive CdZnTe substrates. In addition to the successful demonstration of single color IRFPA on composite HgCdTe/Si substrates, researchers produced MWIR/LWIR dual band FPAs on large area Si substrates. The device structure is based on a triple-layer N-P-N heterojunction (TLHJ) architecture grown by molecular-beam epitaxy (MBE) on 100 mm (211) Si wafers with ZnTe and CdTe buffer layers [62]. The MWIR/LWIR dual band epitaxial wafers have low macro defect densities (<300 cm-2). Inductively coupled plasma etched detector arrays with 640x480 dual band pixels (20 μm) are mated to dual-band readout integrated circuits (ROICs) to produce FPAs (see Figure 19). The measured 80 K cutoff wavelengths are 5.5 μm for MWIR and 9.4 μm for LWIR, respectively. The FPAs exhibit high pixel operabilities in

fill factor characteristic of HgCdTe [61].

174 Optoelectronics - Advanced Materials and Devices

ter than 99.9% operability [61].

*6.2.1. HgCdTe on Silicon Two-Color IRFPAs*

High performance MWIRand LWIR FPAs are normally cooled to cryogenic temperatures at about 80 K, in order to suppress the dark current noise from overwhelming the photo‐ generated signals. If the operating temperature of the FPA is increased without degrading image quality, then smaller coolers can be used and the SWaP and cost of the system could be reduced. Of course, a more significant advantage can result if the operating tem‐ perature is increased to > 200 K where a low cost thermoelectric cooler can be implement‐ ed. There is a growing effort to increase the operating temperature of MWIR and LWIR infrared detectors by: reducing leakage currents; reducing thermal generation rates in the active region and minimizing the active volume of the detector without reducing quan‐ tum efficiency. While a number of strategies can be used to achieve high operating tem‐ perature (HOT) detectors, arecent DARPA program (AWARE-Broadband) focused on reducing detector material volume via a photon trap/photonic crystal approach to reduce dark current without degrading quantum efficiency [63-64].

The principle of volume reduction is demonstrated in Figure 20 which illustrates the effect of reducing the fill factor on device performance for a baseline shrinking mesa and an ideal‐ ized photon trap detector. The fill factor is defined as the volume of material remaining div‐ ided by the volume of the unit cell. The mesa reduction in volume initially reduces the NEDT as noise generating volume is removed, until the volume removed causes the signal to be reduced relative to the noise. Two types of IR photon trapping structures have been investigated: In AsSb pyramidal arrays and HgCdTe pillars and holes. Photon trap detectors on MBE HgCdTe/Si epitaxial wafers (see Figure 21) exhibit improved performance com‐ pared to single mesas, with measured NEDT of 40 mK and 100 mK at temperatures of 180 K and 200 K, with good operability. Large format arrays of these detectors exhibit cut-offs from 4.3 μm to 5.1 μm at 200 K. For the In AsSb pyramidal arrays, the measured dark cur‐ rent at the bias for peaked QE is in the low 10-3 A/cm2 range at 200 K and low 10-5 A/cm2 range at 150 K [64]. The general nBn band diagram and the dark current density curves at various temperatures are shown in Figure 22a and b, respectively. The I-V curves shown in figure 22b are from anCBn design, where the C stands for compound.

**Figure 20.** NEDT for a HgCdTe detector with a 5 μm cutoff at 200 K for baseline reducing mesa approach compared with a photonic crystal approach for 1x mean NEDT operability metric and 2x mean NEDT operability metric [63].

