Kyung Hee University,
Keywords: Quantum-dots, oxide semiconductor, visible-light, phototransistors, ultraviolet photoelectron spectroscopy
Summary:The photocurrent of oxide semiconductor thin film transistors (TFTs) can be observed when the device is exposed to an ultra-violet light, because oxide semiconductor is a wide band gap semiconducting material. However, oxide TFTs can not generate photocurrent with the illumination of low-energy photon such as visible-light. Therefore, we decorated cadmium selenide (CdSe) quantum-dots (QDs) and metal nanoparticles in the oxide TFTs to increase the photocurrent with low-energy light, such as a visible light. A thin layer of QDs or metal nanoparticles were placed on or under the oxide semiconductors. We prepared CdSe QDs with sizes of ~6.3 nm, which can absorb red visible light. Metal nanoparticles were formed on a gate insulator using a thermal evaporator and a post-annealing processes. The prepared devices with QDs showed enhanced photocurrent upon exposure to visible-light. Measurements to construct an energy level diagram were made using ultraviolet photoelectron spectroscopy to determine the origin of the photocurrent, and we found that the small band gap of CdSe QDs enables the increase in photocurrent in the oxide semiconductor TFTs even with the visible-light. The device with metal nanoparticles showed enhanced photocurrent and modulation behavior under the visible-light, as well. The increased photocurrent showed a strong coupling between localized plasmons and electrical carriers in the active channel, where metal nanoparticles were existed. The device characteristics and origin of the photocurrent will be presented in detail. This result is relevant for developing highly transparent visible-light photosensors based on oxide semiconductors.