Reliable Postprocessing Improvement of van der Waals Heterostructures.

Youngwook Kim,Kenji Watanabe,Takashi Taniguchi,Patrick Herlinger,Jurgen H Smet

doi:10.1021/acsnano.9b06992

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. Although 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 postprocessing surface treatment based on thermal annealing and contact mode atomic force microscopy (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 and 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, that is, the detrimental impact of random strain fluctuations.

Highlights

The study of two-dimensional electron systems with extraordinarily low levels of disorder was for a long time the exclusive privilege of the epitaxial thin film research community
By encapsulating the studied 2D material, for instance, graphene or MoS2, between a van der Waals material with a high affinity to the active layer, a self-cleansing effect occurs at the heterointerfaces, causing an aggregation of the contaminants into randomly distributed bubbles.[11]
Couto and co-workers have demonstrated that random strain fluctuations make up the main source of residual disorder in already high-quality graphene samples, such as on Hexagonal boron nitride (hBN).[19]

Summary

Introduction

The study of two-dimensional electron systems with extraordinarily low levels of disorder was for a long time the exclusive privilege of the epitaxial thin film research community. Couto and co-workers have demonstrated that random strain fluctuations make up the main source of residual disorder in already high-quality graphene samples, such as on hBN.[19] In the low-temperature limit, the mobility and the density inhomogeneity at low density, as estimated from the width of the field-effect resistivity peak, are closely correlated. We report a reliable method to boost the quality of such van der Waals heterostructures by means of a final step subsequent to the device processing and a high-temperature annealing step It is based on scanning an atomic force microscope (AFM) tip, operated in contact mode with a well-defined force, across the active device area preselected based on the absence of bubbles in optical microscopy and noncontact AFM. We provide some statistics on the performance of a larger set of devices as relevant for Hall sensing and thereby demonstrate that the ironing technique enhances notably application-relevant room temperature properties

Methods

Results

Conclusion