Effects of in-Depth Inhomogeneities on the Optical Absorption Spectra of a-Si:H Films: Theory and Quantitative Evaluatio
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EFFECTS OF IN-DEPTH INHOMOGENEITIES ON THE OPTICAL ABSORPTION SPECTRA OF a-Si:H FILMS: THEORY AND QUANTITATIVE EVALUATIONS G. Amato*, F. Giorgis* and C. Manfredotti** *National Electrotechnical Institute G.Ferraris, Strada delle Cacce 91, 10135, Torino, Italy "**Experimental Physics Dept., University of Torino, via P. Giuria 1, 10125, Torino, Italy
ABSTRACT The effect of the non-uniform defect profile in a-Si:H films on the subgap photothermal spectra is investigated. A comprehensive theoretical model is suggested. By means of this model it is possible to extract quantitative information about the defects inhomogeneity in the film.
INTRODUCTION Photothermal Deflection Spectroscopy (PDS) is a very well extablished experimental tool to measure the absorption coefficient (x of a-Si:H films. This quantity is closely related to the electronic transitions involving extended, tail and defect states. In detail, the low energy portion of the a(hv) spectra is related to transitions between both occupied and unoccupied defects and the corresponding conduction and valence band. Several methods 1 ,2,3 have been suggested in the past in order to infer quantitative information about the amount of these defects and their energetic distribution. PDS is sensitive to the absorption of surface states, thus the at(hv) spectra obtained in this way can considerably differ from those obtained e.g. by means of photoconductive techniques. The sub-bandgap oa(hv) spectra show interference fringes if twice the optical thickness of the film, 2nd (n is the film refraction index, and d is the film thickness) is less than the coherence length of the monocromatic pump light (~20 gim for arc lamps). In the case of a-Si:H (n~3.4), samples having thickness d
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