**2. Infrared Spectrum and Bands of Interest**

The word "infrared" refers to a broad portion of the electromagnetic spectrum that spans a wavelength range from 1.0 um to beyond 30 um everything between visible light and micro‐ wave radiation. Much of the infrared spectrum is not useful for ground- or sea-based imag‐ ing because the radiation is blocked by the atmosphere. The remaining portions of the spectrum are often called "atmospheric transmission windows," and define the infrared bands that are usable on Earth. The infrared spectrum is loosely segmented into near infra‐ red (NIR, 0.8-1.1um), short wave infrared (SWIR, 0.9-2.5um), mid wave infrared (MWIR, 3-5um), long wave infrared (LWIR, 8-14um), very long wave infrared (VLWIR, 12- 25um) and far infrared (FIR, > 25um), as shown in Figure 1. The MWIR- LWIR wavebands are im‐ portant for the imaging of objects that emit thermal radiation, while the NIR-SWIR bands are good for imaging scenes that reflect light, similar to visible light. Some of the materials technologies and device architectures used for detector fabrication in the various IR bands are discussed in references 1 - 4. Since NIR and SWIR are so near to the visible bands, their behavior is similar to the more familiar visible light. Energy in these bands must be reflected from the scene in order to produce good imagery, which means that there must be some ex‐ ternal illumination source. Both NIR and SWIR imaging systems can take advantage of sun‐ light, moonlight, starlight, and an atmospheric phenomenon called "nightglow," but typically require some type of artificial illumination at night. In lieu of photon starved scenes, arrays of infrared Light Emitting Diodes (LEDs) can provide a very cost effective sol‐ ution for short-range illumination. However, achieving good performance at distances of over hundreds of meters requires more directed illumination, such as a focused beam from a laser or specialized spotlight, although special consideration of eye-safety issues is required.

NIR and SWIR imaging systems often employ sensors that are more exotic than those found in consumer-grade camcorders and digital cameras. Because NIR has a wavelength longer than visible light, and SWIR a wavelength that is longer still, energy in these bands is scattered less by particles suspended in the atmosphere. Consequently, SWIR, and to a lesser extent NIR, systems are tolerant of low levels of obscurants like fog and smoke compared to visible light.

An imaging system that operates in the MWIR and LWIR ranges can be completely passive, requiring no external illumination, because the thermal imager is able to sense the energy that is emitted directly from objects in the scene. The major factors that determine how bright an object appears to a thermal imager are: the object's temperature and its emissivity. As an object gets hotter, it radiates more energy and appear brighter to a thermal imaging system. Emissivity is a physical property of materials that describes how efficiently it radi‐ ates heat. Because cloth has a lower emissivity than skin, the former will appear darker in a thermal imager even when both are exactly at the same temperature.

**Figure 1.** Definition of IR Spectral Band.

nologies (nBn). Each of these technologies has a place in the IR applications where a variety

We also present a discussion on the LWIR band that covers the wavelength range between 8 and 14 microns. The technologies that are addressed are bolometer (Microbolometer Ar‐ rays), HgCdTe arrays, and a variety of very ingenious band-gap engineered devices using

The word "infrared" refers to a broad portion of the electromagnetic spectrum that spans a wavelength range from 1.0 um to beyond 30 um everything between visible light and micro‐ wave radiation. Much of the infrared spectrum is not useful for ground- or sea-based imag‐ ing because the radiation is blocked by the atmosphere. The remaining portions of the spectrum are often called "atmospheric transmission windows," and define the infrared bands that are usable on Earth. The infrared spectrum is loosely segmented into near infra‐ red (NIR, 0.8-1.1um), short wave infrared (SWIR, 0.9-2.5um), mid wave infrared (MWIR, 3-5um), long wave infrared (LWIR, 8-14um), very long wave infrared (VLWIR, 12- 25um) and far infrared (FIR, > 25um), as shown in Figure 1. The MWIR- LWIR wavebands are im‐ portant for the imaging of objects that emit thermal radiation, while the NIR-SWIR bands are good for imaging scenes that reflect light, similar to visible light. Some of the materials technologies and device architectures used for detector fabrication in the various IR bands are discussed in references 1 - 4. Since NIR and SWIR are so near to the visible bands, their behavior is similar to the more familiar visible light. Energy in these bands must be reflected from the scene in order to produce good imagery, which means that there must be some ex‐ ternal illumination source. Both NIR and SWIR imaging systems can take advantage of sun‐ light, moonlight, starlight, and an atmospheric phenomenon called "nightglow," but typically require some type of artificial illumination at night. In lieu of photon starved scenes, arrays of infrared Light Emitting Diodes (LEDs) can provide a very cost effective sol‐ ution for short-range illumination. However, achieving good performance at distances of over hundreds of meters requires more directed illumination, such as a focused beam from a laser or specialized spotlight, although special consideration of eye-safety issues is required. NIR and SWIR imaging systems often employ sensors that are more exotic than those found in consumer-grade camcorders and digital cameras. Because NIR has a wavelength longer than visible light, and SWIR a wavelength that is longer still, energy in these bands is scattered less by particles suspended in the atmosphere. Consequently, SWIR, and to a lesser extent NIR, systems are tolerant of low levels of obscurants like fog and smoke compared to visible light. An imaging system that operates in the MWIR and LWIR ranges can be completely passive, requiring no external illumination, because the thermal imager is able to sense the energy that is emitted directly from objects in the scene. The major factors that determine how bright an object appears to a thermal imager are: the object's temperature and its emissivity. As an object gets hotter, it radiates more energy and appear brighter to a thermal imaging

