**8. Nanostructured detector technology for MWIR and LWIR bands**

EO/IR Sensors and imagers using nanostructure based materials are being developed for a variety of Defense Applications. In this section, we will present recent work under way for development of next generation carbon nanostructure based infrared detectors and arrays. We will discuss detector concepts that will provide next generation high performance, high frame rate, and uncooled nano-bolometer for MWIR and LWIR bands [52-55]. The critical technolo‐ gies being developed include carbon nanostructure growth, characterization, optical and electronic properties that show the feasibility for IR detection. Experimental results on CNT nanostructures will be presented. Further discussion will be provided for the path forward to demonstrate enhanced IR sensitivity and larger arrays.

requires that we minimize the thermal capacitance Cth of the device and thereby minimize the

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193

*Temperature Dependent Electrical Characteristics:* In addition to considering the DC and transient thermal characteristics of the absorbing material, we need to optimize the electrical response as well. To achieve this, we associate the electrical response with the thermal response by considering the temperature dependent voltage-current characteristics of the electrical

A good response is obtained by utilizing a material that has an effectively large variation of electrical resistance with temperature. However, at the same time we want the material to have a temperature coefficient of resistance that is relatively independent of the absolute value of the resistance itself. Therefore, we look at materials that have an exponential relationship between electrical resistance and temperature. For such materials, the Temperature Coefficient of Resistance (TCR) is not strongly dependent on the absolute value of the electrical resistance

In this work we are assessing the possibility of designing a bolometer using carbon nanotubes as both the IR absorbing material and the electrical response material. Thus, our aim is to first determine the thermal response of the bolometer absorber that is composed of CNTs, and then determine how the electrical characteristics of the CNT material depend on its changes in

While there are numerous geometries of CNT based material we can consider, for this work we will focus on an absorbing material composed of a CNT film. The film is taken to consist

*Heat Flow Equation:* To determine the temperature of the material in the presence of IR radiation, we start with the heat flow equation. This is a partial differential equation relating the time rate of change in temperature to the position and the rate of net heat that is absorbed by the

diffusion coefficient in watts/degree-cm). *H* is the net power absorbed by the material in watts per unit volume. To solve this equation for the CNT bolometer, we have to first determine *C*<sup>v</sup>

*Thermal Capacitance of CNT Absorber:* To determine the heat capacity of a carbon nanotube, we first determine the internal vibrational energy of the CNT, and then take the derivative with respect to temperature. The internal energy is found by determining the energy of each vibrational mode, multiplying by the probability that the mode is populated using Bose-

) and *κ* is the thermal

thermal RthCth time constant for the absorbing material.

*8.1.1. Calculating the thermal response of the CNT bolometer film*

of a random placement of CNTs that is two nanotubes thick.

In the equation *C*v is the thermal capacitance (joules/degree-unit cm3

Einstein statistics, and then summing over all of the allowed modes.

material.

*Cv* ∂*T*

<sup>∂</sup>*<sup>t</sup>* <sup>=</sup>*κ*∇2*<sup>T</sup>* <sup>+</sup> *<sup>H</sup>*

of the material itself.

temperature after IR absorption.

material as a function of time and position.

and *κ* for the CNT and the CNT film.

The microbolometer based on Si-MEMS device structure has been under development for over 20 years with support from DARPA and the US Army. Two most common Si-MEMS based structures utilize VOx and amorphous silicon based technologies. Several companies such as BAE systems and DRS Technologies are developing and producing 17 micron unit cell 640x 480 and larger arrays using VOx [48-51]. Similarly, L3Communications and other groups are developing and producing 640x480 with 17 micron unit cell using amorphous-Silicon tech‐ nology [50-51].

We will discuss the use of carbon nanostructures for use as the high performance bolometric element of the MWIR and LWIR bands. As part of this effort, we are exploring development of smaller unit cell bolometer.i.e. 5-10 micron unit cell, with higher TCR and higher frequency response in the 1 to 10 KHz range. The feasibility of such an array can open up a larger number of defense and commercial applications. This section will discuss the efforts under way to explore these possibilities.

#### **8.1. Design and modeling of CNT-based bolometer characteristics**

To optimize bolometer design, we need to consider several physical key phenomena. From a fundamental point of view, we take the absorbing material to have an extremely large response to infrared radiation. The phonon modes of the material need to be able to easily couple to infrared radiation. Furthermore, once this coupling has been achieved, the absorbed radiation should greatly increase the population of the phonon modes thereby significantly increasing the lattice temperature [52-53].

On a macroscopic scale, this large temperature increase is typically described in terms of a large thermal resistance. The higher the thermal resistance of the material, in general, the higher the resulting temperature will be after absorbing IR. Of course, higher thermal resist‐ ance may also give rise to larger thermal noise. This mitigating factor must be balanced with signal response in order to optimize IR sensitivity, or the minimum detectable IR signal.

In summary, for DC operation, we work to maximize the thermal resistance Rth while achieving an acceptable noise, which maximizes the minimum signal that we can detect. In addition to DC operation, we try to maximize the thermal frequency response of the bolometer. This requires that we minimize the thermal capacitance Cth of the device and thereby minimize the thermal RthCth time constant for the absorbing material.

*Temperature Dependent Electrical Characteristics:* In addition to considering the DC and transient thermal characteristics of the absorbing material, we need to optimize the electrical response as well. To achieve this, we associate the electrical response with the thermal response by considering the temperature dependent voltage-current characteristics of the electrical material.

A good response is obtained by utilizing a material that has an effectively large variation of electrical resistance with temperature. However, at the same time we want the material to have a temperature coefficient of resistance that is relatively independent of the absolute value of the resistance itself. Therefore, we look at materials that have an exponential relationship between electrical resistance and temperature. For such materials, the Temperature Coefficient of Resistance (TCR) is not strongly dependent on the absolute value of the electrical resistance of the material itself.

