Fabrication and characterization of μc-Si:H/a-Si:H superlattice and its application to solar cell

Abstract

Spectroscopic ellipsometry investigated the optical characteristics of thin (＜22 nm) hydrogenated amorphous silicon (a-Si:H) films. For comparison, I prepared $H_2$ diluted as well as undiluted a-Si:H samples on c-Si substrates. As the thickness decreases, the peak-positions of dielectric functions ($\epsilon_r$, $\epsilon_i$) shifted to the higher-energy sides both in the $H_2$ diluted and the undiluted a-Si:H films. In addition, noticeable difference of growth behaviors between the $H_2$ diluted and the undiluted a-Si:H films was observed. To preclude effects caused by the incorporated hydrogens, the samples were annealed for sufficient dehydrogenation, which was verified by Fourier Transformed Infrared Spectroscopy. Even after this dehydrogenation, there still remained the difference between the $H_2$ diluted and the undiluted samples, also, the evolution of the ($\epsilon_r$, $\epsilon_i$) peak-positions remained in the same trend as before the annealing. A simple chemical-alloy effect of silicon-and-hydrogen bonds cannot explain these phenomena that are viewed to be an amorphous network change. It is strongly presumed that orderings of the amorphous network are highly deviated from the bulk ones during initial growth, and that $H_2$ addition causes the more intense reorganization of the amorphous network. The microcrystalline silicon/amorphous silicon (μc-Si:H/a-Si:H) superlattice showed an enhanced vertical photo-sensitivity (photo-conductivity/dark-conductivity), whereas it reserved a lateral photo-sensitivity nearly unchanged. The film was fabricated by alternating the mixing of $SiH_4$ and $H_2$ in a photo-chemical vapor deposition system. The fact that a high vertical photo-sensitivity and an obvious crystalline volume fraction can be obtained at the same time distinguishes the μc-Si:H/a-Si:H superlattice from the bulk μc-Si:H. The change of the vertical dark -conductivity with the sublayer thickness was explained by the change of the a-Si:H sublay...