Frequency Dependence of Modulated Surface Photovoltage (SPV) to Characterize Surface Defects

International audienceThe modulated surface photovoltage (SPV) experiment in metal-insulator- semiconductor configuration (MIS) uses application of a chopped light beam upon the surface of a sample. The induced variation of the surface potential is recorded by a transparent conductive probe placed on top of the sample. We investigate the impact of the light modulation frequency on the response of untreated p-type silicon, used as a model system. Due to the absence of buried heterojunctions in the sample, we assume that the SPV response originates at the Si surface due to the presence of a defect-induced space charge region (SCR). Experimentally, we observe an increase of the phase shift of SPV with respect to the incoming beam, and a reduction in the SPV amplitude, when we increase the modulation frequency from 29 to 494 Hz. This trend is qualitatively explained by the timescale of charge carrier trapping and detrapping at the surface of the sample. We have employed drift-diffusion simulations to quantitatively evaluate the impact of key surface defects’ properties, and to show the power of frequency modulation in the MIS- SPV technique. Following the Shockley–Read-Hall non-radiative recombination mechanism [1,2], parameters such as defect density and carrier capture cross sections have been evaluated. From our preliminary results, a large asymmetry in the role of the electron and hole capture cross sections arise. This can be ascribed to the doping nature of the semiconductor. Under illumination, photogenerated electrons are driven towards the surface, where they subsequently trap, with a speed which depends on the electron capture cross section. During the dark semi-period, these electrons are released by recombining with holes in the valence band, a process which mainly depends on the hole capture cross section. On the one hand, dynamics under light are impacted by the large photogenerated carrier density, which increases the electron trapping rate on the surface. On the other hand, during the dark semi-period, the hole density at the surface quickly decreases. This translates into a much higher impact on the SPV phase shift of the hole capture cross section, with respect to the electron one