Mobile ion drift in Langmuir-Blodgett films

Abstract

The temperature dependence of the drift of polymerized 10,12-nonacosadiyinoic acid based metal-Langmuir-semiconductor field effect transistors (MLSFETs) is examined. It was concluded that the drift was due to metal ions, probably in the Langmuir-Blodgett (LB) film. It was deduced through elimination that the ions were introduced by the use of low purity (less than 500 kΩ resistivity) water for solid state processing and LB film deposition. The devices used to monitor the drift were n-channel silicon MLSFETs with the gate and drain shorted together to ensure operation in the saturation region. The drain current as a function of time under constant applied bias was measured and used to extract information about the polarity and activation energy of the drifting ions. The results were found to be best modeled by the first-order drift of two species of positive ions. The activation energies were found to be approximately 0.22 eV for the dominant species and 0.15 eV for the second species. These activation energy values are quite different from known values for sodium and potassium ions in SiO 2, leading to the suspicion that most of the drift is occurring in the polymer film. The final proof of the drift's being associated with mobile ions is the fact that by switching to high purity water for the processing (greater than 7 MΩ cm resistivity) and deposition (better than 14 MΩ cm resistivity) the drift was entirely eliminated.