A method and device for determining the quality of the interface surface between a layer of a dielectric material and the top surface of the semiconductor substrate are disclosed. In one aspect, the method comprises providing a semiconductor substrate with a top surface whereon a layer of a dielectric material is deposited thereby forming an interface surface, the surface of the layer of the dielectric material being or not in direct contact with the semiconductor substrate defining a top surface. A charge is then applied on a dedicated area of the top surface. A voltage Vs is measured on the top surface. The dedicated area is illuminated to define an illuminated spot. The photovoltage is measured inside and outside the determined illuminated spot during the illumination of the area.