Some electrical properties of thin films of polypropylene formed by vacuum evaporation
The electrical properties of films of metal-polypropylene-metal structures with polypropylene thicknesses in the range 600 - 2600Å were studied for possible use as thin dielectric films and some standard and reproducible results of electrical measurements were obtained. As a consequence of using different electrode materials, systematic differences in the behaviour of these films were found, even though the voltage-current characteristics remained ohmic. In these measurements there was evidence of electron trapping and the trap depth was estimated by using the thermally-stimulated current techniques, and was found equal to 0.33 eV. Thin films of evaporated polypropylene having thicknesses less than 1000Å, showed electroforming effects and electron emission into vacuum under suitable conditions. The results were found to be consistent with a theory based on the generation of conducting filaments during the forming process. The increase in the a.c. conductivity of the sample after electroforming also indicated the presence of filaments in the formed device. Another useful application of the a.c. measurements was to identify the hopping conduction both at low and at room temperatures. A linear decrease in the capacitance of the device with increasing frequency was observed and was associated with the growing leakage current. A vacuum heat treatment of the films at 110°C led to an increase in resistivity. The transmission electron diffraction patterns of several evaporated layers of polypropylene having thicknesses between 150 to 1600Å certainly indicated the amorphous structure of these films. However, the examination by electron microscopy and by electron diffraction showed an aggregation of particulate material of more crystalline nature as a result of the heat treatment but the connecting chains were broken and dispersed as a result of the same treatment. Some observations on the general character of evaporated polymers are made.