Schottky Contacs on Silicon Nanowires

Licentiate thesis, 2007

This thesis demonstrates the effect of charge on Schottky barrier height for metal contacts at the end surfaces of silicon nanowires. It is shown, by measurements and analytical models, how the effective electron barrier is lowered by a positive charge introduced into an oxide embedding the wire. As the scaling of MOSFET devices continue, the increasing contact resistance in source and drain have proven to be a difficult problem to overcome by conventional methods. A feasible option is to use metal electrodes forming Schottky contacts to the channel region. The main requirement for such a contact will be to achieve a very low energy barrier. Many different materials and methods have been studied in order to achieve a lower barrier but there is no optimal solution. The fabrication of a silicon nanowire device with palladium silicide Schottky contacts at the end surfaces of the wire is described. The Schottky contacts function as source and drain electrodes and the backside of an SOI wafer carrying the sample structures is used as a gate to achieve transistor behavior. Patterning of the nanowire is done by E-beam lithography and etching. The silicon core of the wires is surrounded by a thermally grown oxide. By measuring current vs. voltage at different temperatures the effective barrier heights of the Schottky contacts are extracted. Positive point charge is introduced into the oxide surrounding the wire by UV radiation. This charge changes the effective barrier height as demonstrated by measurements and theory. An electrostatic model is derived demonstrating the effect of charge on the total potential in the wire and in particular close to the contacts. It is demonstrated how the effective Schottky barrier for electrons is lowered due to the positive charge as an effect of increased tunneling while the hole barrier is correspondingly increasing.