Modeling For Contact Resistance Between Carbon Nanotubes And Substrates Of Field Emission Devices

Due to its fantastic characteristics in structure and performance, Carbon Nanotube (CNT)has been paid more and more attention since its field emission property has been found in1995. Applications like Field Emission Display, X-ray Tube, Backlight, etc. based on CNTshave already become a research hotspot. Along with the development of CNT field emissiondevices, exploring methodologies to increase emission current is of great importance. Apotential barrier will be formed inevitably at the interface between different materials, leadingto a contact resistance which might cause a significant influence on emission current. Soresearch on modeling contact resistance between CNTs and substrates could help improve theperformance of field emission devices and promote the accomplishment of some novel ideason applications of CNT field emission like portable CT.In this study, a common case that field emission device with metallic CNTs andsemiconducting substrates is considered. Model of contact resistance is deduced according tothe principles of metal-semiconductor contact and Fowler-Nordheim theory, and then themodified FN equation is obtained and verified to calculate emission current more accurately.At last, suggestions on optimizing field emission devices are proposed through the discussionof factors that impact contact resistance and emission current.It could be told from simulation of the obtained model by Matlab that contact resistancecould reach1MΩ. Impact on emission current brought by this resistance is obvious at highelectric field region, when the electric field reaches5V/μm or higher, current density woulddrop by about75%. However, contact resistance could hardly affect the open electric field,which has only changed0.2V/μm. Comparison with experimental data shows that themodified current equation is closer to reality, which verified the effectiveness of the proposedcontact resistance model and the modified FN equation.Discussion on factors that impact contact resistance demonstrates that contact resistancewill get smaller, current density higher along with the decrease of barrier height ΦBand theincrease of substrate doping density Nd, radius of CNTs r and temperature T. Each of the factors above will cause a shift on current density of2-3mA/cm2under the supply electricfield of4V/μm. When ΦBis lower than0.3eV, or Ndis higher than5×1019cm-3, excellentohmic contact could be formed which means contact resistance could be neglected.Furthermore, since the open voltage would increase with the increase of CNT radius, andexcessive temperature would cause a breakdown on electron source. Thus, CNT radius shouldin the range of15-20nm, and the temperature should be relatively higher than roomtemperature, but lower than500K.