Blake S. Simpkins

Surface depletion effects in semiconducting nanowires

The impact of surface depletion on the electronic properties of semiconductor nanowires (NWs) is explored both theoretically and experimentally. The impact of dopant concentration, surface barrier height, and NW radius on surface depletion and extracted material properties are determined by solving Poisson’s equation for the cylindrical system. The theoretical results reveal a size-dependent systematic error in carrier concentration extraction, which is verified through experiment. Interrogation of GaN NWs with radii from 15 to 70 nm exposed an error that reaches over an order of magnitude for the samples studied. These data compared favorably to an analytical treatment assuming physically reasonable material properties. While this manuscript focuses on GaN, the systematic error discussed will be present for any semiconducting NW, which exhibits surface band bending and therefore influences the behavior and characterization of a wide range of semiconducting nanoelements.

Diameter control of gallium nitride nanowires

Gallium nitride (GaN) nanowires are grown with controlled diameter and position by combining electron-beam lithography and naturally occurring surface tension forces. Lithographically defined particle diameters were held constant while only the film thickness was varied. Annealing drives as-deposited metal disks toward hemispheres according to conservation of volume constraints, resulting in well-controlled catalyst particles with radii smaller than those of the as-deposited particles. Transmission electron microscopy and electron diffraction confirm that the nanowires are highly crystalline wurtzite GaN. The ability to structurally control the GaN nanowire size yields effective modulation of NW-FET conductivity.

Polarization fields in III-nitride nanowire devices

Control of the polarization fields is the most important parameter in designing III-nitride thin-film devices, and herein we show that the polarization fields may be equally, if not more, important in devising III-nitride nanowire devices. One common approach to produce III-nitride nanowires is via a vapor-liquid-solid approach that, in general, yields nanowires with the major (growth) axis in the <1120> direction. The cross section of this wire is an isosceles triangle with two {1101} facets and one {0001} facet. In this work, we analyze the polarization fields that arise in two distinct sets of crystal planes that can manifest in this triangular nanowire geometry: (0001), (1101), (1101) or (0001), (1101), (1101). Calculations show that the polarization field at the {0001} facet is much larger than at the two opposing {1101} facets, although the sign of the field at each facet has a complicated dependence on the orientation and structure of the nanowire. An undoped nanowire transistor was fabricated that displayed p-type operation based solely on polarization-induced hole carriers at the (0001) AlGaN/GaN interface, consistent with our field calculations.

Electronic conduction in GaN nanowires

Conductivity mechanisms in unintentionally doped GaN nanowires (NWs) are studied. Gated current-voltage measurements and threshold voltage modeling demonstrate the unique impact of device parameters on NW field-effect transistors as compared to conventional systems. Temperature-dependent resistivity results, acquired with a scanning tunneling microscope equipped with multiple tips, reveal only mild temperature dependence at higher temperatures, with temperature-independent resistivity observed below ∼100K indicating impurity band conduction. The likely origins and implications of these results are discussed.

Pitch-dependent resonances and near-field coupling in infrared nanoantenna arrays

We investigate coupling in arrays of nanoparticles resonating as half-wave antennas on both silicon and sapphire, and find a universal behavior when scaled by antenna length and substrate index. Three distinct coupling regimes are identified and characterized by rigorous finite-difference time domain simulations. As interparticle pitch is reduced below the oft-described radiative to evanescent transition, resonances blue shift and narrow and exhibit an asymmetric band consistent with a Fano lineshape. Upon further pitch reduction, a transition to a third regime, termed here as near-field coupling, is observed in which the resonance shifts red, becomes more symmetric, and broadens dramatically. This latter regime occurs when the extension of the resonant mode beyond the physical antenna end overlaps that of its neighbor. Simulations identify a clear rearrangement of field intensity accompanying this regime, illustrating that longitudinal modal fields localize in the air gap rather than in the higher index substrate at a pitch consistent with the experimentally observed transition.