#### *8.1.1. Calculating the thermal response of the CNT bolometer film*

In this work we are assessing the possibility of designing a bolometer using carbon nanotubes as both the IR absorbing material and the electrical response material. Thus, our aim is to first determine the thermal response of the bolometer absorber that is composed of CNTs, and then determine how the electrical characteristics of the CNT material depend on its changes in temperature after IR absorption.

While there are numerous geometries of CNT based material we can consider, for this work we will focus on an absorbing material composed of a CNT film. The film is taken to consist of a random placement of CNTs that is two nanotubes thick.

*Heat Flow Equation:* To determine the temperature of the material in the presence of IR radiation, we start with the heat flow equation. This is a partial differential equation relating the time rate of change in temperature to the position and the rate of net heat that is absorbed by the material as a function of time and position.

$$Cv \frac{\partial \, T}{\partial t} = \kappa \nabla^2 \, T \, \star H$$

**8. Nanostructured detector technology for MWIR and LWIR bands**

demonstrate enhanced IR sensitivity and larger arrays.

192 Optical Sensors - New Developments and Practical Applications

**8.1. Design and modeling of CNT-based bolometer characteristics**

nology [50-51].

explore these possibilities.

the lattice temperature [52-53].

EO/IR Sensors and imagers using nanostructure based materials are being developed for a variety of Defense Applications. In this section, we will present recent work under way for development of next generation carbon nanostructure based infrared detectors and arrays. We will discuss detector concepts that will provide next generation high performance, high frame rate, and uncooled nano-bolometer for MWIR and LWIR bands [52-55]. The critical technolo‐ gies being developed include carbon nanostructure growth, characterization, optical and electronic properties that show the feasibility for IR detection. Experimental results on CNT nanostructures will be presented. Further discussion will be provided for the path forward to

The microbolometer based on Si-MEMS device structure has been under development for over 20 years with support from DARPA and the US Army. Two most common Si-MEMS based structures utilize VOx and amorphous silicon based technologies. Several companies such as BAE systems and DRS Technologies are developing and producing 17 micron unit cell 640x 480 and larger arrays using VOx [48-51]. Similarly, L3Communications and other groups are developing and producing 640x480 with 17 micron unit cell using amorphous-Silicon tech‐

We will discuss the use of carbon nanostructures for use as the high performance bolometric element of the MWIR and LWIR bands. As part of this effort, we are exploring development of smaller unit cell bolometer.i.e. 5-10 micron unit cell, with higher TCR and higher frequency response in the 1 to 10 KHz range. The feasibility of such an array can open up a larger number of defense and commercial applications. This section will discuss the efforts under way to

To optimize bolometer design, we need to consider several physical key phenomena. From a fundamental point of view, we take the absorbing material to have an extremely large response to infrared radiation. The phonon modes of the material need to be able to easily couple to infrared radiation. Furthermore, once this coupling has been achieved, the absorbed radiation should greatly increase the population of the phonon modes thereby significantly increasing

On a macroscopic scale, this large temperature increase is typically described in terms of a large thermal resistance. The higher the thermal resistance of the material, in general, the higher the resulting temperature will be after absorbing IR. Of course, higher thermal resist‐ ance may also give rise to larger thermal noise. This mitigating factor must be balanced with signal response in order to optimize IR sensitivity, or the minimum detectable IR signal.

In summary, for DC operation, we work to maximize the thermal resistance Rth while achieving an acceptable noise, which maximizes the minimum signal that we can detect. In addition to DC operation, we try to maximize the thermal frequency response of the bolometer. This In the equation *C*v is the thermal capacitance (joules/degree-unit cm3 ) and *κ* is the thermal diffusion coefficient in watts/degree-cm). *H* is the net power absorbed by the material in watts per unit volume. To solve this equation for the CNT bolometer, we have to first determine *C*<sup>v</sup> and *κ* for the CNT and the CNT film.

*Thermal Capacitance of CNT Absorber:* To determine the heat capacity of a carbon nanotube, we first determine the internal vibrational energy of the CNT, and then take the derivative with respect to temperature. The internal energy is found by determining the energy of each vibrational mode, multiplying by the probability that the mode is populated using Bose-Einstein statistics, and then summing over all of the allowed modes.

The number of allowed modes will depend on the diameter and wrapping angle of the CNTs present, so we take a statistical sample. Multiplying the individual CNT heat capacity by the number of CNTs in the film provides a reasonable value for the heat capacity of the film. After following this procedure and inserting numerical values for physical constants, we arrive at the following average numerical value for thermal capacity of a CNT *C*vt, where the length *L* is in microns and the diameter *d* is in nanometers

 $\mathcal{C}\_{\rm vt} \sim 1.4 \text{ x } 10^{-18} \text{ ( $L$ )} \,(\, d).$ 

*CNT Thermal Diffusion Coefficient:* In addition to the thermal capacitance, we need to determine the thermal diffusion coefficient, and eventually the thermal resistance of a single CNT. Experiments on isolated CNTs have fit the coefficient of thermal diffusion to data obtaining the following expression [52-53]:

$$\kappa \kappa (L \ll T) = \left| 3.7 \times 10^{-7} T + 9.7 \times 10^{-10} T^{-2} + \frac{9.3}{T^{-2}} \left[ 1 + \frac{0.5}{L} \right]^{-1} \right| $$

From *κ* we can obtain the thermal resistance *R*T of a single CNT using the following definition:

$$\mathcal{R}\_T = \frac{4L}{\kappa \pi d^{\frac{\kappa}{2}}}$$

Where L, d are the length and diameter of the CNT

Using average values for CNTs gives the following numerical value for the thermal resistance of a CNT

$$\mathbf{R\_{T}} \sim \mathbf{5} \times 10^{8} \text{(L/d}^{2}\text{)}$$

Where *R*T is in units of degrees K/Watt, *L* is in units of microns and *d* is in units of nanometers. So a tube that is one micron long and one nanometer in diameter will have a thermal resistance *R*T of approximately 5 x 108 K/W.

