On the Problem of Determining the Bulk Lifetime by Photoconductivity Decay on the Unpassivated Samples of Monocrystalline Silicon

In the indirect band gap semiconductors, and in particular, in silicon, the lifetime of nonequilibrium charge carriers is determined by recombination through impurity centers and it is inversely proportional to their concentration. Therefore, this is the most important parameter for determining the quality of the material. The noncontact methods of its measurement, in particular, the noncontact measurements of the decay of the photoconductivity (PC) constant, are the most important ones. The surface recombination strongly influences the shape of the curve. The calculation of the lifetime in the bulk by the decay constant remains relevant since there is no single-valued analytic solution of the continuity equation for this case. In the samples of monocrystalline silicon with unpassivated surfaces, the relaxation of PC is analyzed by numerical methods. The applicability of the known formulas for estimating the contribution of surface recombination to the effective PC relaxation time is discussed. It is shown that the period of time for which the fast exponents disappear depends on the relative thickness of the sample to be measured. The effective decay time is determined by the maximum value of the surface component of the relaxation time and is described by wellknown formulas only in this piece of the relaxation curve. The effective relaxation time attains saturation by the time when the signal intensity reaches 45% of the maximum value (the origin of the effective decay time according to the recommendation of the SEMI MF 1535 standard) only for the samples with a thickness of less than 3–5 diffusion lengths. For larger thicknesses, the contribution of the fast exponents to the effective PC relaxation time is observed right up to 5% of the maximum signal (i.e., until reaching the noise level of the signal to be measured). In this case using the recommended by SEMI MF 1535 standard formulas leads to a sufficiently large systematic error (up to 20%) in evaluating the free charge carrier’s lifetime.