Stable Field Emission from Nanoporous Silicon Carbide
Myung-Gyu Kang, Henri Lezec, Raymond L. Kallaher and Fred Sharifi
Field emitters, whereby electrons are extracted through quantum mechanical tunneling under an applied electric field, have advantages such as lower emittance, mitigation of thermal power management, and the ability of fast turn on-off times compared to conventional thermionic emission. Potential uses of field emitters include electron sources for microwave electronics, travelling wave tubes, sources for X-rays production, and display applications.
We present the results of a new technique for fabrication of nanostructured field emitters with the aim of increasing the emission capability of the cathode so as to enable some of the above applications. Developed emitters are monolithic structures fabricated from silicon carbide wafers, where the wafers were electrochemically anodized to form a continuous nanoporous structure providing several advantages as high current density and stable field emitters. First, the resulting monolithic structure has no failure points such as the interfaces found in CNT-based field emitters. Second, the structure is homogenous in depth, and as the emitting surface wears, there is a continuous supply of emission points serving as replacements. Furthermore, the structure is wafer-based resulting in compatibility with standard micro/nanofabrication processes and allowing the structures to be patterned into discrete emission structures such as pillars and meshes.
Fabricated porous SiC structures, where the C-faces of SiC wafers were formed into semi-columnar nanoporous structures by electrochemistry techniques using aqueous HF and ethanol, have typical pore and wall size of about 150 nm and 30 nm, respectively. The field emission property of these two-dimensional structures was obtained and compared to unprocessed wafer using a diode configuration in a UHV chamber at a base pressure less than 7 x 10-9 Torr. Stable emission current was observed for nanoporous structures with current densities reaching up to 0.4 A/cm2. To further enhance emission, nanoporous wafers were patterned into discrete square pillars approximately 2 mm per side, 10 mm period and 20 mm height using a focused ion beam (FIB). This structure produced an impressive current density in excess of 6 A/cm2 at less than 8 V/mm. We also performed preliminary lifetime tests demonstrating stable emission for several hours with no signs of degradation. We believe the emitters are capable of higher current densities through optimization of electrochemistry and geometric design, and may allow for new applications of nanotechnology in electronics.