Polycrystalline silicon thin films on glass deposited from chlorosilane at intermediate temperatures

Abstract

We show that commercial float glass can be used as a substrate to deposit polycrystalline silicon (poly-Si) by chemical vapor deposition from trichlorosilane at temperatures between 740 and 870 • C. By using scanning electron microscopy an average grain size lower than 0.4 μm was observed, with a columnar structure suitable for the electrical conduction in photovoltaic cells. X-ray diffraction reveals a strong (2 2 0) preferential orientation of the films, which is indicative of a low density of intra-grain defects. Atomic force microscope images reveal a conical structure, with a root mean square roughness of ∼65 nm for samples of around 3 μm in thickness. This natural texture is a positive characteristic from the point of view of light trapping. By using boron tribromide as a doping agent, degrees of doping ranging from intrinsic to clearly p-doped were obtained, as shown by dark conductivity measurements as a function of temperature. The process, the reactants and the substrate used are of low cost and proved to be adequate for direct poly-Si deposition.

Characterization of thin polycrystalline silicon films deposited on glass by CVD

“…The basic structural characterization of the films has been presented in [7]. As a summary, we can say that the films are dense and uniform, with a good coverage and adhesion to the substrate.…”

“…In a previous work [7], we have shown that a commercial aluminosilicate glass (Schott AF37 [8]) can be used as a substrate to deposit poly-Si by AP-CVD from SiHCl 3 at intermediate temperatures between 735 and 870 • C. By using boron tribromide (BBr 3 ) as a doping agent, degrees of doping ranging from intrinsic to clearly p-doped were obtained. The structural properties of the films were evaluated by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray diffraction and reflectance in the ultraviolet region.…”

Doped Polycrystalline Silicon Thin Films Deposited on Glass from Trichlorosilane**

“…The average grain size is about 190 nm, independent of the BBr 3 concentration. Within the doping range of our experiments, this average grain size is lower than that of undoped samples (around 270 nm) deposited with the same procedure and the same parameters . This means that the addition of boron increases the nucleation rate.…”

“…TLS reduces at high temperature in the presence of hydrogen, resulting in the deposition of silicon and the release of hydrochloric acid. This reaction is reversible, meaning that, under certain circumstances, hydrogen chloride can etch the silicon deposited . Boron tribromide or phosphorous trichloride also reduce in the presence of hydrogen, leading to the incorporation of boron or phosphorous into the silicon lattice, and the release of hydrobromic or hydrochloric acid, respectively.…”

“…In previous works, we have shown that intrinsic poly‐Si layers, exhibiting good structural and optical properties, can be deposited by APCVD on glass substrates . The next step towards the production of solar cells is the deposition of doped layers.…”