CVD GROWTH AND CHARACTERIZATION OF 2D NbS2
Open Access
- Author:
- Kozhakhmetov, Azimkhan
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 26, 2017
- Committee Members:
- Joan Marie Redwing, Thesis Advisor/Co-Advisor
Joshua Alexander Robinson, Committee Member
Suzanne E Mohney, Committee Member - Keywords:
- NbS2
TMD
2D materials - Abstract:
- Metallic NbS2 is a member of the 5B group dichalcogenides. NbS2 is the only compound that shows low temperature superconductivity, but does not also exhibit charge density waves (CDW) unlike other layered transition metal dichalcogenides. Superconductivity in NbS¬2 shows a strong polytype dependence. There are two known polytypes of NbS2: three-rhombohedral (3R) and two-hexagonal (2H). The 2H polytype shows superconductivity and has a transition temperature at 6.2 K whereas no superconductivity was reported in 3R NbS2 down to 1.7K. To achieve precise thickness controlled crystal films of NbS2 over large areas and on different substrates, a chemical vapor deposition (CVD) synthesis method is desired. Prior work on CVD of NbS2 films was primarily performed in hot-wall reactors under Ar and N2 environment using a variety of substrates such as bare Si, SiO2/Si, glass, hBN/sapphire and graphene. Almost always NbCl5 was used as the metal precursor, but different source materials ranging from elemental sulfur to complex organic compounds were used as the chalcogen precursor. The main goal of the thesis research-was to develop a controlled process to grow monolayer and few layer films of NbS¬2 in a cold-wall CVD reactor and investigate the substrate effect on the film growth. Growth of NbS2 was broadly investigated on C-plane sapphire and epitaxial graphene substrates in the temperature range from 400 to 800 ⁰C. NbCl5 and H2S were used as precursor materials with H¬2 as a carrier gas and different S: Nb ratios from 1200 to 6000 were studied. The 3R polytype of NbS¬2 was obtained in all experiments. The films grown on C-plane sapphire substrates always had a fine grain structured morphology and did not change as a function of the different S: Nb ratio and temperature whereas triangular-shaped well-faceted domains of NbS2 formed on epitaxial graphene substrates. The effect of the S: Nb ratio on the film growth was studied on C-plane sapphire substrates including an evaluation of the electronic properties of the films using room and low temperature resistivity measurements. Measured resistivity values were in a good agreement with previous reports and confirmed the metallic nature of the NbS¬2 films. At low temperatures, an anomalous increase in resistivity was observed which indicated an insulating nature of the films. The origin of the unexpected resistivity increase likely due to oxidation of the films. Oxide layer formation was observed in all the films on C-plane sapphire substrates and a depth profile study revealed that oxidation of NbS¬2 films occurred only on the top surface of the films. The films on sapphire substrates had a fine grain structured morphology whereas the planar growth with the well-faceted domains was obtained on the epitaxial graphene substrates. The probable origin of the irregular shaped morphology on the sapphire substrates was suggested due to dangling bonds on the sapphire surface. Further, the film growth on dangling bond free substrates such as hBN, WS2, WSe2 and other van der Waals surfaces will enable to investigate the substrate effect on the film growth.