Atomic assessment of paramagnetic defects in 2-dimensional semiconductor layers: MoS2

For decades, the semiconductor industry has pursued the extensive miniaturization of the transistor. Conventional Si-based devices are however reaching their scaling limits since undesired ‘physics’ effects arise when Si-based transistors are ultra-scaled. A possible solution to overcome these electrostatic control issues and lithography challenges is to incorporate novel 2D materials in future nanoelectronic devices. In this regard, TMDs, and in particular MoS2, emerged as promising candidates to create novel low-power devices with enhanced functionalities.

Crucially, two paramount issues need to be solved before MoS2 can be widely applied as a 2D channel material in future devices: A reliable synthesis method and a robust doping procedure need to be developed. In this respect, ESR serves as a most useful non-destructive technique to assess the quality and doping properties of 2D materials by characterizing intrinsic and extrinsic point defects. An extensive investigation of defects in synthetic and geological MoS2 has therefore been performed in this work by ESR, a technique that achieves exceptional sensitivity and possesses eminent discriminative and quantitative capabilities.

This work presents a detailed multi-frequency ESR analysis of a newly observed impurity in a CVD-grown bulk 2H MoS2 crystal. The previously unreported signal of axial symmetry exhibits a g anisotropy typical of acceptors and is, after careful consideration, identified as originating from N acceptor dopants, which are found to be substituting for S sites in bulk MoS2 with a density of ~ 2E17 cm-3, thus predominantly accounting for the p-type sample doping. The thermal stability, spatial distribution, and activation energy (~ 45 meV) of the N acceptor is also studied in detail. Ultimately, substantial N contamination is revealed to be an inherent trait of the specific CVD method applied for the synthesis of the studied MoS2 material. 

The thermal stability, activation energy (~ 0.7 meV), and temperature dependence of the ESR spectral characteristics of the As acceptor dopant (As substituting for S) in geological bulk 2H MoS2 is also assessed in this work. In general, As is confirmed as a promising candidate for stable covalently bonded p-type doping of MoS2. Additionally, these findings indicate that the As acceptor emerges as a shallower and more robust dopant than the N acceptor.

Next, this work deals with an ESR investigation of point defects present in transferred synthetic few-layer MoS2. The ESR investigation is closely combined with an in-depth analysis by an assortment of other experimental techniques, including AFM, RBS, XPS, and TEM, to ultimately result in the assignment of the ESR signal to a defect of intrinsic nature, most likely a Mo vacancy related defect located at MoS2 grain edges or boundaries. The oxidation of the 2D material at grain edges and boundaries combined with the applied water-based transfer procedure is demonstrated to play a crucial role in the generation of the newly observed defect, thus exposing a weakness in the process method.

The final part of this thesis presents a comparative multi-frequency ESR analysis of various geological MoS2 crystals which reveals numerous kinds of bulk and surface contamination related defects. Different dopant regimes (n,p, and mixed) are uncovered, with As, N, and Re being traced as effective dominant impurity dopants. These observations emphasize the necessity of rigorous surface cleaning and even removing surface layers to obtain a pristine MoS2 parent crystal suitable for the exfoliation of high-quality flakes intended for fundamental analysis or state-of-the-art applications.