Effect of alloying of Si with hydrogen on the electronic structure of spherical Si nanocrystals

Electronic states of spherical Si dots (of similar to1 and similar to2 nm in diameter) passivated by hydrogen are calculated by the extended Huckel-type nonorthogonal tight-binding method. The calculated results demonstrate that with strengthening the Si-H bonds the lowest unoccupied molecular orbital (LUMO) shifts up, whereas the highest occupied molecular orbital (HOMO) rises in energy contrary to an expectation based on the quantum confinement model. It is found that H passivation causes the effect of alloying of Si with hydrogen, inducing the Si s component as well as H contribution into the HOMO, and thereby reduces the size of the Si p-p interactions, downshifting the HOMO energy. It is concluded that not only quantum confinement effects but also alloying effects contribute to the widening of the energy gap in the Si nanocrystals.