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The morphological evolution of a GaAs surface induced by a focused ion beam (FIB) has been investigated by in situ electron microscopy. Under off-normal bombardment without sample rotation, Ga droplets with sizes from 70 to 25 nm in diameter on the GaAs surface can self-assemble into a highly ordered hexagonal pattern instead of Ostwald ripening or coalescence. The mechanism relies on a balance between anisotropic loss of atoms on the surface of droplets due to sputtering and an anisotropic supply of atoms on the substrate surface due to preferential sputtering of As. The ratio of wavelength to the droplet diameter predicted by this model is in excellent agreement with experimental observations.

Transparent MgAl2O4 spinel nanoceramics have been sintered at relatively low-temperature (500–700°C) under high pressure (2–5GPa) using a hydrostatic press with high-temperature-calcined nanopowders. The morphology, nanostructure, optical property, and density of ceramics were analyzed by scanning electron microscopy, transmission electron microscopy, UV-VIS-IR transmission spectrum, and Archimedes draining method. The average grain size (<100nm for all samples sintered) depends on the sintering pressure and temperature. The nanoceramics are highly transparent even though their relative densities are all less than 99%, due to the low or negligible light scattering from the nanosized grains and pores.

Low-energy-ion bombardment of semiconductors can lead to the development of complex and diverse nanostructures. Of particular interest in these structured surfaces is the formation of highly ordered patterns whose optical, electronic, and magnetic properties are different from those of bulk materials and might find technological uses. [1][2][3][4][5] Compared to the low efficiency of lithographic methods for mass production, this self-organized approach offers a new route for fabrication of ordered patterns over large areas in a short processing time on the nanometer scale, beyond the limits of lithography. [1,4] This technique is based on the morphological instability of a sputtered surface driven by a kinetic balance between roughening and smoothing. [6,7] Thus mechanisms that control the species concentration on the surface can make contributions to structure formation. [3,[7][8][9][10][11][12] It is now established that well-ordered quantum dots can be generated on the surface of semiconductors (Si, Ge, GaSb) under certain irradiation conditions. [1,13,14] For a long time it has been expected that the instability of a surface can also lead to well-ordered hole formation. However, to date experimental observation of such features has been lacking. In this Communication, we report that a hexagonally ordered, honeycomb-like structure of holes 35 nm across and 45 nm apart on the Ge surface can be formed under focused ion beam (FIB) bombardment at normal incidence. The structured Ge fabricated by FIB bombardment shows a high surface area and a considerably blue-shifted energy gap. We found that interplay between ion sputtering, redeposition, viscous flow, and surface diffusion is responsible for ordered pattern formation. Simulations of the evolution of the surface morphology on the basis of the damped Kuramoto-Sivashinsky (DKS) growth model have been performed to facilitate the interpretation of the experimental findings. [15][16][17][18][19] As an indirect energy-gap semiconductor, germanium is a poor light emitter, which makes it challenging to create efficient Ge-based light-emitting devices. Significant effort has been devoted to the development of the optical properties of Ge based on changing the surface morphology.[20] In the work reported here, we focused on the use of ion beam radiation to fabricate nanostructures on the Ge surface.The ion-induced nanostructures were fabricated on commercially available Ge with (100) orientation by FIB bombardment. Under normal bombardment with ion energy greater than 5 keV, worm-like structures were developed on Ge surface with large aspect ratio. When the energy was 5 keV, however, highly ordered hole arrays could be achieved. Figure 1 shows scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of a typical nanohole pattern induced on a Ge(100) surface by 5 keV (Ga þ ) FIB bombardment for 5 min. A perfect hexagonal arrangement of holes is observed within domains of ca. 500 nm. Like polycrystalline structure, there are ''grain boundaries'' separati...

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An analytical formula is developed for the evolution of angular dependence of sputtering yields by extending the theory of sputtering yield proposed by Sigmund. We demonstrate that the peak of sputtering yield at oblique incidence can be attributed to a balance between the increased energy deposited on the surface by incident ion which enhances the sputtering yield and the decreased depth travelled by recoil atom which reduces the sputtering yield. The predicted dependence of sputtering yield on the incident angle is in good agreement with experimental observations.

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