Electronic structures of silicon quantum dots as nanoclusters for single electron transistors

Abstract

Nanostructures such as quantum dot and nanocluster have occupied the centre of scientific interest because of their unique electronic nature. In this research, the electronic structures of silicon quantum dot were studied. The quantum dot was homologized with nanocluster since there is no strict distinction between these two nanostructures. The simulations in this research were carried out by using VASP (Vienna Ab-Initio Software Package) which utilizes the method of density functional theory and plane wave basis set. In order to speed up the computational time, parallelization was implemented on VASP. First, silicon clusters with surface passivated by hydrogen, SinHm were simulated and the density of states (DOS) as well as bandstructure for each cluster was yielded. From the DOS graphs, discrete spectrum was observed instead of bulk-like continuous DOS which is the evolvement from bulk to nano-size. Bandstructure graphs also showed the discrete energy level in consistence with the discrete energy spectrum from DOS. It was found that the bandgap for hydrogenated silicon clusters increases with the decrease in size. Bare silicon clusters, Sin were also simulated from 1 to 15 number of silicon atom (n). Optimization was performed to obtain the ground state structure. The bandgaps for the ground state silicon clusters do not show a decreasing trend with the increment of cluster size as that of hydrogenated silicon cluster. The electronic structures of optimized clusters are affected by the surface orientation of the clusters. A comparison of the bandgap values for SinHm and Sin was made. Finally, the currentvoltage (I-V) characteristic and conductance-voltage spectrum (G-V) of singleelectron transistor (SET) were studied with a simple toy model. These transport properties have shown the relativity of the electronic structure and the electron transport, where the conductance gap increases with the energy difference between Fermi level of the gold lead and the nearest molecular energy level of silicon cluster.