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John Robertson1 Kazi Islam1 Maxwell Woody1 Jacqueline Failla1 Jiang Wei1 Matthew Escarra1

1, Tulane University, New Orleans, Louisiana, United States

Optoelectronic devices featuring 2D transition metal dichalcogenide (TMDC) semiconductors, such as MoS2, WS2, and MoSe2, hold great promise for the miniaturization of future light emitting and collecting devices. In order for 2D optoelectronic devices to be developed into viable technologies, it is necessary to produce large-area films and vertical device architectures. Vertical devices are expected to increase device performance by reducing the path length of charge carriers by ~100x and by enabling lateral scaling on the order of centimeters. In response, we propose and demonstrate a device architecture that uses a molybdenum film as both the growth substrate for 2D MoX2 (X = S, Se, Te) and also as the bottom contact for vertical MoX2-based optoelectronic devices. This architecture allows for TMDC growth directly on top of its bottom contact, simplifying and improving the device fabrication process. Using a Thermal Vapor Sulfurization (TVS) technique developed in previous works, we show that a high-quality 2D film of MoS2 can be grown on top of molybdenum by reacting the top of the molybdenum layer with sulfur vapor; the remaining molybdenum underneath the MoS2 is used as a conducting bottom contact. This contact scheme requires no external transfers of the MoS2 layer and results in an intimate bottom contact interface relatively free of contaminants. Preliminary multilayer MoS2 photodetector devices on a 150nm molybdenum film were fabricated and characterized. Strong A1g and E2g raman peaks, spaced 25cm-1 apart, indicate a 10-15 layer MoS2 growth. In addition, diffuse and specular reflection measurements indicate an absorption of up to 85% of visible light, as aided by internal reflection off of the MoS2/Mo interface. A 30µm × 20µm Ti/Au grid finger array is used as the top contact. When illuminated from 400nm to 700nm using a supercontinuum laser and laser line tunable filter under 6V source-drain bias, the device shows a 3 order of magnitude increase in spectral photocurrent relative to comparable lateral photodetectors. Dark and illuminated IV curves reveal diode-like behavior, with an induced photocurrent of up to 300nA under monochromatic illumination of ~0.05mW. Palladium and other hole-selective contacts will be used to fabricate large-area 2D MoX2-based Schottky-type photovoltaics with collection areas on the order of cm2. Device simulation using the AFORS-HET software package is used to computationally interpret the performance of the devices and to explore the parameter space of device properties. We also emphasize the potential for these devices to be fabricated on micron-thick molybdenum foils, enabling roll-to-roll fabrication on ultra-lightweight and flexible substrates. This presented technique reveals a viable route for industrial-scale synthesis of nm-thick 2D TMDC photovoltaics, with great potential for applications requiring ultra-lightweight photovoltaics, such as spacecraft and vehicle energy collection.

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