Ultraviolet Light Emission Research Articles

Are AlN and GaN substrates useful for the growth of non‐polar nitride films for UV emission? The oscillator strength perspective

AbstractThe authors present results of a perturbation theory study of the effect of strain on the oscillator strengths of interband transitions in wurtzite group‐III‐nitride films suitable for ultraviolet light emission applications. Ternary alloy films are investigated, which can be pseudomorphically grown on GaN and AlN substrates with nonpolar M ‐plane (1$ \bar 1 $00) and A ‐plane (11$ \bar 2 $0) orientations. Valence band mixing, induced by the anisotropic in‐plane strain that arises in these films, can have a dramatic influence on the optical polarization properties of the transitions. An increased efficiency of light emission in the 0.21 μm to 0.25 μm spectral range is best achieved using AlN substrates, on which both Al1–x Gax N and Al1–x Inx N films experience compressive strain. On GaN substrates, Al1–x Gax N films experience tensile strain and will exhibit poor light emission efficiency. However Al1–x Inx N films on GaN substrates can emit light efficiently in the 0.3 μm to 0.36 μm spectral range. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Combustion synthesis of β-Ca 1.95P 2O 7:0.05Eu 2+ blue phosphor for near ultraviolet excitation

Synthesis of a near ultraviolet light emitting phosphor β-Ca 1.95P 2O 7:0.05Eu 2+ for white LEDs by combustion method is reported. The photoluminescence, powder analysis and particle morphology of the phosphors were investigated using a luminescence spectrometer (LS), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). It was found that when the initiating combustion temperature reached 1000 °C, the phase of γ-Ca(PO 3) 2 which begun to appear at 700 °C, transformed into the target material – the phase of β-Ca 2P 2O 7. And the phase transition leads to a red shift of the emission peak about 9 nm. TEM images also confirm those results. The emission spectrum shows a single intense band centered at 421 nm while the excitation spectrum is a broad band extending from 250 nm to 410 nm, with two bands centered at 330 nm and 387 nm. The band at 387 nm is a main excitation peak matching the emission of near ultraviolet light (370–410 nm). The investigated phosphor is suitable for a NUV LED excitation.

Characteristics of high‐quality homoepitaxial ZnO (0001) films grown by the plasma‐assisted reactive evaporation method

AbstractHomoepitaxial growth of high‐quality non‐doped and nitrogen‐doped ZnO (0001) films could be achieved by the plasma‐assisted reactive evaporation (PARE) method under the conditions of constant oxygen gas flow rate and several nitrogen gas flow (Nf) rates for nitrogen doping. The values of FWHM of XRC for (0002) and (10‐10) planes of the non‐doped ZnO thin film were 31 and 35 arcsec., respectively. The values of FWHM of XRC for (0002) and (10‐10) planes of the nitrogen‐doped ZnO thin film were 49 and 38 arcsec., respectively. The RMS roughness of surfaces of those films measured by AFM was about 0.1‐0.2 nm. In PL spectra (5 K) of the films, the intensity of D0X emission was decreased with increase in Nf rate. However, A0X and DAP emissions were clearly observed in the films grown under conditions of nitrogen gas flow. The intensity ratio between A0X and D0X emissions increased with increase in Nf rate. A‐free exciton (FEA) and n=2 state of FEA emissions were observed at around 3.378 and 3.424 eV, respectively, independently of the Nf rate. Again, in PL spectra (300 K) of the films, the intensity of visible light emission decreased and that of near‐band‐edge emission also decreased with increase in Nf rate. PL spectra of nitrogen‐doped ZnO thin films and emission of ultraviolet light from diodes fabricated using the nitrogen‐doped ZnO films showed that the homoepitaxial films grown by the PARE method were of high quality and that nitrogen atoms fulfilled oxygen vacancies and acted as acceptors in the nitrogen‐doped ZnO thin films. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Treatment of Streptococcus mutans bacteria by a plasma needle

A dielectric barrier discharge plasma needle was realized at atmospheric pressure with a funnel-shaped nozzle. The preliminary characteristics of the plasma plume and its applications in the inactivation of Streptococcus mutans (S. mutans), the most important microorganism causing dental caries, were presented in this paper. The temperature of the plasma plume does not reach higher than 315 K when the power is below 28 W. Oxygen was injected downstream in the plasma afterglow region through the powered steel tube. Its effect was studied via optical-emission spectroscopy, both in air and in agar. Results show that addition of 26 SCCM O2 does not affect the plume length significantly (SCCM denotes cubic centimeter per minute at STP). The inactivation of S. mutans is primarily attributed to ultraviolet light emission, O, OH, and He radicals.

