Schottky Contacts on Polarity-Controlled Vertical ZnO Nanorods

Polarity-controlled growth of ZnO by chemical bath deposition provides a method for controlling the crystal orientation of vertical arrays of nanorods. The ability to define the morphology and structure of the nanorods is essential to maximising the performance of optical and electrical devices such as piezoelectric nanogenerators; however, well-defined Schottky contacts to the polar facets of the structures have yet to be explored. In this work, we demonstrate a process to fabricate metal-semiconductor-metal device structures from vertical arrays with Au contacts on the uppermost polar facets of the nanorods and show the O-polar nanorods (~0.44 eV) have a greater effective barrier height than the Zn-polar nanorods (~0.37 eV). Oxygen plasma treatment is shown by Cathodoluminescence (CL) spectroscopy to reduce mid-gap defects associated with radiative emissions that improves the Schottky contacts from weakly-rectifying to strongly-rectifying. Interestingly, the plasma treatment was shown to have a much greater effect in reducing the number of carriers in O-polar nanorods through quenching of the donor-type substitutional hydrogen on oxygen sites (HO) when compared to the zinc vacancy related hydrogen defect complexes (VZn, Hn) in Zn-polar nanorods that evolve to lower coordinated complexes. The effect on HO in the O-polar nanorods coincided with a large reduction in the visible range defects producing a lower conductivity and creating the larger effective barrier heights. This combination can allow radiative losses and charge leakage to be controlled enhancing devices such as dynamic photodetectors, strain sensors, and LEDs while showing the O-polar nanorods can outperform Zn-polar nanorods in such applications

An Effective State? But Effective for Whom?

Summary Neoliberal rhetoric has urged the reduction of the state's role, but in many ways state power and spending have merely been redirected rather than reduced. WDR97 urges states to become effective, but does not say for whom. The World Bank's biases are seen in the WDR's economistic approach to political issues, limited view of participation, blindness to corporate power and support for the use of international mechanisms to lock in government policies. The views expressed in WDR97, combined with the poor record of the World Bank's privatisation and public sector reform loans, suggest the need for great caution as the World Bank seeks to move further into institutional reform

Stability of Schottky and Ohmic Au Nanocatalysts to ZnO Nanowires

Manufacturable nanodevices must now be the predominant goal of nanotechnological research to ensure the enhanced properties of nanomaterials can be fully exploited and fulfill the promise that fundamental science has exposed. Here, we test the electrical stability of Au nanocatalyst-ZnO nanowire contacts to determine the limits of the electrical transport properties and the metal-semiconductor interfaces. While the transport properties of as-grown Au nanocatalyst contacts to ZnO nanowires have been well-defined, the stability of the interfaces over lengthy time periods and the electrical limits of the ohmic or Schottky function have not been studied. In this work, we use a recently developed iterative analytical process that directly correlates multiprobe transport measurements with subsequent aberration-corrected scanning transmission electron microscopy to study the electrical, structural, and chemical properties when the nanowires are pushed to their electrical limits and show structural changes occur at the metal-nanowire interface or at the nanowire midshaft. The ohmic contacts exhibit enhanced quantum-mechanical edge-tunneling transport behavior because of additional native semiconductor material at the contact edge due to a strong metal-support interaction. The low-resistance nature of the ohmic contacts leads to catastrophic breakdown at the middle of the nanowire span where the maximum heating effect occurs. Schottky-type Au-nanowire contacts are observed when the nanowires are in the as-grown pristine state and display entirely different breakdown characteristics. The higher-resistance rectifying I-V behavior degrades as the current is increased which leads to a permanent weakening of the rectifying effect and atomic-scale structural changes at the edge of the Au interface where the tunneling current is concentrated. Furthermore, to study modified nanowires such as might be used in devices the nanoscale tunneling path at the interface edge of the ohmic nanowire contacts is removed with a simple etch treatment and the nanowires show similar I-V characteristics during breakdown as the Schottky pristine contacts. Breakdown is shown to occur either at the nanowire midshaft or at the Au contact depending on the initial conductivity of the Au contact interface. These results demonstrate the Au-nanowire structures are capable of withstanding long periods of electrical stress and are stable at high current densities ensuring they are ideal components for nanowire-device designs while providing the flexibility of choosing the electrical transport properties which other Au-nanowire systems cannot presently deliver

Modifying the Interface Edge to Control the Electrical Transport Properties of Nanocontacts to Nanowires

