Investigation of the Infrared Radiation Detection Mechanism for Antenna-Coupled Metal-(Oxide)-Metal Structures

Abstract

At room temperature (300 K), the electromagnetic (EM) radiation emitted by humans and other living beings peaks mostly in the long-wavelength infrared (LWIR) regime. And since the atmosphere shows relatively little absorption in this EM band, applications such as target detection, tracking, active homing, and navigation in autonomous vehicles extensively use the LWIR frequency range. Antennas are scalable, frequency selective, and polarization sensitive and hence present themselves as good candidate for such detectors. The research work presented in this chapter is focused on developing an antenna-based, uncooled, and unbiased detector for the LWIR regime.

LWIR is a high-frequency EM radiation (∼30 THz), and once detected by an antenna, induces a corresponding high-frequency antenna current in the antenna. Hence to get a useful detector output, rectification of high-frequency antenna currents is required. We used an asymmetric-barrier metal-oxide-metal (MOM) diode as the rectifier since it operates based on quantum-mechanical tunneling of electrons with tunneling times on the order of femto-seconds, and since the MOM diode is asymmetric, the detector does not need to be biased. In the first part of the chapter, we discuss operating principles, fabrication techniques, and electrical and infrared measurements of such an antenna-coupled metal-oxide-metal diode (ACMOMD). Although the ACMOMDs that we fabricated behaved as LWIR detectors and operated based on classical antenna theory, the electrical measurements on these ACMOMDs revealed that these detectors did not exhibit the characteristic behavior of an asymmetric-barrier MOM diode. Our further experimentation showed that these detectors operated based on Seebeck effect. So, the second part of this chapter presents the evolution of our understanding of the detection mechanism of these sensors, and presents relevant electrical measurements. Further in the chapter, we discuss the design, fabrication, and extensive electrical and infrared characterization of detectors based on Seebeck effect. As per Seebeck effect, also known as thermoelectric effect, if two different metals are joined together at one end and their other ends are open-circuited, and if a non-zero temperature difference exists between the joined end and the open ends, then a non-zero open-circuit voltage can be measured between the open ends of the wires.

The antenna-based thermoelectric detectors fabricated in this research are antenna-coupled nano-thermocouples (ACNTs). In ACNTs, radiation-induced antenna currents produce polarization-dependent heating of the joined end of the two metals whereas the open ends remain at substrate temperature. This polarization-dependent heating induces polarization-dependent temperature difference between the joined end and the open ends of the metals, leading to a polarization-dependent open-circuit voltage between the open ends of the metals. A CW CO ₂ laser tuned at 10.6 μm wavelength has been used for infrared characterization of these sensors. For these sensors, average responsivity of 22.7 mV/W, signal-to-noise (SNR) ratio of 29 dB, noise equivalent power (NEP) of 1.55 nW, and specific detectivity (D*) of 1.77 × 10⁵ cm Hz^1/2 W⁻¹ were measured. ACNTs are expected to operate at frequencies much beyond 400 KHz.