Photoluminescence of as-grown and annealed ZnO films on Ti-buffered Si(111) substrates

ZnO thin films with strong c-axis preferred orientation have been deposited on Ti buffered Si （111） substrate by reactive ratio-frequency magnetron sputtering. With X-ray diffraction analysis and photoluminescence （PL） measurement， the structure， residual stress and PL emission properties of Ti-buffered ZnO films are studied and effects of Ti-layer thickness and annealing temperature are discussed. It is found that the PL properties of the films are improved after the introduction of Ti buffer layer and an optimized thickness of Ti buffer layer is given. Annealing treatment can improve the properties of Si（111）/Ti-buffer/ZnO films further. The residual stress is found to be an important factor influencing the emission properties of ultraviolet light. It is helpful for the emission of ultraviolet light if the films have a small residual stress. The residual stress can also change the transition energy of excitons. With the increase of annealing temperature， the increase of tension strain in films reduces the band gap of ZnO， hence results in the shift of the excitonic peak to lower energy.

Selective Growth of ZnO Nanorod Arrays on a GaN/Sapphire Substrate Using a Proton Beam Written Mask

ZnO nanorod arrays were grown on a GaN/sapphire substrate in a controllable way using a hydrothermal growth method. Proton beam writing (PBW), a direct-write 3D lithographic technique, was used to pattern a polymethyl methacrylate (PMMA) mask spin-coated on to a GaN substrate. ZnO, which has the same wurtzite crystal structure and a low lattice misfit of about 1.9% compared with GaN, nucleated and grew into vertical rods at positions where the GaN was exposed. The structural and optical characteristics of the ZnO nanorods were further investigated using X-ray diffraction (XRD), and microphotoluminescence spectroscopy (μ-PL). Annealing of the ZnO nanorods in nitrogen gas significantly improved the ultraviolet (UV) light emission at 380 nm wavelength, and successfully decreased the yellow and green band emission considerably. These results show new potential applications for devices based on ZnO nanostructures.

Laser wavelength dependence of extreme ultraviolet light and particle emissions from laser-produced lithium plasmas

Maximum extreme ultraviolet (EUV) conversion efficiencies (CEs) of 2.3% and 1.8% were achieved in planar Li targets by using pulsed 2ω and 1ω Nd:YAG laser irradiation, respectively. In a forced recombination scheme, the total CE can be expected to be about 4%. The maximum kinetic energy of the lithium ion debris was found to be less than 1 keV, indicating that mirror damage caused by lithium ion debris is more easily mitigated by using a magnetic field than for tin ions. These results suggest that a Li target is a reasonable candidate for an EUV lithography source.

Electrical and light‐emitting properties of homoepitaxial diamond p–i–n junction

AbstractWe have investigated the electrical and light‐emitting characteristics of (001)‐oriented homoepitaxial diamond p–i–n junction diodes with the boron‐doped p‐type, non‐doped intrinsic, and phosphorus‐doped n‐type layers formed by applying an optimized homoepitaxial growth technique based on microwave plasma‐enhanced chemical vapor deposition. High‐performance p–i–n junction characteristics were confirmed from current–voltage and capacitance–voltage properties. A strong ultraviolet light emission at around 240 nm due to free exciton recombination was observed at a forward current of over 6 mA, while the broad visible light emission from deep levels was significantly suppressed compared to that of reported electroluminescence in diamond p–n junctions. It was elucidated that the excitonic emission intensity increases superlinearly for the current dependence and is stable even at the high temperature of 200 °C. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Linear and nonlinear optical characteristics of ZnO-SiO_2 nanocomposites

