A formation process for semiconductor quantum dots based on a surface instability induced by ion sputtering under normal incidence is presented. Crystalline dots 35 nanometers in diameter and arranged in a regular hexagonal lattice were produced on gallium antimonide surfaces. The formation mechanism relies on a natural self-organization mechanism that occurs during the erosion of surfaces, which is based on the interplay between roughening induced by ion sputtering and smoothing due to surface diffusion.To date two approaches for the fabrication of semiconductor quantum dots have been pursued. In the top-down approach, lithographic methods are used for direct patterning of quantum dots, whereas the bottom-up approach relies on self-organized processes. In contrast to serial electron-beam lithography, self-organization phenomena open the way for the formation of a regular array of quantum dots on large areas in a single technological process step. Self-organized semiconductor quantum dots have been produced by the Stranski-Krastanow growth mode in molecular beam epitaxy and metal-organic vapor phase epitaxy, in which coherent island formation occurs during the growth of lattice-mismatched semiconductors (1). Here we present a controlled and costeffective method for the production of wellordered quantum dots by ion bombardment of semiconductor surfaces (2) that is based on a self-organization mechanism induced by ion sputtering of solid surfaces, where the formation kinetics is determined by etching instead of growth.There has been great effort to interpret the microscopic dynamics of surface roughness and pattern formation induced by ion sputtering in which the formation of coherent ripples has been observed on metal, semiconductor, and insulator surfaces under ion bombardment at off-normal angles of incidence (3-7). The characteristic period in the submicrometer to nanometer range is defined by the sputtering conditions (for example ion energy, ion flux, and substrate temperature) and by the material properties. An explanation of the underlying mechanism was proposed by Bradley and Harper (8) in which the sputtering yield, which is the number of surface atoms removed per incident ion, depends on the surface curvature. Under certain conditions this dependence gives rise to a surface instability where the erosion is greater in a depression than on an elevation. The ion-induced surface instability can be described by a specific term in the erosion equation that is proportional to the negative Laplacian of the surface. The proportionality factor is called negative surface tension because it tends to maximize the surface, in contrast to surface tension that minimizes the surface. It is the competition between this roughening instability and diffusive smoothing mechanisms that governs the buildup of a regular pattern with a characteristic wavelength. Under off-normal incident ions the instability is anisotropic, giving rise to characteristic ripple patterns. Their direction was found to be either parallel or perpen...

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Amorphous and crystalline phases of Ge 2 Sb 2 Te 5 films are investigated by coherent phonon spectroscopy. By heating amorphous films above specific temperatures, the coherent phonon signatures exhibit pronounced changes due to the crystallization of the amorphous phase into a cubic lattice and the transition from the cubic to a hexagonal crystal structure. The phonon modes observed are identified by comparison with coherent phonon spectra of the binary Sb 2 Te 3 and GeTe constituents.