GaN Nanowire Schottky Barrier Diodes

A new concept of vertical gallium nitride (GaN) Schottky barrier diode based on nanowire (NW) structures and the principle of dielectric REduced SURface Field (RESURF) is proposed in this paper. High-threading dislocation density in GaN epitaxy grown on foreign substrates has hindered the development and commercialization of vertical GaN power devices. The proposed NW structure, previously explored for LEDs offers an opportunity to reduce defect density and fabricate low cost vertical GaN power devices on silicon (Si) substrates. In this paper, we investigate the static characteristics of high-voltage GaN NW Schottky diodes using 3-D TCAD device simulation. The NW architecture theoretically achieves blocking voltages upward of 700 V with very low specific on-resistance. Two different methods of device fabrication are discussed. Preliminary experimental results are reported on device samples fabricated using one of the proposed methods. The fabricated Schottky diodes exhibit a breakdown voltage of around 100 V and no signs of current collapse. Although more work is needed to further explore the nano-GaN concept, the preliminary results indicate that superior tradeoff between the breakdown voltage and specific on-resistance can be achieved, all on a vertical architecture and a foreign substrate. The proposed NW approach has the potential to deliver low cost reliable GaN power devices, circumventing the limitations of today's high electron mobility transistors (HEMTs) technology and vertical GaN on GaN devices

Nonequilibrium ferroelectric-ferroelastic 10-nm nanodomains : wrinkles, period-doubling, and power-law relaxation

Since the 1935 work of Landau and Lifshitz and of Kittel in 1946 all ferromagnetic, ferroelectric, and ferroelastic domains have been thought to be straight-sided with domain widths proportional to the square root of the sample thickness. We show in the present work that this is not true. We also discover period doubling domains predicted by Metaxas et al. (Phys. Rev. Lett. 2008, 217208) and modeled by Wang and Zhao Q. (Sci. Rpts. 2015, 5, 8887). We examine non-equilibrium ferroic domain structures in perovskite oxides with respect to folding, wrinkling, and relaxation and suggest that structures are kinetically limited and in the viscous flow regime predicted by Metaxas et al. in 2008 but never observed experimentally. Comparisons are made with liquid crystals and hydrodynamic instabilities, including chevrons, and fractional power-law relaxation. As Shin et al. [Soft Mat. 2016, 12, 3502] recently emphasized: “An understanding of how these folds initiate, propagate, and interact with each other is still lacking.” Inside each ferroelastic domain are ferroelectric 90-degree nano-domains with 10-nm widths and periodicity in agreement with the 10-nm theoretical minima predicted by Feigl et al. (Nat. Commun. 2014, 5, 4677). Evidence is presented for domain-width period doubling, which is common in polymer films but unknown in ferroic domains. A discussion of the folding-to-period doubling phase transition model of Wang and Zhao is included.PostprintPeer reviewe

Metodología de elaboración de mapas acústicos como herramienta de gestión del ruido urbano - caso medellín

Dada la naturaleza de la variable ruido y su variabilidad espacio – temporal, se tomó como base fundamental la aplicación de la metodología Geoestadística para la predicción y valoración de la distribución del ruido en las zonas de estudio del proyecto “Elaboración del Mapa Acústico para el Municipio de Medellínâ€�, realizado en el año 2006, mediante Convenio 680 de 2005, entre el Área Metropolitana del Valle de Aburrá y el Politécnico Colombiano Jaime Isaza Cadavid, con la participación de la Universidad Nacional de Colombia, Sede Medellín. Con base en esta metodología se concluyó que Medellín presenta un nivel de ruido diurno global de 73dB(A), indicando que su zona urbana tiene niveles de ruido apropiados solo para sectores industriales. De igual manera en la noche, se presenta un nivel de ruido global de 68 dB(A), apropiado solo para el mismo tipo de sector

Mapping grain boundary heterogeneity at the nanoscale in a positive temperature coefficient of resistivity ceramic

Despite being of wide commercial use in devices, the orders of magnitude increase in resistance that can be seen in some semiconducting BaTiO3-based ceramics, on heating through the Curie temperature (TC), is far from well understood. Current understanding of the behavior hinges on the role of grain boundary resistance that can be modified by polarization discontinuities which develop in the ferroelectric state. However, direct nanoscale resistance mapping to verify this model has rarely been attempted, and the potential approach to engineer polarization states at the grain boundaries, that could lead to optimized positive temperature coefficient (PTC) behavior, is strongly underdeveloped. Here we present direct visualization and nanoscale mapping in a commercially optimized BaTiO3-PbTiO3-CaTiO3 PTC ceramic using Kelvin probe force microscopy, which shows that, even in the low resistance ferroelectric state, the potential drop at grain boundaries is significantly greater than in grain interiors. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy reveal new evidence of Pb-rich grain boundaries symptomatic of a higher net polarization normal to the grain boundaries compared to the purer grain interiors. These results validate the critical link between optimized PTC performance and higher local polarization at grain boundaries in this specific ceramic system and suggest a novel route towards engineering devices where an interface layer of higher spontaneous polarization could lead to enhanced PTC functionality