We have studied the time constants involved in photoreflectance from several GaAs surface-intrinsic-n+ structures. The rise and fall times were determined from digital oscilloscope traces. We find that they depend on the intensity and wavelength of the pump and probe beams. The observed photoreflectance feature does not always follow a single exponential decay. The dependence of rise and fall times on intensity and wavelength of pump and probe beams can be accounted for by a theory based on majority-carrier flow. The characteristic time obtained can be used to determine the potential barrier height.
The use of multilayer analysis schemes for calculating resistivity profiles from spreading resistance measurements in semiconductors is well established. All of the techniques make use of the approximation that the current density at a perfectly conducting probe can in all cases be represented by the current density which would exist in a medium of unitorm resistivity. We present improved approximate current density profiles for the case of a probe in contact with a single layer over an infinite substrate for a wide range of layer thickness to probe radius ratios and layer resistivity to substrate resistivity ratios. Using these we find the errors incurred using the standard approximation to be limited to 10% when the layer thicknesses become smaller than 10% of the probe radius and less for larger thicknesses. We find-there is a large class of current density profiles for which the situation is no worse, which may be desirable for reasons other than accuracy.
The efficiency of multilayer analysis in calculating resistivity profiles from spreading resistance measurements depends on the rapid numerical evaluation of the well‐known correction factor integral first introduced by Schumann and Gardner. We present a new approximate form for the correction factor which allows its numerical evaluation with only 22 integrand values for each evaluation of the integral. When this technique is used in our multilayer analysis program, we unfold spreading resistance profiles at the real time rate of less than 3 sec/point on a desktop computer. Its results match those of analytic evaluations of special two layer cases or with more elaborate numerical evaluations of graded structures to within 1%.
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