Evolution of microstructure and opto-electrical properties in boron doped nc-Si:H films deposited by HW-CVD method

Abstract

Demand for an efficient window layer for a-Si:H based solar cells in terms of electrical conductivity and optical band gap is ever increasing since the inception of single junction and tandem solar cells. In this paper, we report synthesis of highly conducting boron doped p-type nc-Si:H films by HW-CVD using the mixture of silane (SiH4) and diborane (B2H6) without hydrogen (H2) dilution. Variation in film characteristics with B2H6 gas-phase ratio was studied, and revealed that the boron doping induces amorphization in nc-Si:H film structure. The AFM analysis show increase in rms surface roughness and micro-void density while FTIR spectroscopy analysis show shift of hydrogen bonding from Si–H2 and (Si–H2)n complexes to Si–H configuration on boron doping. The hydrogen content was found <2.66 at.% while the band gap remain as high as 1.8 eV or more. At optimized B2H6 gas-phase ratio, we have obtained p-type nc-Si:H films having high band gap (∼2.0 eV), high dark conductivity (∼1.2 S/cm) with low hydrogen content (∼1.78 at.%) at reasonably high deposition rate (∼9.2 A/s). The obtained films can be used as a window layer in a-Si:H based p–i–n and tandem solar cells and color light detectors.