EFFECTS OF LATTICE MISMATCH DUE TO PARTIALLY RELAXED BUFFER LAYERS ININGAAS ALGAAS STRAIN BALANCED QUANTUM-WELL MODULATORS/

This letter describes the performance of electro-absorption optical mo dulator structures, based on strain-balanced InGaAs/AlGaAs p-i-n multi ple quantum wells (MQWs), It specifically considers the influence of l attice mismatch with the graded InGaAlAs buffer arising from unrelaxed material forming the substrate upon which the p-i-n structure is grow n, We have shown that to achieve a lattice constant corresponding to t he average for the MQW region, it is necessary to grade the buffer up to a higher indium concentration than the average indium concentration of the MQW. We have found that mismatched samples exhibit poorer modu lation on the long wavelength side of the exciton peak as a result of a greater reduction in the exciton peak with applied field, and we att ribute this to screening and nonuniform fields resulting from the ioni zation of defects at high fields. We have also shown that the similar effect induced by going to lower barrier potentials is relatively less important, (C) 1997 American Institute of Physics.