Measurement time reduction for generation lifetimes

The defect properties of thin semiconductor layers (epitaxial and denuded zones as well as silicon on insulator) are not as easily characterized as those of bulk wafers. Recombination lifetime or diffusion length measurements, routinely used for bulk wafers, are unreliable when the layer thickness is significantly less than the diffusion length. Generation lifetime measurements are eminently suitable for such characterization because the characterized thickness of the sample is determined by the space-charge region width. However, the measurement times become inordinately long for high quality layers when pulsed MOS capacitors are used as test structures. Times of hundreds or even thousands of seconds per measurement are not uncommon. We present two methods to reduce this measurement time: optical excitation and elevated temperature. Optical excitation can only be used if the light is pulsed and the lifetime is measured from the remaining "dark" curve. Steady-state light leads to erroneous lifetimes and should not be used. Elevated temperature measurements lead to temperature-varying lifetimes, because the generation lifetime may be temperature dependent.