Semiconductor nanowires of controlled composition and doping provide an exciting platform for new science and new device technologies. The continued advancement of nanowire-based devices will depend critically on knowledge of their atomic-scale structure, as compositional fluctuations as small as a single dopant atom can affect performance. Recently, we have made significant advances in quantitative nanoscale characterization that can provide a rational basis for engineering new or improved technologies based on semiconductor nanowires. Specifically, we have used atom probe tomography (APT) to map the distribution of dopant atoms in individual vapor-liquid-solid (VLS) grown Si and Ge nanowires.[1] We find that the VLS doping efficiency using phosphine and diborane is, in general, lower than expected, with the P doping of Ge nanowires being particularly suppressed. A thermodynamic model of doping from the ternary (Au:Ge:P) liquid catalyst has been developed that provides insight into the kinetic control of doping levels. In addition, we find evidence of enhanced surface doping even in the absence of notable vapor-solid deposition on the nanowire surface.[2] Surface doping produces a radially and axially non-uniform carrier concentration profile, which has important implications for device performance and modeling. Quantitative potential profiles extracted from scanning photocurrent microscopy measurements confirm intrinsic variations in carrier concentration associated with non-uniform doping,[2] and establish distinct operating principles of nanowire field effect transistors.[3] In particular, we show that surface doping can lower contact resistances while preserving the ability to modulate the channel conductivity, particularly when the surface doping is removed by selective etching of the device channel. Enhanced surface doping can also screen surface states that would otherwise pin the Fermi level. The quantitative dopant and unintentional impurity concentrations provided by APT enable the relative influence of surface states and bulk defects on majority and minority carrier characteristics to be established.[4] More generally, integrated efforts in synthesis, atom-by-atom composition mapping, and functional imaging close the loop on processing-structure-property relationships in nanowires and provide a sound basis for engineering of materials and devices.
[1] D. E. Perea, E. R. Hemesath, J. L. Lensch-Falk, and L. J. Lauhon, “Direct Measurement of Dopant Distribution in an Individual Vapor-Liquid-Solid Nanowire,” Nature Nanotechnology, doi:10.1038/nnano.2009.51
[2] J. E. Allen, E. R. Hemesath, D. E. Perea, and L. J. Lauhon, “Non-Uniform Nanowire Doping Profiles Revealed by Scanning Photocurrent Microscopy,” Adv. Mater., in press.
[3] J. E. Allen, E. R. Hemesath, D. E. Perea, and L. J. Lauhon, “SPCM analysis of Si nanowire field effect transistors fabricated by surface etching of the channel,” Nano Letters ASAP.
[4] J. E. Allen, E. R. Hemesath, D. E. Perea, J. L. Lensch-Falk, Z.Y. Li, F. Yin, M. H. Gass, P. Weng, A. L. Bleloch, R. E. Palmer, and L. J. Lauhon, "High-resolution detection of Au catalyst atoms in Si nanowires," Nature Nanotechnology 3, 168-173 (2008).