**2. UV, Visible and infrared spectrum and bands of interest**

The Ultraviolet spectrum has been of interest for a variety of sensors for defense and com‐ mercial applications. The UV band is from 250-400 nanometers as shown in the figure 1. This band can be further divided into UVA and UVB bands. Each of these bands has applications for sensors, detectors and LED applications.

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 imaging 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 infrared (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 2. The MWIR- LWIR wavebands are important for the imaging of objects that emit thermal radiation, while the NIR-SWIR bands are good for imaging

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167

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 external illumination source. Both NIR and SWIR imaging systems can take advantage of sunlight, 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 solution 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

Imagery for identification of targets at various distances uses visible cameras, image intensi‐ fiers, shortwave IR cameras and long wave uncooled cameras. Each have distinct advantages and disadvantages and are each useful under specific sets of conditions such as light level, thermal conditions, and level of atmospheric obscuration. The shortest wavelength is desired

Visible cameras, if adequate light level is present, can provide high resolution, but for long range identification even under moonlit and starlit illuminations, long integration times and large optics are required and dust, smoke and fog easily defeat a single visible camera. Image intensifiers and SWIR cameras are useful in many conditions as the SWIR penetrates fog easily but requires fairly clear night skies for the upper atmospheric airglow light source, and image intensifiers require a certain level of celestial (starlight, moonlight) or light pollution irradiance. Both the SWIR and image intensifiers are limited by the diffraction resolution of the NIR to

scenes that reflect light, similar to visible light.

consideration of eye-safety issues is required.

**3.1. Applications of UV imaging technology**

SWIR wavelengths [5-6].

**3. Ultraviolet nanostructured detector array development**

for spatial resolution which allows for small pixels and large formats. [2- 6]

**Figure 1.** Overview of UV and Visible Spectral Band [1]

**Figure 2.** Definition of IR Spectral Band [1].

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 imaging 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 infrared (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 2. The MWIR- LWIR wavebands are important 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.

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 external illumination source. Both NIR and SWIR imaging systems can take advantage of sunlight, 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 solution 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.

#### **3. Ultraviolet nanostructured detector array development**

#### **3.1. Applications of UV imaging technology**

**2. UV, Visible and infrared spectrum and bands of interest**

for sensors, detectors and LED applications.

166 Optical Sensors - New Developments and Practical Applications

**Figure 1.** Overview of UV and Visible Spectral Band [1]

**Figure 2.** Definition of IR Spectral Band [1].

The Ultraviolet spectrum has been of interest for a variety of sensors for defense and com‐ mercial applications. The UV band is from 250-400 nanometers as shown in the figure 1. This band can be further divided into UVA and UVB bands. Each of these bands has applications

> Imagery for identification of targets at various distances uses visible cameras, image intensi‐ fiers, shortwave IR cameras and long wave uncooled cameras. Each have distinct advantages and disadvantages and are each useful under specific sets of conditions such as light level, thermal conditions, and level of atmospheric obscuration. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. [2- 6]

> Visible cameras, if adequate light level is present, can provide high resolution, but for long range identification even under moonlit and starlit illuminations, long integration times and large optics are required and dust, smoke and fog easily defeat a single visible camera. Image intensifiers and SWIR cameras are useful in many conditions as the SWIR penetrates fog easily but requires fairly clear night skies for the upper atmospheric airglow light source, and image intensifiers require a certain level of celestial (starlight, moonlight) or light pollution irradiance. Both the SWIR and image intensifiers are limited by the diffraction resolution of the NIR to SWIR wavelengths [5-6].

For optimal resolution, the visible or ultraviolet spectrum is preferable; however, active (laser) illumination is required for long-range night imaging. Covert UV illumination is preferred over the visible and the atmosphere transmits fairly well at the longer UV wavelengths. The covert active system for high-resolution identification modeled in this paper consists of a UV laser source and a silicon CCD, AlGaN or AlGaN APD focal plane array with pixels as small as 4 microns that are spectrally tuned for the solar-blind region of the UV spectrum. The solarblind region is optimal as virtually all of the solar radiation is absorbed at the higher altitudes leaving a pitch dark terrain even in bright day, yet for sea-level path lengths of 1 km and shorter; the UV atmospheric transmittance is still acceptable.

*eLret*,*ti* = *ti*

Nbins/f:

region.

*ητ<sup>o</sup> PLcw*

If we allow for frame summing:

*λ hc τa Ωpix Ω<sup>L</sup>*

*ρtarτ<sup>a</sup>*

*ητ<sup>o</sup>*

*ητ<sup>o</sup>*

*ApixAo <sup>π</sup> <sup>f</sup>* <sup>2</sup> *<sup>τ</sup><sup>a</sup>*

*ApixAo <sup>π</sup> <sup>f</sup>* <sup>2</sup> *<sup>τ</sup><sup>a</sup>*

For pulse laser operation and using tbin which equals tpulse and the number of bins per frame

2

To model the sensor and system performance, we have assumed the pixel size for a high sensitivity, detector size of 5-20 microns for the UV detector array. The fill factor of 70% is assumed typical for these small pixels. Typical quantum efficiencies have been assumed to be in the 70% range for the PIN diode and APD [5-6]. The model uses as default, an amp noise of 15 electrons per frame time, a dark current of 1e-15 amps for a 5 micron pixel or 4 nA/cm2

200 electrons or about 14 noise electrons, and scene noise is effectively zero in the solar-blind

The model from the MODTRAN runs shown in figure 3, the daytime irradiance in the UV is insignificant in the solar-blind region. The drop-off from 0.30 microns to 0.26 microns illus‐ trates the requirement for a UV detector with spectral response is in the solar-blind region. Figure 4 shows the UV spectral radiance at midday and the comparative laser illumination of the target at 1 km for a 6 milliradian beam divergence for powers of 1 mW and 10 mW. The left plot in the figure shows that the transmittance improves with longer UV wavelengths for

To achieve high-resolution day-night imaging and identification of targets, the following conditions and requirements must be met. While linear detection (no APD and no laser illumination) is fine for muzzle flashes and images of nearby combatants illuminated by live

all three levels of aerosols and is sufficient for 1 km lengths in our solar-blind region.

*PLpulse*(*λ* / *hc*) *<sup>θ</sup>fdiv* <sup>2</sup>

*eLret*,*ti* = *ti*

*eLret*,*ti*<sup>+</sup> = *N fs ti*

*eLimage* = *Nbins*/ *<sup>f</sup> ητ<sup>o</sup>*

*3.1.2. Systems performance metrics for UV systems design*

<sup>2</sup> *Ao <sup>π</sup><sup>R</sup>* <sup>2</sup> <sup>=</sup> *ti*

Or when separated into detector/optics, atmosphere, laser and target attributes:

*ApixAo <sup>π</sup> <sup>f</sup>* <sup>2</sup> *<sup>τ</sup><sup>a</sup>*

2 *ΦLcw θfdiv* 2

> 2 *ΦLcw θfdiv* 2

*ητ<sup>o</sup> PLcw*

*λ hc τa*

4*ρtar*

4*ρtar*

4*ρtar*

4*Apix πθfdiv*

Nanostructured Detector Technology for Optical Sensing Applications

<sup>2</sup> *<sup>f</sup>* <sup>2</sup> *<sup>ρ</sup>tarτ<sup>a</sup>*

*<sup>π</sup><sup>R</sup>* <sup>2</sup> (5)

*<sup>π</sup><sup>R</sup>* <sup>2</sup> (6)

*<sup>π</sup><sup>R</sup>* <sup>2</sup> (7)

or

<sup>2</sup> *Ao*

http://dx.doi.org/10.5772/58349

*<sup>π</sup><sup>R</sup>* <sup>2</sup> (4)

169

This combination is ideal for exploitation by a UV illuminator and UV FPA sensor. Current UV lasers can provide either continuous or pulsed energy at levels detectable by solarblind UV detectors under relatively small optics and at 30 Hz frame rates, providing realtime high-resolution (on the order of 1 cm at 1 km) imagery. At these illumination levels and target ranges, both standard PN, PIN and APD UV detectors and silicon CCD's can be used for target identification. The model has been developed and used to include the combined effects of detector and electronics, atmospheric transmittance and UV back‐ ground radiance, target size, range and reflectance, and UV laser attributes to simulate and predict both CW and pulsed laser imaging performance and to assist in the design of this prototype system [6].

