Electrical transport and noise in polycrystalline 2D van der Waals materials and their grain boundaries

Abstract

The successful exfoliation of graphene in 2004 ushered in the era of 2D van der Waals materials, paving the way for a wide variety of applications in the field of electronics, optoelectronics, sensors, catalysis, flexible and wearable electronics. Chemical vapour deposition (CVD) is a low-cost, scalable technique which allows the direct and controllable synthesis of large-area uniform films of 2D materials for wafer-scale technologies. A major drawback of this technique lies in the polycrystalline nature of the films due to the inevitable presence of lattice point defects and extended line defects such as grain boundaries (GBs) which is a serious detriment to electronic transport. Recent improvements in CVD growth have shown that it is possible to tailor the growth conditions specifically in order to obtain high quality grains by grain boundary engineering. This thesis work is focused mainly on the intrinsic electrical properties of GBs in polycrystalline 2D material films and their role in hindering the device performance of 2D field effect transistors (FETs).

In the first part of the thesis, we describe the use of a symbiotic two-pronged approach of time-averaged conductance and low frequency noise measurements to establish the extent to which GBs enhance carrier localization in transition metal dichalcogenide (TMDC) systems which manifests itself not only in the reduction of the localization length by 30% − 70% in the GBs as compared to the single crystalline region but also leads to an overall noise enhancement factor of nearly five orders of magnitude. In doing so, our work also serves to extend the range of application of the well-established McWhorter's noise model to localized systems by explaining the origin of the uncharacteristic exponential dependence of low frequency noise with temperature. In the second half of the thesis, we describe how our study of time-dependent universal conductance fluctuations (UCF) via low frequency noise measurements at ultra-low temperatures point towards the spontaneous breaking of time reversal symmetry across graphene grain boundaries. The magnetic ordering is found to be gate-tunable and is attributed to the formation of localized magnetic moments at the octagon-pentagon defects of the disordered GB region, which is further enhanced by the inbuilt lattice strain leading to the dephasing of spins.