Electrical Conduction Processes Through Thin Organic Layers.

Available from UMI in association with The British Library. New techniques for fabricating Ag/LB/Mg structures have been developed which yield junctions having insulating properties in the absence of an oxide layer. This has allowed an original study of the conduction processes through 22-tricosenoic acid down to a thickness of only two layers of this material. Capacitance measurements from two to twelve layers of 22-tricosenoic acid established that the insulating properties of the junctions were due solely to the LB material. The room temperature conductance (G) was found to be independent of frequency from 0.001Hz to 0.1Hz, but changed to a G(omega) ~ f^{0.7 } dependence for higher frequency a.c. signals from 10Hz to 10^5 Hz. The capacitance was also independent of frequency for the low frequency range but started to fall off gradually for the higher frequencies, due to a decrease in the dielectric constant of the LB material. The low and high bias results showed a change in the conduction mechanism with change in layer thickness, associated with possible differences in the LB structure between the first and subsequently deposited monolayers. The high bias results for greater than six monolayers showed that the logarithm of the current was proportional to the square root of the voltage, characteristic of a Schottky or Poole-Frenkel mechanism. Assuming a Schottky mechanism to describe this dependence the height of the insulating barrier was calculated to be 1.05 +/- 0.05 V. Temperature measurements from room temperature up to 62^circ C showed that thermally stimulated currents could be generated from the LB material. By successive heating and cooling of the substrate the trapped charges were released from the LB layers. Time integration of this thermally stimulated current gives an equivalent trapped charge density of about one electron per ten molecules. After the thermally stimulated current had decayed an equilibrium current, of current density (J), is measured and a plot of ln(J) against {1over T} yielded a value of 1.10 +/- 0.04 eV for the excitation energy. Both the thermally stimulated low bias current and the Schottky injection high bias current are suggestive of a set of trapped carriers about 1.10 +/- 0.04 eV below the conduction band.