STRUCTURAL CHARACTERIZATION OF GAAS GAP SUPERLATTICES/

Powder and double-crystal x-ray diffraction were used to study the str uctural properties of highly strained (GaAs)N/(GaP)M short-period supe rlattices grown on GaAs (001) substrates. In spite of the large lattic e mismatch (f = 3.6%) between GaAs and GaP and the competition for inc orporation between As and P high-quality short-period superlattices of GaAs/GaP have been grown by a development of conventional molecular b eam epitaxy named atomic layer molecular beam epitaxy. The in-plane la ttice parameter (a(parallel-to)) of the different superlattices was me asured and studied as a function of the GaP content. We found that, fo r a given total superlattice thickness of 4000 angstrom, the critical lattice mismatch is f(c) congruent-to 0.5% (corresponding to an averag e GaP content of 13.6% in the superlattice). This means that for an av erage misfit or lattice mismatch above f(c) the superlattice starts to relax. This experimental result is compared with predictions of criti cal thickness theories based on energy criteria. A dear relation of th e degree of relaxation with peak width of the superlattice zeroth-orde r diffraction peak is found. High-resolution transmission electron mic roscopy has been performed to analyse the type of dislocations that re lax the mismatched layers.