PhD position | TMDC materials: tuning transport properties by substitutional doping and phase transition

2D layered Transition Metal Di-Chalcogenides (TMDCs) are envisioned for replacing silicon in advanced CMOS logic technology nodes, in the form of stacked nanosheet heterostructures. The nanofabrication of such devices, using VLSI compliant processing technology, represents a tremendous challenge for the community. Logic systems require to build NMOS and PMOS transistors, using the same starting channel material; this is done by creating, artificially, additional negative charges (electrons) or positive charges (holes) in the semiconducting material – this is called doping. In silicon this is done by implanting donor or acceptor elements using a high energetic beam, followed by an annealing to fix/stabilize the dopants in the crystalline structure.

Although doping of bulk silicon is well understood, the doping of an intrinsically 2D layer, with a thickness of only a few Angströms, represent immense difficulties. This is even more challenging in a stacked nanosheet configuration, where multiple stacked TMDC layers need to be doped simultaneously. In the last few years, extrinsic doping by surface charge transfer (gas adsorption, self-assembled organic molecules, metal oxide) was explored and demonstrated some doping effect, however these methods are unstable with time and temperature, difficult to control and/or impossible to integrate within a CMOS process flow. Substitutional doping, i.e. the replacement of metal atoms (cations) or chalcogen atoms (anions) by donor or acceptor elements, does not, a priori, suffer from these problems. In addition, structural change of the crystalline structure of the 2D layer (2H to 1T phase transition) could also be considered to induce more severe conductivity changes, particularly suitable for contact formation (locally turning the TMDC layer from semiconductor to metallic).

The PhD topic described here will address the challenge of controlled substitutional doping and controlled TMDC phase transitions. The work will rely on three different approaches. First, anion substitutional doping will be explored by developing a downstream plasma-based approach for creating controlled chalcogen vacancies, subsequently filled by group VII or group V atoms for n-type and p-type doping respectively. Second, doping using low energy phosphorus implantation will be studied, controlling the implantation depth by means of sacrificial polymer films. As a third approach, plasma-induced 2H-1T phase transitions will be explored using soft conditions (pulsed plasma discharges) leading to controlled crystallinity changes possibly combined with chemical doping. The effect of capping layers(s) on this structural transition will also be part of the research activity.

As a PhD student, you will learn to work in a highly dynamic and multicultural environment and be exposed to a large variety of analytical techniques and experimental methods. You will use VLSI-compliant downstream plasma systems as well as laboratory scale flexible platforms. You will learn micro/nano-fabrication methods and run them by yourself or use imec’s pilot line and/or infrastructure at the KULeuven. You will learn how to measure transport properties (carrier density, mobility, Ion/Ioff ratio…) on samples that you will fabricate yourself. Surface chemistry and physics will be studied both ex situ and post operando by x-ray photoelectron spectroscopy (XPS), Secondary ion mass spectroscopy (SIMS) and high-resolution angle-resolved photoemission spectroscopy (ARPES). Other complementary physical characterization techniques like elastic recoil detection analysis (ERDA), atomic force microscopy (AFM), scanning and transmission electron microscopy (SEM, TEM) and low-energy ion scattering will be made available to support your mechanistic studies.

At imec, we have a large expert team with various backgrounds in materials and deposition techniques, state of the art 200mm and 300mm process lines, laboratories, device learning and modelling. The PhD candidate will be part of the Unit Process Module department at imec, with strong links to the Material Engineering department of KU Leuven.

Required background: Master in engineering technology / Master in engineering science / Master in materials science / Master in science

Type of work: 40% experimental, 30% characterization, 20% data analysis, 10% literature study and writing

Supervisor: Clement Merckling

Co-supervisor: Alexis Franquet

Daily advisor: Jean-Francois de Marneffe, Rita Tilmann

The reference code for this position is 2024-023. Mention this reference code on your application form.

Find all the information on doing a PhD at imec, as well as all required documents for your application file, here: Imec - PhD at imec (Belgium) (imec-int.com)