Synthesis of Few-layer Tungsten Diselenide Films

Open Access
Author:
Browning, Paul Nathan
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 29, 2014
Committee Members:
  • Joshua Alexander Robinson, Thesis Advisor
Keywords:
  • Two-dimensional materials
  • chemical vapor deposition
  • synthesis
  • electronic materials
  • Raman spectroscopy
Abstract:
Ultrathin films of two-dimensional materials have attracted attention in the past decade due to the unique properties they display in comparison to their bulk counterparts. While research of these materials focused for many years upon investigations into the properties and processing of graphene, attention has now begun to shift towards examination of other two dimensional material systems, including hexagonal boron nitride (h-BN) and the transition metal dichalcogenide (TMD) compounds. These compounds possess a wide range of optical, electrical, thermal, and chemical properties that make them intriguing for use in many applications ranging tunneling field effect transistors (TFETs) to hydrogen evolution catalysts. Tungsten diselenide (WSe2) has recently become of particular interest due to its many technologically advantageous properties including a direct band gap in the visible spectrum, modest carrier mobility, high degree of thermal anisotropy, and catalytic properties. Unfortunately, a method capable of producing WSe2 in the form of large-scale, uniform films of controllable thickness remains undiscovered, making commercialization of WSe2 currently impossible. The purpose of this thesis is to explore the possibility of producing wafer-scale films of WSe2 using methods involving oxygen-selenium exchange mechanisms. WSe2 films were synthesized using several different techniques including a batch cold-wall reactor process involving reaction of elemental selenium with a tungsten oxide (WOx) film deposited by thermal evaporation onto sapphire (Al2O3) and silicon dioxide (Si/SiO2) substrates, a continuous hot-wall process capable of supplying a continuous selenium overpressure, a cold-wall reactor process using reaction between WOx and the metal organic precursor dimethyl selenide (DMSe), and a reaction process between elemental selenium and the metal organic tungsten hexacarbonyl (W(CO)6). Studies of the role of selenium overpressure, reaction temperature, system pressure, heat treatment, substrate, and treatment of the oxide film were conducted to gain an understanding of the role of different experimental variables on film growth. Films were characterized through the use of Raman spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). Using the batch synthesis process it was found that WSe2 could be obtained under a number of experimental conditions, however best results were obtained at elevated pressures and temperatures near 750 °C (the maximum system temperature), with low hydrogen to nitrogen carrier gas ratios. Contrary to previous reports in literature, it was found possible to generate WSe2 films even without the presence of hydrogen. Based on cross-sectional TEM imaging and discrepancies between sample results, the process was deemed too unreproducible for practical use and unable to fully selenize the tungsten oxide due to the departure of the selenium vapor over time. Synthesis using the hot-wall reactor process revealed films synthesized at temperatures up to 900 °C were capable of producing WSe2, however open raising system temperature to 1050 °C no WSe2 Raman spectra was detected of the film, placing a limit on maximum processing temperature. Pressures near 10 Torr were found to produce superior films than those generated at 100 Torr, suggesting the improvement at higher pressure found for the batch process was due to reduced depletion of selenium with time rather than film growth thermodynamics. Low concentrations of hydrogen to argon or nitrogen gas were again found to produce optimal film growth results. Cross-sectional TEM revealed full selenization of the deposited oxide films, however an amorphous interlayer was found between the WSe2 film and sapphire substrate, suggesting damage of the substrate by prolonged exposure of selenium and possible formation of aluminum selenide. Synthesis using DMSe exposure over WOx films reveled the metal-organic could be successfully used to generate carbon-free films at pressures near 10 Torr and temperatures as high as 800 °C, with low hydrogen concentrations again being found optimal for growth of films. Studies of prolonged exposure of films to selenium vapor revealed no increase in grain size, suggesting grain growth was limited by the concentration of WOx present in the system. Activation of DMSe at temperatures near 500 °C was found to greatly improve film roughness and morphology, suggesting a selenium overpressure serves to prevent delamination of WOx films as tungsten oxide is known to participate in evaporation-condensation growth mechanisms. Atomically-smooth, wafer-scale films of WSe2 were obtained by this method and the hot-wall selenization methods, however the grain size of the films was found to be on the order of several hundred nanometers in both cases. Use of rapid thermal annealing treatments on the oxide films was found to produce films of increased grain size, as well as improve film uniformity, highlighting that final film morphology was controlled by the morphology of the oxide prior to selenization, and suggesting little to no solid-state diffusion of tungsten atoms within the films. Furthermore, film thickness of WSe2 was found to be controlles by the thickness of the deposited oxide film, allowing for deposition of WSe2 films of thickness 2 nm and greater. Using these treatments, heterostructures of directly-grown WSe2 on epitaxial graphene and hexagonal boron nitride were demonstrated for the first time, with results indicating a clean interface between the epitaxial graphene and WSe2 film and AFM showing van der Waal’s epitaxy between the WSe2 film and epitaxial graphene or h-BN substrates. Finally, reaction of W(CO)6 with selenium was determined to be a promising synthesis route for development of WSe2 films, although the batch nature of the process used in the study made the reaction incapable of producing the ultrathin films desired.