Ellipsometry investigation of the amorphous-to-microcrystalline transition in a-Si:H under hydrogen-plasma treatment

Abstract

We have investigated by ellipsometry the structural evolution of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) thin films during their exposure to a hydrogen plasma in conditions of chemical transport. The real-time kinetic ellipsometry trajectory at 4.2 eV allowed a precise determination of the amorphous-to-microcrystalline (a→μc) phase transition time. In situ spectroscopic ellipsometry spectra were collected at both sides of the transition to identify the hydrogen induced changes in the film structure and its time evolution under hydrogen-plasma exposure. The whole ellipsometry diagnostics reveal that, while intrinsic and phosphorus-doped a-Si:H present a similar trend during the plasma treatment, boron-doped a-Si:H differs by the following special features: (i) a rapid formation of the hydrogen-rich subsurface layer with a high hydrogen excess. (ii) An early amorphous-to-microcrystalline phase transition. (iii) A less porous and small-grains formed transition layer. Such a particular behavior of the p-type material is attributed to the effects of boron which, on the one hand, reduces the etching rate by hindering the formation of volatile species such as SiH4 and, on the other hand, promotes quick and repeated nucleation of nanoparticles and/or crystallites which contribute to the growth film.