SPONTANEOUS LATERAL COMPOSITION MODULATION IN ALAS INAS SHORT-PERIOD SUPERLATTICES VIA THE GROWTH FRONT/

The spontaneous formation of lateral composition modulation in AlAs/In As short period superlattices on InP (001) substrates has been investi gated. Transmission electron microscopy and x-ray diffraction reciproc al space mapping show that the lateral modulation is very regular, wit h a periodicity along the [110] direction on the order of 180 Angstrom . A surprising result is that this material system also exhibits a lat eral modulation along the \1 (1) over bar 0\ direction, with a periodi city of 330 Angstrom. Reflection high energy electron diffraction perf ormed during the deposition revealed that the reconstruction changed f rom (2 x 1) during the InAs deposition cycle to (1 x 2) during the AlA s cycle, which may be related to the presence of the modulation in bot h <110> directions, High magnification transmission electron micrograp hs show that the surface is undulated and that these undulations corre late spatially with composition modulation. Detailed analysis of the i mages shows that the contrast observed is indeed due to composition mo dulation. Photoluminescence from the modulated layer is strongly polar ized and red-shifted by 220 meV.