**5. SWIR Detector Technologies**

low cost substrates such as GaAs and control N- and P-type doping levels. In comparison to an MBE system, the overall maintenance and operational costs of an MOCVD system is lower.

Thin film deposition by MBE enables the growth of large area epilayers with sophisticated multilayer structures having abrupt and complex compositions and doping profiles. Growth of MBE-MCT is carried out under an ultra-high vacuum environment with Knudsen-type ef‐ fusion source cells charged with Hg, Te2, and CdTe [31-32]. MBE-MCT deposition tempera‐ ture plays a critical role in the introduction of extended defects. Typically, growth is carried

The low growth temperature and the ability to rapidly shutter the sources are key features that allow MBE to produce sharp interfaces for multilayered IR devices that operate in two or three different spectral bands. The ultra-high vacuum growth chamber allows for in-situ analytical tools to monitor and control the MCT growth process and evaluate the properties

At the lower temperature range, a Hg-rich condition prevails at the substrate because the sticking coefficient of Hg increases as the temperature is reduced. The condition with excess Hg results in the formation micro-twins that are detrimental to the performance of the MCT IR focal plane array. Typical etch pit densities (EPD) of material grown under such Hg-rich

Hg is incorporated in the film only by reacting with free Te, thus the MCT composition is contingent on the Te to CdTe flux ratio. The structural perfection of the film depends strong‐ ly on the Hg to Te flux ratio and growth is usually restricted to a tight temperature range. By optimizing the Hg to Te flux ratio, the concentration of voids is about 100 cm–2 which may be attributable to dust particles or substrate related imperfections. The EPD values for

Indium is the most widely used N-type extrinsic dopant in MCT epitaxial layers and is well activated. At low Indium doping levels, Hg vacancies can compensate some of the N-type impurities and affect dopant control. P-type dopants, such as Arsenic, are less conveniently incorporated into the epilayer. Significant efforts are being expended to improve the incor‐ poration of As and Sb during the MBE process and to reduce the temperature required for activation. The metal saturation conditions cannot be reached at the temperatures required for high-quality MBE growth. The necessity to activate acceptor dopants at high tempera‐ tures diminishes the gains of low-temperature deposition. Nearly 100% activation has been achieved for a 2 × 10E18 cm−3 As concentration, with as low as 300 °C activation anneal, fol‐

Because of its various advantages, MBE-MCT technology is becoming more attractive than the other epitaxial technologies and is required for the fabrication of IR detectors with ad‐ vanced architectures. The MBE-MCT technology has developed to the point where MBE lay‐

cm–2). If the growth temperature is raised to about 190 °C, then a

cm–2 ranges.

*4.1.5. Molecular Beam Epitaxy (MBE)*

160 Optoelectronics - Advanced Materials and Devices

of the grown layers [33-34].

conditions are high (106

out at 180 °C–190 °C on (211) CdZnTe substrates.

–107

epilayers grown under these conditions are in the low 105

lowed by a 250 °C stoichiometric anneal [35].

deficiency of Hg leads to the formation of voids in the MCT layer.

The SWIR band (0.9-2.5 um) bridges the spectral gap between the visible and thermal bands in the electromagnetic spectrum. In this spectral band, the primary phenomenology of inter‐ est is the reflectance signature of the target, manifested as either its variations in brightness or spectral reflectance, or both.

Infrared imaging in the SWIR band offers several advantages: can detect reflected light, offer‐ ing more intuitive, visible-like images; better suited for imaging in adverse environments and weather conditions, including fog, dust, and smoke; can also see in low light conditions, and use eye safe 1550 nm illumination that is totally undetectable by regular night vision equip‐ ment; and can generate digital video outputs and thus offering more advantages than tradi‐ tional image intensifier night vision equipment. Under low light conditions, the sensitivity of the focal plane array is ultimately determined by the R0 A product of the photodiode.

