Optical Identification of a DNA-Wrapped Carbon Nanotube: Signs of Helically Broken Symmetry

High intrinsic mobility and small, biologically-compatible size make single-walled carbon nanotubes (SWNTs) in demand for the next generation of electronic devices. Further, the wide range of available bandgaps due to changes in diameter and symmetry give SWNTs greater versatility than traditional semiconductors. Single-stranded DNA has been employed to make these desirable properties accessible for large scale fabrication of devices. Because single-stranded DNA can helically wrap a SWNT, forming a stable hybrid structure, DNA/polymer wrapping has been used to disperse bundles of intrinsically hydrophobic SWNTs into individual tubes in aqueous solution. The ability to isolate individual tubes, make them soluble, and separate them according to symmetry would enable fabrication of SWNT optoelectronic devices that benefit from the unique electronic properties of specific nanotube structures. Envisioning optoelectronic applications of nanotubes, we investigate whether the optical properties of DNA-wrapped SWNT materials are different than those of pristine SWNTs. Our previous work found that bandstructures of DNA-SWNTs were indeed affected by the charged wrap. That is, the direct optical bandgap, $E_{11}$, decreases, but changes are fairly small. This is consistent with the available experimental data in standard experimental geometry in which incident light is polarized along the SWNT axis. Here we consider optical absorption of light with perpendicular (or circular) polarization with respect to the tube axis, which has been measured experimentally for SWNTs dispersed using a surfactant. In this geometry we find qualitative changes in the absorption spectra of SWNTs upon hybridization with DNA, including strong optical circular dichroism in non-chiral SWNTs.Comment: 3 color figures, 4 pages, accepted for Small journa

Large radius exciton in single-walled carbon nanotubes

The spectrum of large radius exciton in an individual semiconducting single-walled carbon nanotube (SWCNT) is described within the framework of elementary potential model, in which exciton is modeled as bound state of two oppositely charged quasi-particles confined on the tube surface. Due to the parity of the interaction potential the exciton states split into the odd and even series. It is shown that for the bare and screened Coulomb electron-hole (e-h) potentials the binding energy of even excitons in the ground state well exceeds the energy gap. The factors preventing the collapse of single-electron states in isolated semiconducting SWCNTs are discussed.Comment: 14 pages, 1 figure, 5 table

Modeling and simulation of bulk gallium nitride power semiconductor devices

Bulk gallium nitride (GaN) power semiconductor devices are gaining significant interest in recent years, creating the need for technology computer aided design (TCAD) simulation to accurately model and optimize these devices. This paper comprehensively reviews and compares different GaN physical models and model parameters in the literature, and discusses the appropriate selection of these models and parameters for TCAD simulation. 2-D drift-diffusion semi-classical simulation is carried out for 2.6 kV and 3.7 kV bulk GaN vertical PN diodes. The simulated forward current-voltage and reverse breakdown characteristics are in good agreement with the measurement data even over a wide temperature range

Contact-induced spin polarization in carbon nanotubes

Motivated by the possibility of combining spintronics with molecular structures, we investigate the conditions for the appearance of spin-polarization in low-dimensional tubular systems by contacting them to a magnetic substrate. We derive a set of general expressions describing the charge transfer between the tube and the substrate and the relative energy costs. The mean-field solution of the general expressions provides an insightful formula for the induced spin-polarization. Using a tight-binding model for the electronic structure we are able to estimate the magnitude and the stability of the induced moment. This indicates that a significant magnetic moment in carbon nanotubes can be observed.Comment: To appear in Phys. Rev. B (2003

Photoluminescence and terahertz generation in InGaN/GaN multiple quantum well light-emitting diode heterostructures under laser excitation

In this paper the results of experiments on terahertz generation from nitride light-emitting diode heterostructures under twophoton excitation by femtosecond laser pulses are reported. Dependencies of the photoluminescence and terahertz spectra on structural properties of the samples and intensity of laser pulses have been studied. It was found that the terahertz pulse amplitude increases and its spectrum shifts towards higher frequencies with an increasing number of quantum wells in the heterostructures. Photoluminescence spectral shape change at high excitation intensities was observed. The probable mechanisms explaining the observed experimental dependencies are discussed