of detector configurations can be used.

150 Optoelectronics - Advanced Materials and Devices

III-V compound semiconductor materials.

**2. Infrared Spectrum and Bands of Interest**

At the MWIR and LWIR wavelengths, infrared radiation behaves differently from visible light. For example, glass is transparent to wavelengths less than 3.0 μm, so glass optics can be used and windows can be seen through at these wavelengths. However, glass is opaque in the LWIR band and blocks most energy in the MWIR band. Consequently, the optics in LWIR and MWIR imaging systems cannot use inexpensive glass lenses, but are forced to use more expensive materials, such as germanium. Because glass windows are not transparent at the longer wavebands, they can appear to be brighter or darker according to their temper‐ atures. Another difficulty with radiation in the MWIR and LWIR bands is that it is not trans‐ mitted through water. Imaging of a water (rain) coated scene with MWIR-LWIR wavelengths can wash out much of the scene's thermal contrast, resulting in a duller image.

The choice of wavelength band to exploit for IR imaging depends on the type of atmospher‐ ic conditions/obscurants between the target and the imager. Generally, atmospheric obscur‐ ants, such as haze or conventional smoke, cause much less scattering in the MWIR and LWIR bands than in the VIS-NIR or SWIR bands. This is because the haze or smoke particle size (~0.5 um) is much smaller than the IR wavelength (Rayleigh scattering). Obscurants such as fog and clouds can cause more scattering, since the particle size is comparable with the IR wavelength (Mie scattering). Infrared cameras sensitive to the longer wavelengths are more tolerant to smoke, dust and fog. In addition to obscurants, atmospheric turbulence can dictate the choice of IR wave band for a given application. The effects of optical turbulence, due to the fluctuations in the refractive index of the atmosphere, can add up over very long distances to impact range performance (blurring and image motion), allowing LWIR an edge over MWIR. As a rule of thumb, longer the wavelength better is the transmission through the earth's atmosphere.

According to Wien's Law, hotter objects emit more of their energy at shorter wavelengths. A blackbody source at 300 K has a peak exitance (power per unit area leaving a surface) at a wavelength of about 9.7 μm. For a source at 1000 K, the maximum exitance occurs at 2.9 μm. Therefore, detectors operating in the LWIR band are well suited to image room temperature objects (people, buildings etc.), while MWIR band imagers are good for viewing objects at higher temperatures (hot engines and exhaust gasses). In general, LWIR and MWIR bands will produce thermal images if small temperature changes or varying emissivities exist within a scene. However, while the LWIR band imagery may exhibit a higher sensitivity for room temperature objects, the MWIR band imagery presents a better resolution.

