The interface states parameters of density, distribution and capture cross-section for Si 1− x− y Ge x C y /TiW Schottky contacts has been found from forward bias capacitance measurements using the Schottky capacitance spectroscopy (SCS) technique. It was found that the interface states parameters strongly depend on the composition and quality of the Si 1− x− y Ge x C y alloy layer. `Negative capacitance' effects, observed for low measurement frequencies, limit the application of the Schottky diode model used to calculate the interface states parameters.