Evolution of the Er environment in a-Si:H under annealing: ion implantation versus co-deposition

The evolution of the chemical environment of Er in hydrogenated amorphous silicon (a-Si:H) prepared by co-sputtering and by ion implantation under cumulative annealing steps was studied by extended X-ray absorption fine structure (EXAFS) at the Er LIII-edge. Samples were prepared by rf-sputtering. In one sample small chunks of metallic Er were attached to the Si target during deposition, resulting in an Er concentration [Er]/[Si]∼0.2 at.%. In the other sample a similar Er concentration was ion-implanted. Annealing was performed in 20 min steps between 215 and 1100°C. In the as-co-sputtered sample (which had 7.6 at.% [O]/[Si] intentionally added to improve the Er3+ luminescence) the Er environment consists of a 3-fold co-ordinated oxygen shell. It smoothly evolves towards an Er2O3-like 6-fold co-ordinated shell. In the as-implanted sample the Er environment consists of a 10-fold co-ordinated Si shell. By annealing to 450°C the Er neighborhood evolved towards a smaller coordination. Above this temperature the Er coordination increased, indicating the formation of ErSix domains around the Er atoms. Only at 750°C the Er coordination starts to decrease, due to the onset of oxidation. The Er oxidation is completed between 850 and 1100°C.