  Semiconducting 2D materials, such as molybdenum disulfide (MoS2) and other
members of the transition metal dichalcogenide family, have emerged as
promising materials for applications in high performance nanoelectronics that
exhibit excellent electrical and optical properties. Highly efficient
photocurrent (PC) generation is reported in vertical few layer MoS2 devices
contacted with semitransparent metallic electrodes. The light absorption of the
device can be improved by fabricating vertical photodevices using few layer
flakes, achieving a photoresponse of up to 0.11 A/W and an external quantum
efficiency (EQE) of up to 30%. Because of the vertical design, the distance
between electrodes can be kept in the range of a few nanometers, thus
substantially reducing the collection time of photogenerated carriers and
increasing the efficiency of the devices. The wavelength dependent PC,
photoresponsivity, and EQE are measured over a photon energy range from 1.24 to
2.58 eV. Compared to previous in plane and vertical devices, these vertical few
layer MoS2 photodevices exhibit very short response time, 60 ns, and a cutoff
frequency of 5.5 MHz, while maintaining high photoresponsivity.
