Studies on Amorphous Silicon Thin Films Doped with Aluminium
Amorphous Silicon(a-Si) films have attracted the attention of several investigators as it is an economical material for devices. One of the problems that is addressed is the doping of these films after they are prepared. In this thesis, we investigated the effects of doping amorphous Silicon films(prepared by r.f. sputtering) with Aluminium(Al) by thermal diffusion. Amorphous Silicon films have been prepared on glass substrates at optimal process parameters. Then, the a-Si films are coated with Al by vacuum evaporation and subjected to heating in N2 atmosphere in the temperature range 300°C to 600°C for different durations. After etching Al layer, it has been found that some of the films which are heated around 550°C contain filament like polycrystalline regions surrounding islands of a-Si. This structure has been confirmed through Scanning Electron Mi-croscope(SEM) photographs and electrical conductivity measurements. SEM photographs indicate that, bright regions of amorphous material are surrounded by dark regions of relatively higher conducting boundaries. The electrical conductivity study shows that there is sharp increase in conductivity of Al doped films, which is attributed to the conducting polycrystalUne filament. A simple model has been proposed to explain the variation of conductivity of these transformed films, with process parameters and with temperature. Schottky barrier diodes have been fabricated using these transformed materials and their characteristics explained.