Influence Of Bulk And Surface Properties On Measurable Steady-State Carrier-Lifetime

Minority carrier lifetime measurements are a major characterization technique regarding the material quality of semiconductors. In particular, a large variety of electrically active defects can be detected at room temperature even if present in low concentrations down to 1010cm-3 only. Transient and (quasi) steady-state methods as well as combinations of both have been established. However, in either case, the measurable lifetime is influenced not only by the bulk lifetime itself but by a number of additional sample properties such as surface recombination, sample thickness, and wavelength of the carrier-generating light.An analytical model is investigated that relates the measurable lifetime to the bulk lifetime for samples of arbitrary thickness and surface quality when measured under steady-state conditions. It thus provides a generalization for the frequently used limiting expressions for thin Si-wafers and thick Si-blocks. In particular it allows the interpretation of data obtained for samples of intermediate thickness. Furthermore, it is shown under which conditions the surface properties can influence the lifetime measurements even for thick blocks. Finally, an approach is proposed that allows the simultaneous extraction of both bulk lifetime and surface recombination velocity. It is argued that reliable measurements of bulk properties are possible only for samples being thicker than some critical thickness while surface properties are best determined on samples thinner than this critical thickness. A quantitative estimate for this critical thickness is given. Experimental results are shown that demonstrate the applicability of the model.