### **5.1. Inx Ga1-x As Detector Array Development**

For SWIR imaging, InGaAs is one of the widely used detector materials due to its low dark current. The detector material can be prepared using any of the following techniques: Mo‐ lecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), liquid phase epitaxy (LPE), hydride-transport vapor phase epitaxy (VPE), and atomic layer epitaxy (ALE). InGaAs layers are typically grown on lattice matched InP substrates using an alloy composition of x = 0.53.

The spectral response typically covers 0.9-1.7μm at room temperature. By increasing the composition to x=0.82, InGaAs is able to extend its cutoff to 2.6 μm. However, the crystal defects due to epitaxy and the decreased shunt resistance, due to a smaller band gap, de‐ grade performance at the longer cutoff wavelengths.

noise. Dark current consists of unwanted thermally generated carriers that can cause the de‐

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It is associated with interfacial, diffusional, G-R, and tunneling currents. The temperature dependence of the dark current is primarily due to the intrinsic carrier concentration which depends exponentially on the temperature. The dark current of the detector can be reduced through appropriate fabrication processes and device design. The impact of dark current noise as a function of read noise is shown in Figure 5, where the curves for different pixel

For a given read noise, the required dark current density increases as the pixel pitch is de‐ creased. The challenge is to maintain a low dark current density as the pixel pitch is re‐ duced. Simultaneously, the challenge for the read out circuit is to reduce the read noise. If the limitation is due to the detector and its noise level overwhelms the source signal, the sol‐ ution may be to use an external illuminator or cool the detector. The choice of either solution

**Figure 6.** Dark current density at different temperatures using test structures on the wafer. Test arrays have 225 pixels

Applications involving situational awareness require FPAs to have more pixels (large format/high resolution) for increased surveillance coverage. For soldier portable and some airborne platforms, it is desirable to reduce the size of the pixel in order to satisfy the constraints of low SWaP and cost, without sacrificing performance. However,as the pixel pitch is scaled to smaller geometries, there is a tendency for the dark current den‐ sity to increase. The reduced pixel diameter can cause the sidewall related dark current to become more pronounced and overwhelm the area related contribution, resulting in

will depend on a tradeoff between size, weight, and power requirements (SWaP).

tector to produce a random varying output signal.

(15 μm pitch) and the guard ring is not biased [46].

pitch map the dark current noise into an equivalent read noise.

The band gap [42] of the strained Inx Ga1-x As:InP structure can be tailored by varying the alloy composition during crystal growth according to the equation:

$$E\_g\left(eV\right) = \left(E\_{g\_{\rm Gain}} - \frac{\alpha\_{\rm Gads}T^2}{T + \beta\_{\rm Gads}} + \left(E\_{g\_{\rm Au}} - \frac{\alpha\_{\rm dust}T^2}{T + \beta\_{\rm Auts}} - E\_{g\_{\rm Gads}} + \frac{\alpha\_{\rm Gads}T^2}{T + \beta\_{\rm Gads}}\right)\mathbf{x} - 0.475\mathbf{x}(1 - \mathbf{x})\right) \tag{7}$$

Where Eg is the band gap in (eV), α and β are fitting parameters, and x is the In:As ratio. The cut-off wavelength can be calculated from the expression λco =hc / Egap .

The response can be extended to include the visible wavelength range by removing the InP substrate. There has been an intensive effort to develop InGaAs arrays for Low Light Level (LLL) SWIR imaging [42-47]. An example is in astrophysical space based observatories that are very demanding on the detectors due to the very low IR flux levels. Such low flux levels represent the detection of few photons over long integration times and, therefore, require extremely low dark current photodiodes hybridized to a high performance ROIC stage. For such LLL applications there are challenges ahead to further lower noise, reduce pixel size, fabricate larger arrays, achieve higher operating temperatures, and reduce production cost.

