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Design and Fabrication of Nano Antenna for Carbon Nanotube Infrared Detector

Carmen Kar Man Fung*, Ning Xi, Balasubramaniam Shanker, King Wai Chiu Lai, Jiangbo Zhang, Hongzhi Chen and Yilun LuoElectrical and Computer Engineering,

Michigan State University, USAE-mail: [email protected]

Abstract- This paper reports on fabrication of a novelnanoscale antenna with carbon nanotubes (CNTs) based sensorfor sensitive infrared detection. CNTs are used as sensing elementfor detecting both near and middle wave infrared. By using thenanoassembly of CNTs and standard photolithography processes,a nanosized antenna is designed and integrated to enhance theelectric field intensity at the position of the sensing elementcreating a very sensitive infrared nanosensor. The efficiency of nano antenna was studied experimentally by measuring andcomparing the photocurrent response of the sensor to one withoutthe antenna. The photocurrent was increased 10 fold afterintegrating the nano antenna. This implies that the electric field inthe vicinity of the detector is enhanced by the nano antenna. Theproposed fabrication process enables easy and direct integrationof the nano antenna into the manufacturing of infrared devices.Therefore, this opens the possibility of developing high fidelityinfrared sensors with a wide sensing range. Moreover, a novelapproach was employed to consider and include quantum effectsin the analysis and design for the nano antenna, which uses theGreen’s function for finding the near field effect from the antenna.As a result, these effects were verified and demonstratedexperimentally in this paper.

I. I NTRODUCTION

The development of infrared (IR) detectors usingnanostructural materials has attracted increasing interest inrecent years. CNTs have been found to be promisingcandidates for photodetection due to their low thermal noise. Itis well known that the electronic properties of CNTs aredifferent and they can exist in two forms, conducting and semi-conducting. Semi-conducting CNTs are natural candidates for IR detector as they have a bandgap in the IR range. Our grouphas already developed a single CNT based nano infrareddetector [1], which exhibited IR response and small dark current. However, the photocurrent response of CNT sensorshas been relatively low because of the small sensing area.Thus, an enhancement technology is developed in this paper toincrease the electric field at the sensing area. Since the size of the CNT is in nano scale, the technique to enhance the electricfield that is incident on such small sensing element ischallenging. In this paper, a nano antenna is used to increasethe response of the sensor. These are not antennas in thetraditional sense, in that we are not interested in the radiatedfield. However, this terminology has been used extensively inthe optics literature to refer to metallic field enhancementstructures in the near field, especially in development of

plasmonic devices. On the other hand, dipole antennas have been widely used in electromagnetics because of their

simplicity and special features. Recently, antennas in nanoscale have been investigated theoretically [2] and explored invarious applications such as optoelectronics and nanophotonics[3]. However, the experimental realization of practical nanoantenna for quantum infrared detector has not been reported.

In this paper, the fabrication of the ultra-small and sensitiveCNT based IR detector with the nano antenna will be

presented. To investigate the electric field enhancement effectof the antenna, experiments have been performed to measure

the photocurrent response of the sensor with and without thenano antenna. Preliminary results showed the photocurrent wasincreased by 10 times, and the I-V characteristic of the devicehas also been obtained. Several issues such as the conductivityof the nano sized antenna and factors to affect the antenna gainwill also be discussed. The novelty of our work includes theanalysis of the near field and quantum effect of the nanoantenna, by applying Schrödinger’s equation and Green’sfunction to find the maximum field intensity near the antenna,where the classical Maxwell’s method cannot be fully appliedfor the antenna in nano environment. In addition, our experimental results show the nano antenna is efficient in near field. These main contributions will be presented in this paper.