**Figure 22.** a) nBn band diagram illustrating the carrier flow [http://www.photonics.com/Article.aspx?AID=27744]. b) Dark current density curves as a function of temperature for an nCBn pyramidal InAs1-xSbx detector with a cutoff of

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The nBn [65] detector design consists of a n-type absorption layer, a conduction band offset barrier layer and a n-type contact layer. This design suppresses majority carrier currents (electrons in this case) while maintaining low electric fields. The ideal design would require a flat valence band as shown in the inset of figure 22(a). However, in practice and depend‐ ing on the choice of materials used, a small valence band offset may or may not exist. The conduction band large potential barrier blocks the flow of electrons while the flat valence band allows easy flow of holes. As a result, the thermally generated majority carrier, which contributes to dark current, is suppressed. Because the nBn architectures suppress the ther‐ mal noise, it is very suitable to operate this device at higher temperatures. As mentioned above (also see figure 22(b)), operation as high as 150K with excellent performances have

been demonstrated under the DARPA AWARE program.

5.05 μm at 200 K [64].

**Figure 21.** x 512 30 μm array consisting of unit cell design with photonic crystal holes on a 5 μm pitch in a MWIR HgCdTe [63].

pared to single mesas, with measured NEDT of 40 mK and 100 mK at temperatures of 180 K and 200 K, with good operability. Large format arrays of these detectors exhibit cut-offs from 4.3 μm to 5.1 μm at 200 K. For the In AsSb pyramidal arrays, the measured dark cur‐

range at 150 K [64]. The general nBn band diagram and the dark current density curves at various temperatures are shown in Figure 22a and b, respectively. The I-V curves shown in

**Figure 20.** NEDT for a HgCdTe detector with a 5 μm cutoff at 200 K for baseline reducing mesa approach compared with a photonic crystal approach for 1x mean NEDT operability metric and 2x mean NEDT operability metric [63].

**Figure 21.** x 512 30 μm array consisting of unit cell design with photonic crystal holes on a 5 μm pitch in a

MWIR HgCdTe [63].

range at 200 K and low 10-5 A/cm2

rent at the bias for peaked QE is in the low 10-3 A/cm2

176 Optoelectronics - Advanced Materials and Devices

figure 22b are from anCBn design, where the C stands for compound.

**Figure 22.** a) nBn band diagram illustrating the carrier flow [http://www.photonics.com/Article.aspx?AID=27744]. b) Dark current density curves as a function of temperature for an nCBn pyramidal InAs1-xSbx detector with a cutoff of 5.05 μm at 200 K [64].

The nBn [65] detector design consists of a n-type absorption layer, a conduction band offset barrier layer and a n-type contact layer. This design suppresses majority carrier currents (electrons in this case) while maintaining low electric fields. The ideal design would require a flat valence band as shown in the inset of figure 22(a). However, in practice and depend‐ ing on the choice of materials used, a small valence band offset may or may not exist. The conduction band large potential barrier blocks the flow of electrons while the flat valence band allows easy flow of holes. As a result, the thermally generated majority carrier, which contributes to dark current, is suppressed. Because the nBn architectures suppress the ther‐ mal noise, it is very suitable to operate this device at higher temperatures. As mentioned above (also see figure 22(b)), operation as high as 150K with excellent performances have been demonstrated under the DARPA AWARE program.

#### **6.4. Type II Strained Layer Superlattices (T2SL)**

Proposed by Smith and Mailhiot [66] in 1987, detectors based on InAs/GaSb strained layer superlattice (SLS) have attracted a lot of attention over the past few years as a possible alter‐ native to the II-VI based IR sensors. The motivation for pursuing the III-V based SLS result‐ ed from two major difficulties with LWIR MCT detectors: large tunneling currents and precise compositional control for accurate cutoff wavelengths. The InAs/GaSb SLS is engi‐ neered to achieve small bandgap materials with thin repeating layers for enhanced optical absorption and good electrical transport in the growth direction. The SLS structure typically consists of alternating layers with thicknesses varying from 4-20 nm.

tion of tunneling currents and higher operating temperatures compared to HgCdTe. The large splitting between heavy-hole and light-hole valence sub-bands, due to strain in the SLs, contributes to the suppression of Auger recombination rate. The maturity of the III-V materials technology offers technological advantages to the SLS effort by providing a source of commercially available low defect density substrates and recipes for very uniform proc‐ esses utilizing large area substrates. This makes detectors based on SLs an attractive technol‐