*Net IR Radiation Power Absorbed:* Now that we have the thermal diffusion and capacitance we are almost ready to begin solving the above heat flow equation to determine the temperature of the bolometer. However, before doing so, we need to determine *H*, the net IR power absorbed by the bolometer. We determine this power using the Stefan-Boltzmann Law of blackbody radiation, which relates the net power absorbed to the temperatures of the subject and the bolometer using the following expression:

$$H\_{net} = \sigma A \varepsilon \left( T\_{\alpha b \dot{\gamma}}^4 - T\_{\dot{b}}^4 \right)$$

Where *Hnet, σ, A, ε, Tobj* and *Tb* are heat absorbed by the bolometer, the Stefan-Boltzmann constant, cross-sectional area, emissivity, object of interest temperature and bolometer absorber temperature, respectively. Figure 1 below shows the net IR power absorbed by the absorber as a function of bolometer temperature for radiating objects at 20o C and 36.5o C. Cooling the bolometer by 30o C below room temperature allows for significantly more power to be absorbed, which can give rise to a much stronger signal.

*Calculating the Bolometer Temperature Distribution:* Using the aforementioned expressions for *Cvt, K* and *H*, we expand on previous work and convert the heat flow equation above into a thermal network, illustrated in Figure 2 [6]. In actuality, there are thousands of nodes in the

**Figure 30.** Illustration of thermal network superimposed on bolometer for calculating temperature map of CNT bol‐

In Figure 30, each resistor represents the thermal resistance of a CNT in series with the thermal resistance between adjacent CNTs. In addition, the capacitors represent the thermal capacity of a CNT, while the current sources represent the net IR radiation absorbed by each CNT. This thermal network contains thousands of nodes, and there is an equation relating the thermal

Figure 30: Illustration of thermal network superimposed on bolometer for calculating temperature map of

30

network for which we calculate the temperature for each.

CNT bolometer absorber.[55]

ometer absorber.[55]

**Figure 29.** Net power received by bolometer as a function of bolometer temperature.[54]

function of bolometer temperature for radiating objects at 20<sup>o</sup>

network for which we calculate the temperature for each.

Net IR Radiation Power Absorbed: Now that we have the thermal diffusion and capacitance we are almost ready to begin solving the above heat flow equation to determine the temperature of the bolometer. However, before doing so, we need to determine H, the net IR power absorbed by

4 4 ( ) H AT T net = − σ ε obj b

Calculating the Bolometer Temperature Distribution: Using the aforementioned expressions for Cvt, K and H, we expand on previous work and convert the heat flow equation above into a thermal network, illustrated in Figure 2 [6]. In actuality, there are thousands of nodes in the

C and 36.5<sup>o</sup>

Nanostructured Detector Technology for Optical Sensing Applications

C below room temperature allows for significantly more power to be absorbed,

C. Cooling the

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195

Where Hnet, σ, A, ε, Tobj and Tb are heat absorbed by the bolometer, the Stefan-Boltzmann constant, cross-sectional area, emissivity, object of interest temperature and bolometer absorber temperature, respectively. Figure 1 below shows the net IR power absorbed by the absorber as a

the bolometer. We determine this power using the Stefan-Boltzmann Law of blackbody radiation, which relates the net power absorbed to the temperatures of the subject and the

bolometer using the following expression:

which can give rise to a much stronger signal.

bolometer by 30<sup>o</sup>

**Figure 29.** Net power received by bolometer as a function of bolometer temperature.[54] network for which we calculate the temperature for each.

The number of allowed modes will depend on the diameter and wrapping angle of the CNTs present, so we take a statistical sample. Multiplying the individual CNT heat capacity by the number of CNTs in the film provides a reasonable value for the heat capacity of the film. After following this procedure and inserting numerical values for physical constants, we arrive at the following average numerical value for thermal capacity of a CNT *C*vt, where the length *L*

*CNT Thermal Diffusion Coefficient:* In addition to the thermal capacitance, we need to determine the thermal diffusion coefficient, and eventually the thermal resistance of a single CNT. Experiments on isolated CNTs have fit the coefficient of thermal diffusion to data obtaining

> 0.5 *<sup>L</sup>* } −1

From *κ* we can obtain the thermal resistance *R*T of a single CNT using the following definition:

Using average values for CNTs gives the following numerical value for the thermal resistance

Where *R*T is in units of degrees K/Watt, *L* is in units of microns and *d* is in units of nanometers. So a tube that is one micron long and one nanometer in diameter will have a thermal resistance

*Net IR Radiation Power Absorbed:* Now that we have the thermal diffusion and capacitance we are almost ready to begin solving the above heat flow equation to determine the temperature of the bolometer. However, before doing so, we need to determine *H*, the net IR power absorbed by the bolometer. We determine this power using the Stefan-Boltzmann Law of blackbody radiation, which relates the net power absorbed to the temperatures of the subject

Where *Hnet, σ, A, ε, Tobj* and *Tb* are heat absorbed by the bolometer, the Stefan-Boltzmann constant, cross-sectional area, emissivity, object of interest temperature and bolometer absorber temperature, respectively. Figure 1 below shows the net IR power absorbed by the

*Calculating the Bolometer Temperature Distribution:* Using the aforementioned expressions for *Cvt, K* and *H*, we expand on previous work and convert the heat flow equation above into a

C below room temperature allows for significantly more power

C and 36.5o

C.

absorber as a function of bolometer temperature for radiating objects at 20o

to be absorbed, which can give rise to a much stronger signal.

