(Invited) Transition Metal Dichalcogenide Semiconductor Growth and Large Area Devices for Optoelectronics and Sensing

Monday, 2 October 2017: 09:30
Chesapeake E (Gaylord National Resort and Convention Center)
S. Garg, A. S. Mollah, J. L. Waters, S. M. Kim, and P. Kung (University of Alabama)
Atomically-thin transition metal dichalcogenides (TMDCs), such as MoS2 and WS2, are topological materials with physical characteristics of great interest for the realization of nano-electronic and photonic devices, with promising applications for chemical and biological sensing. These semiconductors possess a bandgap that changes from indirect to direct as a result of the disappearance of inversion symmetry when the material is reduced into a sub-nanometer monolayer form (2D). Although TMDC monolayers were originally and are continuing to be widely realized through mechanical exfoliation, there are accelerating material synthesis efforts to achieve more controllably sized and shaped TMDC in order to facilitate the realization of devices with greater consistency.

In this work, we present the growth and characterization of continuous monolayer TMDC followed by the realization and measurement of both photoconductive and Schottky detectors, as well as chemical sensor devices. The materials were grown by chemical vapor deposition on sapphire substrates. The monolayers were subsequently characterized using Raman spectroscopy, photoluminescence, fluorescence imaging, optical absorption, atomic force microscopy, and electron microscopy. Large area metal-2D semiconductor-metal interdigitated devices were realized by conventional photolithography, with 20 μm wide and 4000 μm long metal fingers separated by a distance ranging from 5 to 50 μm.

The electrical and photoresponse characteristics of MoS2 based photoconductors were measured at room temperature in nitrogen ambient. The spectral response was measured in the 400-800 nm range and exhibited a rapid drop for wavelengths longer than 675 nm, corresponding to the bandgap of MoS2. The spectral response revealed clear A and B band-edge exciton related peaks, as well as a peak arising from the van Hove singularities. At a bias of 1 V, monolayer MoS2 devices exhibited a responsivity of 11 A/W corresponding to a specific detectivity of 8x1011 Jones. The high responsivity was associated with a large photoconductive gain arising from trap states/surface states in the large area MoS2 material and device. The resulting persistent photoconductivity and photoconductive time decay were subsequently studied. The use of MoS2 devices for chemical sensing was investigated. The changes in electrical resistance in the dark was monitored as a function of the concentration of the gas sensed in an inert nitrogen ambient. When tested for CO2, concentrations down to 200 ppm were able to be detected.