Microscopic electronic and structural analysis of femtosecond laser sulfur hyperdoped silicon

Sulfur hyperdoping by fs laser irradiation is a promising process for improving absorption properties of crystalline silicon which indicates its suitability for manufacturing intermediate band silicon solar cells. Here, first results of a novel approach to study the electrical and structural properties of sulfur hyperdoped regions of p-type silicon substrates are presented. In addition to electron beam induced current (EBIC) and transmission electron microscopy (TEM) in cross-section geometry, shallow-angle bevel polishing is used to employ surface sensitive techniques, such as atomic force microscopy (AFM) and Kelvin force microscopy (KFM). The latter reveals high lateral electric fields in regions beneath the crests of the surface topography, while EBIC shows an enhanced recombination activity at the location of the voltage drops. Both these effects are related to extended defects revealed by TEM and a possible lateral variation of sulfur concentrations. AFM image (top) and KFM map (bottom) showing a voltage drop on the surface of a terrace produced by bevel polishing of sulfur hyperdoped silicon.