The influence of the character of laser source on the minority carrier lifetime in silicon lifetime measurement

Minority carrier lifetime is one of the most important parameters for judging the quality for silicon samples. Many methods have been used to measure it. In the measurement process, pulse laser is often used to generate the excess electron-hole pairs in the detected silicon samples. Ordinarily people consider all the pulse excitations as the ideal infinitesimal impulse, neglecting that the real pulse excitation often has finite pulse width and decline tail. In this paper, the distinctions among the Gaussian pulse, the square pulse and an infinitesimal pulse were discussed theoretically. We find that for the Gaussian pulses, only the pulse width less than 2ns were considered as an impulse and for the square pulses, the pulse width less than 10ns were assumed to be an impulse. Otherwise the existence of the separation factor Delta will influent the results of bulk lifetime and surface recombination velocities which were extracted from the photo conductance decay curve, though it does not affect the value of the effective minority lifetime. For the samples less influenced by the surface condition, the requirement of pulse width can be broader.