Electrode-dielectric Interface Research Articles

Electrical Conduction in Very Thin Polybutadiene Films Formed in a Glow Discharge

Films in the thickness range 100 to 500 Å are studied. Ohmic contacts (gold electrodes) are formed only sporadically. In the best example of Ohmic contact, space-charge-limited current (SCLC) occurs at a field of 3×104 V/cm. SCLC is obtained in this and other specimens at fields great enough for field emission (3 to 5×105 V/cm) where trap densities about 1023/m3 were found, greater by a factor of ten than the lower field case. In less conclusive results and in general agreement with theory, a greater dependence on thickness as compared to voltage is found. The establishing of SCLC at fields great enough for field emission suggests a causative relation, perhaps tunneling through the blocking contact at the electrode-insulator interface. The change in the ac resistivity is lower than that using dc by about three orders of magnitude over the temperature range of 20° to 95°C. Three activation levels are found with transition points at 20° and 110°C. It is necessary to use a layer of SiO at the crossover region of the electrodes to prevent dielectric breakdown in this area. This protective layer is made sufficiently thick not to interfere significantly with the measurements on the polybutadiene.

Plastic Electrets

A brief discussion of proposed explanations for the electret effect is presented. Plexiglas, Lucite, and Nylon electrets were prepared under fields ranging from 19 to 36 kv/cm and observed for as long as 2000 hours. The surface charge densities were measured by electrostatic induction using a commercial electronic electrometer and a shunt capacitance to reduce the readings to a maximum of 20 volts. The initial state was a heterocharge or a homocharge depending on the strength of the forming field. The steady state was a homocharge in every case. Charge densities as high as 5.5×10−9 coulomb/cm2 (16.5 esu) were observed. A Plexiglas disk was charged at room temperature under a strong field and its subsequent decaying homocharge recorded. The experimental results support the ideas of Mikola and Gross, i.e., the existence of two decaying polarizations of opposite sense due to ionic migration, one occurring within the dielectric, the other, across the electrode-dielectric interface.