9.3 *<sup>T</sup>* <sup>2</sup> <sup>1</sup> <sup>+</sup>

is in microns and the diameter *d* is in nanometers

194 Optical Sensors - New Developments and Practical Applications

*T* + 9.7*x*10−10*T* <sup>2</sup> +

Where L, d are the length and diameter of the CNT

K/W.

and the bolometer using the following expression:

*Cvt ~ 1.4 x 10-18 ( L)( d)*

*κ*(*L* , *T* )={3.7*x*10−<sup>7</sup>

*RT* <sup>=</sup> <sup>4</sup>*<sup>L</sup> κπd* <sup>2</sup>

of a CNT RT ~ 5 x 108

*Hnet* =*σAε*(*Tobj*

(L/d2 )

*R*T of approximately 5 x 108

<sup>4</sup> <sup>−</sup>*Tb* 4 )

Cooling the bolometer by 30o

the following expression [52-53]:

Figure 30: Illustration of thermal network superimposed on bolometer for calculating temperature map of **Figure 30.** Illustration of thermal network superimposed on bolometer for calculating temperature map of CNT bol‐ ometer absorber.[55]

thermal network, illustrated in Figure 2 [6]. In actuality, there are thousands of nodes in the network for which we calculate the temperature for each. CNT bolometer absorber.[55] In Figure 30, each resistor represents the thermal resistance of a CNT in series with the thermal

resistance between adjacent CNTs. In addition, the capacitors represent the thermal capacity of a CNT, while the current sources represent the net IR radiation absorbed by each CNT. This thermal network contains thousands of nodes, and there is an equation relating the thermal

30

In Figure 30, each resistor represents the thermal resistance of a CNT in series with the thermal resistance between adjacent CNTs. In addition, the capacitors represent the thermal capacity of a CNT, while the current sources represent the net IR radiation absorbed by each CNT. This thermal network contains thousands of nodes, and there is an equation relating the thermal resistance, capacitance and net power for each node. This system of equations is then solved for the temperature as a function of position and time throughout the bolometer absorber [54].

resistance, the pixel temperature can be determined. Therefore, in addition to having a large thermal resistance which translates into higher temperature rises, a large temperature coefficient of electrical resistance (TCR) is desirable to achieve a higher temperature

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197

Here TCR is defined as the change in electrical resistance per degree Kelvin divided by the

Thus, the pixel electrical resistance after it reaches a temperature that is ∆T above its ambient becomes *Re(T) = Re(To)(1+TCR)*. Using this relationship, the pixel temperature is calculated.

To obtain a high temperature resolution, a large change in electrical resistance is needed upon heating. To achieve this, a substantial increase either in electron concentration or velocity (for a given electric field) is necessary. And to this end, materials with junctions where thermionic emission or tunneling are the electrical current bottlenecks offer a good solution. As the tunneling current exponentially rises with temperature, the effective change in their electrical resistance due to temperature becomes large compared to those observed in bulk materials

Here, a film of CNTs is proposed as the bolometer pixel material, since it is likely to have large thermal resistance and TCR values simultaneously. Both of these favorable properties are partially owed to the junctions between the tubes. As the electrical current flows along the mat,

At this intersection, the carriers see a potential barrier that they need to tunnel through which gives rise to exponential increase in current upon heating. Assuming that the electron transport across this barrier is governed by a Fowler-Nordheim-type tunneling or thermionic emission,

Where *Tt, v, DOS* and *f*, are the transmission coefficient, thermal velocity, density of states and distribution function, respectively for electrons in the CNT. We perform this calculation a function of barrier height and electric field. The results are shown in figure 4. The figure on

The TCR is plotted as a function of electric field between the tubes and the barrier height. Theoretical calculations predict an extremely large TCR, which can be attributed to the relatively large barrier height between adjacent CNTs. If a lower barrier height is assumed, on the order of 0.06eV, then a TCR of approximately 2.5% is obtained. It is also worth pointing out that such a large TCR comes at the price of extremely low output currents. The bolometer current densities, as a function of barrier height and electric field are shown in Figure 32, on

absolute electrical resistance measured at the quiescent point, as follows:

where the change is proportional *T<sup>γ</sup>* and *γ* is generally < 2.

it needs to jump from one tube to the next where they intersect.

*Tt*(*E*)*v*(*E*)*DOS*(*E*) *f* (*E*, *T* )*dE*

the expected TCR values can be calculated using the following expression:

the left shows a contour plot of the theoretical values of the TCR for a CNT film.

resolution.

*TCR* <sup>=</sup> <sup>1</sup> *Re*

*I* =*q∫* 0 *∞*

the right.

*dRe dT*

Results of these calculations for are shown in Figure 31 for different types of CNT networks. Here, we assumed that the net absorbed power is 1 nW, and the pixel is tightly packed with the CNTs. The entire pixel's temperature map is obtained with a 100×100 tempera‐ ture resolution. For the tubes, we used two different thermal resistance values: 5×108 and 1×109 K/W.

As expected, higher the thermal resistance, higher the temperature difference from the ambient. We note that in general thermal resistance also rises with increasing temperature, resulting in further heating of hot spots compared to the case that this dependency is ignored. The temperature gradient of the contacts legs connecting the film to the readout IC (ROIC) is clearly shown.

**Figure 31.** Temperature map of a bolometer pixel when the net absorbed power *H* is 1 nW, and the CNT thermal resistance is 5×108 K/W (left) and 1×109 K/W (left). We assumed that the pixel is tightly packed with the CNTs. The temperature gradient of the contacts legs connecting the film to the ROIC is clearly shown [54]

#### *8.1.2. Calculating the electrical response of the CNT film*

To read the temperature that the bolometer pixel reaches after an exposure to infrared radiation, one needs to measure the electrical resistance of the pixel. By comparing this resistance to a look-up table or using the a-priori knowledge of temperature coefficient of

resistance, the pixel temperature can be determined. Therefore, in addition to having a large thermal resistance which translates into higher temperature rises, a large temperature coefficient of electrical resistance (TCR) is desirable to achieve a higher temperature resolution.

Here TCR is defined as the change in electrical resistance per degree Kelvin divided by the absolute electrical resistance measured at the quiescent point, as follows:

$$TCR = \frac{1}{R\_e} \frac{d \, R\_e}{dT}$$

In Figure 30, each resistor represents the thermal resistance of a CNT in series with the thermal resistance between adjacent CNTs. In addition, the capacitors represent the thermal capacity of a CNT, while the current sources represent the net IR radiation absorbed by each CNT. This thermal network contains thousands of nodes, and there is an equation relating the thermal resistance, capacitance and net power for each node. This system of equations is then solved for the temperature as a function of position and time throughout the bolometer absorber [54].

