Infrared tomography of the charge-carrier lifetime and diffusion length in semiconductor-grade silicon ingots

Abstract

A nondestructive method for measuring the three-dimensional distribution of the charge-carrier lifetime and diffusion length in silicon ingots with a length of up to 1 m and a diameter as large as 0.3 m is presented. Physically, this method is based on infrared crossed-beam probing of an ingot with polished surface areas. One of the beams is repetitively pulsed, has a wavelength of 1.15–1.28 µm, and generates excess charge carriers in a rodlike zone along the beam trajectory in the ingot. Other beams are continuous and have longer wavelengths; these beams detect the temporal and spatial kinetics of excess charge carriers in a small portion of the rodlike zone and in the vicinity of it (the free-carrier absorption is measured). By virtue of the fact that the investigated zone is at a distance from the ingot surface, there is no need to consider the surface recombination. The capabilities of the method are demonstrated for an ingot with known spatial nonuniformity of the charge-carrier lifetime. Spatial resolution amounting to several millimeters was attained.