**Figure 5.** Dark current density versus read noise for different pixel pitches [46].

The spectral response of InGaAs diodes at room temperature is in the 0.9 – 1.67 μm wave‐ length range which matches the ambient night glow spectrum. Imaging under such low light conditions requires that the noise of the detector be extremely low. A significant por‐ tion of the noise is contributed by the dark current of the InGaAs detector and the readout noise. Dark current consists of unwanted thermally generated carriers that can cause the de‐ tector to produce a random varying output signal.

defects due to epitaxy and the decreased shunt resistance, due to a smaller band gap, de‐

The band gap [42] of the strained Inx Ga1-x As:InP structure can be tailored by varying the

( 0.475 (1 ) *GaAs InAs GaAs*

aa

æ ö = - + - -+ - - ç ÷ + ++ è ø (7)

bb

*GaAs InAs GaAs*

Where Eg is the band gap in (eV), α and β are fitting parameters, and x is the In:As ratio. The

The response can be extended to include the visible wavelength range by removing the InP substrate. There has been an intensive effort to develop InGaAs arrays for Low Light Level (LLL) SWIR imaging [42-47]. An example is in astrophysical space based observatories that are very demanding on the detectors due to the very low IR flux levels. Such low flux levels represent the detection of few photons over long integration times and, therefore, require extremely low dark current photodiodes hybridized to a high performance ROIC stage. For such LLL applications there are challenges ahead to further lower noise, reduce pixel size, fabricate larger arrays, achieve higher operating temperatures, and reduce production cost.

The spectral response of InGaAs diodes at room temperature is in the 0.9 – 1.67 μm wave‐ length range which matches the ambient night glow spectrum. Imaging under such low light conditions requires that the noise of the detector be extremely low. A significant por‐ tion of the noise is contributed by the dark current of the InGaAs detector and the readout

*GaAs InAs GaAs*

*T TT E eV E <sup>E</sup> <sup>E</sup> x xx T TT*

grade performance at the longer cutoff wavelengths.

*g g g g*

b

a

162 Optoelectronics - Advanced Materials and Devices

alloy composition during crystal growth according to the equation:

( ) 2 22

cut-off wavelength can be calculated from the expression λco =hc / Egap .

**Figure 5.** Dark current density versus read noise for different pixel pitches [46].

It is associated with interfacial, diffusional, G-R, and tunneling currents. The temperature dependence of the dark current is primarily due to the intrinsic carrier concentration which depends exponentially on the temperature. The dark current of the detector can be reduced through appropriate fabrication processes and device design. The impact of dark current noise as a function of read noise is shown in Figure 5, where the curves for different pixel pitch map the dark current noise into an equivalent read noise.

For a given read noise, the required dark current density increases as the pixel pitch is de‐ creased. The challenge is to maintain a low dark current density as the pixel pitch is re‐ duced. Simultaneously, the challenge for the read out circuit is to reduce the read noise. If the limitation is due to the detector and its noise level overwhelms the source signal, the sol‐ ution may be to use an external illuminator or cool the detector. The choice of either solution will depend on a tradeoff between size, weight, and power requirements (SWaP).

**Figure 6.** Dark current density at different temperatures using test structures on the wafer. Test arrays have 225 pixels (15 μm pitch) and the guard ring is not biased [46].

Applications involving situational awareness require FPAs to have more pixels (large format/high resolution) for increased surveillance coverage. For soldier portable and some airborne platforms, it is desirable to reduce the size of the pixel in order to satisfy the constraints of low SWaP and cost, without sacrificing performance. However,as the pixel pitch is scaled to smaller geometries, there is a tendency for the dark current den‐ sity to increase. The reduced pixel diameter can cause the sidewall related dark current to become more pronounced and overwhelm the area related contribution, resulting in an effective increase in the dark current density. The sidewall contribution can be avoid‐ ed with appropriate surface passivation of the exposed PN junction.