Results of these calculations for are shown in Figure 31 for different types of CNT networks. Here, we assumed that the net absorbed power is 1 nW, and the pixel is tightly packed with the CNTs. The entire pixel's temperature map is obtained with a 100×100 tempera‐ ture resolution. For the tubes, we used two different thermal resistance values: 5×108 and

As expected, higher the thermal resistance, higher the temperature difference from the ambient. We note that in general thermal resistance also rises with increasing temperature, resulting in further heating of hot spots compared to the case that this dependency is ignored. The temperature gradient of the contacts legs connecting the film to the readout IC (ROIC) is

**Figure 31.** Temperature map of a bolometer pixel when the net absorbed power *H* is 1 nW, and the CNT thermal resistance is 5×108 K/W (left) and 1×109 K/W (left). We assumed that the pixel is tightly packed with the CNTs. The

To read the temperature that the bolometer pixel reaches after an exposure to infrared radiation, one needs to measure the electrical resistance of the pixel. By comparing this resistance to a look-up table or using the a-priori knowledge of temperature coefficient of

temperature gradient of the contacts legs connecting the film to the ROIC is clearly shown [54]

*8.1.2. Calculating the electrical response of the CNT film*

1×109 K/W.

196 Optical Sensors - New Developments and Practical Applications

clearly shown.

Thus, the pixel electrical resistance after it reaches a temperature that is ∆T above its ambient becomes *Re(T) = Re(To)(1+TCR)*. Using this relationship, the pixel temperature is calculated.

To obtain a high temperature resolution, a large change in electrical resistance is needed upon heating. To achieve this, a substantial increase either in electron concentration or velocity (for a given electric field) is necessary. And to this end, materials with junctions where thermionic emission or tunneling are the electrical current bottlenecks offer a good solution. As the tunneling current exponentially rises with temperature, the effective change in their electrical resistance due to temperature becomes large compared to those observed in bulk materials where the change is proportional *T<sup>γ</sup>* and *γ* is generally < 2.

Here, a film of CNTs is proposed as the bolometer pixel material, since it is likely to have large thermal resistance and TCR values simultaneously. Both of these favorable properties are partially owed to the junctions between the tubes. As the electrical current flows along the mat, it needs to jump from one tube to the next where they intersect.

At this intersection, the carriers see a potential barrier that they need to tunnel through which gives rise to exponential increase in current upon heating. Assuming that the electron transport across this barrier is governed by a Fowler-Nordheim-type tunneling or thermionic emission, the expected TCR values can be calculated using the following expression:

$$I = q \int\_0^\infty T\_t(E) v(E) DOS(E) f(E, T) dE$$

Where *Tt, v, DOS* and *f*, are the transmission coefficient, thermal velocity, density of states and distribution function, respectively for electrons in the CNT. We perform this calculation a function of barrier height and electric field. The results are shown in figure 4. The figure on the left shows a contour plot of the theoretical values of the TCR for a CNT film.

The TCR is plotted as a function of electric field between the tubes and the barrier height. Theoretical calculations predict an extremely large TCR, which can be attributed to the relatively large barrier height between adjacent CNTs. If a lower barrier height is assumed, on the order of 0.06eV, then a TCR of approximately 2.5% is obtained. It is also worth pointing out that such a large TCR comes at the price of extremely low output currents. The bolometer current densities, as a function of barrier height and electric field are shown in Figure 32, on the right.

for SWCNT and MWCNT samples with various sample treatments. We have carried out some

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199

**Figure 34.** The composite figure shows prototype CNT film bolometer test fixture to evaluate the CNT film quality. Some preliminary data on reflectivity measurements for SWCNT and MWCNT are shown along with preliminary results

Figure 35 shows scanning electron microscopy SEM. images of representative MWCNT films in the unsuspended left. and suspended.right. forms, respectively. Unlike their SWCNT counterparts, the MWCNT films contain substantial uncovered substrate areas. In addition, some minor deformation of recess is visible on suspended MWCNT films, which is similar to the SWCNT film case in the same thickness range. Figure 35.b includes a transmission electron microscopy.TEM. image of a representative individual MWCNT, which has a large hollow center of approximately 10–11 nm in diameter and contains approximately 40–50 CNT shells.

on TCR measurements.[55]

preliminary measurements of TCR on CNT samples.

**Figure 32.** Left figure is a contour plot of TCR versus electric field and barrier height between CNTs of the film. The right figure shows the bolometer current also as a function of electric field and barrier height. The scales are the color bars on the right of each contour plot in units of %TCR and amperes, respectively.

#### *8.1.3. CNT growth and charcterization*

In this section, we will discuss growth and characterization of carbon nanotubes with single wall (SWCNT) and multiwall (MWCNT) for use as the high performance bolometric element for development of MWIR and LWIR sensitive detector elements.

**Figure 33.** Growth of multiwall CNT forest with the ability to separate the form growth substrate with good length / diameter uniformity and the MWCNT released from the template.[55]

Figure 33 presents growth of dense oriented multi-walled CNT "forest like growth". The figure shows the CNT growth can be easily separated from the growth substrate. We have shown good length/diameter uniformity. Further work on the growth optimization is underway.

Figure 34 shows the prototype fixture to evaluate the CNT films for bolometric application. This fixture is being used for quick evaluation of both electrical and optical characteristics of the CNT samples. The figure also shows the preliminary results of reflectivity measurements for SWCNT and MWCNT samples with various sample treatments. We have carried out some preliminary measurements of TCR on CNT samples.

**Figure 32.** Left figure is a contour plot of TCR versus electric field and barrier height between CNTs of the film. The right figure shows the bolometer current also as a function of electric field and barrier height. The scales are the color

In this section, we will discuss growth and characterization of carbon nanotubes with single wall (SWCNT) and multiwall (MWCNT) for use as the high performance bolometric element

**Figure 33.** Growth of multiwall CNT forest with the ability to separate the form growth substrate with good length /

Figure 33 presents growth of dense oriented multi-walled CNT "forest like growth". The figure shows the CNT growth can be easily separated from the growth substrate. We have shown good length/diameter uniformity. Further work on the growth optimization is underway.

