Field emission from metal covered by film of polymer insulator

Abstract

Summary form only given. We obtained a low-threshold field emission from polished surface of metallic electrode which was covered by thin film of non-conjugated polymers such as: i) imide/siloxane copolymer; ii) aryl-polycarbonate; iii) Nylon 66. The films were prepared by deposition of a solution on the polished Mo or Nb electrodes. Then films were heated in air at a constant temperature to remove the major part of the polymer solvent. Field emission measurements were carried out in vacuum chamber at pressure of about 4 /spl times/ 10/sup -6/ Torr in construction with accelerating grid. We obtained a stable field emission at 4 kV/mm. However, for initial activation of the cathode, the threshold field should be 2.5-3 times more than 4 kV/mm. We observed a homogeneous fluorescence from the phosphorus glass anode with current-voltage characteristic corresponds to Fowler-Nordheim law at small electric fields but at high electric fields, there is deviation to smallest values of current. The field emission from polished metals we did not observe up to 25 kV/mm. Topography of the polymer film surfaces was analysed by an atomic force microscope before and after action of threshold field. Initially, polymers had smooth surfaces but after a threshold field emission, the surfaces were modified very strong: spikes were formed up to 400 nm high and its density was about 10 cm. Apart from conducting organic polymer such as poly (3-octylthiophene) where polymer bulk material emits electrons itself (I. Musa et al., 1998) in our case, there are two critical parameters where field emission effect exists: i) critical film thickness depending from polymer (about or less one micron) and ii) critical temperature of heat treatment for polymer film. We consider that polymer accepts the charge from metallic cathode due to relatively large effect of electrification (J. Lowell and A. C. Rose-Innes, 1980). Under the action of external electric field, the soft polymer surface was modified and polymer's needles emit electrons. We also discuss small electron affinity and field emission time stability. It should be also noted that there is correlation between field emission effect and conductivity effect that is spontaneously arisen when thin polymer films were placed between two metallic electrodes. For example, the imide/siloxane copolymer films exhibit high conductivity and supercurrents being placed between two normal metals and superconductors correspondingly (A. N. Ionov et al. 2003). There, it was also confirmed that the conductivity occurs through many channels orientated perpendicular to the polymer surfaces and connecting the two metallic electrodes. The cross-section of the conductive channels grows with increasing current as well as number of conductive path. The smallest observed value of the cross-section was less than 10/sup -10/ cm/sup 2/.