Metalorganic Chemical Vapor Deposition and In-Situ Characterization of Transition Metal Dichalcogenide Thin Films
Restricted (Penn State Only)
- Author:
- Mc Knight, Thomas
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 16, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Suzanne Mohney, Major Field Member
Joan Redwing, Chair & Dissertation Advisor
Joshua Robinson, Major Field Member
Douglas Wolfe, Major Field Member
Danielle Hickey, Outside Unit & Field Member - Keywords:
- Metalorganic Chemical Vapor Deposition
MOCVD
2D Materials
MoS2
WTe2
WS2
In-situ Characterization
Spectroscopic Ellipsometry
SE
Transition Metal Dichalcogenides
Thin Films
TMDs
WSe2 - Abstract:
- In recent years, transition metal dichalcogenides (TMDs) have garnered much interest for state-of-the-art computing applications and device architectures, taking advantage of both their exceptional electronic properties and ultrathin structure. However, expanding the large area growth capabilities for less studied TMDs is a growing necessity for new device fabrications to take advantage of the full range of properties TMD’s offer. Additionally, the ability to directly monitor these grows in real-time is a critical objective for the realization of these materials in industrial applications and continues to be a challenge. Here two main studies are presented to aid in addressing these issues. Tungsten ditelluride (WTe2) is a layered, type-II Weyl semimetal TMD typically observed in a distorted 1T (1T’) phase with an orthorhombic crystal structure comprised of planes of distorted triangular lattices of tungsten atoms sandwiched by tellurium atoms. The distortion pushes tungsten atoms closer together in the x-axis than the y or z-axis, generating quasi-one-dimensional chains of these atoms, leading to strongly anisotropic electronic properties throughout the material. It has been shown to have extraordinary physical properties, such as a high magnetoresistance, anisotropic ultra-low thermal conductivity, and metal-insulator transition. It also exhibits interesting quantum phenomena such as the quantum spin-Hall effect and pressure-driven superconductivity. These unique properties make WTe2 an exciting candidate for emerging applications, including phase change memory electrodes, magnetic field sensors, biosensors, microelectromechanical systems, hard disk drives, and quantum computing. While bulk crystals of the material have been available for many years, to date, thin films of WTe2 have only recently been synthesized using techniques such as molecular beam epitaxy and powder source chemical vapor deposition (CVD) that utilize Te powder and W-feedstock. These techniques, however, exhibit a number of challenges including tellurium dimer formation that can severely hinder growth with high dissociation energies, and the use of salt assisted growth promotors during CVD growth which could contaminate future device fabrication. Metalorganic CVD (MOCVD) growth is of interest as it enables precise delivery of precursors to the substrate at moderate growth pressures (100-700 Torr), while proper precursor selection mitigates the Te dimer formation, but has not yet been studied in detail. In this first study, the use of MOCVD for the growth of WTe2 on c-plane sapphire substrates is investigated. Studies were carried out in a vertical cold wall MOCVD reactor using tungsten hexacarbonyl (W(CO)6) and diethyltelluride (DETe) as precursors for W and Te, respectively, in a H2 carrier gas. Initial studies demonstrate the growth of centimeter scale WTe2 as confirmed by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Additional studies were then undertaken to elucidate the growth mechanism and properties of MOCVD grown WTe2. The growth rate was found to decrease with increasing temperature over the range of 350 - 600oC. Similarly, higher growth rates were observed at lower growth pressures when examined over a range of 50 - 300 Torr. The effect of precursor flow rates was also investigated and demonstrated that the DETe flow rate had little effect on growth rate while changes in W(CO)6 flow rate can be used to directly control growth rate of films. Peaks at ~1340 cm-1 and ~1600 cm-1 were present in the Raman spectra of some films indicating the presence of carbon in the layers resulting from the DETe source. Higher carbon content was observed at higher growth temperatures and growth pressures. This suggests that WTe2 growth is becoming limited by parasitic pre-reactions of precursors in the gas phase. When precursors react in the gas phase before reaching the substrate surface, they form carbon containing species which deposit, thus limiting the available reactants to interact on the surface and form WTe2. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis further indicate that films are hexagonal and exhibit the layered 1T’ structure. XPS studies over a period of a few weeks reveals that films do not readily oxidize and are quite stable. The ability to monitor the growth rate of TMD monolayer and few-layer films in-situ is an area of great interest to achieve tight control of layer number and to gain insight into the fundamental mechanisms of film growth. Optical characterization techniques such as laser reflectivity are widely used for real-time measurements of growth rate during thin film deposition by MOCVD but cannot be readily extended to the growth of 2D materials which require sensitivity in the sub-monolayer regime. Spectroscopic ellipsometry (SE), on the other hand, is widely used to measure the dielectric function and thickness of thin films and is a powerful in-situ technique for atomic layer deposition providing information on film thickness per cycle and insight into the initial mechanisms of nucleation. In the second study, the use of SE as an in-situ technique to monitor the growth of MoS2 monolayer and few-layer films by MOCVD on c-plane sapphire substrates is investigated. The studies were carried out in a vertical cold wall MOCVD reactor equipped with a J.A. Woollam M2000XI ellipsometer integrated using purged optical ports on the reactor. Molybdenum hexacarbonyl (Mo(CO)6) and hydrogen sulfide (H2S) were used as precursors with H2 as the carrier gas. Initial studies were carried out by keeping growth parameters constant and varying growth time to develop a series of samples with varying surface coverage. After each growth, SE was performed as a function of temperature during cooldown to room temperature under H2S. Atomic force microscopy (AFM) was used to measure the film coalescence and quantify monolayer and bilayer surface coverage. An optical model of the layer structure was developed using an effective medium approximation to consider partial film coverage where the effective medium is a variable combination of void and the MoS2 film. The models were first used to fit the room temperature data to extract the dielectric function of MoS2, which compared favorably to prior literature reports. The optical model was then used to predict the variation in the ellipsometric parameters (Ψ and Δ) as a function of surface coverage demonstrating sensitivity from the sub-monolayer regime to significant bilayer coverage. Additional studies were undergone to determine the temperature dependence of the dielectric function in order to refine the models to accurately measure film coverage at growth temperature. The features of the ellipsometric curves soften at elevated temperatures compared to room temperature measurements, though marked differences are observed when comparing samples of varying coverage, expounding the potential for SE to discern sub-monolayer coverages at growth temperatures. The refined optical models were then applied to monitor MoS2 film growth in-situ, demonstrating the effectiveness of SE to allow precise measurement and control of film thickness and properties in real time through layer-by-layer tailoring of film deposition conditions. The ability to directly assess the instantaneous film growth rate throughout the deposition process gives invaluable insight into the fundamental nucleation and lateral growth mechanisms at work and their underlying relationship with film deposition parameters. Further studies using SE enable the ability to monitor potential changes in optical properties as a function of lateral growth.