#### *3.1.1. Model development for passive and active UV systems design*

The general equations for SNR prediction for laser illumination and APD are derived.

$$\text{SNR} = \frac{G e\_{Lret}}{\left[\text{F}^2 \text{G}^2 (e\_{Lret} + e\_{bk} + e\_{dk}) + (\hat{e}\_{n,amp})^2\right]^{1/2}} \tag{1}$$

Where G is the APD gain, F is the excess noise, the noise electron terms are the laser return shot noise, the scene noise, the dark current noise and the amp noise

Two special and frequently occurring cases are (2) for the laser power noise limited case and (3) for the amp noise limited case:

$$\text{SNR} = \frac{\mathbf{I} \mathbf{c}\_{Lrt} \mathbf{J}^{1/2}}{F} \tag{2}$$

$$\text{SNR} = \frac{\text{G}e\_{Lret}}{(\tilde{e}\_{n,amp})} \tag{3}$$

The laser return in electrons for cw assuming lambertian reflection is:

$$\boldsymbol{e}\_{L\boldsymbol{\pi}t,\boldsymbol{\mu}} = \mathsf{I}\_{i}\boldsymbol{t}\_{l}\boldsymbol{\eta}\boldsymbol{\tau}\_{o}\mathbf{II}\boldsymbol{P}\_{L\boldsymbol{\alpha}}\frac{\boldsymbol{\lambda}}{\boldsymbol{h}\boldsymbol{c}}\mathbf{I}\boldsymbol{\tau}\_{a}\frac{\boldsymbol{\Omega}\_{pi}}{\boldsymbol{\Omega}\_{L}}\boldsymbol{\rho}\_{\mathrm{tar}}\boldsymbol{\tau}\_{a}^{2}\frac{\boldsymbol{A}\_{o}}{\boldsymbol{\pi}\boldsymbol{\mathsf{R}}^{2}} = \mathsf{I}\_{i}\boldsymbol{t}\_{l}\boldsymbol{\eta}\boldsymbol{\tau}\_{o}\mathbf{II}\boldsymbol{P}\_{L\boldsymbol{\alpha}}\frac{\boldsymbol{\lambda}}{\boldsymbol{h}\boldsymbol{c}}\mathbf{I}\boldsymbol{\tau}\_{a}\frac{4A\_{\mathrm{pix}}}{\pi\boldsymbol{\theta}\frac{2}{\boldsymbol{h}\boldsymbol{h}\boldsymbol{v}}f^{2}}\boldsymbol{\rho}\_{\mathrm{tar}}\boldsymbol{\tau}\_{a}^{2}\frac{A\_{\boldsymbol{o}}}{\pi\boldsymbol{\mathsf{R}}^{2}}\tag{4}$$

Or when separated into detector/optics, atmosphere, laser and target attributes:

$$e\_{Lret,ti} = \mathsf{f}\,t\_i\eta\,\tau\_o \frac{A\_{pix}A\_o}{\pi f^{\frac{2}{2}}}\,\mathsf{I}\,\tau\_a^2 \frac{\mathsf{I}\,\mathcal{O}\_{Luc}\mathsf{J}}{\mathcal{O}\_{fdiv}^2}\frac{4\rho\_{tar}}{\pi R^2} \tag{5}$$

If we allow for frame summing:

For optimal resolution, the visible or ultraviolet spectrum is preferable; however, active (laser) illumination is required for long-range night imaging. Covert UV illumination is preferred over the visible and the atmosphere transmits fairly well at the longer UV wavelengths. The covert active system for high-resolution identification modeled in this paper consists of a UV laser source and a silicon CCD, AlGaN or AlGaN APD focal plane array with pixels as small as 4 microns that are spectrally tuned for the solar-blind region of the UV spectrum. The solarblind region is optimal as virtually all of the solar radiation is absorbed at the higher altitudes leaving a pitch dark terrain even in bright day, yet for sea-level path lengths of 1 km and

This combination is ideal for exploitation by a UV illuminator and UV FPA sensor. Current UV lasers can provide either continuous or pulsed energy at levels detectable by solarblind UV detectors under relatively small optics and at 30 Hz frame rates, providing realtime high-resolution (on the order of 1 cm at 1 km) imagery. At these illumination levels and target ranges, both standard PN, PIN and APD UV detectors and silicon CCD's can be used for target identification. The model has been developed and used to include the combined effects of detector and electronics, atmospheric transmittance and UV back‐ ground radiance, target size, range and reflectance, and UV laser attributes to simulate and predict both CW and pulsed laser imaging performance and to assist in the design of this

The general equations for SNR prediction for laser illumination and APD are derived.

Where G is the APD gain, F is the excess noise, the noise electron terms are the laser return

Two special and frequently occurring cases are (2) for the laser power noise limited case and

1/2

*SNR* <sup>=</sup> *eLret*

*SNR* <sup>=</sup> *GeLret*

(*eLret* <sup>+</sup> *ebk* <sup>+</sup> *edk* ) <sup>+</sup> (*e*˜ *<sup>n</sup>*,*amp*)<sup>2</sup> 1/2 (1)

*<sup>F</sup>* (2)

(*e*˜ *<sup>n</sup>*,*amp*) (3)

shorter; the UV atmospheric transmittance is still acceptable.

168 Optical Sensors - New Developments and Practical Applications

*3.1.1. Model development for passive and active UV systems design*

*F* 2 *G* <sup>2</sup>

*SNR* <sup>=</sup> *GeLret*

shot noise, the scene noise, the dark current noise and the amp noise

The laser return in electrons for cw assuming lambertian reflection is:

prototype system [6].

(3) for the amp noise limited case:

$$\sigma\_{Lret,ti+} = \mathbf{N}\_{fs} \mathbf{L} t\_i \eta \tau\_o \frac{A\_{pix} A\_o}{\pi f^2} \mathbf{J} \tau\_a^2 \frac{\mathbf{L} \otimes\_{Lew} \mathbf{J}}{\Theta\_{fdiv}^2} \frac{4 \rho\_{tur}}{\pi \mathbf{R}^2} \tag{6}$$

For pulse laser operation and using tbin which equals tpulse and the number of bins per frame Nbins/f:

$$\sigma\_{Limage} = \mathsf{IN}\_{bins/f} \eta \tau\_o \frac{A\_{pix} A\_o}{\pi f^2} \mathsf{I} \tau\_a^2 \frac{P\_{Lpulse}(\lambda \mid hc)}{\Theta\_{fdiv}^2} \frac{4 \rho\_{tar}}{\pi R^2} \tag{7}$$

#### *3.1.2. Systems performance metrics for UV systems design*

To model the sensor and system performance, we have assumed the pixel size for a high sensitivity, detector size of 5-20 microns for the UV detector array. The fill factor of 70% is assumed typical for these small pixels. Typical quantum efficiencies have been assumed to be in the 70% range for the PIN diode and APD [5-6]. The model uses as default, an amp noise of 15 electrons per frame time, a dark current of 1e-15 amps for a 5 micron pixel or 4 nA/cm2 or 200 electrons or about 14 noise electrons, and scene noise is effectively zero in the solar-blind region.

The model from the MODTRAN runs shown in figure 3, the daytime irradiance in the UV is insignificant in the solar-blind region. The drop-off from 0.30 microns to 0.26 microns illus‐ trates the requirement for a UV detector with spectral response is in the solar-blind region. Figure 4 shows the UV spectral radiance at midday and the comparative laser illumination of the target at 1 km for a 6 milliradian beam divergence for powers of 1 mW and 10 mW. The left plot in the figure shows that the transmittance improves with longer UV wavelengths for all three levels of aerosols and is sufficient for 1 km lengths in our solar-blind region.

To achieve high-resolution day-night imaging and identification of targets, the following conditions and requirements must be met. While linear detection (no APD and no laser illumination) is fine for muzzle flashes and images of nearby combatants illuminated by live fire (a millisecond event), laser illumination is required for cold targets (facial recognition, profile recognition). A continuous laser and 33 msec integrations are adequate if enough laser power is available. If not, a pulsed laser with nanosecond integrations and APD detectors are required to reduce atmosphere scatter and improve detector sensitivity.

