Vibrational Spectra of a-Si:H Obtained by Pds
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VIBRATIONAL SPECTRA OF a-Si:H OBTAINED BY PDS E. LOTTER, M. B. SCHUBERT, M. HEINTZE, AND G. H. BAUER, Institut ffir Physikalische Elektronik, Universitit Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart 80, Federal Republic of Germany
ABSTRACT In the energy range below mid-gap, absorption spectra obtained by photothermal deflection spectroscopy (PDS) show much higher values than data evaluated from photo-current measurements. This difference can now be explained as an additional absorption arising from combined vibrational excitation of molecular hydrogen and Si-H stretching modes, which can occur when the H 2 molecule is deformed in a collision with Si-H. Down to lower energies, our samples absorb even stronger than in mid-gap. Extension of spectral scans down to photon energies of 0.4 eV shows that this results from absorption of the high energy wings of the collision induced H 2 absorption, which is superimposed by the first overtone of the Si-H and Si-H 2 stretching modes. The analysis of the H2 absorption profile and strength on samples as grown and annealed gives evidence that molecular hydrogen is enclosed in small cavities and polarized by anisotropic environment in as-grown material while it is concentrated in larger voids under high pressure after annealing above 350 °C. EXPERIMENTAL The PDS measurements were carried out in a standard setup with a 250 W tungsten iodine lamp as a light source, which is preferable over an arc lamp in the infrared region. For the same reason and to avoid chromatic aberration effects of lenses, only reflecting optics were used for collecting and focusing the pumping beam. Infrared transmission spectra were taken on a Bruker IFS 48 Fourier Transform spectrometer. The a-Si:H samples were deposited by rf-glow discharge from Sill4 at a substrate temperature of 250 'C. The pressure was held at 0.2 mbar at a flow rate of 10 sccm; rf power was 5 W for a total area of - 100 cm 2 yielding growth rates of about 0.3 nm/s. Sample thicknesses were varied from 3 to 32 4m. As substrate fused silica and simultaneously crystalline silicon were used. It should be noted, that only infrared-optimized quartz such as Infrasil can be used in the range of 2.5 pm. Glasses such as Coming 7059 absorb strongly in that region and additionally in the 0-H overtone region at 1.4 pzm and hence can only be used for calibration of the PDS experiment. The Tauc band-gap of the films, determined by optical transmission measurements, was found to be 1.76 eV independent of sample thickness. The hydrogen content, evaluated from the Si-H wagging mode, amounted from 8 to 10%. For investigation of changes in structure and hydrogen bonding, samples were annealed at temperatures from 200 to 500 °C in steps of 50 'C for 16 hours at each temperature.
Mat. Res. Soc. Symp. Proc. Vol. 219. ©1991 Materials Research Society
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RESULTS 10 In Fig. 1 example PDS spectra are shown for 3 films with thicknesses of 3.2, 16, and 32 pm. The defect absorption decreases with thickness due to reduced relative surface contribution and
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