Switching in coplanar amorphous hydrogenated silicon devices

Switching has been observed in a wide variety of materials and devices. Hyd rogenated amorphous silicon has become one of the most important cases beca use of interest in neural network applications. Although there are many rep orts regarding this phenomenon, not all of the physical processes involved are still determined precisely. Therefore, some more experimental informati on is needed in order to achieve this task. Much of the behavior of the dev ices has been ascribed to the existence of a filamentary region which is pr oduced after the first switching process, called forming. We observed this filamentary region in its full extension by producing forming in amorphous silicon devices with coplanar metallic contacts placed near each other (sim ilar to 5 mu m). The I-V characteristics, filament optical and atomic force microscopy images and chemical etching led us to correlate changes in resi stance to metal inclusion into the amorphous film. There are two stages: th e first is related to contact stabilization, the second to metal transport into the film bulk. Optical images show a permanent filamentary region afte r forming. AFM images of these filaments showed that they are formed essent ially by material accumulation between the contacts. This material tends to get some atomic arrangement, becoming a polycrystalline solid. If the devi ce was led to breakdown, such accumulation becomes either a hillock or a th in conducting channel connecting both contacts. In the case of a switching filament, the accumulation tends to be a chain of smaller hillocks along th e conduction path. Metal from the contacts remains in the conduction path a fter forming and chemical etching indicated that it is placed near the path core. Before forming, a tunneling transport process can be ascribed to the non-ohmic behavior of the samples during the first stage of metallic inclu sion. (C) 2000 Elsevier Science Ltd. All rights reserved.