METALORGANIC CHEMICAL VAPOR DEPOSITION OF TWO-DIMENSIONAL LAYERED CHALCOGENIDES

Open Access
- Author:
- Zhang, Xiaotian
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 13, 2019
- Committee Members:
- Joan M. Redwing, Dissertation Advisor/Co-Advisor
Joan M. Redwing, Committee Chair/Co-Chair
Joshua A. Robinson, Committee Member
Thomas N. Jackson, Committee Member
Susan B. Sinnott, Outside Member - Keywords:
- Two-dimensional Materials
Transition Metal Dichalcogenides
2D Layered Chalcogenides
Chemical Vapor Deposition
Metalorganic Chemical Vapor Deposition
Thin Film Deposition
2D Materials
CVD
MOCVD
WSe2
In2Se3 - Abstract:
- Two dimensional (2D) materials have attracted wide interest because of their layered crystal structures and anisotropic properties which leads to potential new diversity of function in nanoelectronics, photonics, sensing, energy storage, and optoelectronics. Among them, the family of 2D layered chalcogenides (2DLCs) including transition metal dichalcogenides (TMDs) (in the form of MX2 where M=Mo, W, etc. and X=S, Se, Te) and group IIIA metal chalcogenides (MCs) (typically in the form of MX and M2X3 where M=Ga, In and X=S, Se) have been a focus of increasing interest due to their number of intriguing properties. Monolayer TMDs have exhibited unique optical and electical properties such as indirect-to-direct band gap, spin valley polarization, interlayer exciton coupling, single photon emission. On the other hand, monolayer and few-layer MCs also demonstrate novel properties such as high carrier mobilities (~1000 cm2/Vs) for γ-InSe and ultrasensitive photoresponse as well as in-plane or/and out-of-plane ferroelectricity for α- and β-In2Se3. Therefore, the rapid development of device technologies based on 2DLCs causes increasing demand for synthesis of high-quality wafer-scale single crystal monolayer and few layer films. Among techniques for thin film deposition, gas source chemical vapor deposition (CVD) /metalorganic CVD (MOCVD) is emerging as a promising method for wafer-scale growth of TMDs and related 2DLCs due to the ability to grow at high temperatures (>700 ℃), moderate reactor pressures (10-700 Torr) and high chalcogen/metal gas phase ratios which are needed to achieve epitaxy and film stoichiometry. However, the effects of precursor chemistry on film properties have not been examined and methods to achieve uniform, fully coalesced epitaxial monolayer TMD films are required. In addition, the growth of group III MCs by MOCVD is largely unexplored. This dissertation focuses on a comprehensive study of gas source CVD/MOCVD growth of WSe2 and In2Se3 which represent the TMDs and group IIIA MCs, respectively. In the MOCVD growth of WSe2, a defective graphene layer was found to form on the sapphire substrate simultaneously at high growth temperature and high Se:W ratio when using tungsten hexacarbonyl (W(CO)6) and dimethyl selenide ((CH3)2Se, DMSe) as precursors. The graphene layer alters the surface energy of the substrate reducing the lateral growth and coalescence of WSe2 domains. By switching to hydrogen selenide (H2Se) instead of DMSe, the defective graphene layer was eliminated and a multi-step diffusion-mediated process was developed for the epitaxial growth of coalesced monolayer WSe2 films on c-plane sapphire. The multi-step process consists of an initial nucleation step which used a high W(CO)6 flow rate along with H2Se at 800℃ and 700 Torr to promote nucleation. The W(CO)6 was then switched out of the reactor and the sample was annealed in H2Se to promote surface diffusion of tungsten-containing species to form oriented WSe2 islands with uniform size and controlled density. The W(CO)6 was then switched back into the reactor at a lower flow rate to suppress further nucleation and laterally grow the WSe2 islands to form a fully coalesced monolayer film in less than one hour. Reflection high energy electron diffraction (RHEED) and in-plane X-ray diffraction (XRD) measurements further confirm that the coalesced WSe2 monolayer film is epitaxially oriented on sapphire as [10-10] WSe2 ‖ [10-10] α-Al2O3. High resolution annular dark field scanning transmission electron microscopy (ADF-STEM) and selected area diffraction analysis of WSe2 removed from the sapphire also indicate that the films are predominately single crystal with inversion domain boundaries (IDBs) that result from merging of 0º and 60º oriented domains. The process also provides fundamental insights into the 2D growth mechanism. The evolution of domain size and cluster density with annealing time follows a 2D ripening process, enabling an estimate of the tungsten-species surface diffusivity of 1.2×10-14 cm2/s. The lateral growth rate of domains was found to be relatively independent of substrate temperature over the range of 700-900 oC suggesting a mass transport limited process, however, the domain shape (triangular versus truncated triangular) varied with temperature over this same range due to local variations in the Se:W adatom ratio. The results provide an important step toward atomic level control of the epitaxial growth of WSe2 monolayers in a scalable process that is suitable for large area device fabrication. To pursue the scalable growth of “single crystal” TMDs, a defect-controlled approach for the nucleation and epitaxial growth of WSe2 on hexagonal boron nitride (hBN) was investigated. The WSe2 domains exhibit a preferred orientation of over 95% leading to a reduced density of IDBs upon coalescence. First-principles calculations and experimental studies as a function of growth conditions and substrate pre-treatment confirm that WSe2 nucleation density and orientation are controlled by the hBN surface defect density rather than thermodynamic factors. Detailed TEM analysis provides support for the role of single-atom vacancies on the hBN surface which trap W atoms and break surface symmetry leading to a reduced formation energy for one orientation of WSe2 domains. Through careful control of nucleation and extended lateral growth time, fully coalesced WSe2 monolayer films on hBN were achieved. Low temperature photoluminescence (PL) measurements and transport measurements of backgated field effect transistor (FET) devices fabricated on WSe2/hBN films show improved optical and electrical properties compared to films grown on sapphire under similar conditions. These results reveal an important nucleation mechanism for epitaxial growth of van der Waals (vdW) heterostructures and demonstrate hBN as a superior substrate for single crystal transition metal dichalcogenide (TMD) films. The final chapter of the dissertation demonstrates the initial attempts at low-temperature (T<450oC) MOCVD growth of WSe2 for 2D materials and the properties of field effect transistors (FETs) fabricated using these layers. However, the FET mobility was found to be low (~0.01 cm2/Vs) due to the nanocrystalline nature of the film obtained at low growth temperature. In comparison, the MOCVD growth of group IIIA MCs, β-In2Se3 thin films shows the potentials as an alternative low temperature material for 2D FETs. The growth of β-In2Se3 thin films on various substrates at 400 ℃ using trimethylindium (TMIn) and H2Se in a H2 carrier gas was investigated. The β-In2Se3 films were identified by Raman spectroscopy and their epitaxial relationship with both sapphire and Si (111) substrates were confirmed by XRD technique. Top-gated thin film transistors (TFTs) fabricated on β-In2Se3 thin films exhibited higher mobility (~1 cm2/Vs) and promising current on/off ratio. The results demonstrate the potential of group IIIA MCs for silicon BEOL-compatible process integration and extend the potential of 2D materials into low-temperature electronic, optoelectronic, and ferroelectric applications.