The measurement of bulk and surface recombination by means of modulated free carrier absorption

A measurement technique is described which makes it possible to monitor the lifetime and surface recombination velocity of the starting wafer as well as a partially or completely processed wafer in the Si solar-cell fabrication process. This technique uses an infrared laser to monitor the carrier concentration via free carrier absorption while periodically exciting free carriers by means of a visible laser. The excited laser is sinusoidally modulated with an electro-optical modulator at frequencies of 100 Hz to 100 kHz. The free carriers generated by the exciter beam attenuate the probe beam, and the resultant output is detected with a phase-sensitive lock-in amplifier. The quantities measured are the amplitude and the phase of the detected signal relative to the exciter beam. The measured data are then fitted to theoretical expressions, and the bulk lifetime and surface recombination velocities are determined. The amplitude and phase are independent quantities, and the computed values from the two sets of data provide a self-consistency test.