Multi-scale design and analysis for developing 1-nm-scale neosilicon quantum information devices

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Electronics states and quantum transport properties of 'neosilicon' nanostructures (Si nanodots and nanorods) have been investigated theoretically by combining DFT (density-functional theory) based ab-initio simulation SIESTA and nonequilibrium quantum transport simulation TranSIESTA-C. For Si nanodot structures we found that a new icosahedral structure is stable with diameter of about 1 nm, and we investigated quasi-molecular electronic states formed in a strongly-coupled double Si nanodots for realizing a charge-based qubit. We observed that the two-level splitting between the bonding-like and anti-bonding-like states formed in the double dot structure largely depends on the atomistic distance and configuration of the individual dots. We therefore proposed a atomic-wire interconnect to fix both the configuration in an atomistic manner and revealed that the wavefunction coupling is strengthened via the atomic wire interconnect and the two-level splitting is enlarged. We also found that the external electric field applied parallel to the double dots enables to tune the two-level splitting.For Si nanorod structures, we simulated for the first time nanoscale Si transistors consisting of a Si nanorod with the surface terminated by hydrogen atoms as a channel and Au (111) nanoelectrodes. We analyzed the effects of interface states between the Si nanorod and Au nanoelectrods on the transmission spectra, density of states, current-voltage characteristics and quantum transport properties. We revealed that I-V characteristics of Si nanorod transistors changes from semiconductor-like nature from metallic nature as the Si nanorod length decreases. This is because the electronic states of Au atoms in the nanoleads penetrates into the Si nanorod and dominates the entire transport properties when the channel length is reduced down towards 1 nm. We also found that the Si nanorod transistor may show unique ambipolar behaviour when the gate bias is applied.