**7. SWIR detector technologies**

**Figure 22.** Schematic of detector array structure consisting of a SiGe /Si strained layer Superlattice grown on (001)

**Figure 23.** SEM image (45° tilt) of a Ge QD layer deposited on Si. The QDs are ~60 nm in diameter with a density of 1020 cm2. Also shown is a Cross-sectional TEM image of Ge/Si QDSL grown. Ge QDs appear with dark contrast com‐

silicon [38].

186 Optical Sensors - New Developments and Practical Applications

pared to Si barriers. [38].

The SWIR band (0.9-2.5 µm) bridges the spectral gap between the visible and thermal bands in the electromagnetic spectrum. In this spectral band, the primary phenomenology of interest 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: it can detect reflected light, offering more intuitive, visible-like images; is better suited for imaging in adverse environ‐ ments 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 equipment; and can generate digital video outputs and thus offering a more dynamic range than traditional image intensifier night vision equipment. Under low light conditions, the sensitivity of the focal plane array is ultimately determined by the R0A product of the photodiode.

#### **7.1. Inx Ga1-xAs 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: Molec‐ ular 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 [40-42].

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, degrade performance at the longer cutoff wavelengths. [43].

The band gap [7-1] of the strained Inx Ga1-x As: InP structure can be tailored by varying the alloy composition during crystal growth according to the equation:

$$E\_{\mathcal{g}}\text{ (e}V\text{)}=\left(E\_{\mathcal{g}\_{\text{CaAs}}} - \frac{a\_{\text{CaAs}}T}{T + \beta\_{\text{CaAs}}} + \left(E\_{\mathcal{g}\_{\text{InAs}}} - \frac{a\_{\text{InAs}}T}{T + \beta\_{\text{InAs}}} - E\_{\mathcal{g}\_{\text{CaAs}}} + \frac{a\_{\text{CaAs}}T}{T + \beta\_{\text{CaAs}}}\right)\text{x} - 0.475\text{x}\left(1 - \text{x}\right)\right)$$

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

detectors [5-6]. Figure 26 presents a plot of dark current density, measured at 20 °C, for eight different, 300 x 10 pixel test arrays distributed across a 3" wafer. The average dark current

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Further effort is underway to demonstrate large format (>1Kx1K) and small pixel (<20µm) InGaAs focal plane arrays (FPAs) for a variety of low light level (LLL) imaging applications such as night vision. These applications demand extremely low detector dark current and Si

Recent work [47] has demonstrated significant progress in InGaAs detector array development on a 4" wafer as shown in figure 27; and also reducing dark current density for 10-20µm pixel arrays, (3) developing sub-10µm pixel array technology and demonstrating the feasibility of making 5µm pixel arrays, and (4) reducing the capacitance of small pixels [47]. Figure 28 demonstrates recent results for spectral quantum efficiency (QE) as a function of wavelength measured on backside illuminated InGaAs photodiodes test array at different temperatures

demonstrating Visible-Near IR response with InP substrate removed [47].

**Figure 24.** Dark current density versus read noise for different pixel pitches [44].

density at -100 mV was 2.95 nA/cm2.

read-out integration circuit (ROIC) noise [47].

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 [40-46]. 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 wavelength range which matches with the ambient night glow spectrum. Imaging under such low light conditions requires that the noise of the detector be extremely low. A significant portion 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 detector to produce a random varying output signal.

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 5.1, 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 decreased. The challenge is to maintain a low dark current density as the pixel pitch is reduced. 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 solution 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).

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 InGaAs absorption layer, heterointerfaces and the passivation layer, researchers have been able to demonstrate dark current density below 1.5 nA/cm2 at 77°C for 15 µm pitch arrays as shown Figure 24.

In scaling to small pixel pitch, further effort is continued to develop wafer processing param‐ eters and methods that reduce surface related perimeter effects and enable small pixel pitch InGaAs detectors with dark current densities comparable to large (25 µm) pixel pitches detectors [5-6]. Figure 26 presents a plot of dark current density, measured at 20 °C, for eight different, 300 x 10 pixel test arrays distributed across a 3" wafer. The average dark current density at -100 mV was 2.95 nA/cm2.

