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The Use of Secondary Ion Mass Spectrometry to Advance Integrated Process Technology for 2D Materials


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Type

Thesis

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Authors

Veigang-Radulescu, Vlad-Petru 

Abstract

Despite the advancements in 2D material fabrication methods, the development of scalable manufacturing and device integration strategies are critical bottlenecks that currently hinder the commercial application of low-dimensional nanomaterials. Trace contaminants can significantly influence the catalytic growth process, as well as the post-growth processing, and can be deleterious for many applications. There is a need for characterisation techniques with very high elemental resolution combined with surface sensitivity and suitably high spatial resolution. In this work, we employed a highly sensitive technique that is widely used in the semiconductor industry, secondary ion mass spectrometry (SIMS), at different stages along the path of optimizing the fabrication process to maximize the quality of 2D materials for their scalable integration in the next future technologies. Specifically, time of flight secondary ion mass spectrometry (ToF-SIMS) is used to gain insight into the growth of 2D materials, including graphene and hexagonal boron nitride (hBN) on a variety of substrates.

The growth of graphene on copper foil substrates by chemical vapour deposition (CVD) is investigated first. ToF-SIMS reveal that a minute amount of residual oxygen in the bulk of Cu foil catalysts on graphene CVD can significantly deteriorate the quality of as-grown graphene, as highlighted by an increased Raman D/G ratio, increased propensity to post-growth etching, and increased fraction of multilayer graphene nucleation. In the same context, Fe catalysed hBN is investigated. Here, ToF-SIMS is employed to reveal the significant role the bulk dissolved species has on the catalytic growth of hBN. Similarly to graphene, bulk oxygen can act to supress the nucleation density, but also too much of it can be deleterious. This new understanding allowed us to design strategies to retain the beneficial effect of oxygen, while avoiding etching by it through the use of reducing treatments: hydrogen annealing for the graphene CVD and carburizing for the hBN that resulted in high quality, mm-sized crystals.

Next, we investigate contamination occurring from the gas phase during CVD process. In this context, we employ ToF-SIMS to evaluate carbon contamination arising during WS2 synthesis from metallo-organics precursors. Through this new understanding we were able to determine the optimal process parameters to reduce the carbon contamination of the as-grown large-area WS2.

For most applications, the 2D materials cannot be directly used on the metal catalyst and have to be transferred with very low metallic contamination. We investigate an alternative metal-free growth approach in which graphene is synthesized on Ge. Two Ge crystal orientations are employed, Ge(110) and Ge(100), and also epitaxial Ge(100)/Si(100) films. We find that Ge(110) yields the best graphene quality among all Ge orientations, while ToF-SIMS shows that intermixing between Si and Ge occurs at growth temperatures in the case of graphene on Ge(100)/Si(100). We establish the deleterious effects of oxygen impurities present in the gas phase precursors used during the CVD process, resulting in damaged Ge surfaces and poor graphene quality, and we emphasise the need for an oxygen/H2O filter.

Finally, we utilise ToF-SIMS to study contaminations arising from the transfer of 2D materials, either CVD or exfoliated. We reveal typical metallic and organic contamination that results from the wet transfer of Cu catalysed CVD graphene, and we show that a simple post-transfer treatment in heated hydrochloric acid can remove almost completely the Cu contamination. hBN/graphene/hBN heterostructures and devices were probed by ToF-SIMS to determine the typical contamination that arises during their fabrication based on the dry transfer of 2D materials and subsequent lithographic processes. We detect contamination between the 2D materials located in blisters and we investigate two Hall-bar devices where ToF-SIMS measurements are done in conjunction with transport characterisation to address the question of how much contamination is enough to alter device performance and to determine how contamination-tolerant such devices are. Here, we show that specific ions detected by ToF-SIMS can be related to non-uniformity in the heterostructure channel, which can explain the large variation in contact resistance and electron and hole mobilities.

Description

Date

2022-05-18

Advisors

Hofmann, Stephan

Keywords

2D heterostructures, 2D materials, Contamination, Graphene, Hexagonal boron nitride, SIMS, ToF-SIMS, Tungsten disulfide

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
National Physical Laboratory (NPL) (unknown)