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

an effective increase in the dark current density. The sidewall contribution can be avoid‐

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

**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].

ed with appropriate surface passivation of the exposed PN junction.

15 μm pitch InGaAs arrays as shown Figure 6.

164 Optoelectronics - Advanced Materials and Devices

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‐ perature of SWIR MCT detectors [49].

To operate in the SWIR band, the Cd mole fraction in In Hg1-xCdxTe is tailored to the appro‐ priate energy band gap [50] according to the expression:

$$\text{Eg}\left(eV\right) = \text{--} \ 0.302 \ + 1.930\text{x} + \ 5.35 \times 10^{-4}T \ (1 - 2\text{x}) - 0.810x^2 + 0.832x^3 \tag{8}$$

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 very low temperatures at the expense of SWaP.

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 in order to ensure a low dislocation density (mid 104 /cm²). Diodes are based on planar tech‐ nologies, with CdTe and ZnSe passivation layers [51]. Very low, state of the art dark cur‐ rents are observed over a wide temperature range as shown in Figure 8.

vice size and novel device architectures, such as Superlattice, Quantum dot and Buried junc‐ tion designs [54]. Furthermore, some of these approaches have the potential of extending the wavelength of operation beyond 1.8-2.0 microns. The challenge is to take advantage of these

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A proposed diagnostic device structure to evaluate the impact of various fabrication meth‐ odologies to reduce leakage currents and produce higher detector performance in SiGe/Si is shown in Figure 9. The structure can help to assess the following: ability to grow high quali‐ ty/low defect density Ge on Si; layer thickness necessary for minimal topological and defect density requirements; isolation of defect states at the Ge/oxide interface from the signal car‐ rying layers; and optimum doping and thickness of the P-type Ge layer under the oxide to isolate interface states and lateral leakage current that could result between the highly dop‐

p+ - Si Substrate (100)

Dark currents in SiGe detectors can be reduced by reducing the pixel size, since dark cur‐ rents track with thevolume of the pixel. Reductions in size are advantageous for resolution; however, for low light level conditions, such as nightglow, a large pixel size or at least a large collection area is required. The I-V characteristics of photodiodes with different areas fabricated on 2 μm thick intrinsic epitaxial Ge layers are shown in Figure 10(a). The curves indicate that the dark current is lowered as the device area is reduced. The responsivity as a function of wavelength for a 100 μm x 100 μm diode without an anti-reflection coating is given in Figure 10(b). The reverse leakage currentat a reverse bias of 1 vol tis 32 mA/cm2

.

**Figure 9.** SiGe/Si diagnostic structure to evaluate impact of various fabrication methodologies.

p+ - Ge seed layer

Photon Absorbtion Layer

n+ - poly SiGe Layer p - Ge Layer p - Ge Layer Oxide Oxide

n+ - Ge Layer

>1 mm Intrinsic Ge Layer

innovative device designs and reduce the dark currents to 1-10 nA cm-2.

Ge region used for contacts and the lighter doped P-type Ge region.

ed N+

**Figure 8.** a) I-V curves measured on isolated 10 μm N-on-P implanted diodes; Cutoff=2.0 um. b) Measured dark cur‐ rents and comparison with literature data at 2.5 μm cut-off [51].

#### **5.3. Si1-x Gex(SiGe) Detector Arrays**

Like the other two alloy semiconductors mentioned above, SiGe is another example of mate‐ rial that can be used for the fabrication of SWIR detectors. The key attractive feature of SiGe IR detectors is that they can be fabricated on large diameter Si substrates with size as large as 12-inch diameter using standard integrated circuit processing techniques. Furthermore, the SiGe detectors can be directly integrated onto low noise Si ROICs to yield low SWaP, low cost and highly uniform IR FPAs. The primary motivation for SiGe SWIR FPA develop‐ ment is the CMOS-like fabrication allowing for very low cost technology.

Some of the earlier attempts in developing SiGe IR detectors focused on their LWIR applica‐ tionsby using internal photoemmision [52-53]. Renewed efforts are now developing these detectors for application in the NIR-SWIR band [54]. For the SiGe material to respond to the SWIR band, its cutoff wavelength is tuned by adjusting the SiGe alloy composition. Si and Ge have the same crystallographic structure and both materials can be alloyed with various Ge concentration. The lattice constant of Ge is 4.18% larger than that of Si, and for a Si1-x Gex alloy ("a" for alloy) the lattice constant does not exactly follow Vegard's law. The relative change of the lattice constant is given by [55]:

$$\mathrm{aSi}\_{\mathrm{l-x}}\mathrm{Ge}\_{\mathrm{x}} = \begin{array}{c} 0.5431 \ + \ 0.01992\mathbf{x} + \ 0.0002733\mathbf{x}^{2} \text{(nm)} \end{array} \tag{9}$$

