Thin film silicon devices deposited at 100 °C: A study on the structural order of the photoactive layer

The dielectric functions of thin film silicon materials on glass were measured by spectroscopic ellipsometry (SE) and simulated by using the Tauc-Lorentz (TL) dispersion law, which provided information on disorder (C) and density (A). A VHF plasma-deposited sample made at 100 °C with an optimum hydrogen dilution shows density (relative packing density of Si-Si bonds as estimated by SE) and structural disorder that are comparable to samples made at 200 °C. HWCVD materials made at 100 °C at lower hydrogen dilution conditions have a less dense structure and higher roughness compared to the plasma-deposited samples. This can be attributed to the absence of ion bombardment on the growing film. Out of all samples investigated, the HWCVD sample made at a hydrogen to silane flow ratio value of 20 showed a remarkably low structural disorder (C = 1.67) even though the deposition temperature was only 100 °C. A small bond angle variation of ∼6.4° as determined from its Raman spectrum, the presence of small (1-1.5 nm) dispersed crystalline-like islands in the silicon matrix, and sharp rings in the selective area diffraction pattern point towards a special ordered structure. The photoresponse of this material is >10⁵.