Dependence of coil sensitivity on sample thickness in inductively coupled photoconductance measurements

Abstract

It is shown that the sensitivity of widely used inductively coupled photoconductance measurements is significantly dependent on sample thickness in the range of typical silicon wafer thicknesses, due to the decay of the magnetic field strength 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 combine experimental data together with analytical and finite-element modelling of the coil impedance to identify the functional form of the sensitivity decay in the regime of interest and determine the corresponding value of the attenuation length for the WCT-120TS photoconductance measurement system. Simple formulas are provided to allow the experimentalist to correct for sample thickness and lift-off. Application of this procedure to experimental calibration data is shown to resolve apparent scatter, allowing measurements to be performed with an unprecedented degree of confidence.