Doping Effect in Hydrogenated Amorphous Carbon Thin Films by Ion Implantation

Abstract

Hydrogenated amorphous carbon (a-C:H) thin films have been prepared by rf glow discharge decomposition of C₆H₅CH₃ gas at 13.6 MHz. Multi-energy boron or phosphorus implantation into these films has been performed to achieve a relatively flat impurity profile and the doping effect as a function of implanted impurity concentration has been studied. It is found that there are critical values of the implanted boron and phosphorus concentration below which there is no significant change observed in the electrical conductivity activation energy E_a, and the room temperature conductivity σ_RT increases only slightly with the impurity concentration. When the critical values are reached, there are first an abrupt decrease in E and an abrupt increase in Σ_RT. Then E_a decreases and Σ_RT increases gradually with further increase in impurity concentration. The values of the critical concentrations are also found to be dependent on the substrate temperature during deposition. The critical concentrations are lower for lower deposition substrate temperatures. Preliminary results on the comparison between the doping effects by B⁺ and BF⁺ implantation show that the doping efficienty by BF₂⁺ implantation is much² higher.