Penetration depth of different laser wavelength in silicon

In silicon laser light with different wavelength has different penetration depth, hence the right laser should be used for different applications, e.g. for epitaxial layers or investigations of the surface smaller wavelength are ideal. An approximation of the penetration depth is given in the diagram here.

The microwave detected photoconductivity measures the photoconductivity after the irradiation of the sample with light. Usually the light should have an energy that is higher than the bandgap, so that electrons and holes are created. For silicon this means that a wavelength smaller than 1100 nm should be used. If the surface should be investigated or thin epitaxial layers, it might be useful to use even UV or blue light, which has a much smaller penetration depth in silicon. Figure 1 shows the penetration depth in silicon versus the wavelength and gives the user a hint, which wavelength is most useful for his application.