( , , ) [ ] *d d*

*N*

To have a meaningful comparison between different detectors, their respective performance must be reduced to representative conditions, so that the detectivity is often normalized to a

can be interpreted as the signal to noise ration (SNR) out of a detector when 1 watt of radi‐

The performance of low-noise detectors may also be limited by radiative noise arriving at the detector from the background environment. When the background photon flux is much greater than the signal flux, the photodetector is said to be background-limited in perform‐ ance or in the BLIP mode. The resulting detectivity of the photovoltaic detector is called

> *η* 2 *ϕ<sup>B</sup>*


where *λ* is the wavelength, *η* is the quantum efficiency, and *ϕB* is the incident photon flux

ed. For photoconductive detectors that are generation-recombination (G-R) noise limited, the *DBLIP \** is lower by a factor of 2. The variances of the G-R noise are additive, causing an

Background-limited performance of detectors can be improved by reducing the background photon flux, *ϕB* . There are two ways of implementing this: a) use a cooled and/or spectral filter to limit the spectral band, or b) use a cold shield to limit the angular FOV of the detec‐ tor. The former approach eliminates most of the background radiation from spectral regions in which the detector does not need to respond. The best detectors can approach back‐ ground limited detectivities by limiting the field of view with a cold shield.Detectivity curves across the infrared spectrum for various commercially available detectors are shown in Figure 2 [7]. Calculated detectivities for the background-limited performance for ideal

*<sup>P</sup> EA EA NEP T f BW S SV N*

*Detectivity*, *D* = <sup>1</sup>

The inverse of NEP is referred to as the Detectivity:

bandwidth of 1 Hz and a detector area of 1 cm2

increase in the rms noise voltage by a factor of 2.

*D* \*

<sup>=</sup> *Ad BW*

*DBLIP \** <sup>=</sup> *<sup>λ</sup> hc*

photon and thermal detectors are also included in Figure 2 as dashed curves.

ant power is incident on a 1 cm2

*DBLIP \** and is expressed as:

in photons/cm2

*s N*

*V*

*s*

=== *wa t* (2)

*NEP* (3)

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. This figure of merit is called D-star (D\*) and

(5)

*t*

detector area at a noise equivalent bandwidth of 1 Hz.

*NEP* cm Hz / *watt* (4)

### **3. Theoretical Considerations**

IR detectors can be categorized as being either a quantum or thermal device. In a quantum detector, electromagnetic radiation absorbed in a semiconductor material generates electronhole pairs (EHP), which are sensed by an electronic readout circuit (ROIC). In a thermal de‐ tector, on the other hand, the incident IR photons are absorbed by a thermally isolated detector element, resulting in an increase in the temperature of the element. The tempera‐ ture is sensed by monitoring an electrical parameter such as resistivity or capacitance.

Because thermal detectors depend on the quantity of heat absorbed, their response is in‐ dependent of wavelength, however, the sensitivity depends on the material design for sensing. There are two types of quantum detectors: a) photoconductive (PC) where the electrical conductivity of the semiconductor changes as a function of the photon intensity; b) photovoltaic (PV) where a voltage is generated across a PN junction as photons im‐ pinge the semiconductor. Quantum detectors convert photons directly into charge carriers and no intermediate process is involved, such as the heating in a thermal detector to cause a change of a measurable electrical property.

Due to the various mechanisms used by detectors to convert optical to electrical signals, several figures of merit (FOM) are used to characterize their performance [5-8]. The out‐ put of the detector consists of its response signal to the incident radiation and random noise fluctuations. One such FOM is the Responsivity (R) of the detector, defined as the ratio of the root mean squared (rms) value of the signal voltage ( *Vs* ) to the rms power (P in volts/watt) incident on the detector. The total power on an area ( *Ad* ) is associated with an irradiance E (in watts/ cm2 ). Therefore,

$$R = \frac{V\_s}{P} = \frac{V\_s}{EA\_d} = \text{[volts/ watts]}\tag{1}$$

The random fluctuations in a detector's output limit its sensitivity to a certain minimum de‐ tectable power. The power necessary to generate an output signal equal to the noise is known as the Noise Equivalent Power (NEP). NEP is determined by measuring the amount of radiative power from a blackbody that falls on the detector to produce an rms signal Vs, equal to that generated by the detector noise *V <sup>N</sup>* , when it is shuttered from the blackbody. NEP must be specified for a particular source temperature (T), modulation frequency (f), system bandwidth (BW), and detector area ( *Ad* ).

#### Advances in Infrared Detector Array Technology http://dx.doi.org/10.5772/51665 153

$$NEP\left(T, f, BW\right) = \frac{P}{\bigdiamond\_{N}} = \frac{EA\_d}{\bigdiamond\_{N}} = \frac{EA\_d}{V\_s} \Big|\_{V\_N} \tag{2}$$

The inverse of NEP is referred to as the Detectivity:

objects (people, buildings etc.), while MWIR band imagers are good for viewing objects at higher temperatures (hot engines and exhaust gasses). In general, LWIR and MWIR bands will produce thermal images if small temperature changes or varying emissivities exist within a scene. However, while the LWIR band imagery may exhibit a higher sensitivity for

IR detectors can be categorized as being either a quantum or thermal device. In a quantum detector, electromagnetic radiation absorbed in a semiconductor material generates electronhole pairs (EHP), which are sensed by an electronic readout circuit (ROIC). In a thermal de‐ tector, on the other hand, the incident IR photons are absorbed by a thermally isolated detector element, resulting in an increase in the temperature of the element. The tempera‐ ture is sensed by monitoring an electrical parameter such as resistivity or capacitance.

