Nondestructive Measurement of Minority Carrier Lifetimes in Si Wafers Using Frequency Dependence of ac Photovoltages

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Abstract
Frequency dependence of ac photovoltages in p-n junctions has been measured while changing the chopping frequency of a photon beam with a wavelength of 1.15 µm. The chopping frequency was varied between 2 Hz and 200 kHz. When substrate thickness is larger than carrier diffusion length, minority carrier lifetime is obtained from the bending frequency which appears in photovoltage vs frequency curves on a logarithmic scale. The ratio of diffusion length to substrate thickness should be smaller than 0.27 to obtain lifetimes within 10% error due to infinite surface recombination velocity. When the substrate thickness is much smaller than carrier diffusion length, the bending frequency is determined by the substrate thickness. The longest lifetime is thus limited by the substrate thickness and the shortest by the photocurrents generated in surface and junction depletion layers.