Measuring TCR of High Purity Semi-Conducting CNT Networks for Super Sensitive Bolometric Infrared Detectors

Uncooled infrared sensors of bolometer type have a wide range of applications such as security, military, food inspection, health care, and automotive camera. Currently, the highly sensitive and low-cost device development is key issue for further expansion of demand. The bolometer is an infrared detector for radiant heat by means of an infrared absorber having a temperature sensitive electrical resistance material. Infrared radiation strikes the absorber material heating it and thus changing resistor material resistance. Therefore, bolometer’s performance is strongly limited by temperature coefficient of resistance (TCR) of the resistor. The conventional resistor is generally based on vanadium oxide (VO_x) with TCR of about -2%/K [1] and an outstanding resistor is essential to achieve highly sensitive infrared detectors. Recently, single-walled carbon nanotubes (SWCNTs) have been expected as promising materials with high TCR, low resistivity, and high chemical stability. The TCR of -2.1%/K has already been obtained by semiconducting SWCNT films extracted by density gradient method [2]. Further, conventional bolometer which is named microbolometer has a suspended structure for heat separation between the absorbing layer and the substrate, and these are suspended structure with micro electro mechanical system (MEMS) process.<br/><br/>Therefore, we have tried to develop low-cost and high sensitivity bolometer with SWNCTs as resistor material [2]. In this study, we fabricated high TCR bolometer with printed process that was not MEMS process by using the high purity semiconducting SWCNT network films above 98% and the parylene C film which is the low thermal conductivity material as a thermal separation layer and evaluated its device structure and electrical property.<br/><br/>The pristine SWCNTs are fundamentally a mixture of both semi-conducting and metallic SWCNTs. To extract the semi-conducting SWCNTs, “Electric-field inducing Layer Formation (ELF)” method was used, which is our original method [3]. ELF method is the remarkable promising technique to extract semi-conducting SWCNTs with high purity and excellent electrical transportation property by carrier-free electrophoresis using non-ionic surfactant. To show capability of SWCNT bolometers, we measured TCR of the semi-conducting CNT networks which are extracted from various type of commercial SWCNTs.<br/><br/>The sample fabricating process is described below. First, Ti/Au electrodes as a source drain were deposited on a Si substrate, and SiO₂ was sputter-deposited on the surface. Each electrode was exposed by SiO₂ etching. Then a (3-Aminopropyl) triethoxysilane monolayer was formed on the SiO₂ which remaining between the two electrodes, and semi-conducting SWCNTs which is extracted by ELF method were dispersed. The Si chip was mounted on a ceramic carrier, and the temperature dependence of the electrical characteristics of the CNT network between the source and drain electrodes was measured in a liquid nitrogen cryostat at around room temperature. TCR was calculated using each temperature’s resistance. The maximum TCR was close to -6%/K, which was larger than the commercial bolometer’s resistor material. This shows the superiority of high purity semiconducting SWCNTs extracted by the ELF method. Furthermore, the relationship between the morphology of CNT networks and its electrical characteristics was also obtained.<br/><br/>Acknowledgments: This study was supported by Innovative Science and Technology Initiative for Security Grant No. JPJ004596, ATLA, Japan.<br/><br/>[1] C. Chen, et. al., Sen. Act. A. Phys. 90, 2001, 212.<br/>[2] K. Narita, et. al., Sen. Act. A. 195, 2013, 142.<br/>[3] K. Ihara, et. al., J. Phys. Chem. C, 115, 2011, 22827.