Interfaces between two-dimensional transition metal dichalcogenide semiconductors and oxide dielectrics for transistors

This thesis investigates the interfaces between two-dimensional transition metal dichalcogenide (2D TMD) semiconductors and gate oxide dielectrics for field effect transistors (FETs). 2D TMDs, such as molybdenum disulfide (MoS₂), are promising for sub-10 nm channel FETs because they retain their exceptional electronic properties even at a thickness of 0.7nm. However, suitable gate dielectrics are required for making good FETs with 2D TMDs. The main finding of this thesis is that because 2D TMDs are chemically inert and, therefore, do not form covalent bonds with the gate dielectric. The oxide surface is riddled with defects such as unpassivated bonds, dipoles, and vacancies. These oxide surface defects electrostatically dope the 2D TMDs in non-uniform manner – resulting in variable threshold voltages, high subthreshold swing values, and low ON-state currents in FETs. A method to deposit MoS₂ on different substrates without interfacial residue was developed. This allowed the study of the van der Waal interface between the 2D TMDs and dielectric substrates using hard and soft X-ray photoemission at Diamond Light Source. Using X-Ray Photoelectron Spectroscopy (XPS) complemented by Raman and photoluminescence (PL) spectroscopies, it is shown that common gate oxides such as SiO₂ and HfO₂ strongly dope 2D MoS₂ and other 2D TMDs. XPS shows significant band bending of 1.2 eV at the interface between MoS₂ and SiO₂. The band bending at the MoS₂/HfO₂ interface is 0.4 eV. Raman spectroscopy shows a redshift in A1g mode of MoS₂ on HfO₂ and SiO₂, which indicates electron doping from substrates. Such A1g mode redshift is absent in MoS₂ on ZrO₂. PL spectra of MoS₂ on SiO₂ and HfO₂ show trion-dominant peaks from electron doping, while MoS₂ on ZrO₂ shows an exciton-dominant peak due to the absence of electron doping from oxide. The key discovery of this doctoral research is that the interface between ZrO₂ and MoS₂ is ultra-clean and free of defects – allowing the realisation of 2D MoS₂ channel FETs that operate in enhancement mode. Specifically, MoS₂ FETs using ZrO₂ dielectrics achieve small and positive threshold voltage, subthreshold swing of 63 mV/dec and field-effect mobility of 114 cm²V⁻¹s⁻¹. The results provide insight into dielectric/2D TMD interfaces for high performance electronics.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Yan, Han

Advisor dc:contributor.advisor

• Chhowalla, Manish

• Field Effect Transistors
• Dielectrics
• 2D Materials

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/4e6dcacc-0767-47ad-9656-5f3f6c0f1ed0/download
• https://creativecommons.org/licenses/by/4.0/

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.120087

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/387195