**3.2. ZnO / MgZnO nanostructures for UV applications**

potentially include defense and commercial applications [13].

missile plumes and flames emitting in this region.

system design challenges [17-18].

Zinc oxide (ZnO) is a unique wide bandgap biocompatible material system exhibiting both semiconducting and piezoelectric properties that has a diverse group of growth morphologies. Bulk ZnO has a bandgap of 3.37 eV that corresponds to emissions in the ultraviolet (UV) spectral band [7]. Highly ordered vertical arrays of ZnO nanowires (NWs) have been grown on substrates including silicon, SiO2, GaN, and sapphire using a metal organic chemical vapor deposition (MOCVD) growth process [7]. The structural and optical properties of the grown vertically aligned ZnO NW arrays have been characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements [7-10]. Compared to conventional UV sensors, detectors based on ZnO NWs offer high UV sensitivity and low visible sensitivity, and are expected to exhibit low noise, high quantum efficiency, extended lifetimes, and have low power requirements [11-12]. The Photoresponse switching properties of NW array based sensing devices have been measured with intermittent exposure to UV radiation, where the devices were found to switch between low and high conductivity states at time intervals on the order of a few seconds. Envisioned applications for such sensors/FPAs

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171

Zinc oxide is a versatile functional material that provides a biocompatible material system with a unique wide direct energy band gap and exhibits both semiconducting and piezoelectric properties. ZnO is transparent to visible light and can be made highly conductive by doping. Bulk ZnO has a bandgap of 3.37 eV that includes emissions in the solar blind ultraviolet (UV) spectral band (~240-280 nm), making it suitable for UV detector applications [7]. Over this wavelength range, solar radiation is completely absorbed by the ozone layer of the earth's atmosphere, so the background solar radiation at the earth's surface is essentially zero. This enhances the capability of UV sensors in missile warning systems to detect targets such as

ZnO is the basis for the one of the richest families of nanostructures among all materials taking into accounts both structure and properties. ZnO growth morphologies have been demon‐ strated for nanowires, nanobelts, nanocages, nanocombs, nanosprings, nanorings, and nanohelixes [7]. The development of ZnO nanowire (NW) based UV detectors offers high UV sensitivity and low visible sensitivity for missile warning related applications. Demonstration of devices using single ZnO NW strands has been widely reported in literature [7-16]. However, the development of reliable 2D arrays of aligned ZnO NWs has proven more challenging. The demonstration of reliable 2D arrays requires (1) correlation of growth process and growth parameters with the material quality of ZnO NWs, (2) correlation of the electrical and optical performance with growth parameters and fabrication processes, and (3) addressing

With conventional NW growth methods including electrochemical deposition, hydrothermal synthesis, and molecular beam epitaxy (MBE), it is generally difficult to scale up and control NW growth. Electrochemical deposition is well suited for large scale production but does not allow control over the NW orientation. Hydrothermal synthesis is a low temperature and lowcost process that allows growth of NWs on flexible substrates without metal catalysts, but the direction and morphology of the NWs cannot be well-controlled with this method [8-10]. The


**Figure 3.** UV Sensor Model for evaluating UV Sensor Performance [6]

**Figure 4.** UV transmittance vs. wavelength for three aerosol levels (left) and UV radiance at sea level during midday and laser irradiance on the target at 1 km (6 mradian beam) from a 1mW and 10mW UV laser (right) [6]

#### **3.2. ZnO / MgZnO nanostructures for UV applications**

fire (a millisecond event), laser illumination is required for cold targets (facial recognition, profile recognition). A continuous laser and 33 msec integrations are adequate if enough laser power is available. If not, a pulsed laser with nanosecond integrations and APD detectors are

required to reduce atmosphere scatter and improve detector sensitivity.

**SPECTRAL BANDS LASER SCENE / TARGET TIMING**

t bin **20** nsec t pulse 20 nsec image rate 7.5 Hz

gain apd **1** sample rate (max) Hz 3.31E+05 3.00E+05 3.00E+05 300.0

qe **0.70** diameter laser at target 6.56 ft **q factor**

**Figure 3.** UV Sensor Model for evaluating UV Sensor Performance [6]

**UV trans for a 1 km horiz path**

0.25 0.26 0.27 0.28 0.29 0.30 **microns**

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

**trans**

t quench **3000** nsec w pulse 6 meters

170 Optical Sensors - New Developments and Practical Applications

tau opt **0.90** Dlaser tar (cm, cm<sup>2</sup>

lam hi 0.267 um lambda **0.266** um t transit 3.33E-06 sec lin overfill lam lo 0.265 um P laser cont 0.10 W 3333 nsec 1.25 lam mid 0.266 um Pd laser cont 3.18E-06 W/cm2 3.33 usec FPA FOV (ft) del lam 0.002 um Ph laser cont 1.34E+17 pho/sec N pulses/fr 9999 max poss 5.25 **DETECTOR / FPA** Ph laser cont frame 4.46E+15 pho/frame p rate max 300 kHz (set by range and tint) format **256** E laser pulse **5.00E-06** joule/pulse j/frame **p rate used 20** kHz (set by laser specs) dpix **5** um Ph laser pulse 6.69E+12 pho/pulse 3.33E-03 50 usec (available tqnch)

ti cont **33.33** msec 15 div full **4** mrad 0.100 **Nframesum 4** (no. of effective frames)

) 200 3.14E+04 **1**

I surface **1.00E-21** a e laser ret (s) 9713.1 647.5 9.71E-01 98.55 25.45 9.86E-01 98.55 Rload **1.00E+06** ohm e laser ret cont 647.5 *na na* 25.45 *na na* 25.45 resp frame 1.57E-02 elect scene (b) 0 9.73E-17 1.46E-19 0.00 0.00 3.82E-10 0.00 resp bin 9.45E-09 e dark (d) 104 4.16E-02 6.24E-05 10.20 0.20 7.90E-03 10.20 0.63 e surface 2.08E-04 8.32E-08 1.25E-10 0.01 0.00 1.12E-05

**transmitted to ground solar UV in W/cm<sup>2</sup>**

0.25 0.26 0.27 0.28 0.29 0.30 **microns**

**at midday**

**-um**

trans solar UV PL 10 mW 6mr 1km PL 1 mW 6mr 1km

gain **1 bin and pulse rates** det based mission based det/mssion min min in kHz used (Hz) used (kHz)

Fm noise **1** N pulses/frame 11036 9999 9999 300.0 666.6 0.6666 amp noise **5.80E+10** N bins/frame 11036 9999 9999 500.0 666.6 0.6666

eta inj **1.00 ELECTRONS AND NOISE from laser, scene, dark current and amp** 647.5 fill factor **0.75** electrons electrons **electrons** noise e noise e noise e I dark **5.00E-16** a frame for bins **per** frame for bins per J dark 2.00E-09 a/cm2 full int in frame **bin** full int in frame bin

**OPTICS** e kT amp 10.00 0.77 7.74E-03 dopt **20** cm 14.28 0.80 with frame summing Aopt 314.16 cm2 **MODE signal noise s+b+d noise b+d SNR s+b+d SNR b+d SNR s+b+d SNR b+d** focal length **40** cm DDLM cont **647.5 29.2 14.3 22.2 45.3 44.4 90.7** CW fnum 2.00 DDLM bins sum **647.5 25.4 0.3 25.4 2267.2 50.9 4534.3** pulsed

> MLS desert 23kmv MLS urban 5kmv MLS rural 23kmv

1.E-38 1.E-36 1.E-34 1.E-32 1.E-30 1.E-28 1.E-26 1.E-24 1.E-22 1.E-20 1.E-18 1.E-16 1.E-14 1.E-12 1.E-10 1.E-08 1.E-06 1.E-04

**Figure 4.** UV transmittance vs. wavelength for three aerosol levels (left) and UV radiance at sea level during midday

and laser irradiance on the target at 1 km (6 mradian beam) from a 1mW and 10mW UV laser (right) [6]

**rad**

2.50E-07 cm2 Ph laser bin 6.69E+12 pho/bin watts P avg 100 mWatts 3.18 uW/cm2

Zinc oxide (ZnO) is a unique wide bandgap biocompatible material system exhibiting both semiconducting and piezoelectric properties that has a diverse group of growth morphologies. Bulk ZnO has a bandgap of 3.37 eV that corresponds to emissions in the ultraviolet (UV) spectral band [7]. Highly ordered vertical arrays of ZnO nanowires (NWs) have been grown on substrates including silicon, SiO2, GaN, and sapphire using a metal organic chemical vapor deposition (MOCVD) growth process [7]. The structural and optical properties of the grown vertically aligned ZnO NW arrays have been characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements [7-10]. Compared to conventional UV sensors, detectors based on ZnO NWs offer high UV sensitivity and low visible sensitivity, and are expected to exhibit low noise, high quantum efficiency, extended lifetimes, and have low power requirements [11-12]. The Photoresponse switching properties of NW array based sensing devices have been measured with intermittent exposure to UV radiation, where the devices were found to switch between low and high conductivity states at time intervals on the order of a few seconds. Envisioned applications for such sensors/FPAs potentially include defense and commercial applications [13].

