Electron tunneling from the edge of thin single-crystal Si layers through SiO2 film

To investigate the electron tunneling from the nanometer-scale Si, we fabricated tunnel devices with an extremely thin Si-on-insulator layer as a cathode. The devices consisted of a 5-nm-thick Si layer sandwiched between thick SiO2 films, a thin (≊2 nm) SiO2 film (an edge oxide) terminating the Si layer on one side, and a polycrystalline Si (poly-Si) electrode covering the edge oxide. Using this Si/edge-oxide/poly-Si tunnel device, we measured the tunneling current and found that when the poly-Si electrode was positively biased with respect to the Si layer, the current versus voltage characteristics exhibited a steplike feature at 42 K. The current decreased with increasing edge-oxide thickness, indicating that the tunneling at the edge limited the current. The step structure was never observed in the current directly flowing into the poly-Si electrode for devices without the edge oxide. These results indicate that the observed steps reflect the electronic nature of the electron system at the edge of the Si layer. Analysis of the substrate-bias dependence of the tunneling current strongly suggests that electrons are localized at the edge of the Si layer because of the band bending. The effect of one-dimensional subbands on the present tunneling properties is discussed as an origin of the observed steps. Although these steps become less distinct at higher temperatures, they are evident even above 200 K. This indicates that the strong electron confinement occurs at the edge.