Precise Optical Measurement of Carrier Mobilities Using Z-scanning Laser Photoreflectance

Abstract

A simple yet precise optical technique for measuring the ambipolar carrier mobility in semiconductors is presented. Using tightly focused Gaussian laser beams in a photo-reflectance system, the modulated reflectance signal is measured as a function of the Z (longitudinal) displacement of the sample from focus. The modulated component of the reflected probe beam is a Gaussian beam with its profile determined by the focal parameters and the complex diffusion length. The reflected probe beam is collected and input to the detector, thereby integrating over the radial profile of the beam. This results in analytic expressions for the Z dependence of the signal in terms of diffusion length and recombination lifetime. Best fit values for the diffusion length and recombination lifetime are obtained via an iterative fitting procedure. The output diffusion lengths and recombination lifetimes and their estimated uncertainties are combined according to the Einstein relation to yield the mobility and its uncertainty.