Zinc oxide is a versatile functional material that provides a biocompatible material system with a unique wide direct energy band gap and exhibits both semiconducting and piezoelectric properties. ZnO is transparent to visible light and can be made highly conductive by doping. Bulk ZnO has a bandgap of 3.37 eV that includes emissions in the solar blind ultraviolet (UV) spectral band (~240-280 nm), making it suitable for UV detector applications [7]. Over this wavelength range, solar radiation is completely absorbed by the ozone layer of the earth's atmosphere, so the background solar radiation at the earth's surface is essentially zero. This enhances the capability of UV sensors in missile warning systems to detect targets such as missile plumes and flames emitting in this region.

ZnO is the basis for the one of the richest families of nanostructures among all materials taking into accounts both structure and properties. ZnO growth morphologies have been demon‐ strated for nanowires, nanobelts, nanocages, nanocombs, nanosprings, nanorings, and nanohelixes [7]. The development of ZnO nanowire (NW) based UV detectors offers high UV sensitivity and low visible sensitivity for missile warning related applications. Demonstration of devices using single ZnO NW strands has been widely reported in literature [7-16]. However, the development of reliable 2D arrays of aligned ZnO NWs has proven more challenging. The demonstration of reliable 2D arrays requires (1) correlation of growth process and growth parameters with the material quality of ZnO NWs, (2) correlation of the electrical and optical performance with growth parameters and fabrication processes, and (3) addressing system design challenges [17-18].

With conventional NW growth methods including electrochemical deposition, hydrothermal synthesis, and molecular beam epitaxy (MBE), it is generally difficult to scale up and control NW growth. Electrochemical deposition is well suited for large scale production but does not allow control over the NW orientation. Hydrothermal synthesis is a low temperature and lowcost process that allows growth of NWs on flexible substrates without metal catalysts, but the direction and morphology of the NWs cannot be well-controlled with this method [8-10]. The MBE method allows monitoring of the structural quality during NW growth; however, this type of synthesis often requires use of metal catalysts as a seed layer [10], which introduces undesired defects to the structure, decreasing the crystal quality [12-16]. Chemical vapor deposition (CVD) also requires catalysts at the NW tips, and using this method the tips of the grown NWs were observed to be flat, with vertical alignment.

SEM was performed to explore the NWs morphology. Figure 5 show the synthesized ZnO NWs on the various substrates, which can be generally seen to have uniform distribution density. The ZnO NWs grown on sapphire [Figure 5(a)] had approximate diameters of 50-70 nm and lengths in the range of 1-2 µm. NWs grown on SiO2 [Figure 5(b)] had diameters of 150-200 nm and lengths of 1-2 µm, and were the least vertically oriented and associated with a relatively high lattice mismatch. NWs grown on the Si (111) substrate [Figure 5(c)] had a slightly random orientation, also having diameters in the range of 150-200 nm and lengths from 1-2 µm. Finally, the NWs grown on GaN [Figure 5(d)] showed strong vertical orientation,

40 45 50 55 60 65

**34.4 34.8 35.2**

ZnO (002)

Nanostructured Detector Technology for Optical Sensing Applications

2q Angle (deg)

2q Angle (deg)

**Figure 6.** X-ray diffraction (XRD) of ZnO NWs grown using MOCVD on p-Si (solid), GaN/sapphire (square) and SiO2

Figure 6 shows the XRD pattern for the ZnO NWs grown on p-Si, GaN, and SiO2 substrates [10]. The inset of Figure 2 shows dominant peaks related to ZnO (002). The peak at 34º (2θ) for ZnO grown on p-Si and SiO2 substrates incorporated the overlapping of ZnO NWs (002) and

(triangle). The inset shows the ZnO peak associated with ZnO oriented along (002) and GaN [20].

 p-Si GaN SiO2

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173

with diameters of 20-40 nm and lengths of 0.7-1.0 µm [20].

**GaN**

Sapp.

Intensity (a.u.)

**SiO2**

**p-Si**

0

2

4

Intensity (a.u.)

6

#### **3.3. Characterization of ZnO NWs arrays grown on the various substrates**

The samples were characterized by scanning electron microscopy (SEM) utilizing a Quanta FEG 250 system, and X-ray diffraction (XRD) using Bruker D-8 Advance X-ray diffractometer with a wavelength of 1.5406 Å corresponding to the Cu Kα line. In addition, photolumines‐ cence (PL) measurements were performed at room temperature using a Linconix HeCd UV laser emitting at a wavelength of 325 nm. A Si detector in conjunction with at lock-in amplifier and chopper were used to measure the PL from the beam reflected off the sample at the output over the desired wavelength range [18-20].

**Figure 5.** Scanning electron microscope (SEM) images of NWs grown on the various substrates taken at room temper‐ ature, showing NWs grown on (a) ZnO/sapphire; (b) ZnO/SiO2/p-Si; (c) ZnO/p-Si; and (d) ZnO/GaN/sapphire.[ 20 ]

SEM was performed to explore the NWs morphology. Figure 5 show the synthesized ZnO NWs on the various substrates, which can be generally seen to have uniform distribution density. The ZnO NWs grown on sapphire [Figure 5(a)] had approximate diameters of 50-70 nm and lengths in the range of 1-2 µm. NWs grown on SiO2 [Figure 5(b)] had diameters of 150-200 nm and lengths of 1-2 µm, and were the least vertically oriented and associated with a relatively high lattice mismatch. NWs grown on the Si (111) substrate [Figure 5(c)] had a slightly random orientation, also having diameters in the range of 150-200 nm and lengths from 1-2 µm. Finally, the NWs grown on GaN [Figure 5(d)] showed strong vertical orientation, with diameters of 20-40 nm and lengths of 0.7-1.0 µm [20].

MBE method allows monitoring of the structural quality during NW growth; however, this type of synthesis often requires use of metal catalysts as a seed layer [10], which introduces undesired defects to the structure, decreasing the crystal quality [12-16]. Chemical vapor deposition (CVD) also requires catalysts at the NW tips, and using this method the tips of the

The samples were characterized by scanning electron microscopy (SEM) utilizing a Quanta FEG 250 system, and X-ray diffraction (XRD) using Bruker D-8 Advance X-ray diffractometer with a wavelength of 1.5406 Å corresponding to the Cu Kα line. In addition, photolumines‐ cence (PL) measurements were performed at room temperature using a Linconix HeCd UV laser emitting at a wavelength of 325 nm. A Si detector in conjunction with at lock-in amplifier and chopper were used to measure the PL from the beam reflected off the sample at the output

**Figure 5.** Scanning electron microscope (SEM) images of NWs grown on the various substrates taken at room temper‐ ature, showing NWs grown on (a) ZnO/sapphire; (b) ZnO/SiO2/p-Si; (c) ZnO/p-Si; and (d) ZnO/GaN/sapphire.[ 20 ]

grown NWs were observed to be flat, with vertical alignment.

**(a) (b)** 

**(c) (d)** 

over the desired wavelength range [18-20].