For a Si1-x Gex layer with x > 0 on a Si substrate means that the layer is under compressive stress. A perfect epitaxial growth of such a strained heteroepitaxial layer can be achieved as long as its thickness does not exceed a critical thickness for stability. Beyond the critical thickness, the strain is relaxed through the formation of misfit dislocations, which can cause an increase in the dark current. Several approaches have been proposed to reduce the dark current in SiGe detector arrays by several orders of magnitude.These include fabrication methodologies, de‐ vice size and novel device architectures, such as Superlattice, Quantum dot and Buried junc‐ tion designs [54]. Furthermore, some of these approaches have the potential of extending the wavelength of operation beyond 1.8-2.0 microns. The challenge is to take advantage of these innovative device designs and reduce the dark currents to 1-10 nA cm-2.

A proposed diagnostic device structure to evaluate the impact of various fabrication meth‐ odologies to reduce leakage currents and produce higher detector performance in SiGe/Si is shown in Figure 9. The structure can help to assess the following: ability to grow high quali‐ ty/low defect density Ge on Si; layer thickness necessary for minimal topological and defect density requirements; isolation of defect states at the Ge/oxide interface from the signal car‐ rying layers; and optimum doping and thickness of the P-type Ge layer under the oxide to isolate interface states and lateral leakage current that could result between the highly dop‐ ed N+ Ge region used for contacts and the lighter doped P-type Ge region.

**Figure 8.** a) I-V curves measured on isolated 10 μm N-on-P implanted diodes; Cutoff=2.0 um. b) Measured dark cur‐

Like the other two alloy semiconductors mentioned above, SiGe is another example of mate‐ rial that can be used for the fabrication of SWIR detectors. The key attractive feature of SiGe IR detectors is that they can be fabricated on large diameter Si substrates with size as large as 12-inch diameter using standard integrated circuit processing techniques. Furthermore, the SiGe detectors can be directly integrated onto low noise Si ROICs to yield low SWaP, low cost and highly uniform IR FPAs. The primary motivation for SiGe SWIR FPA develop‐

Some of the earlier attempts in developing SiGe IR detectors focused on their LWIR applica‐ tionsby using internal photoemmision [52-53]. Renewed efforts are now developing these detectors for application in the NIR-SWIR band [54]. For the SiGe material to respond to the SWIR band, its cutoff wavelength is tuned by adjusting the SiGe alloy composition. Si and Ge have the same crystallographic structure and both materials can be alloyed with various Ge concentration. The lattice constant of Ge is 4.18% larger than that of Si, and for a Si1-x Gex alloy ("a" for alloy) the lattice constant does not exactly follow Vegard's law. The relative

For a Si1-x Gex layer with x > 0 on a Si substrate means that the layer is under compressive stress. A perfect epitaxial growth of such a strained heteroepitaxial layer can be achieved as long as its thickness does not exceed a critical thickness for stability. Beyond the critical thickness, the strain is relaxed through the formation of misfit dislocations, which can cause an increase in the dark current. Several approaches have been proposed to reduce the dark current in SiGe detector arrays by several orders of magnitude.These include fabrication methodologies, de‐

1x x aSi Ge 0.5431 0.01992x 0.0002733x (nm) - =+ + (9)

2

ment is the CMOS-like fabrication allowing for very low cost technology.

rents and comparison with literature data at 2.5 μm cut-off [51].

change of the lattice constant is given by [55]:

**5.3. Si1-x Gex(SiGe) Detector Arrays**

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**Figure 9.** SiGe/Si diagnostic structure to evaluate impact of various fabrication methodologies.

Dark currents in SiGe detectors can be reduced by reducing the pixel size, since dark cur‐ rents track with thevolume of the pixel. Reductions in size are advantageous for resolution; however, for low light level conditions, such as nightglow, a large pixel size or at least a large collection area is required. The I-V characteristics of photodiodes with different areas fabricated on 2 μm thick intrinsic epitaxial Ge layers are shown in Figure 10(a). The curves indicate that the dark current is lowered as the device area is reduced. The responsivity as a function of wavelength for a 100 μm x 100 μm diode without an anti-reflection coating is given in Figure 10(b). The reverse leakage currentat a reverse bias of 1 vol tis 32 mA/cm2 .

mesas are etched to provide isolation and the substrate contact is formed. The etched mesa can also be passivated to minimize surface recombination as indicated in Figure 11. The de‐ vice shown in Figure 11 uses substrate sideillumination, as is needed for use in FPA arrays,

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**Figure 12.** (a) 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. (b) Cross-sectional TEM image of Ge/Si QDSL. Ge QDs appear with dark contrast compared to Si barriers [54].

