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[19p-S221-6] Single-Layer WS2 Phototransistor with Multi-Layer Graphene Electrodes
on a Flexible Parylene Substrate
Keywords:two-dimensional material,graphene,ws2
Heterostructures of two-dimensional (2D) material, such as graphene and transistion metal dichalcogenides (TMDCs), have attracted great interest [1]. Quantum confinement in 2D regime offers new advantages, for instance, high carrier mobility in graphene and direct band gap in single-layer WS2. Their advantages open a new avenue for novel optoelectronic devices. In the present work, we demonstrate the combination of graphene and WS2 for making flexible and transparent photodetector. The flexible and transparent nature of our device is realized as we utilize the flexible and transparent parylene substrate.
The single-layer WS2 was grown on sapphire substrate by thermal CVD method by using WO3 and sulfur powder as precursor. Furthermore, in order to grow graphene with controlled thickness by CVD, Cu-Ni alloy foil was used as catalyst. Combination of carbon solubility on Cu-Ni foil at high temperature and slow-cooling process yielded the multi-layer graphene (MLG) with ~10 nm thickness. MLG was chosen over single-layer graphene because MLG has lower sheet resistance (~100 Ω/¨) than that of single-layer graphene (600 Ω/¨). Prior to transfer process, the MLG was patterned using lithography process and etched by O2 plasma. Finally, WS2 and MLG were precisely stacked on a 1 mm-thick parylene substrate like the illustration shown in Figure 1a.
The optoelectronic properties were measured in vacuum. We found two interesting outcomes in this study: Firstly, the device worked by applying probes directly on MLG electrodes. Secondly, the interface between MLG electrode and WS2 showed ohmic behavior, as shown in inset in Figure 1b. Moreover, the extracted field effect mobility of WS2 is around 2 cm2/Vs which is reasonably high considering the extremely thin device structure. Figure 1b shows Id-Vd of the device under light illumination. Photoresponsitivity of the device reached as high as 70 mA/W at Vg = 30 V. Additionally, the internal quantum efficiency (IQE) was calculated around 9%. In summary, even though the device is semi-transparent, good optoelectronic properties are observed well.
The single-layer WS2 was grown on sapphire substrate by thermal CVD method by using WO3 and sulfur powder as precursor. Furthermore, in order to grow graphene with controlled thickness by CVD, Cu-Ni alloy foil was used as catalyst. Combination of carbon solubility on Cu-Ni foil at high temperature and slow-cooling process yielded the multi-layer graphene (MLG) with ~10 nm thickness. MLG was chosen over single-layer graphene because MLG has lower sheet resistance (~100 Ω/¨) than that of single-layer graphene (600 Ω/¨). Prior to transfer process, the MLG was patterned using lithography process and etched by O2 plasma. Finally, WS2 and MLG were precisely stacked on a 1 mm-thick parylene substrate like the illustration shown in Figure 1a.
The optoelectronic properties were measured in vacuum. We found two interesting outcomes in this study: Firstly, the device worked by applying probes directly on MLG electrodes. Secondly, the interface between MLG electrode and WS2 showed ohmic behavior, as shown in inset in Figure 1b. Moreover, the extracted field effect mobility of WS2 is around 2 cm2/Vs which is reasonably high considering the extremely thin device structure. Figure 1b shows Id-Vd of the device under light illumination. Photoresponsitivity of the device reached as high as 70 mA/W at Vg = 30 V. Additionally, the internal quantum efficiency (IQE) was calculated around 9%. In summary, even though the device is semi-transparent, good optoelectronic properties are observed well.