Formation of coherent Ge shallow dome islands on Si(001) by ultra-high-vacuum ion beam sputter deposition

Self-assembled Ge coherent islands, with a monolayer wetting layer, are formed on silicon(001) substrates using a high ion energy of 2.25 keV with ultra-high-vacuum ion beam sputter deposition. The growth evolution of the coherent islands is studied by both atomic force microscopy and transmission electron microscopy (TEM), and appears to differ from the conventional pyramid-to-dome transition. A new shaped island, surrounded by {310} and {510} facets with lower aspect ratio, is found without trench formation and alloying, and the aspect ratio is determined by the area ratio of these two facets. At 500 degrees C, the shape of the islands is transformed from a small dome of low aspect ratio to a large dome of high aspect ratio. The largest shallow dome islands can only relax about 4% of the lattice mismatch from the TEM strain measurements based on two-beam dark-field imaging.