Highly Conducting Phosphorous Doped nc-Si:H Thin Films Deposited at High Deposition Rate by Hot-Wire Chemical Vapor Deposition Method

In this paper, we report the synthesis of highly conducting phosphorous doped hydrogenated nanocrystalline silicon (nc-Si:H) films at substantially low substrate temperature (200 °C) by hot-wire chemical vapor deposition (HW-CVD) method using pure silane (SiH₄) and phosphine (PH₃) gas mixture without hydrogen dilution. Structural, optical and electrical properties of these films were investigated as a function of PH₃ gas-phase ratio. The characterization of these films by low-angle X-ray diffraction, Raman spectroscopy and atomic force microscopy revealed that, the incorporation of phosphorous in nc-Si:H induces an amorphization in the nc-Si:H film structure. Fourier transform infrared spectroscopy analysis indicates that hydrogen predominately incorporated in phosphorous doped n-type nc-Si:H films mainly in di-hydrogen species (Si–H₂) and poly-hydrogen (Si–H₂)n bonded species signifying that the films become porous, and micro-void rich. We have observed high band gap (1.97–2.37 eV) in the films, though the hydrogen content is low (<1.4 at.%) over the entire range of PH₃ gas-phase ratio studied. Under the optimum deposition conditions, phosphorous doped nc-Si:H films with high dark conductivity (σ _Dark∼5.3 S/cm), low charge-carrier activation energy (E _act∼132 meV) and high band gap (∼2.01 eV), low hydrogen content (∼0.74 at.%) were obtained at high deposition rate (12.9 Å/s).