Because thermal detectors depend on the quantity of heat absorbed, their response is in‐ dependent of wavelength, however, the sensitivity depends on the material design for sensing. There are two types of quantum detectors: a) photoconductive (PC) where the electrical conductivity of the semiconductor changes as a function of the photon intensity; b) photovoltaic (PV) where a voltage is generated across a PN junction as photons im‐ pinge the semiconductor. Quantum detectors convert photons directly into charge carriers and no intermediate process is involved, such as the heating in a thermal detector to

Due to the various mechanisms used by detectors to convert optical to electrical signals, several figures of merit (FOM) are used to characterize their performance [5-8]. The out‐ put of the detector consists of its response signal to the incident radiation and random noise fluctuations. One such FOM is the Responsivity (R) of the detector, defined as the ratio of the root mean squared (rms) value of the signal voltage ( *Vs* ) to the rms power (P in volts/watt) incident on the detector. The total power on an area ( *Ad* ) is associated

). Therefore,

*d V V <sup>R</sup> volts watts*

[/ ] *s s*

The random fluctuations in a detector's output limit its sensitivity to a certain minimum de‐ tectable power. The power necessary to generate an output signal equal to the noise is known as the Noise Equivalent Power (NEP). NEP is determined by measuring the amount of radiative power from a blackbody that falls on the detector to produce an rms signal Vs, equal to that generated by the detector noise *V <sup>N</sup>* , when it is shuttered from the blackbody. NEP must be specified for a particular source temperature (T), modulation frequency (f),

*P EA* == = (1)

room temperature objects, the MWIR band imagery presents a better resolution.

**3. Theoretical Considerations**

152 Optoelectronics - Advanced Materials and Devices

cause a change of a measurable electrical property.

system bandwidth (BW), and detector area ( *Ad* ).

with an irradiance E (in watts/ cm2

$$\text{Detativity}\_{\prime} \text{ } D = \begin{vmatrix} \\ \end{vmatrix}\_{\text{NEP}} \tag{3}$$

To have a meaningful comparison between different detectors, their respective performance must be reduced to representative conditions, so that the detectivity is often normalized to a bandwidth of 1 Hz and a detector area of 1 cm2 . This figure of merit is called D-star (D\*) and can be interpreted as the signal to noise ration (SNR) out of a detector when 1 watt of radi‐ ant power is incident on a 1 cm2 detector area at a noise equivalent bandwidth of 1 Hz.

$$D \stackrel{\*}{=} \frac{\sqrt{A\_d BW}}{NEP} \text{[cm\sqrt{Hz}/watt]} \tag{4}$$

The performance of low-noise detectors may also be limited by radiative noise arriving at the detector from the background environment. When the background photon flux is much greater than the signal flux, the photodetector is said to be background-limited in perform‐ ance or in the BLIP mode. The resulting detectivity of the photovoltaic detector is called *DBLIP \** and is expressed as:

$$D\_{BLIP}^{\*} = \frac{\lambda}{hc} \sqrt{\frac{\eta}{2\phi\_B}}\tag{5}$$

where *λ* is the wavelength, *η* is the quantum efficiency, and *ϕB* is the incident photon flux in photons/cm2 -s. Equation (5) is valid for photovoltaic detectors which are shot-noise limit‐ ed. For photoconductive detectors that are generation-recombination (G-R) noise limited, the *DBLIP \** is lower by a factor of 2. The variances of the G-R noise are additive, causing an increase in the rms noise voltage by a factor of 2.

