Properties of amorphous hydrogenated silicon-tin alloys prepared by radio frequency sputtering

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Abstract
Amorphous Si1−xSnx and Si1−xSnx :H (0≤x≤0.3) alloy films have been prepared by radio frequency sputtering from composite Si–Sn targets in Ar or Ar-10% H2 mixtures at dynamic pressures of 4 or 20 mTorr. The structural characteristics of the alloys have been probed with 119Sn conversion electron Mössbauer spectroscopy, x-ray diffraction, and infrared absorption. Most of the tin goes into tetrahedral substitutional Si sites at 4 mTorr whereas significant fractions of crystalline β-Sn are produced at 20 mTorr. The substitutional Sn Mössbauer resonance is systematically characterized with a Lorentzian doublet line shape. Oxygen contamination increases with increasing Sn content in the 20-mTorr films but is attributed to postdeposition oxidation. No evidence is found for Sn–H bonds or interstitial (nonbonded) Sn. The optical band-gap decreases at a rate of −0.056 eV/at. % Sn for alloys containing little or no β-Sn and for x up to 0.13. Both the optical band gap and the Mössbauer isomer shift extrapolate to α-Sn (grey tin) behavior at x≂0.3–0.4. The electrical conductivity increases with x at a rate which is not consistent with the measured reduction in band gap and a fixed relative Fermi level. This and the temperature dependence of the conductivity suggest a transition from extended state free-carrier conduction to localized state hopping conduction. Annealing experiments show that the films are stable at 300 °C but that at 400 °C β-Sn precipitation occurs concurrently with hydrogen evolution and with partial crystallization of the amorphous silicon matrix.