Electron Emission Sources from Quasi-Freestanding Epitaxial Graphene Planar Devices

Abstract

Freestanding graphene nano- and microstructures have been shown to exhibit controllable electron emission when carrying a current and immersed in an external accelerating electric field. Through the mechanism of Phonon-Assisted Electron Emission, these structures 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. Emission currents have been measured primarily out of the plane of the device surface, allowing some directional control before the accelerating field is even considered. Thus far, such demonstrations have tended to involve structural dimensions on the order of only a handful of um, and have commonly been fabricated from CVD transferred graphene, flakes, or carbon nanotubes in various vertical and horizontal arrangements. Applied fields for such devices typically approach hundreds of kV/cm, and emission currents rarely leave the nA range. Herein are presented graphene structures with dimensions up to cm, in fields not exceeding 1 kV/cm, and producing emission currents of nearly 10 UA. Quasi-freestanding epitaxial graphene, synthesized from a silicon carbide substrate, offers several advantages compared to transferred graphene or unzipped nanotubes, including greater structural integrity, lower defect density, ease of handling, and ready compatibility with conventional simple photolithographic 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 exhibit recognizable patterns of emission current performance relative to device and substrate temperature, including an identifiable turn-on point and emission current maintenance. Device behavior relative to substrate heating identifies the electron-phonon interactions that enable function and current emission, both when device temperature is supplied internally and externally. Emission currents are also observed to follow a performance pattern derived from purposeful device morphology, even exhibiting emission currents in accelerating fields as low as 50 V/cm. Graphene electron emission devices present an opportunity for electron emission in a planar 2D heterostructure environment, beyond even the dimensional capabilities of field emission devices, and below the high operating temperatures of thermionic sources. Through refinements in the design and packaging, such devices, which are arbitrarily scalable between the micron and wafer level, could provide the electron sources for novel applications in Xray generation or electron microscopy.