Atomic Layer Processing and Characterization of 2D TMDs

Abstract:

Transition metal dichalcogenides (TMDs, e.g., MoS2) have emerged from the graphene initiatives due to the diverse functionality offered by its tunable bandgap, which creates a broad range of applications spanning from ICT to renewable energy harvesting. With the current technique constraints, device-grade properties of TMDs can only be achieved from flakes that are mechanically exfoliated from high-quality crystals. However, practical device integration requires nucleation, thickness, defect, and dopant controls of TMDs at the pre-manufacturing process-level, i.e., direct growth and processing of TMDs at wafer-level alongside appropriate considerations of thermal budget.

Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are leading candidates for wafer-level atomic layer processing of TMDs, during which dopants can be incorporated into the growth to tune the electrical properties. Fig. 1 illustrates an example of 2D layered polycrystalline non-intentionally doped MoS2 grown by CVD at 550 °C for 2.5 hours [1], which approximates to the back-end-of-line thermal budget limit of complementary metal-oxide semiconductor processes [2]. 

Our group performs atomic layer processing of thin film MoS2 using two pieces of equipment including a manufacturing-compatible Applied Materials 300 mm ALD reactor, which allows for both ALD and CVD modes of deposition, as well as a VEECO plasma enhanced 200 mm ALD system. Both tools offer uniform growth, at 300 mm or 200 mm wafer-level respectively, with a high degree of repeatability. For all processes we conduct, thermal budget is always a key consideration in our investigation. ALD is of particular interest due to its potential low temperature growth, dopant incorporation using nanolaminatedoping technique, as well as its highly conformal growth on high-aspect-ratio structures.

In this presentation, I will give an overview of the CVD of MoS2, contrasting our CMOS compatible methods to the wider CVD literature, the introduction of dopants and our recent developments using ALD. In addition to the widely reported chemical/structural studies, I will present an in-depth electrical characterization of the thin films and devices. 

Acknowledgements The authors acknowledge the financial support of the European Union under Grant Agreement No. 101084261 (FreeHydroCells) and Science Foundation Ireland (SFI) under Grant Number SFI 12-RC-2278_P2

References [1] Lin J et al. 2D Mater. 8 (2021) 025008. [2] Fenouillet-Beranger C et al. 2014 IEEE IEDM (San Francisco, CA, USA, 15–17 December) pp 27.5.1–4.