Effect of dangling bonds of ultra-thin silicon film surface on electronic states of internal atoms

We investigate how dangling bonds at the surface of ultra-thin films affect electronic states inside the film by first principles calculation. In the calculation models, dangling bonds at the surface are directly treated, and the impact on the electronic states of the internal atoms was estimated. Models with a H-terminated surface at both sides have no state in the bandgap. Whereas, new states appear at around the midgap by removing terminated H at surfaces of one or both sides. These mid-gap states appear at all layers, the states of which decrease as the layer moves away from the surface with dangling bonds. The sum of local DOS corresponds to the number of dangling bonds of the model. If the activation rate is assumed as 2.0 × 10-5, which is an ordinary value of thermal oxide passivation on Si (1 0 0) surface, volume concentration and surface concentration at the 18th layer from the surface in a 36-layer model are estimated to be 1.2 × 1014 cm-3 and 1.5 × 109 cm-2, respectively. These numbers are comparable to the values, especially the dopant volume concentration of Si substrate used in current VLSI technology (∼1015 cm-3). Therefore, the midgap states inside ultra-thin films may degrade performance of the FinFETs.