172 Optical Sensors - New Developments and Practical Applications

**3.3. Characterization of ZnO NWs arrays grown on the various substrates**

**Figure 6.** X-ray diffraction (XRD) of ZnO NWs grown using MOCVD on p-Si (solid), GaN/sapphire (square) and SiO2 (triangle). The inset shows the ZnO peak associated with ZnO oriented along (002) and GaN [20].

Figure 6 shows the XRD pattern for the ZnO NWs grown on p-Si, GaN, and SiO2 substrates [10]. The inset of Figure 2 shows dominant peaks related to ZnO (002). The peak at 34º (2θ) for ZnO grown on p-Si and SiO2 substrates incorporated the overlapping of ZnO NWs (002) and ZnO thin film (002). An additional diffraction peak associated with GaN was present for the GaN/sapphire substrate. ZnO NWs oriented along the (002) direction had full-widths at half maxima (FWHM) and c-lattice constants of 0.0498 (θ) and 5.1982 Å at 34.48° (2θ) for p-Si, 0.0497(θ) and 5.1838 Å at 34.58° (2θ) for GaN, 0.0865(θ) and 5.1624° at 34.38º (2θ) for SiO2, and 0.0830˚(θ) and 5.2011 Å at 34.46º (2θ) for sapphire.

The quality of the ZnO epilayers utilized as seed layers to grow ZnO NWs was also charac‐ terized. The ZnO thin films were oriented along (002) and had a maximum at 34.58º with FWHM of 0.0697 (θ) for p-Si, maximum of 34.58º with FWHM of 0.0684 (θ) for GaN, and maximum of 34.43º with FWHM of 0.0557 (θ) for SiO2. Additional shallow diffraction peaks were observed for NWs grown on p-Si and SiO2, which are attributed to ZnO (100, 101, 102 and 110) as can be seen from Figure 6. As shown in Figure 7, for ZnO NW growth on sapphire major peaks were observed for ZnO (002) at 34.46° (2θ) and Al2O at 37.91° (2θ), with a minor peak for ZnO (101) at 36.34° (2θ).

**360 380 400 420 440 460 480 500**

**Wavelength (nm)**

**Figure 8.** Photoluminescence (PL) of ZnO NWs grown on p-Si (100) (solid) with a single peak at 380 nm, GaN (square)

No defects related to Zn or O vacancies were observed, which can be attributed to the confinement of defects at the ZnO thin film/substrate interface. For the ZnO NWs grown on GaN, a predominant peak with a FWHM of 18.18 nm was observed at 378 nm. High stress was evident for ZnO NWs grown on GaN, which can be observed in Figure 2; this can contribute to the broadening of the peak in comparison to p-Si and SiO2. Shallow peaks identified at 474 nm and 490 nm through Lorentzian decomposition are attributed to oxygen interstitial and

A UV LED lamp acquired from Sensor Electronic Technology Inc. was used to characterize the UV Photoresponse of the ZnO NW arrays [20]. The lamp comprises eight separate AlGaN based UV LEDs in a TO-3 package spanning the 240-370 nm wavelength range, with a customized power supply capable of independently monitoring and controlling the current of all or any of the LEDs. The Photoresponse was determined by first applying voltage between indium contacts on the front and back sides of a Si NW sample and measuring the resulting current in the dark, and then repeating this procedure while the sample was exposed to

Figure 9 shows the on-off switching characteristics of a ZnO vertical array NW device when exposed to radiation at 370 nm. This device was found to switch between low and high

**FWHM = 18.1nm**

**378nm**

**FWHM = 14.7nm**

**FWHM = 15.2nm**

**<sup>22</sup>** p-Si

GaN

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Nanostructured Detector Technology for Optical Sensing Applications

SiO2

oxygen vacancies, respectively [20].

radiation from a UV LED at a specific wavelength.

**Intensity (a.u.)**

**\***

**GaN**

with a stronger peak at 378 and SiO2 (triangle) with a single peak at 378 nm [20].

**p-Si**

**\***

**\***

**SiO2**

**380nm**

**378nm**

**Figure 7.** XRD of ZnO NWs grown using MOCVD on sapphire [20]

#### **3.4. Photoluminescence (PL) measurements**

Figure 8 shows the PL spectra for ZnO NWs grown on p-Si, GaN, and SiO2 substrates [10]. The room temperature PL measurements were performed using a ~280 nm light source. Single peaks located at 380 nm having a FWHM of 14.69 nm and at 378 nm having a FWHM of 15 nm were observed for p-Si and SiO2 substrates, respectively, corresponding to the recombination of excitons through an exciton-exciton collision process [18-20].

ZnO thin film (002). An additional diffraction peak associated with GaN was present for the GaN/sapphire substrate. ZnO NWs oriented along the (002) direction had full-widths at half maxima (FWHM) and c-lattice constants of 0.0498 (θ) and 5.1982 Å at 34.48° (2θ) for p-Si, 0.0497(θ) and 5.1838 Å at 34.58° (2θ) for GaN, 0.0865(θ) and 5.1624° at 34.38º (2θ) for SiO2, and

The quality of the ZnO epilayers utilized as seed layers to grow ZnO NWs was also charac‐ terized. The ZnO thin films were oriented along (002) and had a maximum at 34.58º with FWHM of 0.0697 (θ) for p-Si, maximum of 34.58º with FWHM of 0.0684 (θ) for GaN, and maximum of 34.43º with FWHM of 0.0557 (θ) for SiO2. Additional shallow diffraction peaks were observed for NWs grown on p-Si and SiO2, which are attributed to ZnO (100, 101, 102 and 110) as can be seen from Figure 6. As shown in Figure 7, for ZnO NW growth on sapphire major peaks were observed for ZnO (002) at 34.46° (2θ) and Al2O at 37.91° (2θ), with a minor

+

ZnO (00 2)

34.46

34 36 38

Figure 8 shows the PL spectra for ZnO NWs grown on p-Si, GaN, and SiO2 substrates [10]. The room temperature PL measurements were performed using a ~280 nm light source. Single peaks located at 380 nm having a FWHM of 14.69 nm and at 378 nm having a FWHM of 15 nm were observed for p-Si and SiO2 substrates, respectively, corresponding to the recombination

2q

+

ZnO (101)

Al203

0.0830˚(θ) and 5.2011 Å at 34.46º (2θ) for sapphire.

174 Optical Sensors - New Developments and Practical Applications

0

**Figure 7.** XRD of ZnO NWs grown using MOCVD on sapphire [20]

of excitons through an exciton-exciton collision process [18-20].

**3.4. Photoluminescence (PL) measurements**

20

Intensity (a.u.)

40

peak for ZnO (101) at 36.34° (2θ).

**Figure 8.** Photoluminescence (PL) of ZnO NWs grown on p-Si (100) (solid) with a single peak at 380 nm, GaN (square) with a stronger peak at 378 and SiO2 (triangle) with a single peak at 378 nm [20].

No defects related to Zn or O vacancies were observed, which can be attributed to the confinement of defects at the ZnO thin film/substrate interface. For the ZnO NWs grown on GaN, a predominant peak with a FWHM of 18.18 nm was observed at 378 nm. High stress was evident for ZnO NWs grown on GaN, which can be observed in Figure 2; this can contribute to the broadening of the peak in comparison to p-Si and SiO2. Shallow peaks identified at 474 nm and 490 nm through Lorentzian decomposition are attributed to oxygen interstitial and oxygen vacancies, respectively [20].

A UV LED lamp acquired from Sensor Electronic Technology Inc. was used to characterize the UV Photoresponse of the ZnO NW arrays [20]. The lamp comprises eight separate AlGaN based UV LEDs in a TO-3 package spanning the 240-370 nm wavelength range, with a customized power supply capable of independently monitoring and controlling the current of all or any of the LEDs. The Photoresponse was determined by first applying voltage between indium contacts on the front and back sides of a Si NW sample and measuring the resulting current in the dark, and then repeating this procedure while the sample was exposed to radiation from a UV LED at a specific wavelength.

Figure 9 shows the on-off switching characteristics of a ZnO vertical array NW device when exposed to radiation at 370 nm. This device was found to switch between low and high respectively [20].

wavelength.

provide detection in the solar blind region. This device was tested by applying a bias between the top contacts on the pixels, which are apparent in Figure 9(b), and the back contact.

