Non-contacting determination of carrier lifetime and surface recombination velocity using photothermal radiometry

When generating thermal waves in semiconductors the intensity modulated laser beam simultaneously generates a modulation in the free carrier density provided the photon energy is greater than the carrier excitation energy. This carrier modulation is dependent upon the laser power and the electronic properties of the material in the vicinity of the laser beam. If an IR detector is focused to the same point on the sample it is possible to measure the IR emission from the excited region which is distinct from photoluminescence. It can be shown that this radiation is proportional to the optically induced carrier density and the amplitude of the generated thermal wave. The signal dependence upon carrier density provides an alternative contrast mechanism for photothermal radiometry (PTR) which relies on carrier recombination and diffusion. We present a method of determining carrier lifetime and surface recombination velocity using PTR. Theoretical modelling of the PTR effect in semiconductors is discussed in the context of our measurements. Both one- and three-dimensional cases are considered and compared with experimental result for silicon. The accuracy of the method as a function of experimental and material parameters is discussed.