Improving the quality of Cvd graphene based devices and transport studies of few-layer Wse2

Open Access
Author:
Wang, Junjie
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 17, 2015
Committee Members:
  • Jun Zhu, Dissertation Advisor
  • Jun Zhu, Committee Chair
  • Eric W Hudson, Committee Member
  • Theresa Stellwag Mayer, Committee Member
  • Mauricio Terrones Maldonado, Committee Member
Keywords:
  • Graphene
  • CVD
  • Transition metal dichalcogenides
  • Schottky barrier
  • Thermionic field emission
Abstract:
This dissertation consists of two main topics: a) advances on the synthesis and quality of chemical vapor deposited graphene and devices; b) electrical and thermoelectrical transport studies of semiconducting transition metal dichalcogenides (TMDC), in particular few-layer WSe2. Chapter 1 introduces the band structures of graphene and TMDC, as well as the novel physics and potential applications of both materials. Chapter 2 describes the chemical vapor deposition (CVD) synthesis of graphene on copper using gaseous precursors of methane and hydrogen. Experimenting with the etching of carbon by hydrogen gas during and post growth, we demonstrate successful suppression of multilayer graphene growth, which is a common problem of the field. Next, we show that incorporating a diluted SC-2 cleaning step into the transfer process of CVD grown graphene removes metallic contaminations effectively and leads to a significant improvement of carrier mobility from several thousand cm2/Vs to 18,000 cm2/Vs at room temperature. These studies contribute to the improvement of graphene-based devices towards practical applications. Chapter 3 focuses on the characterization of various TMDC materials including MoS2, WS2, and WSe2 using microscopy and spectroscopy. We obtain the crystal structure of WSe2 using high-resolution transmission electron microscopy (TEM) and X-Ray diffraction (XRD). The layer number-dependent intensity ratio of two Raman modes (2LA and A1g) is studied and used to identify the layer number in few-layer WS2. The size of the direct band gap (monolayer) and indirect band gap (few-layer to bulk) of WSe2 is obtained from photoluminescence (PL) spectroscopy. In monolayer WSe2, the width of the A-exciton emission peak decreases with decreasing temperature and saturates at low temperatures to around 15 meV, demonstrating good sample quality. PL spectra of ion irradiated WS2 reveal additional emissions attributed to defect-bound excitons. Various atomic force microscopy (AFM) based imaging techniques, such as topographic AFM, Kelvin probe force microscopy (KPFM), and scanning thermal microscopy (SThM) are employed to study the morphology, work function and thermal conductivity of few-layer WSe2 and WSxSe2-x alloys. These studies provide valuable information on the physical properties of the TMDC materials. Chapter 4 focuses on the electrical transport properties of few-layer WSe2 field-effect transistors. We study the gate-dependent conductance of the transistor in the subthreshold regime and demonstrate that the resistance of the Schottky barrier contacts dominates the two-terminal resistance. The transmission mechanism through the contact is found to be thermionic field emission (TFE), which arises from a large amount of impurity states inside the band gap. Applying the TFE model, we self-consistently determine the density of the impurity states to be approximately 1~2×〖10〗^13 cm2/eV. We determine these impurity states are due to internal contributions rather than originating from the substrate by comparing devices fabricated on different substrates. The large number of impurity states leads to high mobility-edge carrier density (~ 1.1×1013 /cm2 in our devices) and low carrier mobility (~ 200 cm2V-1s-1 in our devices), Our studies shed light on the electrical transport properties of TMDC materials and highlight the material quality challenge of the field. Chapter 5 presents preliminary thermoelectric transport study of few-layer WSe2. Using micro-patterned thermometer and heaters, we obtain preliminary results on the Seebeck coefficient of a 5-layer WSe2 device. The magnitude of the Seebeck coefficient increases as the Fermi level is increased towards the conduction band edge and exceeds 200 µV/K at high carrier densities. Chapter 6 summarizes the studies discussed in this dissertation.