Atomic Layer Deposition of Transition Metal Dichalcogenides and Applications in Water Splitting

A modern research trend is the exploration of potential renewable alternatives to functional device materials. Transition metal dichalcogenides (TMDCs) have attracted much attention due to the 2-dimensional nature of some of them, tunable characteristics, good flexibility, and transparency. As a result of the intensive research in this field, many methods, such as exfoliation, chalcogenization, chemical vapor deposition (CVD), and atomic layer deposition (ALD) have been investigated to fabricate thin films of them. Among them, ALD is the most scalable one and is promising to be used in industry. However, the range of TMDC material types available by ALD is still limited, thus more TMDC ALD methods need to be developed. Furthermore, some aspects have not been studied, one of which is the control over the morphology to meet the various needs for different applications (for transistors, the materials should ideally be conformal and smooth, whereas for catalysts, a rough film is more desirable). In this context, this dissertation deals with the atomic layer deposition method development of one benchmark TMDC MoS₂ and one emerging TMDC HfS₂. Some issues and concerns are addressed, such as control over film morphology during the ALD process and ALD precursor constraints. An application of MoS₂ is also presented. The TMDC ALD is expanded with the low temperature ALD of HfS₂ by reaction of Hf(NMe₂)₄ and H₂S as precursors. Preliminary tests by temperature-programmed infrared reflection absorption spectroscopy (TP-IRAS) from collaborators observed that the surface behavior of the precursors under ultra-high vacuum (UHV) conditions is consistent with the consecutive ALD study. Continuous and conformal HfS₂ films are deposited by the plasma-free ALD process with a GPC (growth per cycle) of ca. 1.2 Å at 100 °C. ALD deposited ZnS is used as an encapsulation layer to protect HfS₂ from oxidization under ambient conditions. Material characterizations by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray analysis (EDX) provide a consistent picture of the success in depositing a highly stoichiometric and pure HfS₂. The well-behaved surface chemistry observed from the ALD study allows us to investigate the area-selective growth of HfS₂ using self-assembled monolayer (SAM) substrates carrying different functionalized groups on the surface. Area-selective growth is achieved with —SH active and —F inert surface sites. The morphology of the material (roughness) can be controlled by different dilution of the surface active sites. The TMDC ALD availability is mainly limited by the precursor. Possible solutions are developing more suitable precursors and pursuing a new strategy based on the existing precursors. Atomic layer deposition in solution phase (sALD) is an emerging ALD deposition method which is promising to overcome the precursor constraints associated with the traditional gaseous atomic layer deposition (gALD) method. The second part of the thesis offers this simple and versatile sALD method to generate HfS₂. The same precursors Hf(NMe₂)₄ and H₂S are used for ease of comparison with the gALD method. A GPC of around 0.5 Å is obtained despite the room temperature and ambient pressure deposition conditions used, whereas the conformity and thickness control advantages are retained. The deposit is characterized on several different oxide substrates by spectroscopic ellipsometry, scanning electron microscopy (SEM), and XPS. The success in sALD expands the range of material classes available by the new method, adding transition metal dichalcogenides to the list containing oxides, cubic sulfides, hydrides, and organics reported so far. MoS₂, as a benchmark TMDC material, is promising to be used in water splitting catalysis as an alternative to noble metals due to the abundance of active sites that its surface features in the amorphous state. In this context, MoS₂ ALD is studied to synthesize thin-film MoS₂ to be u