Applications of the Quasi - Steady - State Photoconductance Technique

The main applications of photoconductance measurements of silicon wafers are the determination of implicit device voltages, bulk minority carrier lifetimes, emitter recombination currents and surface recombination velocities. These applications are illustrated with selected experiments. Multicrystalline and single crystal silicon wafers are used with different surface conditions. The practical situations considered here range from industrial process control to advanced research. Interpreting photoconductance in terms of implicit device voltage is particularly useful: the swept illumination conditions used in a quasi-steady-state photoconductance measurement permit the determination of complete I-V characteristic curves, ideality factors and saturation currents. The more classical interpretation in terms of an effective lifetime τeff allows to discriminate different recombination mechanisms. Shockley-Read-Hall bulk recombination with a large asymmetry between the fundamental electron and hole lifetimes is found to explain the strong variation of τeff at low injection level observed in some samples. Measurements in the high injection range permit the determination of the emitter saturation current density. This saturation current can impose quite restrictive limits on the measurable minority carrier lifetimes at low injection, particularly for low resistivity wafers. The surface recombination velocity of the Si/SiO2 interface can also be a source of variability of τeff.