To achieve the constraints of low SWaP and cost, manufacturers are now developing In‐ GaAs detectors on 4" diameter wafers. For example, 16 - 1280x1024 InGaAs arrays with 15 μm pixels have been demonstrated on 4" InP wafers. To extend the spectral response of these detector arrays down to the UV band, the InP substrates are removed [47]. The test results for a backside illuminated 0.5 mm InGaAs detector is shown in Figure 7 [47]. The Quantum Efficiency (QE) achieved across the 1.2-1.6 μm band is about 80 % over a tempera‐ ture range of -65 °C to 40 °C.As a result of removing the InP substrate (see Figure 7 (b)), the

Continued effort is underway to demonstrate large format (>2Kx2K) and small pixel (<10 μm) InGaAs FPAs fora variety of room temperature, low light level (LLL) imaging appli‐ cations, such as night vision. These applications demand extremely low detector dark current and reduced ROIC noise to maintain performance, since the photon collection

Another approach to accomplish SWIR imaging under low light level (LLL) conditions is to use MCT detectors grown by either MBE or LPE techniques. For Hg1-xCdxTe, the alloy com‐ position can be fixed to provide an energy band gap equal to the longest wavelength to be measuredin the SWIR band. The larger energy band gap enables higher operating tempera‐ tures; MCT arrays operating at near 150 K have achieved BLIP limits at background levels as low as 1011 photons/sec/cm2 [48]. There are continued efforts to increase the operating tem‐

To operate in the SWIR band, the Cd mole fraction in In Hg1-xCdxTe is tailored to the appro‐

( ) <sup>4</sup> 2 3 *Eg eV* 0.302 1.930 5.35 10 (1 2 ) 0.810 0.832 *x Tx x x* - =- + + ´ - - + (8)

The absorption coefficient of MCT is very large and in order to have high responsivity, the rule of thumb is that the thickness of the MCT absorber layer should be at least equal to the cutoff wavelength. For the MCT material system, the choice of P-on-N polarity is generally driven by dark current considerations. The presence of Hg vacancies in the P base layer of an N-on-P diode degrades the minority carrier lifetime, resulting in larger dark currents. Nevertheless, to meet very low dark current requirements, diodes can be cooled down to

Figure 8 presents results for ion implanted P-on-N (MBE with ND= 1E16 cm-3, 4-μm thick) and N-on-P (LPE with NA=3-5E16 cm-3, 7 μm thick) diodes fabricated on lattice matched CZT

nologies, with CdTe and ZnSe passivation layers [51]. Very low, state of the art dark cur‐

/cm²). Diodes are based on planar tech‐

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QE is about 40 % over the entire visible band.

**5.2. SWIR Hg1-xCdxTe (MCT) Detector Arrays**

perature of SWIR MCT detectors [49].

priate energy band gap [50] according to the expression:

very low temperatures at the expense of SWaP.

in order to ensure a low dislocation density (mid 104

rents are observed over a wide temperature range as shown in Figure 8.

area is reduced [47].

As mentioned above, the dark current of the detector can be reduced through appropriate fabrication processes and device designs. By focusing on the growth conditions for the In‐ GaAs absorption layer, heterointerfaces and the passivation layer, researchers have been able to demonstrate dark current densities below 1.5 nA/cm2 at 7 °C and a bias of 100 mV for 15 μm pitch InGaAs arrays as shown Figure 6.

**Figure 7.** Spectral QE vs. wavelength at different temperatures measured for backside illuminated InGaAs photodio‐ des test array demonstrating Visible-Near IR response with a) InP substrate not removed and b) InP removed [47].

To achieve the constraints of low SWaP and cost, manufacturers are now developing In‐ GaAs detectors on 4" diameter wafers. For example, 16 - 1280x1024 InGaAs arrays with 15 μm pixels have been demonstrated on 4" InP wafers. To extend the spectral response of these detector arrays down to the UV band, the InP substrates are removed [47]. The test results for a backside illuminated 0.5 mm InGaAs detector is shown in Figure 7 [47]. The Quantum Efficiency (QE) achieved across the 1.2-1.6 μm band is about 80 % over a tempera‐ ture range of -65 °C to 40 °C.As a result of removing the InP substrate (see Figure 7 (b)), the QE is about 40 % over the entire visible band.

Continued effort is underway to demonstrate large format (>2Kx2K) and small pixel (<10 μm) InGaAs FPAs fora variety of room temperature, low light level (LLL) imaging appli‐ cations, such as night vision. These applications demand extremely low detector dark current and reduced ROIC noise to maintain performance, since the photon collection area is reduced [47].
