In situ investigation of polymorphous silicon deposition

Polymorphous silicon (pm-Si:H) consists of an amorphous silicon matrix in which silicon crystallites with nanometer sizes are embedded. This material combines the optical absorption of amorphous silicon (a-Si:H) with electrical transport properties close to those of microcrystalline silicon (μc-Si:H). pm-Si:H thin films were prepared by radio frequency plasma enhanced chemical vapor deposition. To get a better insight in the growth process, we measured the plasma electrically and by means of optical emission spectroscopy. The optical properties of the films were measured by spectroscopic ellipsometry (SE). The results are consistent with the proposal that the growth precursors of pm-Si:H differ from those of μc-Si:H and a-Si:H and that the silicon particles are formed in the gas phase. High resolution transmission electron microscopy (HRTEM) revealed the presence of crystallites with diamond and hexagonal structure embedded in the amorphous matrix. The ambipolar diffusion length in pm-Si:H is a factor of three longer than in optimized a-Si:H as concluded from diffusion induced time resolved microwave conductivity (DTRMC) measurements.