2D Transition Metal Dichalcogenides (TMDCs) have been actively studied in recent times due to their interesting electronic and optoelectronic properties. In particular for photodetection applications, TMDCs display direct to in-direct band gap transitions and broadband light detection based on layer tuning. Although great effort has been made to design high responsivity photodetectors by discovering new materials, different device structures, material doping, and combining different materials to create heterostructures, an understanding of how to intrinsically enhance the light sensitivity has not yet been discussed. Obtaining a better understanding of modifying the photocurrent generation process can help to provide a tool to design photodetectors for specific types of applications.

In this study, we investigated how to design a dominating photogating or photoconductive effect with TMDC-based phototransistors. The photoconductive effect can provide fast light switching speeds due to collection of photogenerated carriers and the photogating effect can provide high light sensitivity due to photocarrier charge trapping. Although many works have reported photogating behavior for TMDCs, this behavior as a dominating photocurrent generation mechanism is observed in the accumulation mode only. Here, we explored how the dielectric layer properties can directly influence the photocurrent generation process. High-k metal oxides such as hafnium oxide (HfO₂) are well known charge trapping materials which are used in some memory applications. We found that the intrinsic oxygen vacancy defects located in the mid band gap region of HfO₂ offers trap energy level alignment with the valence band edge of MoS₂. As a result, the photogenerated holes tunnel into the HfO₂ dielectric layer to occupy these defect levels enabling a strong photogating behavior as seen in the figure 1(a) below. By providing a trap energy level misalignment, a strong photoconductive effect can be observed with the WSe₂ device in figure 1(b) using the same HfO₂ dielectric. The band energy diagram of MoS₂, WSe₂, and HfO₂ in figure 1 (c) illustrates the valence band alignment with these oxygen vacancies. 

 As a result of creating a dominant photogating photocurrent generation, we were able to achieve a high peak responsivity of 1x10⁶ A/W for intrinsic MoS₂ and photocurrents as high as 2 μA. Our work shows how tuning the valence band edge alignment with known intrinsic dielectric defects can enable a strong photogating or photoconductive effect for the photocurrent generation process in phototransistors.

 If you are interested in learning more about this study, please check out our article published in Communications Materials titled:

“High Responsivity in MoS₂ Phototransistors Based on Charge Trapping HfO₂ Dielectrics”:

 https://www.nature.com/articles/s43246-020-00103-0

 Co-authors:

Dr. Takashi Tsuchiya, Dr. Kasidit Toprasertpong, Dr. Kazuya Terabe, Prof. Shinichi Takagi, and Prof. Mitsuru Takenaka