Electrical properties of graphene, such as conductivity, can vary across a wafer, which can impede the industrial acceptance of the material. Spatial inhomogeneity of conductivity results in devices from the same batch having different performance, which is incompatible with large scale manufacturing. It is hence important to have tools to map electrical properties with sufficient resolution across large areas, ideally over the size of an entire wafer.

Now a team of researchers from Italy and Spain have used Electrical Resistance Tomography (ERT) to map conductivity over large areas of graphene. The mapping was performed with a new dedicated measurement setup coupled with custom reconstruction software. The setup was also used to perform traditional 4-point van der Pauw measurements and the results compared to non-contact scanning terahertz time-domain spectroscopy (THz TDS). The results demonstrate the potential of ERT as an accurate and reliable technique for electrical characterization of graphene samples, with potential for scaling up and high throughput rapid scanning of large areas.

Image: Dedicated setup and resultant conductivity map (adapted from A. Cultrera et al, Sci Rep 9, 10655 (2019) under a Creative Commons 4.0 license).

Measurements were performed with a total of 16 spring-loaded electrodes fixed in a holder that can be lowered such that the electrodes come into contact with the samples. The spacing between the electrodes is 2mm, which gives the resolution of the measurement. The resolution is higher than with traditional 4-point setups, yet lower than with the much more costly THz TDS. Although all three types of measurements yield similar values of average conductivity across the sample, ERT reveals spatial features that cannot be seen with the 4-point setup.

The results were published in the journal Scientific Reports. The work was performed as part of a larger co-ordinated effort to develop electrical characterization methods for future graphene electronics, through project GRACE. The project started in 2017 and organizes 8 partners from 4 countries to develop and compare seven different methods for measuring graphene conductivity, both contact and non-contact.

This project has received co-funding from the European Union's Horizon 2020 research and innovation programme and EMPIR under code No 16NRM01.