Current field effect transistors (FETs), based on Silicon, are approaching the dimensions of few atoms, which degrades their functionality due to the short channel effect. Recently, two-dimensional (2D) semiconductor materials like Transition Metal Dichalcogenides (TMDCs) have shown extensive potential as an alternative for current Si-based transistors. However, several important drawbacks concerning contact, doping and charge carrier transport need to be overcome. Modern plasma processing allows a large variety of nanoscale modifications, with high throughput, that could be effective to treat 2D-TMDCs materials at various stages of device fabrication.
Charge carrier mobility of 2D-TMDCs transistors is degraded by adsorbates, charged impurities, atomic vacancies, interface traps etc. that are acting as scattering site for charge carriers. Etching & cleaning of TMDCs layers have always been encountered many difficulties, because of the ultra-thin nature of the material and the lack of control on the process. Preservation or restoration of the cleanliness of the material is necessary for the realization of efficient devices based on 2D-TMDCs. Plasmas provide the ability to clean 2D materials in a fast, selective and reliable way. Further, to control the carrier type and concentration, by which the carrier mobility of TMDCs devices can be modulated, elemental doping is desirable. Modern plasmas allow to provide specific species with tailored energy and dose to a given surface, they can therefore be used to selectively replace surface atoms or insert extrinsic shallow dopants, providing a simple and scalable route for the doping of monolayer materials such as TMDCs. Contact engineering is essential to realize an efficient transistor. In the fabrication of contacts, the passivation dielectric (high-k's like HfO2, ZrO2 or hBN) is removed prior to metal deposition. Side contacts allow to directly inject carriers in the 2D basal planes but suffer from low active surface area for carrier injection; top contacts allow to maximize the metal-TMDC surface area but need to be engineered to enable efficient carrier injection through the Sulphur planes. Again, plasmas, exploited for etching in the fabrication of TMDCs devices, provide the ability to modify 2D materials in a fast, selective, directional and reliable way.
The proposed PhD work will explore new approaches for low damage plasma-based cleaning of 2D materials, associated with 2D restoration approaches, since the first step of damage occurs mainly through sulfur depletion. Defect engineering and doping will be studied using controlled sulfur replacement with dopants in the vapor phase. Finally, contact patterning will rely on the use of plasma-enhanced atomic layer etching (PE-ALE) concepts, where the removal process is cyclic with time-separated steps, possibly coupled with in-situ restoration or hexafluoroacetylacetone-based vapor etch. The success of the work with largely rely on the fine understanding of fundamental mechanisms of removal, damage and doping, and will rely on the execution of systematic and iterative experimental work, careful data analysis and in-depth discussions. The work will be supported by modeling activities at the university of Antwerp (Prof. E. Neyts). The work will be applied to device manufacturing and involve the learning of the full nanofabrication flow of simple 2D-based devices, as well as electrical measurements of their characteristics. The work will be performed in close collaboration with device and material scientists from IMEC, as well as material and tool suppliers.
The PhD candidate must have excellent hands-on skills, have basic knowledge in inorganic chemistry, plasma technology, be motivated by an international work environment and open to (limited) travel.
Required background: Master in Physics, Chemistry, nanotechnology (Science or Engineering)
Type of work: 70% experimental, 20% modeling, 10% literature
Supervisor: Stefan De Gendt
Daily advisor: Jean-Francois de Marneffe
The reference code for this position is 2020-060. Mention this reference code on your application form.