Background-limited performance of detectors can be improved by reducing the background photon flux, *ϕB* . There are two ways of implementing this: a) use a cooled and/or spectral filter to limit the spectral band, or b) use a cold shield to limit the angular FOV of the detec‐ tor. The former approach eliminates most of the background radiation from spectral regions in which the detector does not need to respond. The best detectors can approach back‐ ground limited detectivities by limiting the field of view with a cold shield.Detectivity curves across the infrared spectrum for various commercially available detectors are shown in Figure 2 [7]. Calculated detectivities for the background-limited performance for ideal photon and thermal detectors are also included in Figure 2 as dashed curves.

the incident radiation needs to be optimized while, simultaneously, minimizing its thermal contacts to the surroundings. In practice, this requires a bolometer with small mass and fine

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Hg1-xCdxTe (MCT) is the most widely used infrared (IR) detector material in military appli‐ cations, compared to other IR detector materials, primarily because of two key features: it is a direct energy band gap semiconductor and its band gap can be engineered by varying the Cd composition to cover a broad range of wavelengths. The direct band gap of MCT allows for a high absorption of IR radiation, yielding high quantum efficiency in a relatively thin detector structure. As the Cd mole fraction, x, increases, the energy gap for MCT increases

The ability to tune the band gap of MCT enables IR detectors to operate in the wavelength bands ranging from SWIR to VLWIR (0.7-30 microns). For low-cost high-performance detec‐ tors, the MCT material must be produced on large diameter wafers with low defect densities and reproducible stoichiometric properties. These requirements are satisfied by a host of crystal growth techniques ranging from high temperature, melt grown bulk crystals, to low

Depending on the detector architecture, the crystal growth strategy could utilize any of the following techniques: Bulk Crystal Growth, Liquid Phase Epitaxy (LPE), Metal-organic Chemical Vapor Deposition (MOCVD), and Molecular Beam Epitaxy (MBE). The sections below will highlight each of these growth techniques with references to publications that

Bulk crystal growth of MCT continues to play an important role in producing IR detector materials for photoconductive arrays, despite the progress made with various epitaxial thin film deposition techniques. Bulk growth process is typically used for large area single detec‐ tors for applications such as spectrometry. However, for photovoltaic arrays there are chal‐ lenges associated with crystal grain boundaries, which are electrically active and contribute to line defects. Also there are limitations in the ingot diameter, which makes bulk growth suitable for only quad or single detector arrays. Several methods have been developed for growing MCT bulk crystals: Solid State Recrystallization (SSR), Traveling Heater Method (THM), Bridgman, Czochralski, Slush Growth, and Zone Melting [9-14]. This section will

The general challenge with melt grown MCT is to maintain a relatively high Hg vapor pres‐ sure during growth; otherwise, it is difficult to control the stoichiometry of the grown crys‐ tal. Also, the large separation between the liquidus and solidus compositions (see Figure 3) across a constant thermal tie line can result in a steady variation in the composition of a

linearly from a semimetal (HgTe) to a wide band gap semiconductor (CdTe).

connecting wires to the heat sink.

temperature, multilayer epitaxial layers.

will provide additional coverage.

cover SSR and THM techniques.

moving growth interface.

**4.1. Bulk Crystal Growth**

**4. IR Material growth Techniques for HgCdTe**

**Figure 2.** Detectivity curves for various commercially available photon and thermal IR detectors. Calculated detectivi‐ ties are indicated by dashed lines [7].

Another frequently quoted figure of merit for a photodiode is its R0A product, where R0 is the dynamic resistance of the photodiode and is equal to the slope of the I-V curve at the zero bias voltage point. This FOM is independent of the junction area, except when the di‐ mensions are comparable to the minority carrier diffusion length.

Thermal detectors require a temperature change to produce a signal and do not generally need cooling, in contrast to photo detectors which are cooled to minimize noise. Absorbed radiation causes a temperature change that alters a temperature sensitive property of the de‐ tector which can be measured externally. A few examples include: electrical resistance in a bolometer, thermal expansion of Golay cells, and polarization in pyroelectric materials. Since these detectors depend on temperature changes resulting from incident radiation, they must be thermally isolated from their surroundings and have low thermal capacities for fast response to the radiation. In the case of a bolometer, the FOM is its thermal time constant which is defined as:

$$
\tau\_{th} = \frac{\mathcal{C}\_{th}}{\mathcal{G}\_{th}} = \mathcal{C}\_{th} \mathcal{R}\_{th} \tag{6}
$$

where *Cth* is the thermal capacity of the detector, *Rth* is the thermal resistance and *Gth* is the thermal coupling of the detector to its surroundings. The interaction of the bolometer with the incident radiation needs to be optimized while, simultaneously, minimizing its thermal contacts to the surroundings. In practice, this requires a bolometer with small mass and fine connecting wires to the heat sink.
