Lifetime determination of epitaxial silicon thin-film layers

In the PV as well as the microelectronic industry there are a lot of applications, were thin epitaxial layers are used. Because of that it is necessary to control the quality of the epitaxial layers with contact less and non-destructive methods to determine the minority carrier lifetime.

Fig. 1: recombination site, which contribute to the measured lifetime
Fig. 2: measured lifetime of samples with different substrate qualities and epitaxial layer thicknesses

With MDP it is possible to measure the lifetime of minority carriers and the photoconductivity in epitaxial layers as fast and exactly as possible with a high resolution.

The measurement of epitaxial layers and the determination of their quality is a huge challenge for lifetime measuring methods, since the signal intensity is very small and the results are very difficult to interpret due to the many recombination sites (fig. 1).

In the MDPmap and MDPingot equipment it is possible to integrate up to 4 lasers with different wavelength and hence different penetration depth. Furthermore it is possible to measure with different pulse length from a very short pulse of only 100 ns, where no carrier diffusion takes place to a pulse length of several ms, where the carriers diffuse into the sample depth. Hence by varying the laser wavelength and the pulse length, it is possible to measure with different penetration depth and deduce from the lifetime results, which recombination site is the most dominant one. Figure 2 shows an example where a sample series with different substrate qualities and epitaxial layer thicknesses were measured, demonstrating the dependence of the measured lifetime from the substrate, interface quality and surface quality.

For more information about the determination of lifetime of epitaxial layers read:

[1] D. Walter, P. Rosenits, F. Kopp, B. Berger, K. Dornich, W. Warta: “Determining the epitaxial carrier lifetime by microwave-detected photoconductance measurements”, Proc. Of the 25th EU PVSEC, submitted (2010)

[2] K. Dornich, T. Hahn, J.R. Niklas: “Non-destructive electrical characterization and topography of silicon wafers and epitaxial layers”, Mater. Res. Soc. Symp. Proc. Vol. 864, 549-554 (2005)