Properties of the bare, passivated and doped germanium nanowire: A density-functional theory study

Using first-principles calculations based on density-functional theory, we systematically investigated the structural, energetic, and electronic properties of bare, hydrogen-passivated as well as doped germanium nanowires oriented along [0 0 1] direction. The bare nanowires with different diameters (from similar to 5.65 angstrom to similar to 19.96 angstrom) initially cut from the bulk Ge crystal in rod-like forms, are all metallic after relaxation. To better mimic the actual growth process, the hydrogen atoms were used to terminate the dangling bonds on the surface of bare wires which have been already relaxed, and subsequently the whole system containing H was relaxed again. Taking H-GeNW(57) as a prototype, we investigated both adsorption and substitution doping of wire by Al (group IIIA) and P (group VA) impurities. In the case of adsorption, the MO and the B-b sites (see text for definitions) are the most preferential sites for Al and P, respectively, and P adatom also breaks its nearest Ge-Ge bond below to form a Ge-P-Ge bond. The impurity-induced band appears and crosses the Fermi level, making the wire become metallic. In the case of substitution, impurity atom substituted at the edge site was seen more favorable than that in the core site, and leads to usual p-type (Al) or n-type (P) behavior similar to the substitutional doped bulk Ge crystal. This behavior is reverse if impurity atom is adsorbed on the wire. Furthermore, for both adsorption and substitution of Ge nanowires, the charge density analysis indicates the formed Al-Ge or P-Ge bond mainly shows a covalent character, and the P-Ge bonding is much stronger than the Al-Ge bonding. The electronic properties of these doped germanium nanowires can be used in nanoscale device applications. (C) 2010 Elsevier B.V. All rights reserved.