Infrared radiation sensors, which can detect infrared electromagnetic radiations (0.7 μm to 1mm), have been widely used in numerous applications. Among the different infrared detectors, thermoelectric (TE) infrared sensors are passive sensors and require no complex cooling system; therefore, they have much less complex structures and are more energy and cost-efficient, making them an attractive solution for infrared detection. However, the conventional planar TE sensor structures are generally quite bulky and they need to make a tradeoff between a high responsivity and a fast response, rendering them unsuitable for high-speed and high-resolution infrared detection.

In this work, we proposed a vertical design based on free-standing micropillars, which can achieve a high responsiv...[ Read more ]

Infrared radiation sensors, which can detect infrared electromagnetic radiations (0.7 μm to 1mm), have been widely used in numerous applications. Among the different infrared detectors, thermoelectric (TE) infrared sensors are passive sensors and require no complex cooling system; therefore, they have much less complex structures and are more energy and cost-efficient, making them an attractive solution for infrared detection. However, the conventional planar TE sensor structures are generally quite bulky and they need to make a tradeoff between a high responsivity and a fast response, rendering them unsuitable for high-speed and high-resolution infrared detection.

In this work, we proposed a vertical design based on free-standing micropillars, which can achieve a high responsivity, a short response time and a small device size simultaneously. Theoretical modeling and numerical simulations were conducted to evaluate the effects of various key design parameters such as the nanowire diameter, length to diameter ratio, absorber area, thermoelectric properties of the TE element, and the number of nanowires, on the performance of the TE sensors. The modeling results show that the vertical design can potentially provide superior performance.

With the optimized device design and absorber structure, we developed the first TE infrared sensing structure based on vertically standing SiGe nanowires and the corresponding fabrication process. We successfully demonstrated the TE infrared sensors with selectively doped nanowires of 1 μm and 300 nm diameters, which have a footprint of 28×28 μm²and are the smallest TE infrared sensors reported in the literature. To achieve a high selective infrared absorption, based on “multi-layer integration” theory, an atmospheric-window matched broadband absorber has been developed and integrated with the developed sensor. Laser and blackbody radiation measurements were conducted to characterize the performance of the fabricated TE infrared sensors. The measured performance of the devices with 1-μm-thick and 2-μm-long nanowires is close to the theoretical predictions, with a responsivity reaching 160 V/W and a response time down to 0.6 ms, which is the fastest response for thermal infrared sensors and may enable high-speed infrared detection using TE sensors. Meanwhile, the devices with 300-nm-thick nanowires show a response time of 6 ms and a responsivity of more than 4000 V/W, achieving both high spatial and temporal resolutions as well as a high responsivity.

These findings demonstrate that it is possible to achieve superior responsivity as well as high spatial and temporal resolutions using a free-standing structure integrated with vertical nanowires. More importantly, these TE infrared sensors can be fabricated using IC-compatible fabrication process and Si-based materials, paving the way for large-scale integration for high performance infrared applications such as high-speed infrared imaging.