Figure 1. Non-destructive, non-contact characterization of electrical properties of graphene. Source: 2D Materials 4, 042003 (2017), creative commons.
Characterizing the electrical properties of graphene and other 2D materials is rapidly becoming a bottleneck for industrial applications.
Even though large-scale production of superior-quality graphene has rapidly progressed, development of practical characterization methods has lagged behind. Common methods are either too slow for industrial use or they damage the device beyond repair.
In a latest publication in the journal 2D Materials, Scientists from Denmark, the UK and Spain have compared the standard methods of measuring and then mapping electronic properties and propose industrially scalable solutions. For the majority of applications of single sheet graphene in optoelectronics, low-power, high-speed electronics and photovoltaics, electronic properties such as sheet resistivity, carrier mobility, and background doping carrier density are vital.
The most commonly used method of measuring these properties in graphene research, standard lithography, despite being useful in the infant stages of graphene technology, contaminates the device with lithographic resist that is problematic or impossible to remove, leading to irreversible damage to the electronic quality of the sample.
Additionally, this method is time-consuming, needing at least half a day for sample preparation. To speed up sample preparation, it is possible to use direct laser lithography (DLL), which takes about 1-2 hours for each wafer.
During this process, metal contacts are deposited at fixed locations using a stencil mask, whereas the graphene devices are defined with laser ablation. Even though it bypasses lithographic resist residues and is faster than standard lithography, DLL yields a fixed device geometry and is therefore applicable to specific applications only, that is, it does not probe the graphene film as-is.
Pristine Graphene Films
Characterizing electronic properties of pristine graphene films and then using the film for a custom application can only be performed with non-destructive methods. Micro four point probe (M4PP) is one such method. M4PP was launched in the year 2000 as an ultra-compact alternative to conventional four point probing used in microelectronics. M4PP is ideal for thin films and fragile surfaces, such as graphene, as it comprises of sensitive micro-fabricated cantilever electrodes on a silicon chip. This method can be used for non-patterned graphene films and carries out measurements on about one device per minute.
Non-contact, non-destructive terahertz time-domain spectroscopy (THz-TDS) appears as a most viable solution for even faster, industrial-scale device testing. THz-TDS is executed by measuring the attenuation of a terahertz pulse by transmission via the sample or by reflection back. This method is capable of measuring electrical properties of graphene sheets within 10 milliseconds per pixel, with a resolution better than 100 µm.
The Authors of the paper, entitled “Mapping the electrical properties of large-area graphene”, conclude that THz-TDS is the perfect candidate for characterizing graphene films with the quality, consistency, and speed needed for the entry of graphene into mass market products.
This information has been sourced, reviewed and adapted from materials provided by Graphenea.
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