II. DESIGN AND WORKING PRINCIPLE To design a nano antenna with maximum electric field

enhancement effect, the location of the maximum fieldintensity is first estimated by the following integral equationfor current density [2]:

' 2 2( )22 ' '

2 ' 2 2

1( ) ( ) 4 ( ( ) ( ))

2( )

L jk z z a

i L

ek I z dz j I z E z

z a z z aπωε

π σ

− − +

−

∂+ = −∂ − +

∫ (1)

where I(z) , a and L are the current, radius, and length of theantenna, respectively, k is the wave vector, ω is the frequencyof the incident electric field E i(z), ε is the permittivity and σ isthe bulk conductivity of the metal. In nano environment, it was

predicted that the antenna with nanometer radius is moreefficient because the conductivity of the nano-sized antenna

plays an important role. When the radius of the antenna is innano scale, the electronic property is different when comparedwith the macroscopic antenna. In the analysis for macroscopicantennas, it considers the antenna is a prefect conductor and sothe conductivity is a constant. However, when the radius of theantenna decreases to nanoscale, the conductivity should betaken into account. Therefore, the classical Maxwell’s methodcannot be fully applied to the antenna at nanoscale. On the

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Frequency (THz)

R e

(σ ) S

I m (σ ) S

Fig. 1. Plot of the conductivity of the antenna (titanium is chosen as thematerial of the antenna) against the frequency of the incident wave.

other hand, the quantum conductivity was considered inequation (1) and can be found by applying Schrödinger’sequation [2]:

2

2( )( )

F e E j

jσ ω

π ω υ = −

−(2)

where e, υ , and are electron’s charge, relaxation frequencyand Planck’s constant, respectively. E F is the Fermi energy of the metal (material of the antenna) and can be found from thefollowing equation [2][4]:

2 2d e

F e

N E

m

π = (3)

where 2d e N and em are the number of electrons per m 2, and

the mass of an electron, respectively.

As a result, the conductivity on the antenna based on theincident electric field was calculated and plotted in Fig. 1.

Based on the current obtained from equation (1), the field inthe vicinity of the nano antenna can then be calculated [5]:

21( ) ( ) ( , ') ( ') 'S

V

E r k g r r J r dV jωε

= +∇∇⋅ ∫ (4)

where V is the volume over the entire space, r is the positionvector from the source point to the observation point, J(r’) isthe current density and g(r,r’) is the electric dyadic Green’sfunction:

'2

1 1( , )

4

jkRe g r r

R Rπ

−⎧ ⎫

= + +⎨ ⎬⎩ ⎭

(5)

where ' R r r = −

By using the above dyadic Green’s function for the electricfield and the Method of Moment, the electric field intensitynear the antenna can be calculated so that the position of themaximum field is found. The nano antenna was then designedfor our system as depicted in Fig. 2. It consists of twosymmetric thin metal wires which are separated by ananometric gap. When the antenna is illuminated with an

infrared source, a standing-wave current pattern is generatedalong two metal wires. The field in the vicinity of theelectrically conducting object is enhanced; the structure can befound from equation (1). The CNT sensing element is thenaligned to the position of the maximum estimated field near theantenna. As discussed before, the maximum radiation occurs atthe point that is perpendicular to the antenna axis.

Fig. 2. Illustration showing the setup of the infrared detection system with thenano antenna.

III. FABRICATION PROCESS

After designing the antenna, the fabrication process of theCNT based IR detector with the nano antenna was developedand the schematic structure is shown in Fig. 3.

Spin on photoresist onquartz substrate

Pattern and develop PR

Deposit Titanium and Gold

Remove photoresist

Photoresist

Gold

Titanium

Quartz

Parylene-C

CNT

CNT formation bynanoassembly

Parylene-C coated on thechip

Deposit Titanium to formthe antenna

(a)

(b)Fig. 3. (a) Fabrication process and (b) schematic structure of a CNT based IR detector with a nanoscale antenna.

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antenna

parylene

quartz

CNT

microelectrode

antenna

1μ m

(a)

parylene

CNT covered byparylene

(b)Fig. 4. (a) Schematic drawing and (b) AFM images of the fabricated CNT IR detector with the nano antenna. Parylene thin film was coated between theantenna and CNT.

