Structural and electronic properties of ternary hydrogenated amorphous silicon-sulfur-selenium alloys

Hydrogenated amorphous silicon-sulfur-selenium ternary alloys are grown by rf glow discharge. Results show that alloying a-Si:H with S and Se results in an increase in the optical gap from a value of 1.85 eV in unalloyed films to approximately 2 eV in films grown with a chalcogenide gas fraction of 0.8. IR absorption spectra reveal a clear Si-S stretching vibrational mode which grows in intensity with increasing S content. In addition, the spectra display a pronounced shoulder in the region of 700 to 760 cm^-1 which is attributed to a superposition of S₂, Se₂ and SiS vibrational modes. Most of the bonded hydrogen in the alloy films occurs in the form of polyhydrides. PDS measurements of the subgap optical absorption in 2.0 eV E_opt material reveal a broad exponential Urbach tail with a slope of approximately 92 meV. The tail saturates into a defect absorption shoulder at a value of approximately 10² cm^-1, indicating that the deep defect density is on the order of 10¹⁷ to 10¹⁸ cm^-3. The alloys display substantial photosensitivities as the dark conductivity values range between 10^-12 and 10^-10 S/cm and the photoconductivities, measured at 100 mW/cm² illumination intensity, lie at approximately 10^-8 S/cm.