Controllable doping and wrap-around contacts to electrolessly etched silicon nanowire arrays.

Top-down electroless chemical etching enables non-lithographic patterning of wafer-scale nanostructured arrays, but the etching on highly doped wafers produces porous structures. The lack of defect-free nanostructures at desired doping and the difficulties in forming reliable electrical side-contacts to the nanostructure arrays limits their integration into high performance nanoelectronics. We developed a barrier layer diffusion technique to controllably dope wafer-scale silicon nanowire arrays (10(17)-10(20) cm(-3)) produced by chemically etching lightly doped silicon wafers. In order to achieve low resistance top-side electrical contacts to the arrays, we developed a two step tip-doping procedure to locally dope the tips (∼10(20) cm(-3)) to metallic levels. The dopant concentration is characterized by depth profiling using secondary ion mass spectroscopy and four-point probe electrical measurements. Further, array scale electrical measurements show that the tip-doping lowers the specific contact resistivity (∼10(-5) Ω cm(2)) since the metallic tips enable direct tunneling of electrons across the nickel silicide contacts to the nanowire arrays. This work provides a scalable and cost-effective doping approach to control charge injection and charge conduction in nanowire arrays, thus advancing their integration into various device applications.

Synergetic role of Li(+) during Mg electrodeposition/dissolution in borohydride diglyme electrolyte solution: voltammetric stripping behaviors on a Pt microelectrode indicative of Mg-Li alloying and facilitated dissolution.

We describe a voltammetric and spectroscopic study of Mg electrodeposition/dissolution (MgDep/Dis) in borohydride diglyme electrolyte solution containing Li(+) carried out on a Pt ultramicroelectrode (UME, r = 5 µm). The data reveal Li(+) cation facilitation that has not been previously recognized in studies made using macroelectrodes. While a single broad, asymmetric stripping peak is expected following MgDep on a Pt macroelectrode in 0.1 M Mg(BH4)2 + 1.5 M LiBH4 diglyme solution on a Pt UME, the stripping reveals three resolved oxidation peaks, suggesting that MgDep/Dis consists of not only a Mg/Mg(2+) redox reaction but also contributions from Mg-Li alloying/dissolution reaction processes. Detailed XPS, SIMS, ICP, and XRD studies were performed that confirm the importance of Mg-Li alloy formation processes, the nature of which is dependent on the reduction potential used during the MgDep step. Based on the electrochemical and surface analysis data, we propose an electrochemical mechanism for MgDep/Dis in a borohydride diglyme electrolyte solution that, in the presence of 1.5 M Li(+) ions, proceeds as follows: (1) Mg(2+) + 2e(-) ⇌ Mg; (2) (1 - x)Mg(2+) + xLi(+) + (2 - x)e(-) ⇌ Mg(1-x)Lix, 0 < x ≤ 0.02; and (3) (1 - y)Mg(2+) + yLi(+) + (2 - y)e(-) ⇌ Mg(1-y)Liy, 0.02 < y ≤ 0.09. Most significantly, we find that the potential-dependent MgDep/Dis kinetics are enhanced as the concentration of the LiBH4 in the diglyme electrolyte is increased, a result reflecting the facilitating influences of reduced uncompensated resistance and the enhanced electro-reduction kinetics of Mg(2+) due to Mg-Li alloy formation.