Two-dimensional optoelectronics in transition-metal dichalcogenide heterostructures: controlling donor and acceptor effects on exciton dynamics for novel photodetectors

• Applicant: Dr. Hans Kleemann
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 283779880

Two-dimensional materials such as graphene or transition-metal dichalcogenides represent one of the most exciting scientific discoveries of the last decades. These atomically thin 2D structures give rise to a variety of new and fascinating quantum mechanical phenomena (such as Dirac electrons or the quantum Hall effect at room temperature) which can be utilized for applications in semiconductor devices and sensors. Transition-metal dichalcogenides are particularly interesting for optoelectronic applications since they possess an optical bandgap in the visible to near-infrared regime and exhibit strong light-matter interactions. Moreover, the ability to form heterojunctions using two different transition-metal dichalcogenide materials allows for efficient light harvesting and ultra-fast exciton dissociation. However, these unique material properties are not utilized for tailored applications, yet which requires a deep understanding of the device and process technology.In this project, highly-efficient and ultra-fast photodetectors based on transition-metal dichalcogenide pn-junctions will be developed. In particular, the role of molecular doping will be analyzed in order to control and optimize the optical and electronic properties of the detector devices. The main focus is on the question how doping, and hence the control of the quasi-chemical potential, can improve the efficiency and kinetics of the exciton generation and dissociation processes. A deeper understanding of the influence of adsorbates such as donor or acceptor molecules on the opto-electronic processes in atomically thin transition-metal dichalcogenide films is not only of fundamental interest, but also essential for the optimization of the whole photodetector device. Besides these opto-electronic effects, molecular doping will be used to tune the electrical parameters of the detector device that will be integrated into a thin-film transistor architecture in a second step. In this regard, doping is helping to reduce undesired contact resistances and is a valuable method in order to control the charge carrier concentration and the electric field within the transistor as the key parameters for the detector sensitivity.The exploration of reliable processes for doping and device integration will greatly promote the development of ultra-fast photodetectors and it will deepen the understanding of the underlying physical phenomena. Additionally, it will push the development for transition-metal dichalcogenide based devices in general.Altogether, the work proposed in this project will serve as a starting point for the development of completely new application scenarios for this interesting class of 2D materials.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Feng Wang, Ph.D.