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SLS detectors are fabricated with either a p-on-n or n-on-p photodiode design. In either case, the optically active area of the photodiode is defined by an etched mesa as shown in Figure 24 (a). During the mesa isolation process, the periodic nature of the idealized crys‐ tal structure ends abruptly at the mesa sidewall surface. Disturbance of the periodic po‐ tential function, due to a broken crystal lattice, leads to allowed electronic quantum states within the energy band gap of the SLS resulting in large surface leakage currents. The suppression of these currents is the most demanding challenge for present day SLS tech‐ nology, especially for LWIR and VLWIR spectral regions, since the dimensions of the SLS pixels have to be scaled to about 20 μm. The limitation imposed by surface leakage cur‐ rents can be avoided by depositing a stable surface passivation layer onto the mesa side‐ wall. Currently, there is a lack of a robust passivating material and approach. The proposed approaches include: deposition of polyimide layer, overgrowth of wide band gap material, deposition of passivation sulphur coating electrochemically and post etch treatment in chemical solutions. However, these methods can affect cut-off wavelength of

**Figure 24.** Schematic of (a) conventionally defined mesa (b) shallow etched isolation nBn device. In the latter case, the

A new heterostructure design to limit the surface leakage currents is depicted in Figure 24 (b) as the nBn architecture. Similar to the nBn discussion above, the SLS-nBn design allows for flexibility in employing band-engineered structures. The nBn detector consists of an n-type narrow band-gap contact that is separated from the absorber layers by a 50-100 nm thick, wide band-gap barrier layer. Unlike a conventional photodiode fabrication, the size of the nBn de‐

ogy for realization of high performance single element detectors and FPAs [67-68].

the device or complicate the fabrication process of the detectors [67-68].

area is defined by the by diffusion length (DL) of the minority carriers (holes) [68].

These InAs/GaSb heterostructures are characterized by the broken-gap type-II alignment where the conduction band of the InAs layer is lower than the valence band of the GaSb lay‐ er as illustrated in Figure 23. The bandgap is the energy difference between the top of the heavy-hole mini-band (HH1) and the bottom of the electron miniband (C1), as indicated in Figure 23. The overlap of electron (hole) wave functions between adjacent InAs (GaSb) lay‐ ers results in the formation of an electron (hole) minibands in the conduction (valence) band. For IR sensing, optical transitions between holes localized in GaSb layers and electrons con‐ fined in InAs layers are employed. As the layer thickness decreases, the wave-function over‐ lap increases causing a more favorable optical absorption. As the thickness is increased beyond about 5 nm, the wavefunction overlap is reduced with a corresponding decrease in optical coupling. The effective bandgap of the InAs/InGaSb SLs can be tailored from 3 um to 30 um abroption by varying the thickness of the constituent layers, thus enabling detectors spanning the entire IR spectrum [67-68].

**Figure 23.** Schematic bandgap alignment of Type II InAs/GaSb superlattices [69].

The effective mass of the charge carriers in the superlattice is not dependent on the semicon‐ ductor bandgap, as in the case of bulk materials. The larger effective mass of the electrons and holes in SLs combined with the slower Auger recombination rate can lead to a reduc‐ tion of tunneling currents and higher operating temperatures compared to HgCdTe. The large splitting between heavy-hole and light-hole valence sub-bands, due to strain in the SLs, contributes to the suppression of Auger recombination rate. The maturity of the III-V materials technology offers technological advantages to the SLS effort by providing a source of commercially available low defect density substrates and recipes for very uniform proc‐ esses utilizing large area substrates. This makes detectors based on SLs an attractive technol‐ ogy for realization of high performance single element detectors and FPAs [67-68].

**6.4. Type II Strained Layer Superlattices (T2SL)**

178 Optoelectronics - Advanced Materials and Devices

spanning the entire IR spectrum [67-68].

consists of alternating layers with thicknesses varying from 4-20 nm.