We present the spectral and nonlinear optical properties of ZnO-SiO2 nanocomposites prepared by colloidal chemical synthesis. Obvious enhancement of ultraviolet (UV) emission of the samples is observed, and the strongest UV emission of a typical ZnO-SiO2 nanocomposite is over three times stronger than that of pure ZnO. The nonlinearity of the silica colloid is low, and its nonlinear response can be improved by making composites with ZnO. These nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behavior. The observed nonlinear absorption is explained through two photon absorption followed by weak free carrier absorption and nonlinear scattering. The nonlinear refractive index and the nonlinear absorption increase with increasing ZnO volume fraction and can be attributed to the enhancement of exciton oscillator strength. ZnO-SiO2 is a potential nanocomposite material for the UV light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.

Chemical solution deposition of ZnO thin films with controlled crystallite orientation and intense ultraviolet emission

ZnO thin films with controlled crystallite orientation have been grown on seeded glass substrate via a chemical solution deposition. The influence of solvent on the ZnO film deposition has been investigated. We have demonstrated that the film texture can be easily controlled by varying the volume ratio of water to ethanol: the crystal orientation ([001] direction) in the films is found to be gradually tilted with increasing ethanol content. The as-deposited films exhibit a dominant ultraviolet emission at 380 nm, which becomes significantly intense and gives a full width at half maximum as narrow as 112 meV when the samples are post-annealed at 400 °C. A high optical quality of the films prepared by the simple wet-chemical route employed here would allow their possible application for ultraviolet light emission.

AlGaN‐based heterostructures grown on 4 inch Si(111) by MOVPE

AbstractAlGaN is an important material for ultra‐violet light emitters, photo detectors and high breakdown voltage switching devices. In this work, crack‐free AlxGa1‐xN (5% < x < 55%) layers up to 1.5 μm have been grown on 4 inch Si(111) using an AlN/AlyGa1‐yN (y > 70%) template. The structural quality of AlGaN layers is comparable to that of GaN layers grown on silicon(111). For Al0.16Ga0.84N, the FWHM of HR‐XRD (0002) and (‐1102) ω‐scan is around 650 arc sec and 1200 arc sec respectively. Based on this high quality AlGaN buffer, a double heterostructure (DH) FET was demonstrated. The sheet resistance of DH‐FET is 274 ± 4.7 Ω/□ and the uniformity value of 1.7% is also excellent. Some of the wafers have been processed. Devices with a gate length of 2 μm showed current density of 500 mA/mm and transconductance of more than 200 mS/mm. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of ultraviolet-light emitting Si/SiO 2 multilayered films using radio-frequency magnetron sputtering and high-temperature annealing

Abstracts We fabricated Si/SiO 2 multilayered films having nanometer-order thicknesses of Si and SiO 2 layers by using radio-frequency magnetron sputtering and subsequently annealed the samples at high temperatures of 1150 to 1250 °C. We observed ultraviolet photoluminescence having a sharp peak at a wavelength of around 370 nm, generated by using a He–Cd laser (wavelength λ = 325 nm) at room temperature, from a sample annealed at 1200 °C. The peak energy was 3.4 eV ( λ ~ 370 nm), and the full width at half maximum of the peak was 0.20 eV. Ultraviolet-light emission can be useful for light sources of higher-density optical-disk systems.

ZnO and Related Materials for Sensors and Light-Emitting Diodes

ZnO is an attractive material for ultraviolet (UV) light emitters and detectors, and transparent thin-film transistors. It is also readily synthesized in the form of nanostructures. In this paper we discuss the fabrication of a number of ZnO thin-film and nanowire devices, including transistors, diodes, and UV and pH sensors. ZnO has been effectively used as a gas sensor material based on the near-surface modification of charge distribution with surface-absorbed species. The large surface area of the nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for high-density sensor arrays.

