In this work we showed a comparison of the effect of H-2,CF4/O-2, RF dry etching on the properties of the heterostructure device useful for photovoltaic application. To this aim we performed low temperature (50-300 K) capacitance measurements in a wide range of probe signal frequencies (10 Hz-10 kHz). Differences in the capacitance profile between samples with various plasma dry treatments indicated different defect density profile at interface. With the aid of a finite difference model of the capacitance as a function of temperature and frequency we extracted information about the defect distribution at interface, that greatly influence the photovoltaic properties of the devices. As a result, the density and the nature of defects at inter-face has been correlated to the technological parameters: wafer cleaning procedure, hydrogen plasma treatment, type and concentration of dopants at interface.