Characteristics and Tunability of Electron Emission Sources from Quasi-Freestanding Epitaxial Graphene Microstructures

Abstract

Abstract Body: Graphene microstructures have been shown to exhibit controllable directional electron emission when carrying a current under an accelerating electric field. Through Phonon-Assisted Electron Emission (PAEE), graphene microstructures demonstrate electron emission at electric field intensities and lattice temperatures below what would be expected for carbon to demonstrate field emission or thermionic emission, respectively. Furthermore, such emissions tend to be out of the plane of the graphene microstructure, giving some control of the directionality of the emission current even before being directed with an applied electric field Thus far, such arrangements have tended to involve structural dimensions in the range of a handful of μm, commonly with CVD transferred graphene, flakes, or carbon nanotubes, with applied fields that can get up to the order of hundreds of kV/cm, and emission currents rarely exceeding a few nA. Herein are some examples of the characteristics and tunable variables demonstrated by multiple graphene structures with some dimensions up to cm, in fields below 4 kV/cm, and producing >1 μA emission currents. Quasi-freestanding epitaxial graphene on a silicon carbide substrate offers several advantages compared to transferred graphene or unzipped nanotubes, including lower defect density, greater structural integrity, ease of handling, and compatibility with simple photolithography fabrication techniques. The devices presented here were fabricated with a low-cost, wafer scalable, single-photomask process that does not require material transfer to a secondary substrate and is easily tailored to create variations in design parameters, which allowed multiple batches of devices to be rapidly manufactured with precise variations for the purpose of performance testing. The devices exhibited recognizable patterns of emission that seem to be influenced by several factors including, but not limited to shape and orientation, resistivity, input power, active time and on/off cycling frequency, device and substrate temperature, applied electric field, and both total device length and width dimensions (relative to current flow). Emission current seemed to show its most substantial dependence on input power, sometimes as much as doubling output with only a 25% increase in input; it is unknown if this relation holds for devices that vary other parameters alongside power input, necessitating further testing. The devices strongly imply a direct relationship between emission current density and device and substrate temperature, as some devices produced current outputs that exceeded measuring limits. Devices with larger dimensions also tended to produce more significant emission currents. However, the trends between increasing length and increasing width were not symmetrical: increasing device length (along the path of the input current) showed a stronger corresponding increase in output relative to % change in dimension than did changes in device width, and preliminary simulation results indicate both factors to have a roughly linear positive trend. While a general trend of larger dimensions and higher applied power levels tended to accompany higher emission currents, the output variations that may be attributed to other variables will allow for a broader range of device optimization parameters or tailoring of devices for specific purposes. Graphene microstructure electron sources may provide a means for device implementation in a 2D heterostructure environment or as a vehicle for further miniaturization of constructs requiring controllable electron emissions, such as electron microscopy or X-ray generation.