  Intensive efforts have been devoted to exploit novel optoelectronic devices
based on two-dimensional (2D) transition-metal dichalcogenides (TMDCs) owing to
their strong light-matter interaction and distinctive material properties. In
particular, photodetectors featuring both high-speed and high-responsivity
performance are of great interest for a vast number of applications such as
high-data-rate interconnects operated at standardized telecom wavelengths. Yet,
the intrinsically small carrier mobilities of TMDCs become a bottleneck for
high-speed application use. Here, we present high-performance vertical van der
Waals heterostructure-based photodetectors integrated on a silicon photonics
platform. Our vertical MoTe2/graphene heterostructure design minimizes the
carrier transit path length in TMDCs and enables a record-high measured
bandwidth of at least 24GHz under a moderate bias voltage of -3 volts. Applying
a higher bias or employing thinner MoTe2 flakes boosts the bandwidth even to
50GHz. Simultaneously, our device reaches a high external responsivity of
0.2A/W for incident light at 1300nm, benefiting from the integrated waveguide
design. Our studies shed light on performance trade-offs and present design
guidelines for fast and efficient devices. The combination of 2D
heterostructures and integrated guided-wave nano photonics defines an
attractive platform to realize high-performance optoelectronic devices, such as
photodetectors, light-emitting devices and electro-optic modulators.
