Nano-gap planar metal electrodes: Fabrication and I-V characteristics

Abstract

The nanowires/bars and nano-gap electrodes are vital components for emerging electronics and have wide ranging applications in flat-panel displays, sensors, sub-100 nm transistor circuits, and miniaturized computers/devices. Focused ion beam (FIB) has emerged as a powerful and unique tool for nanofabrication. The research work described here is concerned with (a) the FIB fabrication of planar metallic (copper and gold) nanostructures, (b) their current-voltage (I-V) measurements in situ, and (c) a viable method for extracting the realistic values of emission parameters. The planar electrodes with gap of 80-100 nm are realized by FIB milling of thin metal films. The difficulties faced in objective interpretation of their I-V data (based on known mechanisms) are highlighted. For determining the parameters (namely, effective emission area α eff, apparent work function .,, and the field enhancement factor β), Fowler-Nordheim [ln(I/V2) versus 1/V] plots showing a minimum with straight line of negative slope can be used. The striking findings demonstrated are (i) occurrence of emission from a tiny region (<1 nm2) vis-à-vis physical area (400 μm × 200 nm), (ii) significant lowering of barrier height, and (iii) enhancement of local field due to protrusions present. Typical values of α eff, φ, and β deduced are 52.3 Å2, 1.62 eV, and 39.3, respectively for copper planar electrodes (gap 1/4100 nm); the corresponding data for the case of gold (gap 1/480 nm) are 29.1 Å2, 1.97 eV, and 12.1, respectively. Moreover, β lowering observed with bias is accompanied by increase in the emission area due to progressive smoothening of protrusions at the cathode surface. The electrodes are found rough/rocky at the nanoscale with protrusions and varying separations at places. These features make the electron emissive region small and pointed with an enhanced local electric field and effectively of a lower barrier height. The current discrepancy in the Child-Langmuir's space charge regime is attributed to the emission occurring from a restricted area only. These findings are important for futuristic nano-devices like thermo-tunnelling refrigerator, energy harvester, etc.