THE CONTROL AND EVALUATION OF BLUE-SHIFT IN GAINAS GAINASP MULTIPLE-QUANTUM-WELL STRUCTURES FOR INTEGRATED LASERS AND STARK-EFFECT MODULATORS/

We report on the structural and optical characterization of nominally lattice-matched GaInAs/GaInAsP multiple quantum well (MQW) structures grown on (100) InP substrates by metalorganic chemical vapour depositi on (MOCVD) which undergo a 'blue shift' in photoluminescence upon ther mal annealing. Electron microscope and magneto-optical analyses show t hat the shifts are principally due to layer interdiffusion, which resu lts in a change in composition of the well centres. These compositiona l variations are quantitatively measured by high-resolution analytical electron microscopy. This analysis demonstrates that the diffusion of the group V elements in the undoped MQWS is faster than that of the g roup III elements, resulting in the incorporation of excess coherency strain in the material. Analysis of a number of samples grown on a var iety of substrates shows that the wavelength shift is particularly lar ge when the substrates are S doped, although the substrate dopant does not participate directly in the diffusion mechanism. We attribute thi s behaviour to the typically low dislocation density of S-doped substr ates. We report, for the first time, a direct measurement of the corre lation between the spatial variation in the magnitude of the blue shif t and the presence of dislocations in the MQWS. On the other hand, the incorporation of Zn as a dopant in the MQW region reduces the extent to which the blue shift occurs. A model is proposed which explains how the presence of dislocations, as well as the substrate and MQW doping , could indirectly influence the extent of the layer interdiffusion.