Two-dimensional (2D) semiconductors have the potential to complement silicon as channel material in advanced CMOS devices at aggressive technology nodes. As opposed to silicon, the group-VI transition metal dichalcogenides (MX2), such as molybdenum- and tungsten disulfide (MoS2, WS2), are monolayer-thin and semiconducting materials and markedly less sensitive to detrimental effects encountered in channels with strongly reduced physical dimensions. At this point in time, however, experimental device results are not yet on par with their theoretical performance, which is amongst other due to insufficient material quality. Therefore, understanding the effects of defects in the 2D layer, but also the interactions between these atomically thin semiconductors and their surrounding environment becomes of prime importance.
Several factors strongly influence the electronic properties of 2D materials: such as, intrinsic defects in the 2D layers (e.g., vacancies and grain boundaries) and the substrate surface underlying them. With respect to the latter, conventional 3D substrates such as SiO2 can cause increased scattering in both graphene and MX2. In contrast, enhanced electronic properties are achieved when atomically flat 2D dielectric substrates are used, e.g., hexagonal boron nitride (h-BN)1–4. However, to date, a systematic methodology to determine the role of intrinsic and extrinsic defect mechanisms on the electronic performance of 2D layers is lacking. Often, the performance is measured in terms of mobility. Such figure of merit gives a rather indirect quality assessment and provides little physical understanding of the scattering mechanisms.
Current understanding is that other 2D materials are the ideal substrates for both graphene and MX2 devices1,5,6. However, there are still important questions to address in terms of MX2 (interface) defectivity: what lies at the origin of interface defects? Are they coming from the oxide selection (e.g. SiO2, HfO2, SiN, hBN) or from the interface properties (e.g. roughness, moisture, carbon)? How important are intrinsic defects in the MX2 layers, and what is their electrical impact? In order to answer these questions, it is crucial to develop a characterization methodology able to quantitatively assess the defect properties.
Therefore, in this PhD project, we aim to tackle these challenging questions by combining different defect characterization methods (e.g. optical and electrical measurements, scanning probe techniques...). Novel optical measurements, based on carrier concentration-dependent photoluminescence, can quantitatively assess the density and dynamics of defect trap states in the semiconductor and its interfaces. Electrical measurements aim to determine the energetic and spatial position of active defects, by their individual defect signatures in ultra-scaled transistors7. By combining distinct trapping mechanisms and studying them from different points of view, it is expected that the PhD candidate will develop a detailed understanding of the role of 2D interfaces, defects in the 2D layer, substrates, defects and traps. Once this understanding is in-place, it will play a key role for achieving low-defectivity and high device performance.
As the multidisciplinary topic is about understanding defects in the 2D layer and the effect of interfaces, knowledge of some characterization techniques (photoluminescence, Raman, electrical transport measurements, surface probe microscopy (SPM) 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.
(1) L. Banszerus, L. et al. 2D Mater. 2017, 4, 025030.
(2) Y.Y. Illarionov, Y. Y.et al. 2D Mater. 2016, 3 (3), 1–10.
(3) Rhodes, D. et al. Nat. Mater. 2019, 18 (6), 541.
(4) Tran, M. D. et al. ACS Appl. Mater. Interfaces 2018, 10 (12), 10580.
(5) Shin, B. G et al. Adv. Mater. 2016, 28 (42), 9378.
(6) Kretinin, A. V. et al. Nano Lett. 2014, 14 (6), 3270.
(7) Stampfer, B. et al. ACS Nano 2018, 12 (6), 5368.
Required background: Physics, Material engineering, Material science, Nanotechnology, Chemistry
Type of work: 60% experimental, 40% data analysis and theory
Supervisor: Valeri Afanasiev
Daily advisor: Benjamin Groven, Kristof Paredis, Steven Brems
The reference code for this position is 2020-004. Mention this reference code on your application form.
Chinese nationals who wish to apply for the CSC scholarship, should use the following code when applying for this topic: CSC2020-04.