Effect of radio frequency power on the properties of hydrogenated amorphous carbon films grown by radio frequency plasma-enhanced chemical vapor deposition

We have studied the influence of radio frequency (RF) power on material properties of hydrogenated amorphous carbon (a-C:H) films prepared by RF plasma-enhanced chemical vapor deposition (CVD). The RF power has a significant impact on film properties such as electron spin resonance (ESR) spin density, the optical band gap, chemical bonding and photoluminescence (PL). Raman spectroscopy, infrared (IR) absorption and ESR measurements reveal that the residual defect density (ESR spin density) increases with increasing RF power due to the increase of sp² C-H_x components. We obtained an ESR spin density as small as 8 × 10¹⁶ spins/cm³ at a low RF power, which is comparable to that of high-quality amorphous silicon films. Plasma optical emission spectroscopy indicates that the intensity ratio (I_C-H/I_H) between hydrogen peaks (I_H) and hydrocarbon peaks (I_C-H) significantly decreases with increasing RF power. The optical band gap, obtained from UV-visible spectroscopy, varies from 3.0 to 1.9eV with increasing RF power. The broad emission band centered around 2.4 eV was observed in the PL spectra of the samples deposited at lower RF powers. We found a 0.5-0.3 eV Stokes shift by comparing the PL emission and the absorption coefficient spectrum from the films deposited at ≤100W RF powers, whereas, we failed to observe a clear Stokes shift from the films deposited at >100W RF powers. A weak correlation between the PL peak and the optical band gap is demonstrated.