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Analysis of amorphous-nanocrystalline multilayer structures by optical, photo-deflection and photocurrent spectroscopy
|Davor Gracin , Jordi Sancho Parramon , Krunoslav Juraić
Ruđer Bošković Institute, Bijenička 54, Zagreb 10000, Croatia
Nano-crystalline silicon is a promising material for its application in thin film photovoltaic solar cells. For industrial purposes it is of interesting the deposition on large scale by plasma enhanced chemical vapour deposition (PECVD) on glass substrate, what sets upper limit on the substrate temperature. For deposition of amorphous layer, the working gas uses to be silane and plasma is produced by radio frequency excitation. For deposition of nano-crystalline layers, in order to induce formation of crystalline phase, the working gas is highly diluted by hydrogen, which implies strong plasma-surface interactions during growth.
In this work we examine multilayer structure consisting of 100 nanometres-thick layers of nano-crystalline silicon between two thin amorphous silicon layers. First, we measured the optical gap, Urbach slope and estimate the defect level of single layers of nanocrystalline silicon by standard transmittance and reflectance measurements, photo-deflection spectroscopy and photoconductivity. The same characterization has been done on the multilayer structures. The results indicate that the optical properties can not be simply explained in terms of the individual properties of the single layers. As first, the interference fringes disappear, proving the increase of interface roughness. Furthermore, additional absorption appears at the longer wavelength spectral range. The results are compared with structural measurements done by Raman spectroscopy, high resolution transmission electron microscopy (TEM) and grazing incidence small angle x-ray scattering (GISAXS). The observed behaviour can be explained as consequence of strong hydrogen ion bombardment which results in changes of surface structure (roughness) and structural changes in the inside of thin films. In that way it was confirmed that the nano-crystal are formed in the already grown film and not in the plasma or, at least, not all of the nano-crystals are formed before built in the film.
Presentation: Poster at E-MRS Fall Meeting 2007, Symposium J, by Davor Gracin
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-21 19:57 Revised: 2009-06-07 00:44