Understanding transport through molecules on silicon
Abstract
We theoretically investigate the effect of a localized dangling bond (DB) on occupied molecular orbital conduction through a styrene molecule bonded to a n++ H:Si(001)-(2×1) surface. For molecules relatively far from the DB, we find good agreement with the reported experiment using a model that accounts for the electrostatic contribution of the DB, provided we include some dephasing due to low energy vibronic modes. However, for molecules within 10Å to the DB, we have to include electronic contribution as well along with higher dephasing to explain the transport features. Apart from this, we theoretically study the electronic band structure of isolated unpaired and paired dangling bonds (DB), DB wires and DB clusters on H:Si(001)-(2×1) surface using Extended Hückel Theory (EHT) and report their effect on the Si band gap. An isolated unpaired DB introduces a near-midgap state, whereas a paired DB leads to π and π* states, similar to those introduced by an unpassivated asymmetric dimer (AD) Si(001)-(2×1) surface. Such induced states have very small dispersion due to their isolation from the other states, which reside in conduction and valence band. On the other hand, the surface state induced due to an unpaired DB wire in the direction along the dimer row (referred to as [110]), has large dispersion due to the strong coupling between the adjacent DBs, being 3.84Å apart. However, in the direction perpendicular to the dimer row (referred to as [110]), due to the reduced coupling between the DBs being 7.68Å apart, the dispersion in the surface state is similar to that of an isolated unpaired DB. Apart from this, a paired DB wire in [110] direction introduces π and π* states similar to those of an AD surface and a paired DB wire in [110] direction exhibits surface states similar to those of an isolated paired DB, as expected. Besides this, we report the electronic structure of different DB clusters, which exhibit states inside the band gap that can be interpreted as superpositions of states due to unpaired and paired DBs.
Degree
Ph.D.
Advisors
Datta, Purdue University.
Subject Area
Electrical engineering
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