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Graphene Synthesis by Thermal Cracker Enhanced Gas Source Molecular Beam Epitaxy and Its Applications in Flash Memory

Abstract

Because of its unique properties, such as extremely high mobility, high mechanical strength, good optical transparency and high chemical stability, graphene has attracted vast interests in nanotechnology communities, condensed matter physics, and chemistry. The method of synthesizing large-area graphene with good quality is critical for its potential applications. This dissertation reports a new synthesis method of using a home built thermal cracker enhanced gas source molecular beam epitaxy (GSMBE) system. Chapter 2 discusses graphene growth using nickel substrate in the GSMBE system. Hydrocarbon gas molecules were broken by thermal cracker at very high temperature of 1200°C and then impinged on a nickel substrate. High-quality, large-area graphene films were achieved at 800°C, and this was confirmed by both Raman spectroscopy and transmission electron microscopy. A rapid cooling rate was not required for few-layer graphene growth in this method, and a high-percentage single layer and bilayer graphene films were grown by controlling the growth time. The results suggest that in this method, carbon atoms migrate on the nickel surface and bond with each other to form graphene. Few-layer graphene is formed by subsequent growth of carbon layers on top of existing graphene layers. This is completely different from graphene formation through carbon dissolving in nickel and then precipitating from the nickel during rapid substrate cooling in the chemical vapor deposition method. Chapter 3 further discusses using cobalt as graphene growth substrate. Growth conditions including growth temperature and growth time play important roles in the resulting morphology of as-grown films. A narrow growth time window was found for different growth temperatures. Carbon absorption and desorption phenomena were responsible for temperature-dependent and growth time-dependent graphene morphology. Fast cooling rate was not required in this process due to direct growth mechanism under atomic carbon growth condition. Large-area graphene films with high single-layer and bi-layer coverage of 93% were confirmed by Raman spectroscopy and transmission electron microscopy.

Graphene based flash memory was demonstrated by using nickel nanocrystals as storage nodes in chapter 4. First, the graphene channel with a dimension of a 20 µm × 5 µm was acquired by photolithography and oxygen plasma etching. Then, the gate stack formed by the deposition of tunneling oxide, nickel nanocrystal and block oxide. On/off operation of the transistor memory was acquired by static pulse response measurement. The memory window of the device was found up to be 23.1 V by back gate sweep. This memory effect is attributed to charging/discharging of nanocrystals. Furthermore, excellent retention and endurance performance were achieved. Chapter 5 demonstrated a memory capacitor with an embedded graphene nano dots structure. Graphene nano dots were successfully fabricated by using nickel nano crystals as etching masks. By tuning the etching parameters, graphene dots with the size of 10 nm to 100 nm can be acquired. Raman spectra confirms the defects and the edges effect of nano dots. A memory capacitor using a SiO2/graphene dots/Al2O3 sandwich structure was fabricated. A hysteresis from the C-V sweep proves the memory capability of the as-fabricated capacitor.

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