  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.
