Non-equilibrium Green function implementation of boundary conditions for full band simulations of substrate-nanowire structures

The density of states of a nanowire exhibits peaks at energies in which the individual transverse modes begin to propagate. Under ideal conditions these modes are eigenstates solely determined by the cross sectional size and shape of the nanowire. However, for realistic nanowires, which are grown on semiconductor substrates, the density of states of the subsrate-nanowire structure is dependent on the distance from the nanowire endpoints. Near the substrate, the density of states is is nonzero far below the energy corresponding to the first eigenstate of the ideal nanowire. This initial increase in the density of states occurs at energies near the conduction and valence band edges of the semiconductor substrate on which the nanowire is grown. Away from the substrate, the density of states begins to acquire ideal nanowire characteristics. In the present work this effect is captured by imposing boundary conditions, with properties of bulk material, at the nanowire base. The calculations utilize a first neighbor sp(3)s*d(5) orbital basis within the non-equilibrium Green function formalism. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.