Plasma enhanced chemical vapor deposition of nanostructured silicon thin films

Siri Thompson, advisors: Uwe Kortshagen (ME), Jim Kakalios (Physics)

Amorphous hydrogenated silicon films are widely used for the economical production of solar cells. Unfortunately, these films suffer from light-induced creation of defects that causes a rapid deterioration of the solar cell efficiency. It has been shown that thin films consisting of hydrogenated silicon with nanocrystalline silicon inclusions (a/nc-Si:H) exhibit a certain immunity to light-induced defect creation. Thus these nanostructed sillicon thin films hold great potential for a variety of optoelectronic applications, specifically for use in high-efficiency solar cells.

The growth of a/nc-Si:H involves a capacitively-coupled radio frequency silane-hydrogen plasma, under conditions at the boundary of powder (large particle) formation. The plasma is produced at pressures of 1-2 Torr in a silane:helium mixture(5:95) that is further diluted in hydrogen. Under these conditions, it has been observed that particles begin to grow, nucleate, and form small clusters with diameters of a few nanometers.  These particles are then deposited in the growing film substrate.

Images of nanocrystallites in these films, with crystallites as small as ~3 nm being resolved, are obtained using high resolution transmission electron microscopy (HRTEM).  Electron energy loss spectra (EELS) acquired in the HRTEM confirm the presence of silicon in the crystallites. The optoelectronic properties of the silicon thin films are characterized with dark conductivity and optical absorption coefficient measurements. The optical absorption spectra, measured using the constant photocurrent technique, indicate that a/nc-Si:H films with fewer defects than in conventional amorphous silicon films have been obtained.

Some results of this research were presented at the Gaseous Electronics Conference in October 2002 in Minneapolis, MN.