A contactless method for determination of carrier lifetime, surface recombination velocity, and diffusion constant in semiconductors

Abstract

A new optical, contactless method to determine carrier recombination lifetime, surface recombination velocity, and diffusion constant at high-level injection is described. In a measurement, a sample of a few mm3 volume and a thickness above 1.5 diffusion lengths is homogeneously illuminated by a band-gap light pulse, and the subsequent decay of the generated carriers is monitored by infrared absorption. Since the data extracted from a measurement represent averages over the whole sample, the technique is especially attractive for vertical power devices or solar cells, or also for process control using test wafers. In silicon, lifetimes between 0.5 and 1000 μs and surface recombination velocities between a few and 105 cm/s can easily be determined. In the first part, the paper gives the one-dimensional solution of the carrier diffusion problem associated with the experiment. This solution is of wider use, however, e.g., for recessed gate structures and power devices in general. In the second part, results for silicon with various surface preparation procedures are given. For lapped samples, an ‘‘electrical penetration depth’’ of the damage is found to be about 1.5 times the lapping grain diameter. Sodium dichromate is found to reduce an otherwise infinite surface recombination velocity to 245 cm/s in p-type and to 2000 cm/s in n-type silicon.