The strained-layer superlattice and quantum dot superlattice (QDSL) in the SiGe material system have the potential of developing Vis-NIR detector arrays with longer cutoff wave‐ length and potentially lower dark current. The advantage of quantum dots is the potential to exploit the optical properties of Ge while avoiding dislocation formation. Ge QDs grown on Si in Stranski-Krastanov mode can be deposited well beyond the critical thickness without

Figure 12 (a) shows the SEM image of an array of Ge nanodots grown by MOCVD. These dots are typically 50-75 nm in diameter with area coverage of ~20%. To increase optical absorption and sensitivity, MOCVD-based growth techniques are being developed for the deposition of Ge/Si quantum dot superlattices (QDSLs), where Ge QDs are alternated with thin (10-30 nm) Si

**6. MWIR/LWIR Detector Array Technologies and Applications (InSb,**

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‐

barrier layers. A cross-sectional TEM image of QDSLs is shown in Figure 12(b).

dislocation nucleation [56].

**HgCdTe, HOT, SLS and Bolometers)**

and short wavelength response can be improved by thinning the Si substrate.

**Figure 10.** a) Measured room temperature I-V characteristics for large area diodes with 20, 50 and 200 micron unit cell. The inset shows the schematic device cross section. b) The spectral response data for SiGe detector [56].

**Figure 11.** Schematic of detector array structure consisting of a SiGe /Si strained layer Superlattice grown on (001) silicon [54].

Figure 11 shows the Strained-Layer Superlattice (SLS) structure being evaluated for longer detector array response to 2 microns. It consists of SiGe quantum wells and Si barrier layers, grown on p-type (001) Si substrates. Super lattices having differing Si barrier and Ge well thicknesses to control the strain are grown to optimize wavelengthresponse and dark cur‐ rent. The SiGe well thicknesses are kept below the critical layer thickness fordislocation for‐ mation. To complete the structure, the undopedsuperlattice is capped with a thin n+ Si caplayer to form the p-n junction. After growth the devices are patterned with a top contact, mesas are etched to provide isolation and the substrate contact is formed. The etched mesa can also be passivated to minimize surface recombination as indicated in Figure 11. The de‐ vice shown in Figure 11 uses substrate sideillumination, as is needed for use in FPA arrays, and short wavelength response can be improved by thinning the Si substrate.

**Figure 12.** (a) 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. (b) Cross-sectional TEM image of Ge/Si QDSL. Ge QDs appear with dark contrast compared to Si barriers [54].

**Figure 10.** a) Measured room temperature I-V characteristics for large area diodes with 20, 50 and 200 micron unit cell. The inset shows the schematic device cross section. b) The spectral response data for SiGe detector [56].

**Figure 11.** Schematic of detector array structure consisting of a SiGe /Si strained layer Superlattice grown on

Figure 11 shows the Strained-Layer Superlattice (SLS) structure being evaluated for longer detector array response to 2 microns. It consists of SiGe quantum wells and Si barrier layers, grown on p-type (001) Si substrates. Super lattices having differing Si barrier and Ge well thicknesses to control the strain are grown to optimize wavelengthresponse and dark cur‐ rent. The SiGe well thicknesses are kept below the critical layer thickness fordislocation for‐ mation. To complete the structure, the undopedsuperlattice is capped with a thin n+ Si caplayer to form the p-n junction. After growth the devices are patterned with a top contact,

(001) silicon [54].

168 Optoelectronics - Advanced Materials and Devices

The strained-layer superlattice and quantum dot superlattice (QDSL) in the SiGe material system have the potential of developing Vis-NIR detector arrays with longer cutoff wave‐ length and potentially lower dark current. The advantage of quantum dots is the potential to exploit the optical properties of Ge while avoiding dislocation formation. Ge QDs grown on Si in Stranski-Krastanov mode can be deposited well beyond the critical thickness without dislocation nucleation [56].

Figure 12 (a) shows the SEM image of an array of Ge nanodots grown by MOCVD. These dots are typically 50-75 nm in diameter with area coverage of ~20%. To increase optical absorption and sensitivity, MOCVD-based growth techniques are being developed for the deposition of Ge/Si quantum dot superlattices (QDSLs), where Ge QDs are alternated with thin (10-30 nm) Si barrier layers. A cross-sectional TEM image of QDSLs is shown in Figure 12(b).
