EVIDENCE FOR STRAIN RELAXATION VIA COMPOSITION FLUCTUATIONS IN STRAINED QUATERNARY QUATERNARY AND QUATERNARY TERNARY MULTIPLE-QUANTUM-WELL STRUCTURES

The occurrence of large-scale contrast modulations is discussed, which have been observed in the transmission electron microscope incross-se ctional samples containing GaxIn1-xAsyP1-y with a number of different x and y values. 90-degrees-wedge samples were used because of their kn own geometry, in which thin-foil contrast effects can be excluded. The samples exhibiting large-scale modulations of a period greater than 1 00 nm all contained quaternary/quaternary or quaternary/ternary multip le quantum well stacks. Indications for a correlation between the onse t, strength and frequency of the modulations with the well/barrier com position and its proximity to the centre of the miscibility gap are pr esented. The contrast is also found to be influenced by the stack thic kness and strain. In large stacks, the modulations are related to wavi ness in epi-layer growth. The contrast is discussed in terms of compos ition modulation patterns connected with strain fluctuations, which ar e set up in interfacial regions and which evolve into wavy layer growt h as means of misfit strain relief.