Automated null ellipsometer with rotating analyzer

An automated version of a spectroscopic ac null ellipsometer is described. By simultaneous variation of the angle of incidence and the polarizer azimuth, circular polarization of the reflected radiation can be obtained for any sample configuration. This state of polarization is checked by a rotating analyzer detector system with lock-in technique, thus achieving the highest precision that is theoretically possible. The experimental details including the minimization routine (simplex method) are presented. First experiences gained with dielectric function spectra of InP show the high reproducibility and accuracy of the arrangement. From numerically calculated second derivatives the energy and type of critical interband points are determined.