TOPOGRAPHY MEASUREMENTS OF THE CRITICAL THICKNESS OF ZNSE GROWN ON GAAS

Synchrotron-based x-ray topography (XRT) measurements have been used t o study the initial stages of relaxation in ZnSe layers grown by molec ular beam epitaxy on vertical gradient freeze Bridgman GaAs substrates . The formation of the very first strain-relieving misfit dislocations in the grown ZnSe layers has been detected in a layer of thickness 10 0 nm. No such dislocations have been observed in a corresponding layer of 95 nm thickness. The critical thickness for this material system i s therefore estimated to be 97.5+/-2.5 nm, which is markedly lower tha n the widely accepted value of 150 nm. In contrast to the InGaAs/GaAs system, combined XRT and transmission electron microscopy studies indi cate that the initial misfit dislocations observed for ZnSe/GaAs are n ot, in general, formed by the bending over of pre-existing threading d islocations into the interface, but by other mechanisms such as stacki ng fault decomposition. The critical thickness data obtained have been used to infer the maximum critical thickness of CdZnSe quantum wells possible in LI-VI laser diodes. (C) 1998 American Institute of Physics .