Abstract
This thesis is concerned with the formation, characterisation and application of doped surface layers in hydrogenated amorphous silicon, a-Si:H. In this study, a-Si:H films have been deposited using a plasma enhanced chemical vapour deposition (PECVD) system and have been doped using ion implantation. In order to determine the efficiency of implantation doping various ion species, ion doses, and ion energies were made by direct implantation of impurities at low energies. Coupled with the annealing effect, the effect of doping on damage recovery has been studied. It has been shown that there is a good recovery of damage with annealing temperature, the optimum being = 250°C. In addition, it was found that the sequence of preparation steps can affect the electrical properties of the doped samples. Electrical properties of the doped surface layers have been determined from current-voltage measurements. This technique is based on the metal-semiconductor Schottky barrier as a characterisation tool with the aim of relating the current passing through a Schottky barrier to the total electrical activity in a surface layer. This is necessary due to the fact that the surface layer is very shallow, typically <150 A deep, and that there is a small number of free carriers present in the layer. It has been shown that the effective barrier height of Schottky diodes on a-Si:H can be varied over a wide range using ion implantation of the common dopants. For small changes in barrier height, damage effects are negligible and dopant activity is high, leading to minimal changes in ideality factor and leakage current. Sheet resistance measurements of the amorphous silicide layers were made. It has been shown that a thin amorphous silicide can be formed by ion implantation via a radiation enhanced mechanism using low temperature processing.