Simultaneous characterization of bulk impurities and interface states by photocurrent measurements

A new method for evaluating both surface recombination velocity and bulk minority carrier lifetime by photocurrent measurements is discussed and validated by comparison with capacitance–voltage measurements of interface state density. This method is an evolution of the measurement of surface recombination velocity by the Elymat technique, it does not require the oxide to be etched off and consists in measurements of surface recombination velocity under an applied surface bias. The application of a surface bias allows the control of the interface potential and the identification of the suitable interface condition so that surface recombination velocity can be considered as a measurement of interface state density. In addition, it is shown that surface recombination velocity is suppressed when the surface is under accumulation conditions, so the application of a surface bias provides the possibility of a surface passivation by driving the surface into accumulation. This passivation by surface polarization is about as effective as the chemical passivation by HF. Finally, the dependence of surface recombination velocity on the injection level is shown to be reversed when the interface changes from depletion to accumulation or inversion conditions. This technique does not require the formation of a capacitor structure, so it is suitable for the measurement of as-grown interface properties. For this reason, this technique was chosen for a systematic study of the nitridation process of oxide films. Surface recombination velocity was correlated with nitrogen concentration at the oxide–silicon interface.