Electrochemical grafting of doped organic films on platinum: Forming and negative resistance in platinum | doped film | metal capacitors

Abstract

The current-voltage (I-V) characteristics of MIM capacitors with electrochemically prepared organic dielectrics (polytetrahydrofuran (poly-THF) and polyacetonitrile) doped with ionic impurities indicate that forming, negative resistance and electron emission occur. Under d.c. conditions two negative resistance (NR) zones appear in the I p- V plot. The first NR zone is voltage controlled and appears at about 3 V for dielectric thicknesses between 700 and 3000 Å, whereas the second NR zone is field dependent and appears at about 10 6 V cm -1. In the temperature range 77–300 K the first NR zone is independent of temperature, in contrast with the second NR zone which disappears at very low temperatures. When oxygen is present the first NR zone shifts from 3 to 2 V. Under a.c. conditions and at relatively low frequencies (60 Hz) both NR zones disappear; however, when the capacitors are subjected to 60–10 000 Hz, a thermal-voltage memory state becomes apparent and the capacitors convert from a high conductivity state to a low conductivity state. Under d.c. conditions, parallel to the forming process, a first weak electron emission (approximately 10 -9 A) is detected when the voltage is greater than 2.5 V; a much stronger emission (10 -6 A) occurs in the region where the second NR zone occurs. The attenuation length of electrons emitted under vacuum through the gold electrode for platinum | poly-THF | gold and platinum | polyacetonitrile | gold capacitors is about 250 Å and is almost independent of voltage. The retarding potential measurements of the normal component of energy for the electrons emitted indicate that some of the electrons acquire energy within the dielectric. The origins of the electroforming process and of the two NR zones under d.c. conditions are discussed. Electroforming is only observed when the dielectric contains mobile ionic impurities. The first NR zone is explained using Simmons' model which supposes the formation of a localized band of impurities in the band gap of the polymer; the second NR zone warrants a different explanation involving the migration of ionic impurities which is analogous to Barriac's model.