Effect of radio-frequency bias voltage on the optical and structural properties of hydrogenated amorphous silicon carbide

Hydrogenated amorphous silicon carbide (a-Si1-xCx:H) films have been deposi ted using the electron cyclotron resonance chemical vapor deposition proces s under varying negative rf-bias voltage at the substrate. The optical and structural properties of these films are characterized using Rutherford bac kscattering spectroscopy, transmittance/reflectance spectrophotometry, phot othermal deflection spectroscopy, Fourier transform infrared absorption, Ra man scattering, and room temperature photoluminescence (PL). These films de posited using a gas mixture of silane, methane, and hydrogen at a constant gas flow ratio showed a slight increase in the carbon fraction x, but very obvious structural transformation, at increasing rf induced bias voltage fr om -20 to -120 V. Near stoichiometric a-Si1-xCx:H films with a carbon fract ion x of almost 0.5 are achieved at low bias voltage range from -20 to -60 V. Visible PL with relatively low efficiency can be observed from such film s at room temperature. For larger bias voltages from -80 to -120 V, slightl y C-rich a-Si1-xCx:H films (x >0.5) with larger optical gaps are obtained. These films have relatively higher PL efficiency, and the relative quantum efficiency was also found to depend strongly on the optical gap. Structural ly, it was found that there is an increase in the hydrogen content and carb on sp(2) bonding in the films at larger bias voltages. The latter leads to an increase in the disorder in the films. The linear relationship observed between the Urbach energy E-0 and B factor in the Tauc equation suggests th at the local defects related to microstructural disorder resulting from all oying with carbon dominate the overall defect structure of the films. Subst rate biasing is noted to be crucial for the formation of Si-C bonds, as ded uced from the Raman scattering results. (C) 2001 American Institute of Phys ics.