Thickness Dependence of the Optical and Electrical Properties of Thin a-Si:H Films
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THICKNESS DEPENDENCE OF THE OPTICAL AND ELECTRICAL PROPERTIES OF THIN a-Si:H FILMS F. DEMICHELIS°, E. MEZZETTI°, P. MPAWENAYO°, A. TAGLIAFERRO°, E. TRESSO°, S. BOURQUARD°°, P. RAVA°°, G. DELLA MEA- AND P. MAZZOLDIO o Dipartimento di Fisica, Politecnico Torino, C.so Duca Abruzzi 24, 10129 Torino, Italia '0 Elettrorava S.p.A., 10040 Savonera, Torino, Italia "Dipartimento Fisica, Universita Padova, Via F. Marzolo 8, 35131 Padova, Italia ABSTRACT Optical and electrical properties are investigated as a function of thickness for two sets of samples of a-Si:H films deposited respectively by RF magnetron sputtering and by RF glow discharge under the same conditions. The hydrogen concentration, the transmittance and reflectance spectra in the UV-visible-NIR and the resistivity were measured for both sets of samples. Relationships between index of refraction, absorption coefficient, imaginary part of the dielectric constant, optical band gap, resistivity and the thickness are found and interpreted. The thickness dependence of these parameters is related to the larger amount of inhomogeneities present in sputtered samples with respect to glow discharge samples. INTRODUCTION The influence of thickness on the physical properties of a-Si:H films does not appear to have been fully investigated. It is known that sputtered films contain voids and that the variation of their number generates density changes [1]. On the other hand the refractive index might be expected to
depend on the density of the material and its electronic band structureL2]. The density enters through the density of valence band electrons per unit volume and the band structure through the energy gap. Furthermore a possibility to estimate the effect of inhomogeneities is given by the knowledge of the imaginary part of the dielectric constant. A study of the thickness dependence of such optical quantities seems then a useful source of information on the presence of inhomogeneities. A systematic study of sets of thin amorphous hydrogenated silicon films prepared by RF magnetron sputtering and glow-discharge under similar conditions and with the same hydrogen content having different thicknesses (400-3000 A) is presented here. Relationships between index of refraction, extinction coefficient, imaginary part of the dielectric constant, optical band gap, resistivity, activation energy and thickness of the films as well as the interdependence of these properties are reported. EXPERIMENTAL The a-Si:H films of thickness ranging from 400 to 3000 A were prepared by RF Magnetron Sputtering and RF glow-discharge C3-51 . The sputtering atmosphere consisted of Ar with 12 vol % Hz atmosphere at a total pressure of about 5x]0- 3 mbar. In the glow discharge deposition the gas mixture consisted of Ar with 20% SiHý . The films were deposited on fused silica for UV-visible-NIR spectrophotometric measurements, Corning 7059 glass for nuclear reaction and electrical measurements and graphite for Rutherford back-scattering (RBS) measurements . Thicknesses were determined from Stylus di
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