Mapping the electrical properties of large-area graphene

Abstract

The significant progress in terms of fabricating large-area graphene films for transparent electrodes, barriers, electronics, telecommunication and other applications has not yet been accompanied by efficient methods for characterizing the electrical properties of large-area graphene. While in the early prototyping as well as research and development phases, electrical test devices created by conventional lithography have provided adequate insights, this approach is becoming increasingly problematic due to complications such as irreversible damage to the original graphene film, contamination, and a high measurement effort per device. In this topical review, we provide a comprehensive overview of the issues that need to be addressed by any large-area characterisation method for electrical key performance indicators, with emphasis on electrical uniformity and on how this can be used to provide a more accurate analysis of the graphene film. We review and compare three different, but complementary approaches that rely either on fixed contacts (dry laser lithography), movable contacts (micro four point probes) and non-contact (terahertz timedomain spectroscopy) between the probe and the graphene film, all of which have been optimized for maximal throughput and accuracy, and minimal damage to the graphene film. Of these three, the main emphasis is on THz time-domain spectroscopy, which is non-destructive, highly accurate and allows both conductivity, carrier density and carrier mobility to be mapped across arbitrarily large areas at rates that by far exceed any other known method. We also detail how the THz conductivity spectra give insights on the scattering mechanisms, and through that, the microstructure of graphene films subject to different growth and transfer processes. The perspectives for upscaling to realistic production environments are discussed. TOPICAL REVIEWOriginal content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

Atomic Layer Deposition Alumina‐Mediated Graphene Transfer for Reduced Process Contamination

“…Raman spectroscopy was performed in a Thermo Fisher DXRxi microscope equipped with a 455 nm laser (10 mW, 0.01 s × 200 exposure time, 50× objective for measurements on copper foils; 2 mW, 0.02 s × 20 exposure time, 50× objective for measurements on oxidized silicon substrates). THz‐TDS was performed using a commercially available system described in detail previously by Buron et al Spatial maps of sheet conductivity averaging from 0.8 to 0.9 THz were generated by raster scanning the sample with a 400 μm step size in the THz focal plane between the emitter and detector units. Sheet conductivity was extracted from the directly transmitted transient of the THz pulse …”

Towards standardisation of contact and contactless electrical measurements of CVD graphene at the macro-, micro- and nano-scale

“…However, this method is not suitable for high throughput characterisation and often the measured graphene is significantly altered due to the microfabrication processes, in which case the electrical properties of the pristine transferred graphene are different from the ones measured. An alternative method for mapping the sheet resistance of graphene films is using micro four-point probes (M4PP) 15,16 . Recently the use of microwave cavities [16][17][18] and Terahertz time-domain…”

“…An alternative method for mapping the sheet resistance of graphene films is using micro four-point probes (M4PP) 15,16 . Recently the use of microwave cavities [16][17][18] and Terahertz time-domain…”

“…Spectroscopy (THz-TDS) 16,[19][20][21][22][23] measurements provided a non-contact method of measuring the conductivity of graphene films. Despite the attractiveness of these techniques, fluctuations in ambient conditions (such as humidity) affect the electrical properties of graphene 24,25 , resulting in discriminations between measurements at different laboratories.…”

Terahertz Time‐Domain Spectroscopy and Near‐Field Microscopy of Transparent Silver Nanowire Networks

“…[3,6,7] Furthermore, large-area lithography and metal coating techniques are increasingly emerging as promising alternatives. Furthermore, although microprobes have been developed to map the conductance with high spatial resolution, [11] the four-point probe technique requires electrical contact with the surface, leading to the risk of modifying the sample. These spatial modulations of the electrical conductance are impossible to detect with standard characterization techniques such as the four-point probe method, in which the electrical impedance is measured using a pair of separated electrodes carrying a current and sensing the voltage differences.…”