**Figure 23.** Schematic bandgap alignment of Type II InAs/GaSb superlattices [69].

The effective mass of the charge carriers in the superlattice is not dependent on the semicon‐ ductor bandgap, as in the case of bulk materials. The larger effective mass of the electrons and holes in SLs combined with the slower Auger recombination rate can lead to a reduc‐

Proposed by Smith and Mailhiot [66] in 1987, detectors based on InAs/GaSb strained layer superlattice (SLS) have attracted a lot of attention over the past few years as a possible alter‐ native to the II-VI based IR sensors. The motivation for pursuing the III-V based SLS result‐ ed from two major difficulties with LWIR MCT detectors: large tunneling currents and precise compositional control for accurate cutoff wavelengths. The InAs/GaSb SLS is engi‐ neered to achieve small bandgap materials with thin repeating layers for enhanced optical absorption and good electrical transport in the growth direction. The SLS structure typically

These InAs/GaSb heterostructures are characterized by the broken-gap type-II alignment where the conduction band of the InAs layer is lower than the valence band of the GaSb lay‐ er as illustrated in Figure 23. The bandgap is the energy difference between the top of the heavy-hole mini-band (HH1) and the bottom of the electron miniband (C1), as indicated in Figure 23. The overlap of electron (hole) wave functions between adjacent InAs (GaSb) lay‐ ers results in the formation of an electron (hole) minibands in the conduction (valence) band. For IR sensing, optical transitions between holes localized in GaSb layers and electrons con‐ fined in InAs layers are employed. As the layer thickness decreases, the wave-function over‐ lap increases causing a more favorable optical absorption. As the thickness is increased beyond about 5 nm, the wavefunction overlap is reduced with a corresponding decrease in optical coupling. The effective bandgap of the InAs/InGaSb SLs can be tailored from 3 um to 30 um abroption by varying the thickness of the constituent layers, thus enabling detectors

SLS detectors are fabricated with either a p-on-n or n-on-p photodiode design. In either case, the optically active area of the photodiode is defined by an etched mesa as shown in Figure 24 (a). During the mesa isolation process, the periodic nature of the idealized crys‐ tal structure ends abruptly at the mesa sidewall surface. Disturbance of the periodic po‐ tential function, due to a broken crystal lattice, leads to allowed electronic quantum states within the energy band gap of the SLS resulting in large surface leakage currents. The suppression of these currents is the most demanding challenge for present day SLS tech‐ nology, especially for LWIR and VLWIR spectral regions, since the dimensions of the SLS pixels have to be scaled to about 20 μm. The limitation imposed by surface leakage cur‐ rents can be avoided by depositing a stable surface passivation layer onto the mesa side‐ wall. Currently, there is a lack of a robust passivating material and approach. The proposed approaches include: deposition of polyimide layer, overgrowth of wide band gap material, deposition of passivation sulphur coating electrochemically and post etch treatment in chemical solutions. However, these methods can affect cut-off wavelength of the device or complicate the fabrication process of the detectors [67-68].

**Figure 24.** Schematic of (a) conventionally defined mesa (b) shallow etched isolation nBn device. In the latter case, the area is defined by the by diffusion length (DL) of the minority carriers (holes) [68].

A new heterostructure design to limit the surface leakage currents is depicted in Figure 24 (b) as the nBn architecture. Similar to the nBn discussion above, the SLS-nBn design allows for flexibility in employing band-engineered structures. The nBn detector consists of an n-type narrow band-gap contact that is separated from the absorber layers by a 50-100 nm thick, wide band-gap barrier layer. Unlike a conventional photodiode fabrication, the size of the nBn de‐ vice is defined by the lateral diffusion length of minority carriers (holes), as illustrated in Fig‐ ure 24 (b). A 100 K increase in the BLIP temperature has been demonstrated [68]. SLS-based detectors with nBn design and special processing schemes showed dark current reduction of two orders of magnitude (at 77 K) in comparison to conventional photodiode processing tech‐ niques. Quantum efficiency and shot-noise-limited specific detectivity are comparable to cur‐ rent SLS-based p-i-n diodes. While nBn detectors have been demonstrated, focal plane arrays based on InAs/GaSb SLS detectors with nBn designs are being developed [68-69].

