Jonathan E. Allen

High-resolution detection of Au catalyst atoms in Si nanowires

The potential for the metal nanocatalyst to contaminate vapour-liquid-solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour-liquid-solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature. Direct measurements of the minority carrier diffusion length versus nanowire diameter, however, demonstrate that surface recombination controls minority carrier transport in as-grown n-type nanowires; the influence of Au is negligible. These results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.

Near-field scanning photocurrent microscopy of a nanowire photodetector

A near-field scanning optical microscope was used to image the photocurrent induced by local illumination along the length of a metal-semiconductor-metal (MSM) photodetector made from an individual CdS nanowire. Nanowire MSM photodetectors exhibited photocurrents ∼105 larger than the dark current (<2pA) under uniform monochromatic illumination; under local illumination, the photoresponse was localized to the near-contact regions. Analysis of the spatial variation and bias dependence of the local photocurrent allowed the mechanisms of photocarrier transport and collection to be identified, highlighting the importance of near-field scanning photocurrent microscopy to elucidating the operating principles of nanowire devices.

A synergistic assembly of nanoscale lamellar photoconductor hybrids

Highly ordered nanostructured organic/inorganic hybrids offer chemical tunability, novel functionalities and enhanced performance over their individual components. Hybrids of complementary p-type organic and n-type inorganic components have attracted interest in optoelectronics, where high-efficiency devices with minimal cost are desired. We demonstrate here self-assembly of a lamellar hybrid containing periodic and alternating 1-nm-thick sheets of polycrystalline ZnO separated by 2-3 nm layers of conjugated molecules, directly onto an electrode. Initially the electrodeposited inorganic is Zn(OH)(2), but pi-pi interactions among conjugated molecules stabilize synergistically the periodic nanostructure as it converts to ZnO at 150 degrees C. As photoconductors, normalized detectivities (D(*)) greater than 2x10(10) Jones, photocurrent gains of 120 at 1.2 V microm(-1) and dynamic ranges greater than 60 dB are observed on selective excitation of the organic. These are among the highest values measured for organic, hybrid and amorphous silicon, making them technologically competitive as low-power, wavelength-tunable, flexible and environmentally benign photoconductors.

Nonuniform doping distribution along silicon nanowires measured by Kelvin probe force microscopy and scanning photocurrent microscopy

We use Kelvin probe force microscopy and scanning photocurrent microscopy to measure the doping distribution along single phosphorous-doped silicon nanowire grown by the vapor-liquid-solid method. A nonlinear potential drop along biased silicon nanowires is detected both by measuring the surface potential directly via Kelvin probe force microscopy and by integrating the photocurrent measured by scanning photocurrent microscopy. These variations in the potential and field are further analyzed to extract the longitudinal dopant distribution along an individual silicon nanowire. The results show a very good agreement between the two methods to quantitatively detect potential, field, and doping variations within doped silicon nanowires.

Scanning photocurrent microscopy analysis of Si nanowire field-effect transistors fabricated by surface etching of the channel.

High-performance field-effect transistors were fabricated by etching the channel regions of surface-doped Si nanowires. On/off ratios of 10(6) and field effect mobilities up to 525 cm(2)/(V x s) represent significant improvements over transistors fabricated from uniformly doped n-Si nanowires. Analysis by scanning photocurrent microscopy (SPCM) confirmed that the devices function similarly to traditional metal-oxide semiconductor field-effect transistors; in accumulation, the device current is controlled by channel conductance modulation, while n(+)-n junctions determine subthreshold characteristics as the channel is depleted. The principles of operation and the drain current saturation mechanisms were investigated by correlating current versus voltage data with integrated photocurrent profiles from SPCM.