Fabrication and Characterisation of High-Speed Graphene Nanodevices for THz Detection

Abstract

Despite graphene's excellent properties, there have been very few electronic devices which make full use of them due to the lack of bandgap. This research aims for the development of high-performance graphene THz detectors. Towards this goal, two nanodevices are investigated. Both do not require a bandgap, with each being capable of operating into the terahertz (THz) range. The graphene ballistic rectifier (GBR) uses the long mean free paths in graphene to achieve ballistic transport and redirect carriers preferentially towards a single output. Building on the extended Buttiker-Landauer formula for ballistic rectifiers in semiconductors, the operational theory of GBRs is derived and tested taking into account the coexistence of electrons and holes. The theory predicts large responsivities when there is a large disparity in carrier mobilities, while calculations using realistic device dimensions and parameters predict easily achievable responsivities of at least 50,800 V/W and noise-equivalent power (NEP) of 0.51 pW/Hz^(1/2). The second nanodevice is the graphene self-switching diode (GSSD), which uses electrostatic effects to cause asymmetric current flow through a conducting channel. By constructing a bridge rectifier from encapsulated GSSDs a modest peak responsivity of 4,400 V/W is found, with minimum NEP of 5.4 pW/Hz^(1/2) is found if thermal noise is assumed to dominate. Both nanodevices are then tested using graphene grown by chemical vapour deposition (CVD), an important step towards fabrication on a larger scale. A bridge rectifier constructed from CVD GSSDs demonstrates peak responsivity >100 kV/W. However, poor noise performance puts the NEP at 11.7 nW/Hz^(1/2), worse than GBRs and many other room-temperature THz detectors. GBRs using CVD graphene show good results, with responsivity of 10,000 V/W and similar noise to encapsulated graphene ballistic rectifiers. However, combining graphene ballistic rectifiers into arrays gives mixed results. Finally, simulations of THz graphene bowtie antennas using parameters realistic for CVD graphene show that they can be operated with high-impedance detectors such as those investigated here. With a 1 kOhm source they show reflection parameters under -12 dB at THz frequencies, although improvement to CVD graphene quality would give significant improvement. While no THz results are presented here, the next step is combining these antennas with high-frequency graphene rectifiers such as the GBR or GSSD to form graphene THz detectors.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Aimin Song (Supervisor) & Ernest Hill (Supervisor)