The single CNT sensor was first fabricated by usingnanoassembly to form the CNT across micro electrodes. A pair of micro electrodes was evaporated on the quartz substrate byusing thermal evaporation. Titanium was used to improve theadhesion of gold to the substrate. The gap distance between themicro electrodes is from 1 to 3 μm. Then a drop of CNT

suspension was dispersed on the substrate and an ac voltagewas applied. CNTs were then formed between the pair of micro electrodes by the dielectrophoresis force. CNT’sresponsibility to IR has been proposed as the Schottky barrier effect at the CNT-metal contact and reported previously by our group in [1]. When the IR irradiates the CNT, electrons andholes inside the CNT are excited by the photons and results inthe generation of current. As a result, the current from theCNT-metal contact is affected by the concentration of the

photo generated carriers in CNT under the incident IR excitation. Therefore, if the CNT-metal contact is placed to the

position of the maximum field output near the antenna, thenthe current can be increased due to the increasing IR power at

the sensing region. This alignment process can be performed by the following steps. Firstly, the CNT formation between themicroelectrodes was observed by using an atomic forcemicroscope (AFM) so that the position of the CNT-metalelectrode was known and estimated. Afterwards, a parylene Cthin film layer was coated on the CNTs to act as an insulatinglayer for separating the antenna and the sensor. The advantageof using parylene C is that it can be deposited conformally atroom temperature and it can also cover CNTs from

contamination. The IR sensing ability of the CNT detector after packing by parylene has been report by our group in [6]. Theseparation distance between the antenna and the sensor wasdetermined by the thickness of the parylene layer. In principle,the field enhancement effect is larger when the sensor is closer to the antenna. The thickness of the parylene layer is 500nm.

Since the position of the sensing region (CNT-metal contact)was estimated by the AFM, the antenna was then patterned andaligned to the sensing region by using e-beam lithography.During this process, a layer of 950PMMA(polymethylmethacrylate with molecular weight of 950) resistwas spin-coated on the substrate. After the bakeout curing of the PMMA, the nano antenna design pattern was writtendirectly on the resist by an electron beam writer. As mentioned

before, the position of the maximum field output is at thecenter of the antenna. By using the above processes, the

position of the antenna can be precisely controlled and thenano antenna structure can be fabricated. Finally, a pair of thinmetal wires was deposited on top of the parylene C layer by

thermal evaporation and the nano antenna was formed after thelift-off process. The fabricated device is shown in Fig. 4. Asseen from Fig. 4, two nano antennas were fabricated on theCNT-metal contacts respectively. The total length, width andthickness of each antenna are 5 μm, 1 μm and 300nm,respectively. Titanium was used as the material of the antenna.Detailed experimental results will be presented in the nextsection.

IV. EXPERIMENTAL R ESULTS

An IR sensing experiment was performed to validate the

field enhancement effect on the CNT sensor with the nanoantenna. The experiment was performed at room temperature.

0.00E+00

2.00E-09

4.00E-09

6.00E-09

8.00E-09

1.00E-08

1.20E-08

1.40E-08

1.60E-08

1.80E-08

0 5 10 15Time (sec)

C u r r e n

t ( A )

No antennaAntenna

IR light

ONOFF

Fig. 5. Comparison of the temporal photocurrent response plots of a CNT based IR detector with and without nano antenna.

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During the experiment, the device was illuminated by theincident IR laser source (power: 30 mW, wavelength: 830 nm,World Star Tech). The current from the CNT based detector was then measured with an Agilent analyzer. The photocurrentresponse for multiple on/off IR illumination cycles wasobtained and plots of the detector with and without the nanoantenna were compared and shown in Fig. 5. The photocurrentwas taken as the current change when IR is on. It has been seenthat the current was increased when the IR laser was on and itdropped to its original value when IR was off.

-5.00E-05

-4.00E-05

-3.00E-05

-2.00E-05

-1.00E-05

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Voltage (V)

C u r r e n

t ( A )

Fig. 6. I-V characteristics of a CNT IR detector with nano antenna.

Moreover, the I-V characteristic of the CNT-metalelectrodes was investigated and is shown in Fig. 6. Our

preliminary results showed the device exhibited fast responseto IR and the photocurrent change was increased by 10 timesafter adding the nano antenna. Before adding the antenna, thecurrent change of the original CNT based IR detector wassmall, which implied the low sensitivity of the sensor. On theother hand, the results showed that the CNT based sensor withthe nano antenna was more sensitive to IR because the incident

power was enhanced by the antenna.

