231 INVESTIGATION ON MICROVOIDS IN PECVD a-Si:H C. MANFREDOTITI, F. FIZZOTTITI, M. BOERO, P. PASTORINO, E. VITITONE* AND V. RIGATO** *Experimental Physics Dept., University of Torino, Via Giuria 1, Torino, Italy and Consorzio INFM, Italy "**Physics Dept., University of Padova, Via Marzuolo 8, Padova, Italy. ABSTRACT The effect of microvoids on optical properties of a-Si:H deposited by PECVD of Sill 4 has been investigated in the deposition temperature range between 143'C and 266'C. Microvoids seem not to affect the density up to H concentrations CH of 15%. The density is decreased only by H incorporation, which, at the same time, influences directly the optical energy gap Eg. Above CH=15%, microvoids can easily accommodate H and no further variation either of Eg or the index of refraction n can be appreciated. Microvoids are proven to be affected not only by deposition temperature, but also by deposition time and by a further annealing step, even if at a relative extent. INTRODUCTION The role of microvoids in structural, electronic and optical properties of a-Si:H has been investigated in many papersl- 6 . Microvoids are largely responsible for the decrease of the density as a function of hydrogen content CH1, they are widely believed to introduce electronic defects which lower the carrier mobility 7 , they probably influence directly the optical properties of films 8 and they are directly correlated with the behaviour of the index of refraction as a function of density of defects 2 . Moreover, microvoids can be correlated with the mechanisms of hydrogen incorporation in the film 9 and, consequently, with the optical gap of a-Si:H. Microvoids are generally investigated by Small Angle X-ray Scattering (SAXS)3,4.5, positron annihilation experiments 5 , even directly with HRTEM, and they may have particular shapes 4 , but the dimensions are mainly of the order of a few nanometers 3 ,4, 5 . Finally, they could be responsible for gaseous molecular H 2 incorporation at very high pressures10. In the present work, we have investigated the influence of microvoids, as indicated by the measurement of void fraction, on the optical properties of a-Si:H, and particularly on the optical energy gap and on the refraction index. A great deal of attention has been put in order to distinguish between Sill and SiH 2 contributions by deconvoluting the relevant peaks at 2000 and 2100 cm- 1 . Finally, the effect of the total deposition time at different deposition temperatures and the effect of annealing on hydrogen content profile have been investigated in some details. EXPERIMENTAL The samples were deposited by a UHV ELETTRORAVA-MV SYSTEMS PECVD apparatus, keeping fixed the following parameters: Sill 4 flow rate 40 sccm, deposition pressure 0.8 mbar, RF Power 3W. The obtained deposition rates varied from 1.4 to 2.1 A/s as a function of temperature in the range from 143 'C to 266 'C. Coming 7059 glass 20x20 mm 2 and c-Si 20x20 nrm 2 wafers were used as substrates, for optical and IR absorption measurements respectively. For density measurements we used aluminium foils which were subsequently etched in a 3.5% HC1 solution. The density measurements were performed by floating the Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society