*Eg* (*eV* )=(*EgGaAs*

production cost.

Figure 24.


produce a random varying output signal.

map the dark current noise into an equivalent read noise.

*<sup>T</sup>* <sup>+</sup> *<sup>β</sup>GaAs* <sup>+</sup> (*EgInAs*

188 Optical Sensors - New Developments and Practical Applications


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

*<sup>T</sup>* <sup>+</sup> *<sup>β</sup>InAs* - *EgGaAs*

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 [40-46]. 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

The spectral response of InGaAs diodes at room temperature is in the 0.9 – 1.67 µm wavelength range which matches with the ambient night glow spectrum. Imaging under such low light conditions requires that the noise of the detector be extremely low. A significant portion 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 detector to

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 5.1, where the curves for different pixel pitch

For a given read noise, the required dark current density increases as the pixel pitch is decreased. The challenge is to maintain a low dark current density as the pixel pitch is reduced. 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 solution may be to use an external illuminator or cool the detector. The choice of either solution will

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 InGaAs absorption layer, heterointerfaces and the passivation layer, researchers have been able to demonstrate dark current density below 1.5 nA/cm2 at 77°C for 15 µm pitch arrays as shown

In scaling to small pixel pitch, further effort is continued to develop wafer processing param‐ eters and methods that reduce surface related perimeter effects and enable small pixel pitch InGaAs detectors with dark current densities comparable to large (25 µm) pixel pitches

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

<sup>+</sup> *<sup>α</sup>GaAs<sup>T</sup>* <sup>2</sup> *T* + *βGaAs*

)*x* - 0.475*x*(1 - *x*)

Further effort is underway to demonstrate large format (>1Kx1K) and small pixel (<20µm) InGaAs focal plane arrays (FPAs) for a variety of low light level (LLL) imaging applications such as night vision. These applications demand extremely low detector dark current and Si read-out integration circuit (ROIC) noise [47].

Recent work [47] has demonstrated significant progress in InGaAs detector array development on a 4" wafer as shown in figure 27; and also reducing dark current density for 10-20µm pixel arrays, (3) developing sub-10µm pixel array technology and demonstrating the feasibility of making 5µm pixel arrays, and (4) reducing the capacitance of small pixels [47]. Figure 28 demonstrates recent results for spectral quantum efficiency (QE) as a function of wavelength measured on backside illuminated InGaAs photodiodes test array at different temperatures demonstrating Visible-Near IR response with InP substrate removed [47].

**Figure 24.** Dark current density versus read noise for different pixel pitches [44].

**Figure 25.** Dark current density at different temperatures using test structures on the wafer. Test arrays have 225 pix‐ els (15 μm pitch) and the guard ring is not biased [45].

**Figure 27.** Experimental 1280x1024/15μm arrays on 4" wafer surrounded by various test mini-arrays with pitch sizes

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**Figure 28.** Spectral QE vs. wavelength at different temperatures measured for backside illuminated InGaAs photodio‐

des test array demonstrating Visible-Near IR response with InP substrate removed [47].

of 5-20μm [47].

**Figure 26.** Experimental results for InGaAs test array demonstrating dark current density for eight separate 300 x 10, 15 μm pitch pixel test arrays measured across a wafer. The average dark current density for the test arrays at 100 mV reverse bias is 2.95 nA/cm2 at 20 °C [ 45].

**Figure 27.** Experimental 1280x1024/15μm arrays on 4" wafer surrounded by various test mini-arrays with pitch sizes of 5-20μm [47].

**Figure 25.** Dark current density at different temperatures using test structures on the wafer. Test arrays have 225 pix‐

**Figure 26.** Experimental results for InGaAs test array demonstrating dark current density for eight separate 300 x 10, 15 μm pitch pixel test arrays measured across a wafer. The average dark current density for the test arrays at 100 mV

els (15 μm pitch) and the guard ring is not biased [45].

190 Optical Sensors - New Developments and Practical Applications

reverse bias is 2.95 nA/cm2 at 20 °C [ 45].

**Figure 28.** Spectral QE vs. wavelength at different temperatures measured for backside illuminated InGaAs photodio‐ des test array demonstrating Visible-Near IR response with InP substrate removed [47].
