Characterizing the electrical properties of graphene and other 2D materials is quickly becoming a bottleneck for industrial applications. Although large-scale production of high-quality graphene has rapidly advanced, development of practical characterization methods has lagged behind. Common methods are either too slow for industrial use or damage the device beyond repair.
In a recent publication in the journal 2D Materials, scientists from Denmark, the UK, and Spain compare the standard methods of measuring and mapping electronic properties and propose industrially scalable solutions. For the majority of applications of single sheet graphene in optoelectronics, high-speed, low-power electronics and photovoltaics, electronic properties such as carrier mobility, sheet resistivity, and background doping carrier density are essential. The most commonly used method of measuring these properties in graphene research, standard lithography, although useful in the infant stages of graphene technology, contaminates the device with lithographic resist that is difficult or impossible to remove, causing irreversible damage to the electronic quality of the sample. In addition, this method is time-consuming, requiring at least half a day for sample preparation. To hasten sample preparation, one could use direct laser lithography (DLL), which takes about 1-2 hours per wafer. During this process metal contacts are deposited at fixed locations using a stencil mask, whereas the graphene devices are defined with laser ablation. Although it bypasses lithographic resist residues and is quicker than conventional lithography, DLL yields a fixed device geometry and is thus applicable to certain applications only, i.e. it doesn’t probe the graphene film as-is.
Image: Non-destructive, non-contact characterization of electrical properties of graphene. Source: 2D Materials 4, 042003 (2017), creative commons
Characterizing electronic properties of pristine graphene films and later using the film for a custom application can only be done with non-destructive methods. One such method is micro four point probe (M4PP). M4PP was introduced in the year 2000 as an ultra-compact alternative to conventional four point probing used in microelectronics. M4PP is suitable for thin films and fragile surfaces, such as graphene, as it consists of sensitive micro-fabricated cantilever electrodes on a silicon chip. This method can be applied to non-patterned graphene films and performs measurements on about one device per minute.
For even faster, industrial-scale device testing, non-contact, non-destructive terahertz time-domain spectroscopy (THz-TDS) appears as a most viable solution. THz-TDS is carried out by measuring the attenuation of a terahertz pulse by transmission through the sample or by reflection back. This method measures electrical properties of graphene sheets within 10 milliseconds per pixel, with a resolution better than 100 micrometers. The authors of the paper, entitled “Mapping the electrical properties of large-area graphene”, conclude that THz-TDS is the best candidate for characterizing graphene films with the consistency, quality, and speed required for the entry of graphene into mass market products.