Influence of p-and n-type doping gases on nanoparticle formation in SiH₄/H₂ radiofrequency plasma discharges used for polymorphous silicon thin film deposition

We study the effect of the addition of p-or n-type doping gases, trimethylboron (TMB) or phosphine, respectively, on cluster formation in the SiH₄/H₂ plasma used for the deposition of hydrogenated polymorphous silicon thin films. The formation of clusters is monitored using time-resolved measurements of the second harmonic of the radio frequency current J₂. We show that the addition of PH₃ does not change the cluster formation, while the addition of a small amount of trimethylboron strongly affects its charging behavior, preventing to some extent its agglomeration. The most ordered pm-Si:H thin films are obtained under conditions from which not only clusters but also larger silicon agglomerates are formed in the plasma and contribute to the deposition. The inhibiting role of TMB on the agglomeration and powder formation is evidenced by the smoothness of p-type films at high rates, as deduced from the sample surface topography obtained by atomic force microscopy measurements.