Figure 34 shows the prototype fixture to evaluate the CNT films for bolometric application. This fixture is being used for quick evaluation of both electrical and optical characteristics of the CNT samples. The figure also shows the preliminary results of reflectivity measurements

bars on the right of each contour plot in units of %TCR and amperes, respectively.

for development of MWIR and LWIR sensitive detector elements.

diameter uniformity and the MWCNT released from the template.[55]

*8.1.3. CNT growth and charcterization*

198 Optical Sensors - New Developments and Practical Applications

**Figure 34.** The composite figure shows prototype CNT film bolometer test fixture to evaluate the CNT film quality. Some preliminary data on reflectivity measurements for SWCNT and MWCNT are shown along with preliminary results on TCR measurements.[55]

Figure 35 shows scanning electron microscopy SEM. images of representative MWCNT films in the unsuspended left. and suspended.right. forms, respectively. Unlike their SWCNT counterparts, the MWCNT films contain substantial uncovered substrate areas. In addition, some minor deformation of recess is visible on suspended MWCNT films, which is similar to the SWCNT film case in the same thickness range. Figure 35.b includes a transmission electron microscopy.TEM. image of a representative individual MWCNT, which has a large hollow center of approximately 10–11 nm in diameter and contains approximately 40–50 CNT shells.

in the resistivity of MWCNT films is much less than that of SWCNT films with decreasing temperature, as shown in Figure 36. This is not unexpected considering a much smaller band gap in MWCNTs. The reduced temperature dependence also implies smaller TCR absolute value in MWCNTs. For example, the TCR absolute value at room temperature for MWCNT films is about 0.07%/K in contrast to.0.17%/K for SWCNT films. R-T curve after suspending

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**Figure 37.** Photoresponse of unsuspended and suspended CNT films.(a) Unsuspended MWCNT film, f=10 Hz, in IR.3 mW/mm2 ;.(b). suspended MWCNT film, f=10 Hz, in IR.3 mW/mm2 ; (c) unsuspended SWCNT film, f=1/30 Hz, in IR.3.5

Figure 37 compares the Photoresponse R/R0 of MWCNT films in unsuspended (a) and suspended (b) cases, where R0 is the sample resistance before IR radiation was turned on and the change in the resistance caused by IR radiation is defined as.R=R-R0. For comparison, the results of their SWCNT counterparts are also included in Figure 37 (c).unsuspended and Fig.

Two major differences are visible between MWCNT and SWCNT films, a significantly higher.R/R0 and a much shorter response time in the cases of MWCNT. The.R/R0 for MWCNT

mW/mm2 ; and (d) suspended SWCNT film, f =2 Hz, in IR.3.5 mW/mm2 [56].

37(d). suspended..

the MWCNT film has been also measured.

**Figure 35.** SEM images of unsuspended.left (a). and suspended.right (b). MWCNT films. A TEM image of a representa‐ tive MWCNT. The shell number is estimated to be.40–50 for the MWCNTs [56]

**Figure 36.** Resistance versus temperatures curves of SWCNT films and MWCNT films.[56]

All MWCNT films studied in this work [56] show semiconductive resistance-temperature.R-T. behaviors and a representative curves is depicted in Figure 36.. Nevertheless, the increase in the resistivity of MWCNT films is much less than that of SWCNT films with decreasing temperature, as shown in Figure 36. This is not unexpected considering a much smaller band gap in MWCNTs. The reduced temperature dependence also implies smaller TCR absolute value in MWCNTs. For example, the TCR absolute value at room temperature for MWCNT films is about 0.07%/K in contrast to.0.17%/K for SWCNT films. R-T curve after suspending the MWCNT film has been also measured.

**Figure 35.** SEM images of unsuspended.left (a). and suspended.right (b). MWCNT films. A TEM image of a representa‐

tive MWCNT. The shell number is estimated to be.40–50 for the MWCNTs [56]

200 Optical Sensors - New Developments and Practical Applications

**Figure 36.** Resistance versus temperatures curves of SWCNT films and MWCNT films.[56]

All MWCNT films studied in this work [56] show semiconductive resistance-temperature.R-T. behaviors and a representative curves is depicted in Figure 36.. Nevertheless, the increase

**Figure 37.** Photoresponse of unsuspended and suspended CNT films.(a) Unsuspended MWCNT film, f=10 Hz, in IR.3 mW/mm2 ;.(b). suspended MWCNT film, f=10 Hz, in IR.3 mW/mm2 ; (c) unsuspended SWCNT film, f=1/30 Hz, in IR.3.5 mW/mm2 ; and (d) suspended SWCNT film, f =2 Hz, in IR.3.5 mW/mm2 [56].

Figure 37 compares the Photoresponse R/R0 of MWCNT films in unsuspended (a) and suspended (b) cases, where R0 is the sample resistance before IR radiation was turned on and the change in the resistance caused by IR radiation is defined as.R=R-R0. For comparison, the results of their SWCNT counterparts are also included in Figure 37 (c).unsuspended and Fig. 37(d). suspended..

