Improved optical performance of multi-layer MoS₂ phototransistor with see-through metal electrode

In recent years, MoS₂ has emerged as a prime material for photodetector as well as phototransistor applications. Usually, the higher density of state and relatively narrow bandgap of multi-layer MoS₂ give it an edge over monolayer MoS₂ for phototransistor applications. However, MoS₂ demonstrates thickness-dependent energy bandgap properties, with multi-layer MoS₂ having indirect bandgap characteristics and therefore possess inferior optical properties. Herein, we investigate the electrical as well as optical properties of single-layer and multi-layer MoS₂-based phototransistors and demonstrate improved optical properties of multi-layer MoS₂ phototransistor through the use of see-through metal electrode instead of the traditional global bottom gate or patterned local bottom gate structures. The see-through metal electrode utilized in this study shows transmittance of more than 70% under 532 nm visible light, thereby allowing the incident light to reach the entire active area below the source and drain electrodes. The effect of contact electrodes on the MoS₂ phototransistors was investigated further by comparing the proposed electrode with conventional opaque electrodes and transparent IZO electrodes. A position-dependent photocurrent measurement was also carried out by locally illuminating the MoS₂ channel at different positions in order to gain better insight into the behavior of the photocurrent mechanism of the multi-layer MoS₂ phototransistor with the transparent metal. It was observed that more electrons are injected from the source when the beam is placed on the source side due to the reduced barrier height, giving rise to a significant enhancement of the photocurrent.