Self-assembled InAs/InGaAsP/InP(001) nanostructures are investigated using cross-sectional scanning tunneling microscopy. Atomically resolved images at both the (110) and the (110) cleavage surface show InAs quantum dashes with almost binary composition and a truncated pyramidal shape. The quaternary matrix material directly above the InP substrate already shows a tendency toward decomposition, which gradually increases along the [001] growth direction, in particular above quantum dash layers. This decomposition, in turn, leads to an enhanced vertical correlation in the nucleation of further quantum dash layers

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Cross-sectional scanning tunneling microscopy is used to study the spatial structure and composition of self-assembled InAs nanostructures grown on InGaAsP lattice matched to the InP substrate. Images of the (110) and ((1) over bar 10) cleavage surfaces reveal InAs quantum dashes of different lateral extensions. They are found to be about 60 nm long, about 15 nm wide, about 2 nm high, and to consist of pure InAs. Furthermore, the quaternary InGaAsP matrix material below, in between, and above the quantum-dash layers shows a strong lateral contrast variation, which is related to a partial decomposition into columns of more InAs-rich and more GaP-rich regions. The effect is particularly pronounced along the [110] direction. A quantitative analysis of this strain-induced contrast yields a decomposition characterized by variations of the group-III and/or group-V concentrations in the order of +/- 10%. The data strongly indicate that the strain at the growth surface induced by the decomposition of the underlying matrix material plays an important role for the nucleation and formation of the quantum dashes as well as for their unexpected stacking over interlayer distances as large as 40 nm. Despite of the observation that the quantum dashes enforce the decomposition, which was already developed directly at the InGaAsP/InP interface without any influence of the subsequently grown InAs quantum dashes