CONTROLLING THE SIZE, STRUCTURE AND ORIENTATION OF SEMICONDUCTOR NANOCRYSTALS USING METASTABLE PHASE RECRYSTALLIZATION

Materials engineering at the nanometre scale should provide smaller te chnological devices than are currently available(1,2). In particular, research on semiconductor nanostructures with size-dependent optical a nd electronic properties is motivated by potential applications which include quantum-dot lasers and high-speed nonlinear optical switches(3 ,4). Here we describe an approach for controlling the size, orientatio n and lattice structure of semiconductor nanocrystals embedded in a tr ansparent matrix. We form nanocrystalline precipitates by implanting i ons of the semiconductor into a single-crystal alumina substrate and a pplying thermal annealing(5-7). Control over the microstructure of the nanocrystals is achieved using substrate amorphization and recrystall ization. In essence, the substrate microstructure is manipulated using ion beams to induce changes in impurity solubility, crystal symmetry and cation bonding, which exert a profound influence on the microstruc ture of the embedded precipitates-a concept familiar in metallurgy(8). This approach can be extended to exercise control over virtually any type of precipitate (such as metals, insulators or magnetic clusters) as well as epitaxial thin films.