Spectral and nonlinear optical characteristics of nanocomposites of ZnO–Ag

In this Letter we present the spectral and nonlinear optical properties of ZnO–Ag nanocomposites prepared by colloidal chemical synthesis. Obvious enhancement of ultraviolet (UV) emission of the samples is observed and the strongest UV emission is over three times than that of pure ZnO. These nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behaviour which increases with increasing Ag volume fraction. The observed nonlinear absorption is explained through two photon absorption followed by free carrier absorption. ZnO–Ag is a potential nanocomposite material for the UV light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.

Zinc Oxide Nano-Cauliflower Array with Room Temperature Ultraviolet Light Emission

A novel geometry of a ZnO nanocauliflower array has been fabricated by an electrochemical route of the electrophoresis deposition of anionic polystyrene spheres on a conductive glass substrate followed by the electrodeposition of ZnO in aqueous solutions. The structural and optical characteristics were investigated by using X-ray diffraction, field-emission scanning electron microscopy, and photoluminescence spectrum measurements. The ZnO nanocauliflower was constructed of a polystyrene sphere core and a shell of ZnO hexagonal columns radially grown from the sphere surface. The ZnO nanocauliflower array possessed a high quality that emits ultraviolet light at a photon energy of 3.3 eV at room temperature due to the recombination of bound excitons.

Optimization of Extreme Ultraviolet Emission from Laser-Produced Tin Plasmas Based on Radiation Hydrodynamics Simulations

We investigated the plasma conditions for obtaining highly efficient extreme ultraviolet light from laserproduced tin plasmas for lithography of next generation semiconductors. Based on accurate atomic data tables calculated using the detailed configuration accounting code, we conducted 1-D radiation hydrodynamic simulations to calculate the dynamics of tin plasma and its emission of extreme ultraviolet light. We included the photo-excitation effect in the radiation transport. Our simulation reproduced experimental observations successfully. Using our verified code, we found that a CO2 laser can be useful in obtaining higher conversion efficiencies up to 4%.

Fabrication of Ultraviolet-light Emission Si/SiO<SUB>2</SUB> Multilayered Films by Using RF Sputtering

Fabrication of Ultraviolet-light Emission Si/SiO<SUB>2</SUB> Multilayered Films by Using RF Sputtering

Continuous production of fine zinc oxide nanorods by hydrothermal synthesis in supercritical water

Continuous production of highly crystalline ZnO nanorods by supercritical hydrothermal synthesis was reported in this article. Zinc nitrate aqueous solution was pressurized to 30 MPa at room temperature and rapidly heated to 673 K by mixing with supercritical water and then fed into a tubular reactor. Residence time is about 10 s. Production of ZnO nanorod particles with uniform particle size distribution showed a strong ultraviolet light emission at room temperature. This article also reported in-situ surface modification of ZnO nanorods with organic reagents by the supercritical hydrothermal synthesis.

Ultraviolet Light Emission from Si in a Scanning Tunneling Microscope

Ultraviolet and visible radiation is observed from the contacts of a scanning tunneling microscope with Si(100) and (111) wafers. This luminescence relies on the presence of hot electrons in silicon, which are supplied, at positive bias on n- and p-type samples, through the injection from the tip, or, at negative bias on p samples, by Zener tunneling. Measured spectra reveal a contribution of direct optical transitions in Si bulk. The necessary holes well below the valence band edge are injected from the tip or generated by Auger processes.

Compact fiber-optic fluorosensor employing light-emitting ultraviolet diodes as excitation sources

A compact fluorosensor using three different ultraviolet light-emission diodes as excitation sources for fiber-optic recording of fluorescence spectra from samples is described. A compact integrated spectrometer with linear array wavelength recording is used, yielding a spectral resolution of about 8 nm. In two system implementations ultraviolet light-emitting diodes at 300, 340 and 395 nm, or at 360, 385 and 410 nm were used as excitation sources with typical emission halfwidths of 12 nm, each combined with a matching long-path colored-glass filter automatically brought into the fluorescence light flow for suppression of reflected light. Spectra from measurements on vegetation, human skin tumors and a rare-earth ion-based thermographic phosphor were recorded to illustrate the system performance.