Switching effect in chalcogenide glasses

The overall aim of the research programme was to investigate threshold switching, both experimentally and theoretically, in thick devices based on chalcogenide glasses. The experimental study included characterization of the switching material Si12 Ge10 As30 Te48 by measurement of its electrical conductivity as a function of temperature and electric field. The temperature dependence of threshold voltage was measured over a broad temperature range and could be explained in terms of two theoretical models. The models had a common feature in that they were based on the one-dimensional thermal model with an added independent condition for switching involving the presence of electric field. The importance of the restriction to one dimension was illustrated by solving the equations of the thermal model in two dimensions using a digital computer. The validity of the models was limited, however, because no modifications due to the presence of structural changes in the switching material were included in the models. It was concluded that the structural changes have to be studied and understood before a realistic quantitative model of switching can be developed. The experimental study of structural changes in the threshold switching material comprised direct evidence of microcrystallites by the use of electron microscope and observations of changes in switching parameters during and after the forming stage. The variation in threshold voltage and delay time has been analyzed by statistical methods which revealed that the variation has two components: (i) slow semi-reversible ageing taking place throughout the life of the device, (ii) fast fluctuations occurring from operation to operation. It was further shown that the latter variation is not random but that correlation exists between sequential fluctuations. A semi-quantitative theoretical model was presented to explain the observed phenomena in terms of microstructural changes in the glass during switching. This model incorporates some simplifying assumptions but nevertheless it is capable of satisfactorily predicting the experimental observations and of suggesting areas for future research. Some direct experiments aimed at the elucidation of the mechanism of switching were conducted, but no definite conclusion could be reached. However, the overall results indicate that a satisfactory description of the switching phenomena could be achieved in terms of a thermal model with the inclusion of electric field effects and aspects of structural changes