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A New Technique For Measuring Spatially Resolved Minority Carrier Lifetimes Using A Scanning Laser Microscope

A frequency response technique for measuring carrier and trap lifetimes in semiconductors has been developed for use with a scanning laser microscope. Electron-hole pairs are excited in the semiconductor specimen using a square-wave modulated focused laser beam. A vector lock-in amplifier measures the real and quadrature components of the induced photoresponse as a function of frequency. This frequency domain data is corrected to remove the square wave modulation harmonic effects, and Fourier transformed into time domain data to produce the time domain transient decay curve; this curve is then analyzed in the same way as the conventional transient photoresponse data resulting from a pulsed laser experiment. The spatial resolution obtained is two orders of magnitude better than previously obtained in pulsed laser experiments, and the increased signal-to-noise ratio achieved with a lock-in amplifier enables us to measure bulk lifetimes in crystalline semiconductors, or lifetimes near grain boundaries in polycrystalline semiconductors with extremely low sample bias and low laser intensity.