| Use of silicon-on-insulator (SOI), a thin single-crystal silicon layer on silicon-dioxide, is already pervasive in microelectronics and nanoelectromechanical systems. SOI promises, in fact, to become the platform for future high-speed electronics, as well as for a range of sensor technologies. When the Si layer is very thin, the assumption of an effectively infinite number of atoms determining its physical properties no longer applies, and unique electronic, structural, and thermodynamic phenomena arise.;SOI provides a potential new paradigm for surface studies, as these thin membranes become almost "all surface". For example, novel electronic properties emerge with decreasing membrane thickness, as surfaces and interfaces dominate bulk properties. We have investigated the conductivity of very thin (10nm) Si membranes bounded by one or two Si-SiO2 interfaces. Below a certain thickness, free carriers in the membrane will be completely trapped by the Si/SiO2 interface states. For a typical doping level of 1015 cm-3, the depletion thickness is of the order of 100 nm; in other words, a thin Si membrane bounded by two oxide layers will act like intrinsic Si. Using the van der Pauw method we have demonstrated that the conductivity of nanometer-scale SOI is vanishingly small. Therefore scanning tunneling microscopy (STM) on thin membranes should be impossible. On the contrary, we successfully image 10 nm thick Si, when the top native oxide is removed and a clean reconstructed Si (001) surface exposed. We show that electronic conduction in a thin Si membrane is determined not by its "bulk" dopants but by the thermal excitation at 300K of Si valence band charges to the surface band. The bulk dopant concentration is virtually irrelevant for electronic properties of Si nanomembranes. Conductivity in the membrane can be tailored by modifying the surface chemistry. We predict that both electrons and holes can be thermally generated, depending on the exact positions of the HOMO and LUMO bands, relative to the Si nanomembrane conduction and valence bands edges, of molecules adsorbed on the surface of Si. The addition of such layers may provide a practical approach to manufacture nanoscale sensors with high sensitivity and reliability based on electronic readout.;Beyond the novel electronic transport properties, nanometer-scale SOI shows rich thermal agglomeration phenomena. As the Si template layer thickness in SOI is reduced to tens of nanometers, it becomes thermally unstable, decomposing into well ordered silicon islands at high temperatures. (Abstract shortened by UMI.). |