V. D ISCUSSION

The photocurrent response is increased while the electricfield near the antenna is enhanced and it is influenced by thegain of the antenna. The gain increases while the size of theantenna is reducing. This is caused by the increasing theconductance of the antenna when the size of the antennadecreases. The conductivity directly affects the currentdistribution and also the electric field in the vicinity of theantenna. Therefore the gain of the antenna for IR detectionsignificantly increases when the wave length and antennalength get smaller. Moreover, the gain of the antenna is alsoaffected by several factors such as the length of the antenna,the alignment of the sensor to the maximum field output near the antenna, the material of the antenna, and the separationdistance between the sensor and the antenna. It has beenreported that the response maximizes when the antenna lengthis a proper multiple of the half-wavelength of the incident

radiation [7]. Therefore, the antenna length effect on the fieldenchantment by the nano antenna has to be further studied.

VI. CONCLUSION

The design and fabrication process of a novel CNT based IR detector with a nano antenna have been presented. Byintegrating the nano antenna, the incident electric field at the

sensor was increased leading to an order of magnitude increasein the photocurrent of the sensor. The antenna in nanoenvironment is more efficient and can be incorporated into themanufacturing process of present day infrared detectors.Quantum effects were considered and included in the analysisfor finding the maximum field from the antenna in near field.The combination of Schrödinger’s equation and Green’sfunction for near field provides a novel approach to study theantenna in nano environment, and leads to the potential methodfor future complete analysis of nano scale structures anddevices where quantum effects cannot be ignored andtraditional Maxwell’s equation cannot be fully applied. Basedon the theoretical prediction, the practical nano antenna for

quantum infrared detector has been realized experimentally,which successfully demonstrated and verified that the IR response of the sensor can be greatly increased by the electricfield enhancement in the vicinity of the nano antenna. Thisrepresents a major breakthrough in sensor research since thiswill lead to a dramatic improvement in the performance of infrared imaging systems, which are important for futurecivilian and military applications.

ACKNOWLEDGMENT

This research work is partially supported under NSF GrantsIIS-0713346 and DMI-0500372, and ONR Grants N00014-04-1-0799 and N00014-07-1-0935.

R EFERENCES

[1] J. Zhang, N. Xi, K. W.C. Lai, H. Chen, Y. Luo, and G. Li, “SingleCarbon Nanotube based Photodiodes for Infrared Detection”, Proc. of the7 th IEEE International Conference on Nanotechnology (IEEE NANO),

pp. 1156 – 1160, 2007.[2] G. W. Hanson, “Fundamental transmitting properties of carbon nanotube

antennas”, IEEE Trans. on Antennas and Propagation , vol. 53, no. 11, pp. 3426 – 3435, 2005.

[3] Javier Alda, Jose M. Rico-Garcia, Jose M. Lopez-Alonso and GlennBoreman, “Micro- and nano-Antennas for light detection”, Egypt. J.Solids , vol. 28, pp. 1 - 13, 2005.

[4] A. P. Sutton, Electronic Structure of Materials . Oxford, U.K.: Clarendon,1993.

[5] R. S. Elliott, Antenna Theory and Design . Englewood Cliffs, NJ:Prentice-Hall, 1981.[6] King W. C. Lai, Ning Xi, Jiangbo Zhang, Guangyong Li and Hongzhi

Chen, “Packaging Carbon Nanotube Based Infrared Detector,” Proc. of the 7 th IEEE International Conference on Nanotechnology (IEEE NANO),

pp. 778 – 781, 2007.[7] Y. Wang, K. Kempa, B. Kimball and J. B. Carlson, G. Benham, W. Z. Li,

T. Kempa, J. Rybczynski, A. Herczynski, and Z. F. Ren, “Receiving andtransmitting light-like radio waves: Antenna effect in arrays of alignedcarbon nanotubes”, Applied Physics Letters , vol.85, no. 13, 2004.

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