#### **6.5. Microbolometers**

A microbolometer is a resistive element fabricated from a material that has a very small thermal capacity and a large temperature coefficient of resistance (TCR). The absorbed IR ra‐ diation is converted into heat which changes the resistance of the microbolometer such that measurable electrical signals can be detected.The spectral response of the bolometer is flat across the IR spectrum, since the sensing mechanism isindependent of the photoexcited car‐ riers jumping across an energy band-gap. A schematic representation of a bolometer pixel on top of a ROIC is shown in Figure 25 details of which can be found in reference 70. The pixel design shows a resonant cavity formed by an absorbing layer suspended above a re‐ flecting metal layer. The cavity is used to amplify the absorptance of the incident IR radia‐ tion. The microbridge is supported by two beams and is thermally isolated from the ROIC to increase the sensitivity of the microbolometer.

**Figure 26.** SEM images of a) amorphous Si and b) VOxmicrobolometers IR Detector [72-73].

Further advancement in imaging systems requires solutions for many fundamental and technological issues related to wide field of view (FOV), resolution, pixel pitch, optics, mul‐ ticolor, form-factor, low SWaP, and low cost. This section highlights two projects currently fielded through the Microsystems Technology Office (MTO) of the Defense Advanced Project Agency (DARPA) to prepare for these advancements. The first is called AWARE (Advanced Wide Field of View Architectures for Image Reconstruction and Exploitation) and addresses several fundamental issues that will enable technologies for wide field-ofview, pixel scaling, broadband and multiband imaging [75-77]. The second project is LCTI-M (Low Cost Thermal Imager-Manufacturing) which is addressing a cost effective solution

Advances in Infrared Detector Array Technology

http://dx.doi.org/10.5772/51665

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Traditional detector arrays are typically designed for a narrow band of wavelengths due to in‐ adequate absorption and charge collection from photons with varying wavelengths. Broad‐ band absorption is usually inadequate due to quantum efficiency (QE) roll-off. To design a detector with high QE, low NETD and high operating temperature across a very broad band of wavelengths, say visible to 5 um range, traditional detector design would be less than opti‐ mum. A detector design that can accomplish these goals is based on a photonic pillar-type ar‐ chitecture. Photonic crystals are relatively well understood and have been demonstrated for applications like VCSELs, which are similar to photovoltaic detectors. Sub-wavelengthsize semiconductor pillar arrays within a single detector can be designed and structured as an en‐ semble of photon trapping units to significantly increase absorption and QE for a wide band of wavelengths. Each sub-element in each pixel can be a 3D photonic structure fabricated using either a top-down or bottom-up process scheme. The sub-element architecture canbe of differ‐

Using unique pyramidal and pillar topologies etched into the photon absorbing layer, re‐ searchers have demonstrated 3D photon trapping, achieved significant reduction in dark current and established uniform QE (see Figure 27). This is the first demonstration of broad‐

**7. Future IR Technology Directions**

for manufacturing microbolometers.

ent shapes such as pyramidal, sinusoidal or rectangular [75].

**Figure 25.** Schematic cross sectional and top views of Microbolometer IR Detector [70].

The microbolometer based on Si-MEMS device structure has been under development for over 20 years with support from DARPA and the Army. Most microbolometer structures utilize VOx and amorphous silicon thin film technologies. Companies such as Raytheon, BAE Systems and DRS Technologies are developing and producing 17 micron pixels in 640x 480 and larger arrays using VOx [71-72]. L3 Communications and CEA-LETI are developing and producing 640x480 arrays with 17 micron unit cells using amorphous-Silicon technolo‐ gy [73-74]. Figure 26 presents examples of the microbolometer structures using VOx and amorphous silicon technologies.

**Figure 26.** SEM images of a) amorphous Si and b) VOxmicrobolometers IR Detector [72-73].
