Defect Passivation of Transition Metal Disulfides by in-situ/ex-situ treatment

Transition metal disulfides (TMDs) like MoS₂ and WS₂ are crucial for advancing CMOS technology due to their exceptional electronic, optical, and mechanical properties. Their atomically thin layers allow for excellent electrostatic control, essential for scaling down transistors in advanced nodes. However, TMDs have not yet reached their full potential due to challenges such as the presence of defects, particularly sulfur vacancies, which degrade their electronic properties. Additionally, large-scale, defect-free synthesis and integration with existing CMOS processes remain challenging, hindering adoption of TMDs in commercial semiconductor applications. Post-growth defect passivation is one major step to mitigate these issues, enhancing material quality and enabling TMDs to meet the stringent requirements of advanced CMOS applications.

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Intrinsic defects such as vacancies¹ and grain boundaries² within the 2D layers significantly impact the electronic and transport properties of 2D materials as they can act as trap states, leading to charge carrier scattering and recombination. Therefore, in-situ or ex-situ treatments for effective defect passivation are crucial to improving these properties. Several intuitive strategies including encapsulation via van der Waals heterstructures³, O2 plasma-⁴ and thiol- treatment⁵, or unconventional approaches involving functionalization with hydrocarbons/organic molecules⁶ have been proposed. Though this has had a limited success, a systematic methodology to understand the interaction mechanisms between the treatment species and specific defect sites is still lacking. Current assessments often rely on mobility measurements, which does not necessarily reflect the effectiveness of defect passivation.

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Despite recognizing the critical need for defect passivation for optimizing the performance of devices based on TMDs like MoS₂ and WS₂, significant questions remain without answer: What are the precise mechanisms behind defect passivation? Do different passivation treatments, such as exposure to O₂, H₂S, or H₂S/Cl₂, offer varying degrees of effectiveness? How do these treatments interact with intrinsic defects in TMD layers, and what is their impact on electronic and optical properties? How stable are the passivated surfaces/interfaces under conditions of gate stack processing or the bias-temperature stress? How can we accurately measure and quantify the success of different passivation strategies? To address these issues, it is essential to develop a comprehensive approach that combines the systematic design of processing methods with the characterization methodology that can evaluate the effectiveness of various passivation approaches and their influence on TMD properties.

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In this PhD project, we aim to address these critical challenges in TMDs by strategically engineering defect passivation through both in-situ (in Metal Organic Chemical Vapor Deposition reactor) and ex-situ processing, which will be strongly supported by a range of advanced microscopy and spectroscopy techniques. We will utilize optical methods such as low temperature photoluminescence and Raman spectroscopy to investigate the effects of different passivation treatments on defect density and dynamics. Electrical measurements at the nanoscale will be conducted to analyze the impact of passivation on current transport properties. By integrating insights from these diverse approaches, the PhD candidate will develop a comprehensive understanding of how various passivation strategies influence defect behavior and device performance. This knowledge will be pivotal for optimizing defect passivation techniques and enhancing the overall quality and efficiency of TMD-based devices.

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Profile

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As the multidisciplinary topic is about understanding defects in the 2D layer and the effect of interfaces, knowledge of some characterization techniques (photoluminescence, Raman, surface probe microscopy techniques) and/or 2D materials is definitely a plus. A genuine interest in semiconductor physics and surface chemistry is desired. You are a curious, independent and resourceful person. The ability to communicate fluently in English is an absolute requirement in our international environment.

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Required background: Master degree in Physics, Material engineering, Material science, Nanotechnology, Chemistry

Type of work: 60% experimental, 40% data analysis and theory

Promotor: Valeri Afanasiev

Co-Promotor: Michel Houssa

Focus of work: Materials; Semiconductor Physics; Chemistry; Metrology & characterization

Daily advisors: Albert Minj, Pawan Kumar

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(1) Wan, Y. et al. Nat Commun. 2022, 13, 4149

(2) Ly, T. et al. Nat Commun 2016, 7, 10426

(3) Jung, J-W. et al. Adv. Sci. 2024, 11, 2310197

(4) Lee, I. et al. Small 2024, 20, 2305143

(5) Sim, D. M., ACS Nano 2015, 9, 12115