Characterisation and modelling of graphene FET detectors for flexible terahertz electronics

Doctoral thesis, 2020

Low-cost electronics for future high-speed wireless communication and non-invasive inspection at terahertz frequencies require new materials with advanced mechanical and electronic properties. Graphene, with its unique combination of flexibility and high carrier velocity, can provide new opportunities for terahertz electronics. In particular, several types of power sensors based on graphene have been demonstrated and found suitable as fast and sensitive detectors over a wide part of the electromagnetic spectrum. Nevertheless, the underlying physics for signal detection are not well understood due to the lack of accurate characterisation methods, which hampers further improvement and optimisation of graphene-based power sensors. In this thesis, progress on modelling, design, fabrication and characterisation of terahertz graphene field-effect transistor (GFET) detectors is presented. A
major part is devoted to the first steps towards flexible terahertz electronics.

The characterisation and modelling of terahertz GFET detectors from 1 GHz to 1.1 THz are presented. The bias dependence, the scattering parameters and the detector voltage response were simultaneously accessed. It is shown that the voltage responsivity can be accurately described using a combination of a quasi-static equivalent circuit model, and the second-order series expansion terms of the nonlinear dc I-V characteristic. The video
bandwidth, or IF bandwidth, of GFET detectors is estimated from heterodyne measurements. Moreover, the low-frequency noise of GFET detectors between 1 Hz and 1 MHz is investigated. From this, the room-temperature Hooge parameter of fabricated GFETs is extracted to be around 2*10^{-3}. It is found that the thermal noise dominates above 100 Hz, which sets the necessary switching time to reduce the effect of 1/f noise.

A state-of-the-art GFET detector at 400 GHz, with a maximum measured optical responsivity of 74 V/W, and a minimum noise-equivalent power of 130 pW/Hz^{0.5} is demonstrated. It is shown that the detector performance is affected by the quality of the graphene film and adjacent layers, hence indicating the need to improve the fabrication process of GFETs.

As a proof of concept, a bendable GFET terahertz detector on a plastic substrate is demonstrated. The effects of bending strain on dc I-V characteristics, responsivity and sensitivity are investigated. The detector exhibits a robust performance for tensile strain of more than 1% corresponding to a bending radius of 7 mm. Finally, a linear array of terahertz GFET detectors on a flexible substrate for imaging applications is fabricated and tested. The results show the possibility of realising bendable and curved focal plane arrays.

In summary, in this work, the combination of improved device models and more accurate characterisation techniques of terahertz GFET detectors will allow for further optimisation. It is shown that graphene can open up for flexible terahertz electronics for future niche applications, such as wearable smart electronics and curved focal plane imaging.