Flexible two‑dimensional/three‑dimensional material based photodetectors
- Flexible Fotodetektoren basierend auf Zweidimensionale / Dreidimensionale Materialien
Schneider, Daniel Stefan; Lemme, Max C. (Thesis advisor); Vescan, Andrei (Thesis advisor)
Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021
1. Motivation, Goal and Task of the Dissertation Multispectral sensors enable the precise identification of unknown substances, undistorted by the human perception of colors. This makes it possible to carry out contactless and non‑destructive analyses for identification of hazardous substances. Electrically tunable photodetectors based on hydrogenated amorphous silicon (a‑Si:H)are alternatives to image sensors that are based on photodiodes with integrated color filters which are used for such analyses. A specific vertical device structure allows sampling of a wide range of spectral bands in a single photodetector without the need for optical filters. The integration of two‑dimensional (2D) materials with interesting electrical, mechanical, and optical properties, such as graphene and (molybdenum disulfide (MoS₂) into the multispectral sensors, opens the opportunity to extend the wavelength responsivity of these devices. This can overcome a significant limitation of silicon technology which primarily detects the visible region. As part of this PhD thesis, hybrid photodetectors based on 2D/3D heterostructures were simulated, fabricated, and characterized. Monolayer MoS₂ offers high absorptions of up to 10% in the visible part of the electromagnetic spectrum. It is also suitable for the development of high‑performance and highly flexible wearable electronics (wearables) due to its good electrical properties. The integration of MoS₂ in light sensors on flexible substrates can enable the realization of ultra‑thin and highly sensitive sensors which are also interesting for Internet of Things. Standard substrates for semiconductors can also be thinned out to achieve higher device flexibilities, but the bearable strain for such intrinsically rigid and brittle substrate is rather limited. Here, metalsemiconductor‑metal photodetectors with MoS₂ absorbers of only a few atomic layers thick were fabricated on foil substrates and characterized with respect to their optoelectronic properties. 2. Major Scientific Contributions In this PhD thesis, 2D materials were successfully integrated into vertical multispectral sensors based on a‑Si:H for the first time. These 2D/3D hybrid photodetector structures with graphene electrodes as transparent and conductive electrodes show an enhanced UV responsivity compared to devices with conventional transparent conductive oxide (TCO) layers. In addition, electrical measurements under bending cycles have demonstrated that graphene‑ optimized multispectral sensors can be operated under repetitive tensile strain. In contrast, reference TCO‑based devices failed after only a few bending cycles. The maximum spectral responsivity of the photodetectors can be tuned from 310 nm to 520 nm by varying the applied bias. The quantum efficiency of the flexible photodetectors could be further increased by using bilayer graphene due to the reduction of sheet resistances. The research results were presented at international conferences (ESSDERC 2016 and DRC 2016)and published in a journal (Nanoscale 2017). In a collaboration work with the University of Siegen, MoS₂ layers were integrated into vertical a‑Si:H photodetectors and led to a responsivity up to the infrared range (~2 μm). The spectral responsivities of about 50 mA/W were achieved at energies below the bandgaps of the absorber layers which is unexpected, but verified by several optical measurements. This is likely attributed to absorption through defects in the 2D material and at the interfaces. The extension of the spectral sensitivity range can be activated by applying an electric field. The results were published in the journal ACS Photonics in 2019. Laterally arranged metal‑2D semiconductor‑metal photodetectors with additional gate contact were fabricated and characterized. The potentially scalable fabrication of flexible light sensors on flexible substrates with very high spectral responses in the blue wavelength region has been demonstrated by using MoS₂ grown on sapphire wafer. An additional gate contact allows the optimization of the photocurrent in contrast to standard photoconductor structures. These light sensors can be used especially for the detection of blue light hazard caused by modern LED‑based light sources with a high blue light content. Also, these photodetectors are resistant to recurring tensile strain, thus their integration in future wearables is achievable. The results on the 2D photodetectors were published at the international conferences (DRC 2019 and Graphene Week 2019) and in the journal ACS Photonics in 2020.
- Chair of Electronic Devices