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The evolution of self-assembled InGaAs tandem nanostructures consisting a hole and pyramid is demonstrated using droplet epitaxy on various type-A high-index GaAs surfaces: (3 1 1)A (4 1 1)A, (5 1 1)A and (7 1 1)A. Under an identical fabrication condition depending on the index of surfaces, the resulting density and size of nanostructures are characteristic. The variation of density and size of nanostructures is explained with the relationship of the density of monolayer steps. An empirical model that describes the mechanism of self-assembled tandem nanostructures consisting a hole and pyramid is suggested as the concurrent occurrence of intermixing between droplets and substrate, dissolution of substrate and anisotropic surface diffusion.