Accounting for the Dependence of Coil Sensitivity on Sample Thickness and Lift-Off in Inductively Coupled Photoconductance Measurements

Inductively coupled photoconductance measurements are widely used to characterize carrier recombination in crystalline silicon. We show that, contrary to what is usually supposed, the sensitivity of such measurements is significantly dependent on sample thickness in the range of typical wafer thicknesses, due to the attenuation of the magnetic field with distance from the coil. Sample thickness, as well as any separation from the coil, should, therefore, be taken into account in system calibration in order to avoid systematic errors. We investigate the magnitude of this effect both experimentally and via analytical and finite-element modeling for a range of commercial photoconductance measurement systems with varying coil geometry. Finite-element modeling is used to identify the functional form of the attenuation in the regime of interest, and simple formulae are derived which allow the experimentalist to correct for sample thickness and lift-off. Close agreement is found between modeled and experimental attenuation behavior. Finite-element modeling is also used to evaluate the magnitude of skin effects, which are found to have a minor influence on the measured conductance for the most highly conductive samples, and to determine the lateral spatial variation of the coil sensitivity, which is important for lifetime imaging techniques where photoconductance measurements are used for calibration.