Vishal Panchal: Scanning gate microscopy on epitaxial graphene: unexpected carrier-type inversion

Vishal Panchal1, Héctor Corte-León1,2, David M. A. Mackenzie3, Olga Kazakova1 and Dirch H. Petersen3 1. National Physical Laboratory, Teddington, TW11 0LW, United Kingdom 2. Royal Holloway, University of London, Egham, Surrey, TW20 0EX, United Kingdom 3. Center for Nanostructured Graphene, Technical University of Denmark, Lyngby, 2800 Kgs, Denmark Applications that require a large number of graphene devices to behave in a precise manner can benefit greatly from large scale uniformity. Currently, the variability of epitaxial graphene thickness leads to devices formed from a mixture of 1LG/2LG. The exact shape, size and location of 2LG islands have been shown to greatly alter the performance of devices in applications such as resistance metrology [1] and environmental sensors [2]. We present local electric field sensitivity maps of Hall cross devices of 1-2LG imaged using electrical scanning gate microscopy (SGM). These experimental results are compared to theoretical maps obtained from equivalent finite element simulations for qualitative comparison. Using both these techniques, we observe abrupt inversion in the voltage response of the device when locally gating 2LG islands compared to 1LG. These manifests from differences in charge neutrality point of 1-2LG, and the ability of the SGM tip bias to manipulate the carrier density and mobility. We further derive an analytical model to extract directly the carrier type and sheet carrier density from SGM measurements. With this we observe carrier type inversion between 1LG and 2LG. Thus, we demonstrate that SGM is an effective tool for studying gating effects in graphene devices with 10's of nm scale spatial resolution. Moreover, these results prove that strategical device design with specific 2LG structures can be used to enhance sensitivity of devices to electric fields. 1. T. Yager et al., Nano Letters, 13, 9, 4217-4223 (2013) 2. V. Panchal et al., 2D Materials, 3, 1, 15006 (2016) Vishal Panchal received his MSci degree in Physics in 2010 from Royal Holloway, University of London, UK. In 2014, he completed his Ph.D. at the National Physical Laboratory, in affiliation with Royal Holloway, University of London. with a dissertation on "Epitaxial graphene nanodevices and their applications for electronic and magnetic sensing". Since then, Vishal has been hired as a Higher Research Scientist by the Quantum Detection Group at the National Physical Laboratory. Additionally, he completed a 6 month secondment in 2016 at the National Institute of Standards and Technology working on growth and electrical/optical characterization of epitaxial graphene nanoribbons on SiC(0001). Vishal's current scientific interests include nano- and micro-scale studies on graphene and other related 2D materials. His expertise are in local electrical and optical characterizations with functional scanning probe microscopy techniques such as Kelvin probe force microscopy, conductive atomic force microscopy, scanning gate microscopy, scattering scanning near-field optical microscopy, etc.