SrTiO3electroreflectance: Flatband-electrolyte and Schottky-barrier measurements

Abstract

The results of electroreflectance (ER) measurements from approximately 2 to 5 eV are reported for SrTi${\mathrm{O}}_{3}$. Both the electrolyte technique and the metal-semiconductor (MS) configuration were used. The room-temperature-electrolyte ER show strong broad line shapes characteristic of the perovskite oxides and in agreement with other modulation spectra. The flatband condition has been determined and the effect of moving off flatband has been studied. In addition to the usual $\frac{\ensuremath{\Delta}R}{R}$ data, a Kramers-Kronig analysis has been performed and the line shapes are reported in terms of $\ensuremath{\Delta}{\ensuremath{\epsilon}}_{1}$ and $\ensuremath{\Delta}{\ensuremath{\epsilon}}_{2}$. The strong 3.81 eV structure in $\ensuremath{\Delta}{\ensuremath{\epsilon}}_{2}$ is associated with a critical region along $\ensuremath{\Delta}$ rather than any one critical point. The following studies were undertaken in attempts to sharpen the rather broad room-temperature line shapes in order to seek additional structure to aid in the classification of the optical transitions. The low-temperature MS-configuration line shapes were taken from 300 to 53 K. The structure was sharpened at low temperatures but no additional structure was found. The low-temperature polarized spectra for (110) faced samples were studied. The line shapes indicated polarization dependence but no additional structure. The low-temperature MS-configuration ER was taken on pure insulating SrTi${\mathrm{O}}_{3}$ to reduce impurity broadening. This uncovered a shoulder at \ensuremath{\sim}3.4 eV in $\frac{\ensuremath{\Delta}R}{R}$. The temperature dependence of several peaks were individually monitored. Each showed an individual different temperature dependence suggesting that each represents independent transitions. No change was detectable in passing through the cubic to tetragonal second-order phase transition near 110 K.