Two major differences are visible between MWCNT and SWCNT films, a significantly higher.R/R0 and a much shorter response time in the cases of MWCNT. The.R/R0 for MWCNT samples is typically in the range of a few percent, which is more than one order of magnitude higher than that of suspended SWCNT films and two orders of magnitude higher than the unsuspended SWCNT films at a comparable IR power. Considering a lower TCR absolute value in MWCNTs, the much enhanced Photoresponse of MWCNT films should be attributed to the naturally suspended inner CNT shells, which may provide an ideal configuration to enhance the bolometric effect by improving light absorption and reducing thermal link. Physical suspension of the films in both MWCNT.Fig. 37.(b) and SWCNT.Fig. 37(d.) cases results in a further improvement of.R/R0 as compared to their unsuspended counterparts. The improvement is, however, much more pronounced in suspended cases [56].

arrays. Our goal is to develop high performance, high frame rate, and uncooled nanobolometer for MWIR and LWIR bands. We also discussed CNT growth system and its capability to grow samples of various orientations. We have also presented recent results on SWCNT and MWCNT samples that show promise for use of CNT for developing next

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203

In this chapter, we have discussed recent advances in nanostructured based detector technol‐ ogy, materials and devices for optical sensing applications. The chapter has presented an overview of recent work underway on a variety of semiconductors and advanced materials

Optical sensing technology is critical for defense and commercial applications including optical communication. Advances in optoelectronics materials in the UV, Visible and Infrared, using nanostructures, and use of novel materials such as CNT have opened doors for new approaches to apply device design methodology that are expected to offer enhanced perform‐

We have covered the UV band (200-400 nm) and address some of the recent advances in nanostructures growth and characterization using GaN/AlGaN, ZnO/MgZnO based technol‐ ogies and their applications. We have also discussed nanostructure based Si/SiGe technologies (400-1700 nm) that covers various bands of interest in visible-near infrared for detection and optical communication applications. The chapter has also discussed some of the theoretical

Recent advancements in design and development of CNT based detection technologies have shown promise for optical sensor applications. We have presented theoretical and experimen‐ tal results on these device and their potential applications in various bands of interest. It is anticipated that the current research and development presented in this chapter will enable a host of new integrated technologies for a variety of defense and commercial applications.

Although numerous research activities are ongoing in the area of Nanoscience and technology, we briefly made comments on such technologies to make readers aware of various research

The authors gratefully acknowledge the contributions of the many distinguished scientists in the United States for development of nanotechnology based EO/IR detector technology for

such as GaN, ZnO, Si/SiGe, InGaAs and CNT for optical sensing applications.

ance and low cost optical sensors in a wide range of applications.

and experimental results in these detector technologies.

generation high performance small pixel bolometer arrays.

**9. Summary**

activities.

**Acknowledgements**

optical sensor applications.

**Figure 38.** TCR as function of temperature for a 90 nm thick MWCNT film (a) and a 100 nm thick MWCNT film before annealing. (b) TCR versus thickness for MWCNT and SWCNT films with different thicknesses. (c) TCR versus thickness/ diameter ratios for SWCNT and MWCNT films [57].

We have also shown the results of TCR as function of temperature in figure 38, for a 90 nm thick MWCNT film (a) and a 100 nm thick MWCNT film before annealing. (b) TCR versus thickness for MWCNT and SWCNT films with different thicknesses. (c) TCR versus thickness/ diameter ratios for SWCNT and MWCNT films [57].

We have discussed recent efforts for modeling CNT based bolometer and the experimental work for development of next generation carbon nanostructure based infrared detectors and arrays. Our goal is to develop high performance, high frame rate, and uncooled nanobolometer for MWIR and LWIR bands. We also discussed CNT growth system and its capability to grow samples of various orientations. We have also presented recent results on SWCNT and MWCNT samples that show promise for use of CNT for developing next generation high performance small pixel bolometer arrays.

#### **9. Summary**

samples is typically in the range of a few percent, which is more than one order of magnitude higher than that of suspended SWCNT films and two orders of magnitude higher than the unsuspended SWCNT films at a comparable IR power. Considering a lower TCR absolute value in MWCNTs, the much enhanced Photoresponse of MWCNT films should be attributed to the naturally suspended inner CNT shells, which may provide an ideal configuration to enhance the bolometric effect by improving light absorption and reducing thermal link. Physical suspension of the films in both MWCNT.Fig. 37.(b) and SWCNT.Fig. 37(d.) cases results in a further improvement of.R/R0 as compared to their unsuspended counterparts. The

**Figure 38.** TCR as function of temperature for a 90 nm thick MWCNT film (a) and a 100 nm thick MWCNT film before annealing. (b) TCR versus thickness for MWCNT and SWCNT films with different thicknesses. (c) TCR versus thickness/

We have also shown the results of TCR as function of temperature in figure 38, for a 90 nm thick MWCNT film (a) and a 100 nm thick MWCNT film before annealing. (b) TCR versus thickness for MWCNT and SWCNT films with different thicknesses. (c) TCR versus thickness/

We have discussed recent efforts for modeling CNT based bolometer and the experimental work for development of next generation carbon nanostructure based infrared detectors and

diameter ratios for SWCNT and MWCNT films [57].

diameter ratios for SWCNT and MWCNT films [57].

improvement is, however, much more pronounced in suspended cases [56].

202 Optical Sensors - New Developments and Practical Applications

In this chapter, we have discussed recent advances in nanostructured based detector technol‐ ogy, materials and devices for optical sensing applications. The chapter has presented an overview of recent work underway on a variety of semiconductors and advanced materials such as GaN, ZnO, Si/SiGe, InGaAs and CNT for optical sensing applications.

Optical sensing technology is critical for defense and commercial applications including optical communication. Advances in optoelectronics materials in the UV, Visible and Infrared, using nanostructures, and use of novel materials such as CNT have opened doors for new approaches to apply device design methodology that are expected to offer enhanced perform‐ ance and low cost optical sensors in a wide range of applications.

We have covered the UV band (200-400 nm) and address some of the recent advances in nanostructures growth and characterization using GaN/AlGaN, ZnO/MgZnO based technol‐ ogies and their applications. We have also discussed nanostructure based Si/SiGe technologies (400-1700 nm) that covers various bands of interest in visible-near infrared for detection and optical communication applications. The chapter has also discussed some of the theoretical and experimental results in these detector technologies.

Recent advancements in design and development of CNT based detection technologies have shown promise for optical sensor applications. We have presented theoretical and experimen‐ tal results on these device and their potential applications in various bands of interest. It is anticipated that the current research and development presented in this chapter will enable a host of new integrated technologies for a variety of defense and commercial applications.