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177

ZnO nanowires based arrays offer high sensitivity and have potential application in UV imaging systems. ZnO nanowire array based UV detectors have no moving parts, high quantum efficiency, extended lifetimes, low noise, low power requirements, and offer high

ZnO nanowires have also been evaluated for providing remote power for the stand alone sensors. This type of application has been extensively studied by Professor Z.L. Wang and his team at Georgia Tech [21, 22]. They have shown that ZnO nanowires can be used as nanogenerators for providing remote power using the Piezo-electric effect. Photovoltaic cells or solar cells are a popular renewable energy technology, relying on approaches such as inorganic p-n junctions, organic thin films, and organic-inorganic heterojunction. However, a solar cell works only under sufficient light illumination, which depends on the location the devices will

Considering that mechanical energy is widely available in our living environment, They have demonstrated [21] the first hybrid cell for concurrently harvesting solar and mechanical energy through simply integrating a dye-sensitized solar cell (DSSC) and a piezoelectric nanogener‐ ator on the two sides of a common substrate. After this, in order to solve the encapsulation problem from liquid electrolyte leakage in the first back-to-back integrated HC, early in 2011, Xu and Wang improved the prototype design of the HC and developed a compact solid state solar cell. This innovative design convoluted the roles played by the NW array to simultane‐ ously perform their functionality in a nanogenerator and a DSSC. The design and the per‐

Based on these demonstrations of HCs for concurrently harvesting solar and mechanical energies, they have. reported an optical fiber-based three-dimensional (3D) hybrid cell, consisting of a dye-sensitized solar cell for harvesting solar energy and a nanogenerator for harvesting mechanical energy; these are fabricated coaxially around a single fiber as a core– shell structure (Figure 11). The optical fiber, which is flexible and allows remote transmission of light, serves as the substrate for the 3D DSSC for enhancing the electron transport property and the surface area, and making it suitable for solar power generation at remote/concealed locations. The inner layer of the HC is the DSSC portion, which is based on a radically grown ZnO NW array on an optical fiber with ITO as the bottom electrode. The dye-sensitized ZnO NW array was encapsulated by a stainless steel capillary tube with a Pt-coated inner wall as the photo- anode for the DSSC. The stainless steel tube also serves as the bottom electrode for the outer layer of the nanogenerator, with densely packed ZnO NWs grown on its outer wall. Another exciting application of ZnO nanowires is designing, fabricating, and integrating arrays of nanodevices into a functional system are key to transferring nanoscale science into

Recent work [22] on three-dimensional (3D) circuitry integration of piezotronic transistors based on vertical zinc oxide nanowires as an active taxel-addressable pressure/force sensor matrix for tactile imaging. Using the piezoelectric polarization charges created at a metal-

be deployed, as well as the time of the day and the weather.

formance are shown in figure 11.

applicable nanotechnology as shown in Figure 12.

sensitivity.

between indium contacts on the front and back sides of a Si NW sample and measuring the resulting current in the dark, and then repeating this procedure while the sample was exposed to radiation from a UV LED at a specific **Figure 9.** Switching Photoresponse characteristics of ZnO NW device when UV LED source at ~370 nm turned on and off over approximately 10 s intervals.[20]

controlling the current of all or any of the LEDs. The Photoresponse was determined by first applying voltage

conductivity states in approximately 3 s, a faster response than most reported thus far for ZnO NW based UV detectors. Figure 9 shows the on-off switching characteristics of a ZnO vertical array NW device when exposed to radiation at 370 nm. This device was found to switch between low and high conductivity states in approximately 3 s, a faster

response than most reported thus far for ZnO NW based UV detectors.

Figure 10.(a) Mounted solar blind NW UV 3x9 pixel array detector device; (b) close-up of device, showing wire bonded pixels [20 ]. **Figure 10.** (a) Mounted solar blind NW UV 3x9 pixel array detector device; (b) close-up of device, showing wire bond‐ ed pixels [20 ].

Figure 9(a) shows a mounted and wire bonded NW UV 3x9 pixel array detector device. Incorporation of Mg allows the detector response to be shifted to shorter wavelengths to provide detection in the solar blind region. This device was tested by applying a bias between the top contacts on the pixels, which are apparent in Figure 9(b), and the back contact.

ZnO nanowires based arrays offer high sensitivity and have potential application in UV imaging systems. ZnO nanowire array based UV detectors have no moving parts, high quantum efficiency, extended lifetimes, low noise, low power requirements, and offer high sensitivity.

ZnO nanowires have also been evaluated for providing remote power for the stand alone sensors. This type of application has been extensively studied by Professor Z.L. Wang and his team at Georgia Tech [21, 22]. They have shown that ZnO nanowires can be used as nanogenerators for providing remote power using the Piezo-electric effect. Photovoltaic cells or solar cells are a popular renewable energy technology, relying on approaches such as inorganic p-n junctions, organic thin films, and organic-inorganic heterojunction. However, a solar cell works only under sufficient light illumination, which depends on the location the devices will be deployed, as well as the time of the day and the weather.

Considering that mechanical energy is widely available in our living environment, They have demonstrated [21] the first hybrid cell for concurrently harvesting solar and mechanical energy through simply integrating a dye-sensitized solar cell (DSSC) and a piezoelectric nanogener‐ ator on the two sides of a common substrate. After this, in order to solve the encapsulation problem from liquid electrolyte leakage in the first back-to-back integrated HC, early in 2011, Xu and Wang improved the prototype design of the HC and developed a compact solid state solar cell. This innovative design convoluted the roles played by the NW array to simultane‐ ously perform their functionality in a nanogenerator and a DSSC. The design and the per‐ formance are shown in figure 11.

conductivity states in approximately 3 s, a faster response than most reported thus far for ZnO

**Figure 9.** Switching Photoresponse characteristics of ZnO NW device when UV LED source at ~370 nm turned on and

Figure 9 shows the on-off switching characteristics of a ZnO vertical array NW device when exposed to radiation at 370 nm. This device was found to switch between low and high conductivity states in approximately 3 s, a faster

Figure 10.(a) Mounted solar blind NW UV 3x9 pixel array detector device; (b) close-up of device, showing wire bonded pixels [20 ].

**Figure 10.** (a) Mounted solar blind NW UV 3x9 pixel array detector device; (b) close-up of device, showing wire bond‐

Figure 9(a) shows a mounted and wire bonded NW UV 3x9 pixel array detector device. Incorporation of Mg allows the detector response to be shifted to shorter wavelengths to

Figure 9. Switching Photoresponse characteristics of ZnO NW device when UV LED source at ~370 nm turned on and off over

A UV LED lamp acquired from Sensor Electronic Technology Inc. was used to characterize the UV Photoresponse of the ZnO NW arrays [20]. The lamp comprises eight separate AlGaN based UV LEDs in a TO-3 package spanning the 240-370 nm wavelength range, with a customized power supply capable of independently monitoring and controlling the current of all or any of the LEDs. The Photoresponse was determined by first applying voltage between indium contacts on the front and back sides of a Si NW sample and measuring the resulting current in the dark, and then repeating this procedure while the sample was exposed to radiation from a UV LED at a specific

and 490 nm through Lorentzian decomposition are attributed to oxygen interstitial and oxygen vacancies,

NW based UV detectors.

off over approximately 10 s intervals.[20]

response than most reported thus far for ZnO NW based UV detectors.

 **(a) (b)** 

wavelength.

ed pixels [20 ].

approximately 10 s intervals.[20]

respectively [20].

176 Optical Sensors - New Developments and Practical Applications

Based on these demonstrations of HCs for concurrently harvesting solar and mechanical energies, they have. reported an optical fiber-based three-dimensional (3D) hybrid cell, consisting of a dye-sensitized solar cell for harvesting solar energy and a nanogenerator for harvesting mechanical energy; these are fabricated coaxially around a single fiber as a core– shell structure (Figure 11). The optical fiber, which is flexible and allows remote transmission of light, serves as the substrate for the 3D DSSC for enhancing the electron transport property and the surface area, and making it suitable for solar power generation at remote/concealed locations. The inner layer of the HC is the DSSC portion, which is based on a radically grown ZnO NW array on an optical fiber with ITO as the bottom electrode. The dye-sensitized ZnO NW array was encapsulated by a stainless steel capillary tube with a Pt-coated inner wall as the photo- anode for the DSSC. The stainless steel tube also serves as the bottom electrode for the outer layer of the nanogenerator, with densely packed ZnO NWs grown on its outer wall.