Design, Simulations, and Optimizations of Mid-infrared Multiple Quantum Well LEDs

We use eight-band k·p energy band structure model to help design novel GaInSb/AlGaInSb mid-infrared multiple quantum well (MQW) structures with an emitting mid-infrared waveband of 4-5 µm. Simulation results suggest that the number of quantum wells has little influence on the spontaneous emission rate and gain because of no strong coupling between quantum wells and they just simply follow scaling laws. The SiLENSe software module from STR-soft is used to investigate injection efficiency of the designed MQW structures. Simulation results indicate that the MQW structures offer better carrier confinement i.e. higher carrier injection efficiency compared with traditional bulk active regions which are currently used for mid-infrared LEDs and sensors. Experimental investigations show that the MQW LEDs with a seven wells structure show an increase of a factor 2 in wall plug efficiency and output power compared with conventional bulk LEDs at the same wavelength

Elimination of Lateral Resistance and Current Crowding in Large-Area LEDs by Composition Grading and Diffusion-Driven Charge Transport

Peer reviewe

Superior color rendering with a phosphor-converted blue-cyan monolithic light-emitting diode

The future generation of modern illumination should not only be cheap and highly efficient, but also demonstrate high quality of light, light which allows better color differentiation and fidelity. Here we are presenting a novel approach to create a white solid-state light source providing ultimate color rendition necessary for a number of applications. The proposed semi-hybrid device combines a monolithic blue-cyan light emitting diode (MBC LED) with a green-red phosphor mixture. It has shown a superior color rendering index (CRI), 98.6, at correlated color temperature of around 3400 K. The MBC LED epi-structure did not suffer from the efficiency reduction typical for monolithic multi-color emitters and was implemented in the two most popular chip designs: “epi-up” and “flip-chip”. Redistribution of the blue and cyan band amplitudes in the white-light emission spectrum, using the operating current, is found to be an effective tool for fine tuning the color characteristics. (Figure presented.)

Precise Temperature Mapping of GaN-Based LEDs by Quantitative Infrared Micro-Thermography

A method of measuring the precise temperature distribution of GaN-based light-emitting diodes (LEDs) by quantitative infrared micro-thermography is reported. To reduce the calibration error, the same measuring conditions were used for both calibration and thermal imaging; calibration was conducted on a highly emissive black-painted area on a dummy sapphire wafer loaded near the LED wafer on a thermoelectric cooler mount. We used infrared thermal radiation images of the black-painted area on the dummy wafer and an unbiased LED wafer at two different temperatures to determine the factors that degrade the accuracy of temperature measurement, i.e., the non-uniform response of the instrument, superimposed offset radiation, reflected radiation, and emissivity map of the LED surface. By correcting these factors from the measured infrared thermal radiation images of biased LEDs, we determined a precise absolute temperature image. Consequently, we could observe from where the local self-heat emerges and how it distributes on the emitting area of the LEDs. The experimental results demonstrated that highly localized self-heating and a remarkable temperature gradient, which are detrimental to LED performance and reliability, arise near the p-contact edge of the LED surface at high injection levels owing to the current crowding effect

Monolithically integrated white light LEDs on (11-22) semi-polar GaN templates

Carrier transport issues in a (11–22) semi-polar GaN based white light emitting diode (consisting of yellow and blue emissions) have been investigated by detailed simulations, demonstrating that the growth order of yellow and blue InGaN quantum wells plays a critically important role in achieving white emission. The growth order needs to be yellow InGaN quantum wells first and then a blue InGaN quantum well after the growth of n-type GaN. The fundamental reason is due to the poor hole concentration distribution across the whole InGaN quantum well region. In order to effectively capture holes in both the yellow InGaN quantum wells and the blue InGaN quantum well, a thin GaN spacer has been introduced prior to the blue InGaN quantum well. The detailed simulations of the band diagram and the hole concentration distribution across the yellow and the blue quantum wells have been conducted, showing that the thin GaN spacer can effectively balance the hole concentration between the yellow and the blue InGaN quantum wells, eventually determining their relative intensity between the yellow and the blue emissions. Based on this simulation, we have demonstrated a monolithically multi-colour LED grown on our high quality semi-polar (11–22) GaN templates