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

Abstract

Hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films have been deposited using the electron cyclotron resonance chemical vapor deposition process under varying negative rf-bias voltage at the substrate. The optical and structural properties of these films are characterized using Rutherford backscattering spectroscopy, transmittance/reflectance spectrophotometry, photothermal deflection spectroscopy, Fourier transform infrared absorption, Raman scattering, and room temperature photoluminescence (PL). These films deposited 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 from −20 to −120 V. Near stoichiometric a-Si1−xCx:H films with a carbon fraction 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 films at room temperature. For larger bias voltages from −80 to −120 V, slightly 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. Structurally, it was found that there is an increase in the hydrogen content and carbon sp2 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 E0 and B factor in the Tauc equation suggests that the local defects related to microstructural disorder resulting from alloying with carbon dominate the overall defect structure of the films. Substrate biasing is noted to be crucial for the formation of Si–C bonds, as deduced from the Raman scattering results.