Quasi-Steady-State Free Carrier Absorption Measurements of Effective Carrier Lifetime in Silicon

Abstract

The paper presents a semiconductor carrier lifetime measurement technique, quasi-steady-state free carrier absorption (QSS-FCA), that is an all-optical analog of the quasi-steady-state photoconductivity technique. The effective electron–hole recombination lifetime of silicon is measured under quasi-steady-state conditions using an above-bandgap pump and a below-bandgap probe of the free carrier absorption (FCA). A mathematical model is developed to relate the experimentally measured probe signal to the lifetime. QSS-FCA is calibrated self-consistently by measuring the quasi-steady-state signal simultaneously with the true recombination lifetime in the same apparatus. This directly establishes the relationship between the quasi-steady-state signal and lifetime, ensuring the accuracy of QSS-FCA measurements. The calibration constant is the FCA cross section of silicon, which is a material constant. QSS-FCA is a simplification of traditional FCA lifetime measurement approaches where the lifetime is extracted from a time- or frequency-resolved carrier decay. Both of these approaches require a bandwidth commensurate with the inverse carrier lifetime of the wafer and require fitting multiple data points in order to extract the lifetime. In contrast, QSS-FCA is a quasi-steady-state measurement, so it is inherently low frequency, and as a single point measurement technique, fitting is not required. Finally, since QSS-FCA is a purely optical technique, it opens up the possibility of in situ lifetime characterization that would be inaccessible to the traditional RF/microwave-based lifetime measurement techniques.