Evidence for strain relaxation via composition fluctuations in strained quaternary / quaternary and quaternary / ternary multiple quantum well structures

The occurence of large-scale contrast modulations is discussed, which have been observed in the transmission electron microscope in cross-sectional samples containing Ga_xIn_1-xAs_yP_1-y with a number of different x and y values. 90°-wedge samples were used because of their known geometry, in which thin-foil contrast effects can be excluded. The samples exhibiting large-scale modulations of a period greater than 100 nm all contained quaternary / quaternary or quaternary / ternary multiple quantum well stacks. Indications for a correlation between the onset, strength and frequency of the modulations with the well/barrier composition and its proximity to the centre of the miscibility gap are presented. The contrast is also found to be influenced by the stack thickness and strain. In large stacks, the modulations are related to waviness in epi-layer growth. The contrast is discussed in terms of composition modulation patterns connected with strain fluctuations, which are set up in interfacial regions and which evolve into wavy layer growth as means of misfit strain relief.