Thermal effects on the bulk and interface electrical properties of Si0.887Ge0.113 and Si0.887-yGe0.113Cy films annealed at an oxidizing or inert ambient were investigated. We found that while annealing the Si0.887-yGe0.113Cy films at an oxidizing ambient reduced the C content significantly, a significant amount of SiC precipitates was found in films annealed in an inert ambient. The high interface state value of the Al–SiO2–Si0.887Ge0.113 capacitor resulted in a low minority carrier generation lifetime and a frequency dispersion behavior at the depletion region of such a device. A good agreement in the interface state values was obtained from the capacitance versus voltage and the deep level transient spectroscopy measurements performed on the Al–SiO2–Si0.887Ge0.113 capacitors. For the capacitors fabricated on Si0.887-yGe0.113Cy substrates, the significant increase in the apparent substrate doping level was attributed to the SiC precipitates in the film caused by the high temperature oxidation process. The capacitance versus frequency measurements showed the influence of interface state and bulk defects on the frequency dependence at the depletion and inversion regions of capacitors fabricated on Si0.887-yGe0.113Cy substrates. © 2004 American Institute of Physics.