PRADO DA SILVA Gabriel

•    10/2024 - present: LMGP and CEA-Leti, Ph.D. student, Grenoble (France).
Ph.D. thesis: Atomic layer deposition of vanadium sulfide contacts for next-generation transistors based on 2D dichalcogenides.
Main supervisor: H. Renevier (LMGP). Co-supervisors: S. Cadot (CEA-Leti) and N. Gauthier (CEA-Leti).
This project is recipient of a Labex Microelectronics thesis grant and it is being carried out in collaboration between LMGP and CEA-Leti.

Context: Semiconducting transition metal dichalcogenides (STMDs), such as MoS2 or WS2, have witnessed a surge in popularity over the past decade, providing novel perspectives for the fabrication of ultra-downscaled electronic devices. However, contact engineering on such materials remains challenging due to the high contact resistance exhibited by most metals and the damage that is induced at the metal/STMD interface. In this regard, the utilization of metallic transition metal dichalcogenides (MTMDs), such as VS2, emerges as a promising solution for the establishment of efficient and industrially applicable contacts on STMDs [1] [2]. 
It has been demonstrated that VS2 has been successfully used for the realization of field-effect transistors (FETs) based on MoS2 [3]. Moreover, the incorporation of vanadium into MoS2 has been shown to generate p-type doping, enabling the precise modulation of carrier polarity from n-type undoped MoS2 to p-type V-MoS2 through ambipolarity. The low contact resistance of VS2 on MoS2, in conjunction with the capacity of vanadium to reverse the charge carrier polarity in STMDs through doping, signifies a pivotal enabler for the development of next-generation logic devices based on transition metal dichalcogenides [4]. 
Conformal and self-limiting techniques, such as ALD, are preferable to CVD or PVD techniques for the deposition of VS2 contacts because they ensure a continuous coating of the contact hole in any device architecture. The atomic layer deposition of VS2 has been the subject of only a limited number of investigations, in part due to the difficulty of stabilizing the +4 valence of vanadium in the V-S system. The V-S phase diagram is intricate, yet the majority of VSx compounds can be regarded as VS2 derivatives. These derivatives are self-intercalated by ordered V atoms within the van der Waals (vdW) gaps and exhibit metallic properties that could also be of interest for contacting MoS2. It is therefore of great interest to study the electrical properties of these conductive VS2 derivatives, together with the thermal stability of the corresponding VSx/MoS2 heterostructures [5].


Objective and methods: The objective of this study is to gain insight into the mechanisms of growth and decomposition that occur during the deposition and crystallization of vanadium sulfide (VSx) ultrathin films on SiO2 and SiO2/MoS2 substrates, using the one-of-a-kind ALD/MLD reactor of the LMGP, which will allow the synthesis to be monitored and controlled by in-situ ellipsometry, residual gas analysis and in-situ synchrotron studies. 
Furthermore, the chemical composition, structure, and microstructures of the thin films, as well as their electrical and bandgap properties, will be investigated by ex-situ, post-synthesis analysis. This analysis will be performed using Raman micro-spectroscopy, ARXPS (Angle Resolved X-ray Photoelectron Spectroscopy), SIMS (Secondary Ion Mass Spectrometry) and TEM (Transmission Electron Microscopy) at CEA-Leti/PFNC. Conventional XPS will be complemented by the use of a novel lab-based hard X-ray source (HAXPES), which will facilitate the investigation of the chemical composition at the dichalcogenides surface and the in-depth distribution of sulfur within the VSx layer.
The LMGP ALD/MLD reactor will be mounted at the beamline SIRIUS of the SOLEIL synchrotron facility, which is optimized for performing grazing incidence X-ray fluorescence (XRF), X-ray absorption spectroscopy (XAS), and X-ray reflectivity (XRR) and X-ray diffraction (XRD), providing a unique tool to address the interaction of the precursors with the substrate, the chemical-structural quality of the deposits from the very first ALD/MLD cycles and the crystallization process.

References:
[1] Nat Commun (2023). 10.1038/s41467-023-41779-5
[2] Nature (2021). 10.1038/s41586-021-03472-9
[3] Nano Letters (2017). 10.1021/acs.nanolett.7b01914
[4] Adv. Funct. Mater. (2022). 10.1002/adfm.202204760
[5] ACS Nano (2024). 10.1021/acsnano.3c05907

To find out more about CEA-Leti.
To find out more about SOLEIL synchrotron facility