Selecting the electrical properties of nanomaterials is essential if their potential as manufacturable devices is to be reached. Here, we show that the addition or removal of native semiconductor material at the edge of a nanocontact can be used to determine the electrical transport properties of metal-nanowire interfaces. While the transport properties of as-grown Au nanocatalyst contacts to semiconductor nanowires are well-studied, there are few techniques that have been explored to modify the electrical behavior. In this work, we use an iterative analytical process that directly correlates multiprobe transport measurements with subsequent aberration-corrected scanning transmission electron microscopy to study the effects of chemical processes that create structural changes at the contact interface edge. A strong metal-support interaction that encapsulates the Au nanocontacts over time, adding ZnO material to the edge region, gives rise to ohmic transport behavior due to the enhanced quantum-mechanical tunneling path. Removal of the extraneous material at the Au-nanowire interface eliminates the edge-tunneling path, producing a range of transport behavior that is dependent on the final interface quality. These results demonstrate chemically driven processes that can be factored into nanowire-device design to select the final properties

Forming reproducible non-lithographic nanocontacts to assess the effect of contact compressive strain in nanomaterials

The application of electrical nanoprobes to measure and characterize nanomaterials has become widely spread. However, the formation of quality electrical contacts using metallic probes on nanostructures has not been directly assessed. We investigate here the electrical behaviour of non-lithographically formed contacts to ZnO nanowires (NWs) and develop a method to reproducibly form Ohmic contacts for accurate electrical measurement of the nanostructures. The contacting method used in this work relies on an electrical feedback mechanism to determine the point of contact to the individual NWs, ensuring minimal compressive strain at the contact. This developed method is compared with the standard tip deflection contacting technique and shows a significant improvement in reproducibility. The effect of excessive compressive strain at the contact was investigated, with a change from rectifying to ohmic I–V behaviour observed as compressive strain at the contact was increased, leading to irreversible changes to the electrical properties of the NW. This work provides an ideal method for forming reproducible non-lithographic nanocontacts to a multitude of nanomaterials

Ion Thermal Decoupling and Species Separation in Shock-Driven Implosions

Anomalous reduction of the fusion yields by 50% and anomalous scaling of the burn-averaged ion temperatures with the ion-species fraction has been observed for the first time in D[superscript 3]He-filled shock-driven inertial confinement fusion implosions. Two ion kinetic mechanisms are used to explain the anomalous observations: thermal decoupling of the D and [superscript 3]He populations and diffusive species separation. The observed insensitivity of ion temperature to a varying deuterium fraction is shown to be a signature of ion thermal decoupling in shock-heated plasmas. The burn-averaged deuterium fraction calculated from the experimental data demonstrates a reduction in the average core deuterium density, as predicted by simulations that use a diffusion model. Accounting for each of these effects in simulations reproduces the observed yield trends.United States. National Nuclear Security Administration (Grant DE-NA0001857)University of Rochester. Fusion Science Center (Grant 415023-G)National Laser User’s Facility (Grant DE-NA0002035)University of Rochester. Laboratory for Laser Energetics (Grant 415935-G)Lawrence Livermore National Laboratory (Grant B600100

Exploration of the Transition from the Hydrodynamiclike to the Strongly Kinetic Regime in Shock-Driven Implosions

Clear evidence of the transition from hydrodynamiclike to strongly kinetic shock-driven implosions is, for the first time, revealed and quantitatively assessed. Implosions with a range of initial equimolar D[superscript 3]He gas densities show that as the density is decreased, hydrodynamic simulations strongly diverge from and increasingly overpredict the observed nuclear yields, from a factor of ∼2 at 3.1  mg/cm[superscript 3] to a factor of 100 at 0.14  mg/cm[superscript 3]. (The corresponding Knudsen number, the ratio of ion mean-free path to minimum shell radius, varied from 0.3 to 9; similarly, the ratio of fusion burn duration to ion diffusion time, another figure of merit of kinetic effects, varied from 0.3 to 14.) This result is shown to be unrelated to the effects of hydrodynamic mix. As a first step to garner insight into this transition, a reduced ion kinetic (RIK) model that includes gradient-diffusion and loss-term approximations to several transport processes was implemented within the framework of a one-dimensional radiation-transport code. After empirical calibration, the RIK simulations reproduce the observed yield trends, largely as a result of ion diffusion and the depletion of the reacting tail ions.United States. Dept. of Energy (Grant DE-NA0001857)United States. Dept. of Energy (Grant DE-FC52-08NA28752)University of Rochester. Fusion Science Center (5-24431)National Laser User’s Facility (DE-NA0002035)University of Rochester. Laboratory for Laser Energetics (415935-G)Lawrence Livermore National Laboratory (B597367