Optical properties of nanocrystalline silicon deposited by PECVD
Nanocrystalline silicon (nc-Si) thin films were deposited by 13.56 MHz plasma enhanced chemical vapor deposition (PECVD) using silane plasma highly diluted in hydrogen to induce microstructural changes from amorphous to nanocrystalline. Raman spectroscopy measurements showed that nc-Si films can be obtained at hydrogen dilutions greater than 98%. High hydrogen dilution in conjunction with high reactor pressure lead to lower absorption of photon energies less than 1.1 eV, and, consequently, to lower defect density inside nc-Si thin films. The nc-Si dark conductivity, dark conductivity activation energy, and photosensitivity were in the range 10−6 S/cm, 0.43 eV, and 10-20, respectively. Photocurrent spectra showed absorption peaks at photon energies around 2.5, 3.2, and 4.5 eV. Measured optical bandgaps were in the range of 2.3–2.5 eV.
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