Spatially inhomogeneous carrier concentration dependence of the reflectivity of semiconductors

Abstract

The excite-and-probe technique was used to study the optically induced charge carrier concentration dependence of the reflectivity of semiconductors, both experimentally and theoretically, in the case of an exponential carrier distribution. The second harmonic of an Nd : YAG laser (pulse duration 25 ps) was used as the exciting beam; the fundamental wave served as the probe beam. At a carrier concentration of about 2.5×10²¹ cm⁻³ a minimum reflectivity was obtained if the angle of incidence was greater than 40°. For increasing absorption constants of the semiconductor at the exciting frequency, this minimum value of the reflectivity was shown to increase. The dependence of the reflectivity on the exponentially distributed carrier concentration was studied experimentally for thin amorphous silicon films produced by the glow-discharge technique. For an absorption constant of 6×10⁵ cm⁻¹ at 532 nm, good agreement was found between the numerical calculations and the experimental results.