Highly conductive boron doped micro/nanocrystalline silicon thin films deposited by VHF-PECVD for solar cell applications

Graphical abstract: AFM images of boron doped micro/nanocrystalline silicon films at different diborane gas flow. - Highlights: • High deposition rate of 10 Å/s was achieved for boron doped silicon films. • Wide range of optical band gap from 1.32 eV to 1.84 eV observed for the deposited films. - Abstract: Boron doped hydrogenated micro/nanocrystalline silicon (μc/nc-Si:H) thin films have been deposited by plasma enhanced chemical vapor deposition technique (PECVD) using silane (SiH₄) diluted in argon. Diborane (B₂H₆) was used as the dopant gas and deposition was carried out at substrate temperature of 200 °C. The diborane flow (F_B) varied in the range 0.00–0.30. Here, we report the effects of B₂H₆ doping on electronic, optical and structural properties of hydrogenated micro/nanocrystalline silicon films. The structural properties were analyzed by atomic force microscopy (AFM) and X-ray diffraction (XRD). The doped micro/nano crystalline silicon films presented a crystallographic orientation preferentially in the (1 1 1) and (2 2 0) plane. We resolve the deposition parameters that lead to the formation of p-type micro/nanocrystalline silicon thin films with very high value of conductivity and lower optical band gap. Correlations between structural and electrical properties were also studied. Based on temperature dependent conductivity measurements, it has been observed that the room temperature dark conductivity varies in the range 1.45 × 10⁻⁴ Ω⁻¹ cm⁻¹ to 2.02 Ω⁻¹ cm⁻¹ for the B-doped films. Meanwhile, the corresponding value of activation energies decreased to 0.06 eV for the B-doped films, which indicates the doped μc/nc-Si films with high conductivity can be achieved and these films prove to be a very good candidate for application in amorphous and micro/nano crystalline silicon solar cells as a p-type window layer