Silicon Nanomembranes
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Introduction Nanotechnology is almost completely defined by the structures that materials scientists, chemists, and physicists can create. The objects of desire of these researchers have led from buckyballs and quantum dots grown on surfaces to nanotubes, rods, wires, and most recently to strain-engineered nanomembranes. Modern wafer bonding and epitaxial growth techniques have made this latest advance possible. Any material that can be fabricated as a single crystal on a release layer is a candidate nanomembrane. An obvious one is silicon-on-insulator (SOI). SOI provides, beyond its application in the Si device industry,1 a magnificent toolbox for exploring the novel science and technological potential inherent in this class of nanomaterial. In SOI, a SiO2 layer is interspersed between a thin crystalline top Si layer and the bottom Si wafer; the ability to etch this buried oxide selectively creates the membranes. Furthermore, by careful heteroepitaxial growth techniques, lattice strain can be introduced into the layered structure. Strain in Si modifies many properties, including piezoresistivity, charge carrier mobility, atomic transport and defect structure, and the self-assembly of quantum dots. When released from the oxide, this layered structure can form extremely flexible strain-engineered thin nanomembranes with thicknesses from several hundred nanometers to less than 10 nm, and in various shapes (including sheets, tubes, spirals, and ribbons), depending
on how one defines a pattern before release.2–4 The novelty, therefore, of these Si nanomembranes (and by extension, the many other materials that can be grown heteroepitaxially and released from a bulk substrate, including other compound semiconductors, functional oxides such as piezoelectrics and ferroelectrics, and even metals) is several-fold: they are highly strained, they are flexible, they are transferable to many other hosts and bond easily to most, and they can be made to take on a large range of shapes by engineering the strain and the geometry. Properly prepared, they retain the perfect-singlecrystal, dislocation-free nature of the original Si substrate, even when all the layers are significantly strained. They also have a very high interface-tovolume ratio, with surfaces or interfaces potentially being very important contributors to unique membrane properties.5 High-speed complementary metal oxide semiconductor (CMOS) transistor electronics (the basis of all our computers) can be built on one or both sides of a Si nanomembrane. Transferred to flexible polymer hosts, they create exceptional potential for high-performance flexible electronics,6,7 suitable for a number of applications, including solar cells, smart cards, optical displays, and rf tags. Si nanomembranes also represent a new and exciting direction for nanoelectromechanical systems (NEMS). The
MRS BULLETIN • VOLUME 32 • JANUARY 2007 • www.mrs.org/bulletin
electrical conductivity can be increased by means of strain, which increases the mobility of charge carriers; a multilayer structure
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