Another exciting application of ZnO nanowires is designing, fabricating, and integrating arrays of nanodevices into a functional system are key to transferring nanoscale science into applicable nanotechnology as shown in Figure 12.

Recent work [22] on three-dimensional (3D) circuitry integration of piezotronic transistors based on vertical zinc oxide nanowires as an active taxel-addressable pressure/force sensor matrix for tactile imaging. Using the piezoelectric polarization charges created at a metalsemiconductor interface under strain to gate/modulate the transport process of local charge carriers, we designed independently addressable two-terminal transistor arrays, which convert mechanical stimuli applied to the devices into local electronic controlling signals.

The device matrix can achieve shape-adaptive high-resolution tactile imaging and selfpowered, multidimensional active sensing. The 3D piezotronic transistor array may have applications in human-electronics interfacing, smart skin, and micro- and nano-electrome‐ chanical systems.

**Figure 12.** Tactile imaging and multidimensional sensing by the fully integrated 92 × 92 SGVPT array. (A) Metrology mapping (inset) and statistical investigation of the fully integrated SGVPT array without applying stress. (B) Current response contour plot illustrating the capability of SGVPT array for imaging the spatial profile of applied stress. Color scale represents the current differences for each taxel before and after applying the normal stress. The physical shape of the applied stress is highlighted by the white dashed lines. (C) Multidimensional sensing by an SGVPT array exhibits the potential of realizing applications such as personal signature recognition with maximum security and unique iden‐

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179

**4. Development of GaN UVAPD for ultraviolet sensor applications**

High resolution imaging in UV bans has a lot of applications in Defense and Commercial applications. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPD's have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We will discuss model that can predict sensor performance in the UV band

tity. The shape of a "written" letter A is highlighted by the white dashed lines. [22].

**Figure 11.** Design and performance of a 3D optical fiber based hybrid cell (HC) consisting of a dye-sensitized solar cell (DSSC) and a nanogenerator (NG) for harvesting solar and mechanical energy. (a) The 3D HC is composed of an optical fiber based DSSC with capillary tube as counter electrode and a NG on top. (b) Open-circuit voltage (VOC) of the HC when the NG and the DSSC are connected in series, where VOC(HC)= VOC(DSSC)+ VOC(NG). (c) Short-circuit current (ISC) of the HC when the NG and the DSSC are connected in parallel. (d) and (e) Enlarged view of ISC(HC) and ISC(NG), clearly showing that ISC(NG) is 0.13 µA, the ISC(DSSC) is 7.52 µA, and the ISC(HC) is about 7.65 µA, nearly the sum of the output of the solar cell. [21].

semiconductor interface under strain to gate/modulate the transport process of local charge carriers, we designed independently addressable two-terminal transistor arrays, which convert mechanical stimuli applied to the devices into local electronic controlling signals.

The device matrix can achieve shape-adaptive high-resolution tactile imaging and selfpowered, multidimensional active sensing. The 3D piezotronic transistor array may have applications in human-electronics interfacing, smart skin, and micro- and nano-electrome‐

**Figure 11.** Design and performance of a 3D optical fiber based hybrid cell (HC) consisting of a dye-sensitized solar cell (DSSC) and a nanogenerator (NG) for harvesting solar and mechanical energy. (a) The 3D HC is composed of an optical fiber based DSSC with capillary tube as counter electrode and a NG on top. (b) Open-circuit voltage (VOC) of the HC when the NG and the DSSC are connected in series, where VOC(HC)= VOC(DSSC)+ VOC(NG). (c) Short-circuit current (ISC) of the HC when the NG and the DSSC are connected in parallel. (d) and (e) Enlarged view of ISC(HC) and ISC(NG), clearly showing that ISC(NG) is 0.13 µA, the ISC(DSSC) is 7.52 µA, and the ISC(HC) is about 7.65 µA, nearly the sum of

chanical systems.

178 Optical Sensors - New Developments and Practical Applications

the output of the solar cell. [21].

**Figure 12.** Tactile imaging and multidimensional sensing by the fully integrated 92 × 92 SGVPT array. (A) Metrology mapping (inset) and statistical investigation of the fully integrated SGVPT array without applying stress. (B) Current response contour plot illustrating the capability of SGVPT array for imaging the spatial profile of applied stress. Color scale represents the current differences for each taxel before and after applying the normal stress. The physical shape of the applied stress is highlighted by the white dashed lines. (C) Multidimensional sensing by an SGVPT array exhibits the potential of realizing applications such as personal signature recognition with maximum security and unique iden‐ tity. The shape of a "written" letter A is highlighted by the white dashed lines. [22].

#### **4. Development of GaN UVAPD for ultraviolet sensor applications**

High resolution imaging in UV bans has a lot of applications in Defense and Commercial applications. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPD's have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We will discuss model that can predict sensor performance in the UV band using APD's with various gain and other parameters for a desired UV band of interest. SNR's can be modeled from illuminated targets at various distances with high resolution under standard atmospheres in the UV band and the solar blind region using detector arrays with unity gain and with high gain APD's [23-26].

grated with silicon CMOS electronics. Figure 16 presents the Reciprocal Space mapping of AlGaN on AlN substrate and Sapphire substrate. The data for sapphire substrate shows

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181

**Figure 14.** Photograph of New-generation AIXTRON CCS 3x2 " high temperature III-Nitride 3x2 MOCVD growth

increased strain and mosaicity compared with AlN substrate.

Chamber open for loading wafers showing close-coupled showerhead [26]

**Figure 15.** Device Structure Cross-section of prototype Back-Side Illuminated AlGaN UV APD [27]

**Figure 13.** Relationship between alloy composition of AlGaN and the corresponding spectral cutoff for the UV detec‐ tor arrays [23].

Figure 13 presents the relationship between the alloy composition of Gallium and Aluminum in AlxGa1-xN that determines the cut-off wavelength of the UV detector for p-i-n [23-24] and also for UV APD's. Deep Ultra Violet (DUV) will require addition of larger composition of Aluminum in AlxGa1-xN. [25].

#### **5. GaN /AlGaN UV APD growth**

Figure 14 presents the High-Temperature MOCVD system by Aixtron. This new reactor design and capability has the ability to grow high quality GaN and AlGaN material with doping for GaN/AlGaN UVAPD applications [26].

Figure 15 presents the device structure of a back-side illuminated AlGaN UV APD. The substrate in this device structure is double side polished AlN substrate. The use of AlN substrate allows the UV APD device structure to be back-side illuminated and can be inte‐ grated with silicon CMOS electronics. Figure 16 presents the Reciprocal Space mapping of AlGaN on AlN substrate and Sapphire substrate. The data for sapphire substrate shows increased strain and mosaicity compared with AlN substrate.

using APD's with various gain and other parameters for a desired UV band of interest. SNR's can be modeled from illuminated targets at various distances with high resolution under standard atmospheres in the UV band and the solar blind region using detector arrays with

**Figure 13.** Relationship between alloy composition of AlGaN and the corresponding spectral cutoff for the UV detec‐

Figure 13 presents the relationship between the alloy composition of Gallium and Aluminum in AlxGa1-xN that determines the cut-off wavelength of the UV detector for p-i-n [23-24] and also for UV APD's. Deep Ultra Violet (DUV) will require addition of larger composition of

Figure 14 presents the High-Temperature MOCVD system by Aixtron. This new reactor design and capability has the ability to grow high quality GaN and AlGaN material with doping for

Figure 15 presents the device structure of a back-side illuminated AlGaN UV APD. The substrate in this device structure is double side polished AlN substrate. The use of AlN substrate allows the UV APD device structure to be back-side illuminated and can be inte‐

unity gain and with high gain APD's [23-26].

180 Optical Sensors - New Developments and Practical Applications

tor arrays [23].

Aluminum in AlxGa1-xN. [25].

**5. GaN /AlGaN UV APD growth**

GaN/AlGaN UVAPD applications [26].