Although numerous research activities are ongoing in the area of Nanoscience and technology, we briefly made comments on such technologies to make readers aware of various research activities.

## **Acknowledgements**

The authors gratefully acknowledge the contributions of the many distinguished scientists in the United States for development of nanotechnology based EO/IR detector technology for optical sensor applications.

#### **Author details**

Ashok K. Sood1 , Nibir K. Dhar2 , Dennis L. Polla3 , Madan Dubey4 and Priyalal Wijewarnasuriya4


[10] Lee, C. H., Yi, G. C.., Zuev, Y. M., and Kim, P., "Thermoelectric power measurements of wide band gap semiconducting nanowires,"Appl. Phys. Lett. 94, 22106 (2009).

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[11] Falyouni, F., Benmamas, L., Thiandoume, C., Barjon, J., Lusson, A., Galter, P., and Sallet, V., "Metal organic chemical vapor deposition growth and luminescence of

[12] Jeong, M. C., Oh, B.Y., Lee, W., and Myoung, J. M., "Comparative study on the growth characteristics of ZnO nanowires and thin films by metal-organic chemical

[13] Kim, S. W., Fujita, S., and Fujita, S., "ZnO nanowires with high aspect ratios grown by metal-organic chemical vapor deposition using gold nanoparticles," Appl. Phys.

[14] Lee, W., Jeong, M. C., and Myoung, J. M., "Catalyst-free growth of ZnO nanowires by metal-organic chemical vapor deposition (MOCVD) and thermal evaporation,"

[15] Liou, S. C., Hsiao, C. S., and Chen, S. Y., "Growth behavior and microstructure evo‐ lution of ZnO nanorods grown on Si in aqueous solution," Journal of. Crystal.

[16] Dong, J. W., Osinski, A., Hertog, B., Dabiran, A. M., Chow, P. P., Heo, Y. W., Norton, D. P, and Pearton, S. J., "Development of MgZnO-ZnO-AlGaN heterostructures for

[17] Rivera, A., Zeller, J., Sood, A.K., and Anwar, A. F. M., "A Comparison of ZnO Nano‐ wires and Nanorods Grown Using MOCVD and Hydrothermal Processes," J. Elec‐

[18] Ha, B., Ham, H., and Lee, C. J., "Photoluminescence of ZnO nanowires dependent on

[19] Djurišić, A. B., Ng, A.M.C., and Chen, X.Y., "ZnO nanostructures for optoelectronics: Material properties and device applications," Progress Quantum Electronics 34,

[20] Mehdi Anwar, Abdiel Rivera, Anaz Mazady, Hung Chou, John Zeller and Ashok K. Sood, "ZnO Solar Blind Detectors: from Material to System", Proceedings of SPIE

[21] Zhong Lin Wang, Guang Zhu, Ya Yang, Sihong Wang and Caofeng Pan, " Progress in Nanogenerator for Portable Electronics" Materials Today Volume 15, Number 12

[22] Wenzhuo Wu, Xiaonan Wen and Zhong Lin Wang, " Texel- Addressable Matrix of Vertical-Nanowire and Adaptive Tactile Imaging" Science, Volume 340, 24 May

O2 and Ar annealing," Phys. Chem. Solids 69, 2453-2456 (2008).

ultraviolet light emitting applications," J. Electron. Mat. 34, 416-423 (2005).

vapor deposition (MOCVD)," Journal of. Crystal Growth 268, 149-154 (2004).

Lett. 86,153119 (2005).

Growth 274, 438 (2005).

tron. Mat. 42, 894-900 (2013).

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December 2012

2013.

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ZnO micro- and nanowires," Journal Vac. Sci. Technol. B 87, 1662 (2009).

4 Army Research Laboratory, Adelphi, MD, USA

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[10] Lee, C. H., Yi, G. C.., Zuev, Y. M., and Kim, P., "Thermoelectric power measurements of wide band gap semiconducting nanowires,"Appl. Phys. Lett. 94, 22106 (2009).

**Author details**

Ashok K. Sood1

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**Chapter 10**

**Dipping Deposition Study of Anodized-Aluminum**

**Pressure-Sensitive Paint for Unsteady Aerodynamic**

In aerospace engineering, anodized-aluminum pressure-sensitive paint (AA-PSP) has been used in short duration time tests [1 - 12], unsteady flow visualizations, and unsteady pressure measurements [13 – 25]. Because of its nano-open structure (Figure 1), AA-PSP yields high mass diffusion that results in a pressure response time on the order of ten microseconds [26]. This structure enables oxygen gas to interact directly with luminophores on the pore surface, which provides fast response to pressures. By applying an AA-PSP, we can obtain global surface pressure information instead of pointwise information that may result in wide applications in pressure detection fields. AA-PSP is an optical sensor that consists of a molecular pressure probe (luminophore) and an anodized aluminum as a supporting matrix. As schematically shown in Figure 2, the luminophore on the anodized-aluminum surface is excited by an illumination source and gives off luminescence. This luminescence is related to gaseous oxygen in a test gas, a process called oxygen quenching. Because the gaseous oxygen can be described as a partial pressure of oxygen as well as a static pressure, the luminescence from an AA-PSP can be described as a static pressure. See Section 3.2 for a detailed description.

The luminophore is directly related to important parameters of AA-PSPs, such as the lumi‐ nescent signal level, pressure sensitivity, temperature dependency, and response time. Mainly three types of luminophores are commonly used for PSP in general, such as ruthenium complex, porphyrin, and pyrene. Each luminophore has an optimum excitation wavelength, and its peak wavelength of luminescence varies by the luminophore as well. For AA-PSP, the luminophore is applied on the anodized-aluminum surface by the dipping deposition method [27]. This method requires a luminophore, a solvent, and an anodized-aluminum coating. The

> © 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Applications**

Hirotaka Sakaue

**1. Introduction**

http://dx.doi.org/10.5772/57416

Additional information is available at the end of the chapter

