Structural and electrical characterization of monolithic core-double shell n-GaN/Al/p-AlGaN nanowire heterostructures grown by molecular beam epitaxy.

We have studied the epitaxy and structural characterization of monolithic n-GaN/Al/p-AlGaN nanowire heterostructures. It is found that high quality, nearly defect free, full shell epitaxial Al can be grown in situ on Al(Ga)N nanowires and vice versa. Detailed scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) suggest that the Al (111) plane maintains an epitaxial relationship with Al(Ga)N (0001) in the nanowire growth direction. Full ultraviolet composition range (340 nm-210 nm) Al/Al(Ga)N core-double shell nanowire backward diode characteristics were investigated. We have demonstrated a monolithic n++-GaN/Al/p++-Al(Ga)N nanowire backward diode, wherein an epitaxial Al layer serves as the tunnel junction. Such an Al(Ga)N-based n-p-n nanowire backward diode exhibits record low resistivity (<1.5 × 10-4Ω cm2) and a low turn-on voltage of ∼2.7 V.

Complementary cathodoluminescence lifetime imaging configurations in a scanning electron microscope.

Cathodoluminescence (CL) spectroscopy provides a powerful way to characterize optical properties of materials with deep-subwavelength spatial resolution. While CL imaging to obtain optical spectra is a well-developed technology, imaging CL lifetimes with nanoscale resolution has only been explored in a few studies. In this paper we compare three different time-resolved CL techniques and compare their characteristics. Two configurations are based on the acquisition of CL decay traces using a pulsed electron beam that is generated either with an ultra-fast beam blanker, which is placed in the electron column, or by photoemission from a laser-driven electron cathode. The third configuration uses measurements of the autocorrelation function g(2) of the CL signal using either a continuous or a pulsed electron beam. The three techniques are compared in terms of complexity of implementation, spatial and temporal resolution, and measurement accuracy as a function of electron dose. A single sample of InGaN/GaN quantum wells is investigated to enable a direct comparison of lifetime measurement characteristics of the three techniques. The g(2)-based method provides decay measurements at the best spatial resolution, as it leaves the electron column configuration unaffected. The pulsed-beam methods provide better detail on the temporal excitation and decay dynamics. The ultra-fast blanker configuration delivers electron pulses as short as 30 ps at 5 keV and 250 ps at 30 keV. The repetition rate can be chosen arbitrarily up to 80 MHz and requires a conjugate plane geometry in the electron column that reduces the spatial resolution in our microscope. The photoemission configuration, pumped with 250 fs 257 nm pulses at a repetition rate from 10 kHz to 25 MHz, allows creation of electron pulses down to a few ps, with some loss in spatial resolution.

An AlGaN Core-Shell Tunnel Junction Nanowire Light-Emitting Diode Operating in the Ultraviolet-C Band.

To date, semiconductor light emitting diodes (LEDs) operating in the deep ultraviolet (UV) spectral range exhibit very low efficiency due to the presence of large densities of defects and extremely inefficient p-type conduction of conventional AlGaN quantum well heterostructures. We have demonstrated that such critical issues can be potentially addressed by using nearly defect-free AlGaN tunnel junction core-shell nanowire heterostructures. The core-shell nanowire arrays exhibit high photoluminescence efficiency (∼80%) in the UV-C band at room temperature. With the incorporation of an epitaxial Al tunnel junction, the p-(Al)GaN contact-free nanowire deep UV LEDs showed nearly one order of magnitude reduction in the device resistance, compared to the conventional nanowire p-i-n device. The unpackaged Al tunnel junction deep UV LEDs exhibit an output power >8 mW and a peak external quantum efficiency ∼0.4%, which are nearly one to two orders of magnitude higher than previously reported AlGaN nanowire devices. Detailed studies further suggest that the maximum achievable efficiency is limited by electron overflow and poor light extraction efficiency due to the TM polarized emission.

Monolithically Integrated Metal/Semiconductor Tunnel Junction Nanowire Light-Emitting Diodes.

We have demonstrated for the first time an n(++)-GaN/Al/p(++)-GaN backward diode, wherein an epitaxial Al layer serves as the tunnel junction. The resulting p-contact free InGaN/GaN nanowire light-emitting diodes (LEDs) exhibited a low turn-on voltage (∼2.9 V), reduced resistance, and enhanced power, compared to nanowire LEDs without the use of Al tunnel junction or with the incorporation of an n(++)-GaN/p(++)-GaN tunnel junction. This unique Al tunnel junction overcomes some of the critical issues related to conventional GaN-based tunnel junction designs, including stress relaxation, wide depletion region, and light absorption, and holds tremendous promise for realizing low-resistivity, high-brightness III-nitride nanowire LEDs in the visible and deep ultraviolet spectral range. Moreover, the demonstration of monolithic integration of metal and semiconductor nanowire heterojunctions provides a seamless platform for realizing a broad range of multifunctional nanoscale electronic and photonic devices.

Alternating-Current InGaN/GaN Tunnel Junction Nanowire White-Light Emitting Diodes.

The current LED lighting technology relies on the use of a driver to convert alternating current (AC) to low-voltage direct current (DC) power, a resistive p-GaN contact layer to inject positive charge carriers (holes) for blue light emission, and rare-earth doped phosphors to down-convert blue photons into green/red light, which have been identified as some of the major factors limiting the device efficiency, light quality, and cost. Here, we show that multiple-active region phosphor-free InGaN nanowire white LEDs connected through a polarization engineered tunnel junction can fundamentally address the afore-described challenges. Such a p-GaN contact-free LED offers the benefit of carrier regeneration, leading to enhanced light intensity and reduced efficiency droop. Moreover, through the monolithic integration of p-GaN up and p-GaN down nanowire LED structures on the same substrate, we have demonstrated, for the first time, AC operated LEDs on a Si platform, which can operate efficiently in both polarities (positive and negative) of applied voltage.

Removal of heavy metals in an abandoned mine drainage via ozone oxidation: a pilot-scale operation.

The objective of this study was to evaluate the ozone oxidation of dissolved heavy metals in an abandoned mine drainage (AMD) by conducting a pilot-scale operation at two different ozone doses of 7.5 and 24.0 g O(3)/h into an ozone reactor. A portion of the abandoned mine drainage near the Jungam Mine in Samchuck, Korea was pumped into this pilot-scale plant and used as an influent for the ozone oxidation. Some possible precipitates of metal oxides and hydroxides that resulted from the pilot-scale ozone oxidation of the dissolved Fe and Mn ions in the AMD (with a hydraulic retention time of 106 seconds in the ozone reactor) were effectively removed via sand filtration. A six-hour ozone oxidation with an ozone dose of 24.0 g O(3)/h and subsequent sand filtration, before backwashing the sand filter bed, can meet Korean drinking water quality standards (less than 0.3 mg/L) for Fe and Mn in the sand filter effluent under the operating conditions that were used in this study. The SO(4)(-2) concentrations and alkalinities of the influents were not affected by the ozone oxidation. The pH values of the influents were neutral or slightly alkaline, and after the six-hour oxidation, increased very slightly. These experiment results show that the ozone oxidation of dissolved heavy metals and the subsequent sand filtration of metal precipitates are desirable alternatives to removing heavy metals in an abandoned mine drainage.