Space-charge-limited currents and trap characterization in coaxial AlGaN/GaN nanowires

This manuscript presents the first observation of the space-charge-limited current (SCLC) conduction mechanism in individual heterostructure nanowires (NWs). This effect is exploited to extract size-dependent carrier densities and to demonstrate surface-dominated behavior for these technologically relevant nanostructures. Mobile carrier densities were shown to increase from 2.5 × 1016 to 5.6 × 1017 cm−3, as NW width decreased from 200 to 50 nm. This size-dependent behavior is a consequence of the increasing influence of near-surface confined carriers as widths decrease. Traps impact the SCLC response and were characterized as an exponential band edge tail with an average characteristic energy of 75 meV. In addition to the specific materials properties extracted, these results further demonstrate the tendency for low-dimensional materials (1D NWs) to exhibit SCLC at much lower injection fluxes compared to their higher dimensional (2D heterostructure field-effect transistors) counterparts.

Assembly of phosphonic acids on GaN and AlGaN

Self-assembled monolayers of octadecylphosphonic acid and 16-phosphonohexadecanoic acid (PHDA) were formed on the semiconductor substrates gallium nitride (GaN) and aluminium gallium nitride (AlGaN). The presence of the molecular layers was verified through x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Structural information was acquired with infrared spectroscopy which verified the bonding orientation of the carboxyl-containing PHDA. The impact of the molecular layers on the channel conductivity and the surface electronic structure of an AlGaN/GaN heterostructure was measured. Our results indicate that pinning of the surface Fermi level prohibits modification of the channel conductivity by the layer. However, a surface dipole of ~0.8 eV is present and associated with both phosphonic acid layers. These results are of direct relevance to field-effect-based biochemical sensors and metal–semiconductor contact formation for this system and provide a fundamental basis for further applications of GaN and AlGaN technology in the fields of biosensing and microelectronics.

Transmission efficiency of surface plasmon polaritons across gaps in gold waveguides

A far-field microscopy approach is introduced that measures the transmittance of surface plasmon polaritons across gaps in plasmonic waveguides. Local plasmon intensity is monitored through radiation scattered from discontinuities placed before and after the gap. An unusually broad range of gap sizes, 30 nm to 16 μm, is reported for optically thick, 5 μm wide Au stripe-waveguides excited at 860 nm wavelength. Transmittances approach 100% for a 30 nm gap and remain as high as 50% for a 1 μm gap. Finite-element analysis yields transmittances in agreement with experiment and finds gap losses are dominated by radiation scattered into the substrate and air.

Localized and nonlocalized plasmon resonance enhanced light absorption in metal-insulator-metal nanostructures

Multiple absorption bands in a metal-insulator-metal (MIM) nanostructure are comprehensively investigated in the visible and near-infrared (near-IR) regime. The MIM nanostructure consists of patterned gold squares with systematically varying size and period atop a thin aluminum nitride dielectric layer on a thick gold film. Both the transverse-electric (TE) slab-waveguide mode and the transverse-magnetic (TM) surface plasmon-polariton mode can be excited in the nanostructure. Grating coupling of the incident light into nonlocalized traveling waves is found from electromagnetic field patterns and from the linear dispersion of the absorption modes with period. A localized mode strengthens the near-IR absorption band for small square sizes, and the resonance shifts to red for large square sizes.

Propagation length of surface plasmon polaritons determined by emission from introduced surface discontinuities

Flexible far-field microscopy methods suitable for directly measuring surface plasmon polariton propagation along optically thick or buried waveguides are introduced. The methods monitor the local intensity of surface plasmon polaritons by imaging the light scattered when the plasmons encounter discontinuities in the form of (i) the terminal end of the guide, (ii) randomly dispersed nanoparticles, and (iii) nanoholes drilled through the guide. Measurements by these three methods give consistent values to within ∼15% of 39 μm for the propagation length along 5-μm-wide Au-stripe waveguides deposited on an oxidized silicon wafer and excited at a wavelength of 860 nm. This range is due to varying losses associated with the introduction of the nanoholes and nanoparticles. These losses are quantified and could be reduced with realistic experimental improvements. Finite-element computations find that propagation in these optically thick (107 nm) guides is intrinsically limited not only by Ohmic losses, but also ...