Graphene FET terahertz detectors on flexible substrates

Licentiate thesis, 2017

Terahertz (THz) science and technology have developed rapidly over the past decades, extending the THz application areas from spectroscopy and earth and space sciences to communications, biomedicine and security sensing. Many of these emerging applications require shape-conforming, light-weight and low-cost detectors rather than existing solid-state detection technology. Graphene, which possesses impressive electrical and mechanical properties, is a promising material for enabling flexible devices at THz frequencies. This thesis reports on the modelling, design, fabrication and characterisation of graphene THz detectors on plastic substrates. These detectors are based on field-effect transistors (FETs) fabricated using chemical vapour deposition (CVD)-grown graphene and integrated with split broadband bow-tie antennas. A fabrication process has been developed to achieve high-performance THz detectors on plastic substrates. Since the properties of the dielectric film on graphene are very sensitive to the growth conditions, parallel-plate capacitor test structures on graphene on flexible substrates are fabricated for characterising the electrical properties of the dielectric films. THz power detection in the frequency range from 330 GHz to 500 GHz at room temperature is demonstrated. The devices operate well above the cutoff frequency of the transistors. At room temperature, the voltage responsivity is above 2 V/W, and the noise-equivalent power (NEP) is below 3 nW/√Hz at 487 GHz. The effects of bending strain on the dc characteristics, voltage responsivity and NEP of these detectors have been investigated. The mechanical studies reveal robust detector performance with tensile strain more than 1% with a corresponding bending radius of 7 mm. This work provides an important route towards high-performance, low-cost THz flexible technology for future niche applications, such as wearable smart electronics, imaging systems, and communications.