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Noncryogenic, cooled IR sensors based on carbon nanotubes

IR-sensor performance can be improved by integrating a carbon-nanotube structure with a photonic crystal.
5 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201011.003294

IR sensors are remarkably important for various civilian and military applications, including IR imaging, medical diagnosis, night vision, and surveillance. The performance of conventional IR sensors is limited by the sensing materials. High-quality IR detectors require cryogenic cooling to reduce the influence of both the background and different types of radiation. Unlike other existing sensing materials, carbon nanotubes (CNTs) exhibit low noise levels1 and can be used as nanosensing materials for IR detection where cryogenic cooling is not required.

CNTs have recently been used to make IR sensors because of their unique opto-electric properties. We have developed CNT-based IR sensors that can operate at room temperature with low dark current and a low-noise equivalent-temperature difference (14.5mK).2However, the fill factor of our CNT-based IR sensor is still limited by the small sensing area and low incoming electric field. We found that their photocurrent can be increased significantly by locally boosting the electric field at the CNT-metal interface.3Therefore, to increase device performance, we introduced a photonic-crystal (PC) quantum-resonant cavity.

We designed and fabricated IR sensors consisting of a pair of metal electrodes connected by a single CNT. The devices were fabricated using a number of nanomanufacturing processes, including dielectrophoretic manipulation4 and atomic-force-microscopy nanoassembly.5 When IR photons irradiate the CNT-metal contact, electrons and holes are excited in the CNT, which results in photocurrent generation. To enhance the electron-phonon coupling efficiency, we aligned the PC cavity with the top of the CNT-metal contact, because the photocurrent is dominated by photon-generated carriers at this interface. The current increases because of the expanding electric field in the sensing region.

PCs are artificial structures consisting of different dielectric materials that are arranged in periodic order in 2D or 3D to affect photon propagation. By careful design, the incident IR light is confined in the cavity and enhances the electric field locally at the sensor. We designed a 2D PC with periodic holes in a parylene thin film to induce a photonic bandgap and formed a resonant cavity by removing holes from the PC array (see Figure 1).

Figure 1.Schematic structure of a carbon-nanotube (CNT)-based IR detector with a photonic-crystal (PC) cavity.

We calculated the band structure of the 2D PC without any defect and found that a frequency bandgap was formed: see Figure 2(a). Next, we designed the PC cavity by removing a point defect. The resulting cavity exhibits a localized resonance mode. The point defect may be considered a disturbance of the discrete translational symmetry of the PC, so a quantum-resonant mode for incident light is formed at the defect position. Figure 2(b) shows the electric-field profile for a 9×9 lattice geometry. The field is confined to the cavity and is enhanced locally.

Figure 2.(a) PC bandgap diagrams. The bandgap is highlighted in green. a: CNT radius. λ: Wavelength. (b) Electric-field profile of our parylene-based PC with a point defect.

We fabricated a device based on this PC cavity (see Figure 3) using parylene as photonic crystal. The PC's point defect was fabricated on one of the CNT-metal contacts. We measured and compared the photocurrent responses of the sensor with and without the PC resonant cavity (see Figure 4). Our preliminary results show that the photocurrent is enhanced by adding the PC structure.

Figure 3.Atomic-force-microscopy images of the fabricated PC (on top of the CNT-based IR detector).

Figure 4.Comparison of temporal photocurrent response of a CNT-based IR sensor with and without addition of a PC.

In summary, we designed and fabricated CNT-based IR detectors using nanomanufacturing processes. We also integrated these devices with a PC, which can concentrate the incoming-light intensity in a cavity, so that the electric field at the CNT-metal contact is enhanced. Our theoretical analysis and experiments demonstrate that the photocurrent response can be increased by adding a PC. Our future plans involve design optimization of PC structures to improve the field enhancement at various wavelengths, aimed at PC application to spectral IR detection and solar-energy harvesting.

This research is partially supported by the Office of Naval Research and the National Science Foundation.

Carmen Kar Man Fung, Ning Xi, King Wai Chiu Lai, Jianyong Lou, Hongzhi Chen
Robotics and Automation Laboratory Department of Electrical and Computer Engineering
Michigan State University
East Lansing, MI

Carmen Fung received her PhD degree in automation and computer-aided engineering from the Chinese University of Hong Kong. She is a postdoctoral researcher and works on nano-IR detectors and bio-electrical engineering in cellular biology.

Ning Xi is the John D. Ryder professor of electrical and computer engineering and director of the Robotics and Automation Laboratory. He is a fellow of the IEEE. His current Research interests include robotics, manufacturing automation, micro/nanomanufacturing, nanosensors and devices, and intelligent control and systems.

King Lai received his PhD degree in automation and computer-aided engineering from the Chinese University of Hong Kong. He is a laboratory manager. His research interests include development of micro/nanosensors, photovoltaics, and nanobiotechnology.

Jianyong Lou received his PhD degree in 2006 from Xi'an Jiaotong University (China), where he has been teaching since 2007. Since 2009, he has also been a visiting scholar at Michigan State University. His research interests include electromagnetic devices and electromagnetic-wave phenomena in patterned media.

Hongzhi Chen received a BS from Guangdong University of Technology (China) in 2005 and an MS from Queen's University Belfast (UK) in 2006. He is currently pursuing a PhD degree. His research interests include nano-electronics, nano-optoelectronics, and micro/nanofabrication and manufacturing.

2. J. B. Zhang, N. Xi, H. Z. Chen, K. W. C. Lai, G. Y. Li, U. Wejinya, Photovoltaic effect in single carbon nanotube-based Schottky diodes, Int'l J. Nanopart. 1, pp. 108-118, 2008. doi:10.1504/IJNP.2008.020266
3. C. K. M. Fung, N. Xi, B. Shanker, K. Lai, Nanoresonant signal boosters for carbon nanotube based infrared detectors, Nanotechnol. 20, pp. 185201, 2009. doi:10.1088/0957-4484/20/18/185201
4. K. W. C. Lai, N. Xi, C. K. M. Fung, J. Zhang, H. Chen, Y. Luo, U. C. Wejinya, Automated nanomanufacturing system to assemble carbon nanotube based devices, Int'l J. Robot. Res. 28, pp. 523-536, 2009. doi:10.1177/0278364908097585
5. G. Li, N. Xi, M. Yu, Development of augmented reality system for AFM based nanomanipulation, IEEE/ASME Trans. Mechatron. 9, pp. 358-365, 2004. doi:10.1109/TMECH.2004.828651