**Figure 14.** Photograph of New-generation AIXTRON CCS 3x2 " high temperature III-Nitride 3x2 MOCVD growth Chamber open for loading wafers showing close-coupled showerhead [26]

**Figure 15.** Device Structure Cross-section of prototype Back-Side Illuminated AlGaN UV APD [27]

Figure 17 presents the microscopic surface morphology using AFM on GaN p-i-n structure grown on GaN/Sapphire template. No surface defects are observed. These results are encour‐

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**Figure 19.** Current - Voltage characteristics of AlGaN UV APD Unpassivated test structure. Further reduction in the

Figure 18 presents the SIMS analysis of GaN p-i-n structure on GaN/Sapphire template, the data shows controlled Si and Mg doping for n- and p-type layers. The data shows low background doping concentration in GaN layer. The Mg doping is being increased for better

Figure19presents theCurrent-VoltagecharacteristicsofAlGaNUVAPDwithspectralresponse of 300 nm. Further reduction in the dark current will be expected with surface passivation. The futureeffortisunderwaytoimprovethegrowthcharacteristicsLow-defect-densityinsubstrates and high-quality epitaxial growth technologies are the keys to the successful implementation

The Visible-Near Infrared band (0.4-1.7 µm) bridges the spectral gap between the visible and thermal bands in the electromagnetic spectrum. In this spectral band, the primary phenom‐ enology of interest is the reflectance signature of the target, manifested as either its variations

Infrared imaging in the NIR /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

of a robust high-performance APDs for UV focal plane arrays [29-33].

**6. Visible –Near Infrared (NIR) detector technologies**

dark current will be expected with surface passivation [29].

in brightness or spectral reflectance, or both.

p-type conductivity

aging to develop a low cost back-side illuminated UV APD detector array.

**Figure 16.** Reciprocal Space mapping of AlGaN p-n junctions on AlN and Sapphire Substrates [28]

**Figure 17.** Microscopic surface morphology using AFM on GaN p-i-n structure grown on GaN/Sapphire template. No specific surface defects are observed [27]

**Figure 18.** SIMS analysis of GaN p-i-n structure on GaN/Sapphire template, the data shows controlled Si and Mg dop‐ ing for n- and p-type layers. The data shows low background doping concentration in GaN layer [28]

Figure 17 presents the microscopic surface morphology using AFM on GaN p-i-n structure grown on GaN/Sapphire template. No surface defects are observed. These results are encour‐ aging to develop a low cost back-side illuminated UV APD detector array.

**Figure 16.** Reciprocal Space mapping of AlGaN p-n junctions on AlN and Sapphire Substrates [28]

specific surface defects are observed [27]

182 Optical Sensors - New Developments and Practical Applications

**Figure 17.** Microscopic surface morphology using AFM on GaN p-i-n structure grown on GaN/Sapphire template. No

**Figure 18.** SIMS analysis of GaN p-i-n structure on GaN/Sapphire template, the data shows controlled Si and Mg dop‐

ing for n- and p-type layers. The data shows low background doping concentration in GaN layer [28]

**Figure 19.** Current - Voltage characteristics of AlGaN UV APD Unpassivated test structure. Further reduction in the dark current will be expected with surface passivation [29].

Figure 18 presents the SIMS analysis of GaN p-i-n structure on GaN/Sapphire template, the data shows controlled Si and Mg doping for n- and p-type layers. The data shows low background doping concentration in GaN layer. The Mg doping is being increased for better p-type conductivity

Figure19presents theCurrent-VoltagecharacteristicsofAlGaNUVAPDwithspectralresponse of 300 nm. Further reduction in the dark current will be expected with surface passivation. The futureeffortisunderwaytoimprovethegrowthcharacteristicsLow-defect-densityinsubstrates and high-quality epitaxial growth technologies are the keys to the successful implementation of a robust high-performance APDs for UV focal plane arrays [29-33].

#### **6. Visible –Near Infrared (NIR) detector technologies**

The Visible-Near Infrared band (0.4-1.7 µm) bridges the spectral gap between the visible and thermal bands in the electromagnetic spectrum. In this spectral band, the primary phenom‐ enology 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 NIR /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; 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.[ 34-36].

passivated to minimize surface recombination as indicated in Figure 22. The device shown in the figure 22 uses substrate illumination, as is needed for use in FPA arrays, and short

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

n+ - Ge Layer

p+ - Si Substrate (100)

**Figure 20.** SiGe/Si based buried junction approach to be evaluated for reduced surface states and leakage current

**Figure 21.** 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. The spectral response data for SiGe detector is also presented. [38]

[36]

p+ - Ge seed layer

Photon Absorbtion Layer

>1 m Intrinsic Ge Layer

wavelength response can be improved by thinning the Si substrate.

#### **6.1. Si1-x Gex (SiGe) detector arrays**

Like the other two alloy semiconductors mentioned above, SiGe is another example of material that can be used for the fabrication of IR 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 cost and highly uniform IR FPAs.

Some of the earlier attempts in developing SiGe IR detectors focused on their LWIR applica‐ tions [34-36]. Renewed efforts are now developing these detectors for application in the NIR-SWIR band [36]. 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 the lattice constant does not exactly follow Vegard's law. The relative change of the lattice constant is given by [36]:

aSi1-x Gex= 0.5431 + 0.01992x+ 0.0002733x<sup>2</sup> (nm).

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 Superlattice, Quantum dot and Buried junction designs [36-38]. 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. Figure 20 presents the SiGe /Ge detector array using buried junction approach to reduce the surface states and leakage current [36].

Figure 22 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 wavelength response and dark current.

The SiGe well thicknesses are kept below the critical layer thickness for dislocation formation. To complete the structure, the undoped superlattice is capped with a thin n+ Si cap layer 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 22. The device shown in the figure 22 uses substrate illumination, as is needed for use in FPA arrays, and short wavelength response can be improved by thinning the Si substrate.

conditions, and use eye safe 1550 nm illumination; 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

Like the other two alloy semiconductors mentioned above, SiGe is another example of material that can be used for the fabrication of IR 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 cost and highly

Some of the earlier attempts in developing SiGe IR detectors focused on their LWIR applica‐ tions [34-36]. Renewed efforts are now developing these detectors for application in the NIR-SWIR band [36]. 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 the lattice constant does not exactly

follow Vegard's law. The relative change of the lattice constant is given by [36]:

(nm).

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

Several approaches have been proposed to reduce the dark current in SiGe detector arrays by several orders of magnitude; these include Superlattice, Quantum dot and Buried junction designs [36-38]. 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. Figure 20 presents the SiGe /Ge detector array using buried junction approach to reduce the surface states and leakage

Figure 22 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 wavelength response and dark current. The SiGe well thicknesses are kept below the critical layer thickness for dislocation formation. To complete the structure, the undoped superlattice is capped with a thin n+ Si cap layer 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

the R0A product of the photodiode.[ 34-36].

184 Optical Sensors - New Developments and Practical Applications

**6.1. Si1-x Gex (SiGe) detector arrays**

aSi1-x Gex= 0.5431 + 0.01992x+ 0.0002733x<sup>2</sup>

uniform IR FPAs.

the dark current.

current [36].

**Figure 20.** SiGe/Si based buried junction approach to be evaluated for reduced surface states and leakage current [36]

**Figure 21.** 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. The spectral response data for SiGe detector is also presented. [38]

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 wavelength 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 dislo‐

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Figure 23 shows an 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 is being developed for the deposition of Ge/Si quantum dot superlattices (QDSLs), where Ge QDs are alternated with thin (10-30 nm) Si

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

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

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

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

The band gap [7-1] of the strained Inx Ga1-x As: InP structure can be tailored by varying the

barrier layers. A cross-sectional TEM image of QDSLs is shown in Figure 23.b.

cation nucleation [39].

**7. SWIR detector technologies**

**7.1. Inx Ga1-xAs detector array development**

performance at the longer cutoff wavelengths. [43].

alloy composition during crystal growth according to the equation:

spectral reflectance, or both.

photodiode.

of x = 0.53 [40-42].

**Figure 22.** Schematic of detector array structure consisting of a SiGe /Si strained layer Superlattice grown on (001) silicon [38].

**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‐ pared to Si barriers. [38].

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 wavelength 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 dislo‐ cation nucleation [39].

Figure 23 shows an 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 is 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 23.b.
