Effects of trimethylphosphine incorporation on hydrogenated amorphous silicon (a-Si:H) properties

The effects of phosphorus doping on physical, chemical, optical and electric properties of hydrogenated amorphous silicon thin films (a-Si:H(P)), deposited by DC magnetron sputtering technique are presented. Doping was carried out from the use of a nontoxic phosphorus liquid source based on hydrocarbons (trimethylphosphine: TMP, (P (CH3)3) introduced into the deposit chamber by using hydrogen as carrying gas. The a-Si:H(P) films were deposited at different TMP partial pressures and discharge powers. The samples were characterized by optical transmission, electric conductivity measurements and infra-red absorption (FT-IR). IR absorption measurements clearly evidenced that the increase of the TMP partial pressure is accompanied by the appearance of additional bands relating to the constituent elements of the TMP (H, C, P). In the same way, a reduction in the phosphorus and carbon contents with the increase of the discharge power is observed. Optical transmission showed an increase of optical gap and a decrease of refractive index with the increase of the TMP partial pressure correlated to the carbon film content. An increase of the plasma discharge power induced a reduction in optical gap energy value accompanied by an increase of refractive index due to a reduction of Si-H bonds and the carbon content. The effect of annealing temperature shows that a significant increasing of conductivity can take place between 200 and 350 ∘C.