MICROSCOPY & IMAGING
the industry due to the decreasing sizes of semiconductor devices. Complementary atomic force microscopy (AFM) techniques such as conductive AFM allow simultaneous insights into electronic and structural sample information. Conductive AFM (C-AFM) in high vacuum offers an additional improvement in the sensitivity of the electronic measurement signals due to the removal of the surface water layer. Since the discovery of graphene in 2004, two-dimensional (2D) materials have seen a growing interest in the scientific community. Researchers have synthesised and characterised a variety of 2D materials, including the group of transition metal dichalcogenides (TMDs). TMDs attracted the attention of the semiconductor industry, because many of the compounds are direct band gap semiconductors with low dielectric constants and high charge-carrier mobilities. Due to their layer thickness of less than 1nm, TMDs qualify as promising candidates for semiconductor devices below 5nm following the trend of ever lower device sizes according to Moore’s law. However, the integration of TMDs in commercial semiconductor devices still faces some challenges. As such, the desired electronic properties often are limited to individual islands or grains and controlled layer growth,
I
nsights into morphological and electronic properties of two- dimensional materials at the nanometre-scale are of great interest to
Fig. 1. Park NX-Hivac high vacuum AFM and 3D topography image overlaid with the current signal, recorded in high vacuum
Jonathan Ludwig and Kristof Paredis reveal how high vacuum can improve conductive atomic force microscopy
VACUUMBETTER IN
Fig.2. a)-c) Topography measurements on 1-2-layer sample, 3-4-layer sample and multilayer sample, d) schematic of sample structure of 3-4-layer sample and cAFM setup, e) height profile extracted at the position of the red line in b). Scale bars are 500nm
transfer and processing of high-quality TMD films remain complicated.(1,2) Electrical atomic force microscopy (AFM) is ideally suited to visualise local differences in the electronic properties of TMD films with a nanometre-resolution. In AFM, a nanometre-sized tip at the end of a cantilever scans sample surfaces. From the deflection of the cantilever at each point of the sample, the surface structure is then reconstructed and assembled into a topography image. Using a conductive cantilever/tip-system and establishing a mechanical contact between tip and sample facilitates C-AFM measurements with an applied bias. C-AFM allows resolving the spatial distribution of heterogeneous conductivities in addition to the sample topography. For this purpose, C-AFM records the current flow
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