Electroabsorption in semi-insulating materials

Abstract

The electroabsorption properties of a number of semi-insulating materials including indium phosphide, gallium arsenide and Langmuir/Blodgett thin films of anthracene are reported. Single crystals of InP:Cr, InP:Fe and GaAs:Cr were subjected to large electric fields at a variety of temperatures and the resulting small changes upon the optical absorption spectra were carefully recorded. Prior to the performance of the electroabsorption measurements, the specimens were examined using such techniques as ac. and dc. conductivity, photoconductivity and zero-field optical absorption. Electroabsorption measurements were performed as a function both of the frequency and magnitude of the applied electric field. In addition, the presence of odd harmonics in the electroabsorption signal was examined. A correlation between the existence of frequency dependent effects and the specimens' photoconductivity was found. The electroabsorption spectra were analyzed within the context of the one-electron Franz- Keldysh theory, thus allowing the reduced effective masses of InP:Fe, InP:Cr and GaAs:Cr to be calculated. These values are in good agreement with published data for pure material. The lightly substituted anthracene Langmuir films were also examined using ac. and dc. conductivity techniques. In addition, data are reported concerning the fluorescent, electroluminescent and photo- conductive properties of the specimens. The electroabsorption spectra for these materials is interpreted in terms of the quadratic Stark effect. An analysis of these data leads to a value of 36 A for the average static polarizability change of a C4 anthracene film in the vicinity of the first singlet state absorption.