Reliable Postprocessing Improvement of van der Waals Heterostructures

Abstract

The successful assembly of heterostructures consisting of several layers of different 2D materials in arbitrary order by exploiting van der Waals forces has truly been a game changer in the field of low dimensional physics. For instance, the encapsulation of graphene or MoS2 between atomically flat hexagonal boron nitride (hBN) layers with strong affinity and graphitic gates that screen charge impurity disorder provided access to a plethora of interesting physical phenomena by drastically boosting the device quality. The encapsulation is accompanied by a self-cleansing effect at the interfaces. The otherwise predominant charged impurity disorder is minimized and random strain fluctuations ultimately constitute the main source of residual disorder. Despite these advances, the fabricated heterostructures still vary notably in their performance. While some achieve record mobilities, others only possess mediocre quality. Here, we report a reliable method to improve fully completed van der Waals heterostructure devices with a straightforward post-processing surface treatment based on thermal annealing and contact mode AFM. The impact is demonstrated by comparing magnetotransport measurements before and after the AFM treatment on one and the same device as well as on a larger set of treated and untreated devices to collect device statistics. Both the low temperature properties as well as the room temperature electrical characteristics, as relevant for applications, improve on average substantially. We surmise that the main beneficial effect arises from reducing nanometer scale corrugations at the interfaces, i.e. the detrimental impact of random strain fluctuations.

Observation of ballistic upstream modes at fractional quantum Hall edges of graphene

“…Details of the fabrication procedure are in SI (S1). The BLG channel area of the stack was microscopically ironed using an AFM (atomic force microscopy) tip in contact mode 54 , to remove any atomic level strain or ripples or small bubbles from the channel area, which can arise due to the stacking process. After this, for making contacts we used electron beam lithography (EBL).…”

High-quality electrical transport using scalable CVD graphene

“…Apart from modifications in the device structure, increasing the size of the processable (bubble-free) regions within the hBN/CVD-G/hBN heterostructures is a clear priority. Engineering clean interfaces over large areas in CVD-G-based hetero-stacks is also of great technological relevance and might benefit from assembly in vacuum conditions [43] and post-assembly thermal and/or nano-mechanical treatment [29]. [33].…”

Room‐Temperature Low‐Voltage Control of Excitonic Emission in Transition Metal Dichalcogenide Monolayers

“…This naturally challenged the spatial homogeneity of photoluminescence response across a TMD monolayer (Figure , Supporting Information), which could be improved by heterostructure brooming with an AFM tip in contact mode. [ 44,45 ] Higher electron and hole doping densities can be achieved by careful choice of the working electrode of electrochemical cell, while the application of voltage bias over ±2 V caused degradation of ITO working electrode in our experiments.…”