Masahiko Matsubara

Attracting shallow donors: Hydrogen passivation in (Al,Ga,In)-doped ZnO

The hydrogen interstitial and the substitutional Al_Zn, Ga_Zn and In_Zn are all shallow donors in ZnO and lead to n-type conductivity. Although shallow donors are expected to repel each other, we show by first principles calculations that in ZnO these shallow donor impurities attract and form a complex, leading to a donor level deep in the band gap. This puts a limit on the n-type conductivity of (Al,Ga,In)-doped ZnO in the presence of hydrogen.

An Extensive Study of Charge Effects in Silicon Doped Heterofullerenes

We present an analysis of charge effects on the highly silicon doped heterofullerenes C30Si30. Structural and electronic properties are investigated by the inclusion of an extra pos- itive and negative charge in the neutral system. The calculations are performed based on the framework of Car-Parrinello molecular dynamics within the spin density version of density functional theory. Structural properties are not significantly affected by adding to or extracting from the C30Si30 heterofullerene one electron. However, the change of charge states has some ef- fects on the electronic properties of heterofullerenes. In the negatively charged system, negative charges are found in the inner part of the Si region, thereby suggesting potential applications of Si-based heterofullerenes as anionic systems.

Atomic scale modelling of materials:a prerequisite for any multi-scale approach to structural and dynamical properties

We describe two examples of application focusing on first-principles molecular dynamics as an effective tool to unravel the atomic-scale structure of condensed-matter systems. The first application is on disordered network-forming materials and the second is on silicon-doped fullerenes. We show that an accurate modelling of interatomic forces based on density functional theory, when combined with an account of the temperature evolution, is an unavoidable prerequisite for analyzing and interpreting experimental results on a quantitative basis. In the case of disordered systems, we describe the basic structural features of amorphous GeSe4 and highlight the predominant chemical order in this system. The effect of adding or removing an electron charge on the stability of Si-doped fullerenes is exemplified by considering the finite temperature evolution of heterofullerenes.