Effect of dopants on the dynamics of powder formation and the properties of polymorphous silicon thin films

Polymorphous silicon (pm-Si:H) is a nanostructured material produced by the dissociation of silane-hydrogen mixtures under plasma conditions where silicon radicals, clusters and agglomerates contribute to the growth. The dynamics of power formation in a capacitively coupled radio-frequency (RF) discharge has been studied as a function of dopant gas concentration through the analysis of the evolution of the second harmonic of RF current. The intrinsic and doped films were characterized by spectroscopic ellipsometry, dark conductivity and Raman spectroscopy measurements. We have demonstrated the possibility of obtaining doped pm-Si:H films with transport properties comparable to those of standard amorphous silicon using trimethylboron or phosphine. We have found that the addition of trimethylboron reduces cluster and agglomerate concentration while phosphine has no prominent effect on particle formation. The best ordered polymorphous films were deposited under conditions where not only 1-2 nm clusters but even larger (#~10 nm) agglomerates contribute to the growth. For these conditions the deposition rate reaches 5 A/s for i- and n-type, and 7 A/s for p-type materials. The dark conductivity at room temperature was 10-2-10-3 *W-1 cm-1 and 10-6 *W-1 cm-1 for the n- and p-type films, respectively.