B. S. Swartzentruber

Scanning tunneling microscopy studies of structural disorder and steps on Si surfaces

Scanning tunneling microscopy observations of several forms of disorder on Si surfaces are presented. These include dimer vacancies on Si(001), step bunches associated with a morphological phase transition on vicinal Si(111), and step structure on vicinal Si(001). A recipe for cleaning of Si surfaces to produce a minimum amount of disorder is presented.

Unusually strong space-charge-limited current in thin wires.

The current-voltage characteristics of thin wires are often observed to be nonlinear, and this behavior has been ascribed to Schottky barriers at the contacts. We present electronic transport measurements on GaN nanorods and demonstrate that the nonlinear behavior originates instead from space-charge-limited current. A theory of space-charge-limited current in thin wires corroborates the experiments and shows that poor screening in high-aspect ratio materials leads to a dramatic enhancement of space-charge limited current, resulting in new scaling in terms of the aspect ratio.

Scanning tunneling microscopy study of diffusion, growth, and coarsening of Si on Si(001)

The growth, diffusion, and coarsening of Si on Si(001) have been investigated with scanning tunneling microscopy (STM). A diffusion coefficient for Si has been determined. Anisotropy in the island shapes during epitaxy is shown to be principally a growth structure due to an anisotropic accommodation coefficient. Diffusional anisotropy is small. An ordered ‘‘diluted‐dimer’’ structure is observed at low coverages and temperatures.

Diameter-Dependent Electronic Transport Properties of Au-Catalyst/Ge-Nanowire Schottky Diodes

We present electronic transport measurements in individual Au-catalyst/Ge-nanowire interfaces demonstrating the presence of a Schottky barrier. Surprisingly, the small-bias conductance density increases with decreasing diameter. Theoretical calculations suggest that this effect arises because electron-hole recombination in the depletion region is the dominant charge transport mechanism, with a diameter dependence of both the depletion width and the electron-hole recombination time. The recombination time is dominated by surface contributions and depends linearly on the nanowire diameter.

Donor-acceptor biomorphs from the ionic self-assembly of porphyrins.

Microscale four-leaf clover-shaped structures are formed by self-assembly of anionic and cationic porphyrins. Depending on the metal complexed in the porphyrin macrocycle (Zn or Sn), the porphyrin cores are either electron donors or electron acceptors. All four combinations of these two metals in cationic tetra(N-ethanol-4-pyridinium)porphyrin and anionic tetra(sulfonatophenyl)porphyrin result in related cloverlike structures with similar crystalline packing indicated by X-ray diffraction patterns. The clover morphology transforms as the ionic strength and temperature of the self-assembly reaction are increased, but the structures maintain 4-fold symmetry. The ability to alter the electronic and photophysical properties of these solids (e.g., by altering the metals in the porphyrins) and to vary cooperative interactions between the porphyrin subunits raises the possibility of producing binary solids with tunable functionality. For example, we show that the clovers derived from anionic Zn porphyrins (electron donors) and cationic Sn porphyrins (electron acceptors) are photoconductors, but when the metals are reversed in the two porphyrins, the resulting clovers are insulators.

Enhanced thermoelectric figure of merit in SiGe alloy nanowires by boundary and hole-phonon scattering

We report the thermoelectric characteristics of individual p-type SiGe alloy nanowires for diameters of 100 to 300 nm and temperatures between 40 to 300 K. A technique that allows for electrical and thermal characterization on the same nanowire was developed in this work. Experimental data provide evidence of the scattering of low-frequency phonons by the boundary of the nanowires. The thermal conductivities for SiGe alloy nanowires with different free carrier concentrations reveal that the long free path phonons are also scattered by hole-phonon interactions. Combined boundary and hole-phonon scattering mechanisms with alloy scattering resulted in thermal conductivities as low as 1.1 W/m-K at 300 K, which is one of the lowest measured for SiGe alloys and is comparable to that of bulk silica. The enhanced thermal properties observed in this work yielded ZT close to 0.18 at 300 K—more than a factor of 2 higher than the bulk SiGe alloy.

Tunneling microscopy of silicon and germanium: Si(111)7×7, SnGe(111)7×7, GeSi(111)5×5, Si(111)9×9, Ge(111)2×8, Ge(100)2×1, Si(110)5×1

The tunneling microscope has been used to study the low‐index faces of silicon and germanium. Si(111) 7×7 and 9×9, GeSi(111) 5×5, and SnGe(111) 7×7 are discussed in the light of the dimer–adatom–stacking fault structural model. Ge(111) 2×8 is compared to Si(111), Ge(100) is shown and compared to Si(100), and new structures on the Si(110) 5×1 surface are shown.

Nanometer‐scale lithography on Si(001) using adsorbed H as an atomic layer resist

We describe nanometer‐scale feature definition in adsorbed hydrogen layers on Si(001) surfaces by exposure to low energy electrons from a scanning tunneling microscope tip. Feature sizes range from <5 to ≳40 nm as a function of bias voltage (5–30 V) and exposure dose (1–104 μC/cm). We show that the cross section for electron stimulated desorption of hydrogen has a threshold at 6–8 eV and is nearly constant from 10 to 30 eV, so that above threshold the feature profiles are a direct reflection of the electron flux profile at the surface. Radial flux distributions are best fit by a simple exponential function, where the decay length is dependent primarily on the tip–sample separation. Low intensity tails at large radius are also observed for high bias emission. Comparison to field emission simulations shows that our tip has an ‘‘effective radius’’ of approximately 30 nm. Simulations demonstrate that tip geometry and tip–sample separation play the dominant role in defining the electron flux distribution, and ...

Selective area growth of metal nanostructures

Nanometer‐scale metal lines are fabricated onto Si(100) substrates by scanning tunneling microscope (STM) based lithography and subsequent chemical vapor deposition. An STM tip is first used to define areas for metal layer growth by electron stimulated desorption of adsorbed hydrogen. Exposure to Fe(CO)5 at 275 °C results in preferential deposition of Fe onto Si dangling bond sites (i.e., depassivated areas defined by the STM tip), while the monohydride resist remains intact in surrounding areas. Fe metal lines with widths ∼10 nm are constructed using this selective‐area, autocatalytic growth technique.

Tuning of defects in ZnO nanorod arrays used in bulk heterojunction solar cells.

With particular focus on bulk heterojunction solar cells incorporating ZnO nanorods, we study how different annealing environments (air or Zn environment) and temperatures impact on the photoluminescence response. Our work gives new insight into the complex defect landscape in ZnO, and it also shows how the different defect types can be manipulated. We have determined the emission wavelengths for the two main defects which make up the visible band, the oxygen vacancy emission wavelength at approximately 530 nm and the zinc vacancy emission wavelength at approximately 630 nm. The precise nature of the defect landscape in the bulk of the nanorods is found to be unimportant to photovoltaic cell performance although the surface structure is more critical. Annealing of the nanorods is optimum at 300°C as this is a sufficiently high temperature to decompose Zn(OH)2 formed at the surface of the nanorods during electrodeposition and sufficiently low to prevent ITO degradation.