Fabrication of 380 nm ultra violet light emitting diodes on nano-patterned n-type GaN substrate

380 nm ultraviolet (UV) light emitting diodes (LEDs) were grown on patterned n-type GaN substrate (PNS) with silicon dioxide (SiO2) nano pattern to improve the light output efficiency. Wet etched self assembled indium tin oxide (ITO) nano clusters serves as dry etching mask for converting the SiO2 layer grown on n-GaN template into SiO2 nano patterns by inductively coupled plasma etching. Three different diameter of ITO such as 200, 250 and 300 nm were used for SiO2 nano pattern fabrication. PNS is obtained by n-GaN regrowth on SiO2 nano patterns and UV LEDs were grown on PNS template by MOCVD. Enhanced light output intensity was observed by employing SiO2 nano patterns on n-GaN. Among different PNS UV LEDs, LED grown on PNS with 300 nm ITO diameter showed enhancement in light output intensity by 2.1 times compared to the reference LED without PNS.     

Characteristics of dielectrophoretically aligned UV-blue GaN nanowire LEDs

This is a report on the characteristics of UV-blue light emitting diodes (LEDs) containing homojunction gallium nitride (GaN) nanowires (NWs). These LEDs were prepared by the dielectrophoresis assisted assembly deposition (DAAD) method. The incorporation of an additional silicon dioxide (SiO2) and a Au capping metal layer was found to improve the electrical and optical properties of the DAAD-prepared GaN NW LEDs. These LEDs exhibited a parasitic series resistance of 120-180 komega with a sharp turn-on forward voltage of 3.4-4.0 V and had as low as approximately approximately 7 x 10(-7) A of leakage current for a reverse bias of -10 V. Typical electroluminescence (EL) spectra, observed from these LEDs under a forward bias, had a broad ultraviolet (UV)-blue emission with a wavelength of 388-422 nm. These LEDs could be seen with the naked eye. We concluded that the high-brightness EL spectra resulted from the enhancement of the carrier injection due to the size effect of the n-GaN nanowires on the p-GaN substrate.     

Improvement in light output intensity of InGaN/GaN multiple-quantum-well blue light-emitting diode by SiO2/Si3N4 distributed Bragg reflectors and silver back mirror

In this article, the light output intensity of InGaN/GaN multiple-quantum-well (MQW) light emitting diodes (LEDs) is improved by using SiO₂/Si₃N₄ distributed Bragg reflectors (DBRs) as window layer and Ag back mirror. The SiO₂/Si₃N₄ DBRs can take several advantages, such as high reflectance with less number of DBR, passive characteristics, and high reliability due to growth in one pump down growth system. The experimental results indicated that InGaN/GaN LEDs with the 3-pair of SiO₂/Si₃N₄ DBRs show a maximum light output intensity of 64 mcd under 20 mA driving current and an improvement of 42% as compared to that of InGaN/GaN LEDs without SiO₂/Si₃N₄ DBRs. In addition, the turn-on voltage, forward resistance, and full width at half maximum (FWHM) of the emission spectra for InGaN/GaN LEDs with the 3-pair of SiO₂/Si₃N₄ DBRs and Ag back mirror are 3.23 V, 16 Ω, and 22.4 nm under 20 mA forward current.     

A cost-effective growth of SiO(x) thin films by reactive sputtering: photoluminescence tuning

We present a new cost-effective method to produce substoichiometric SiO2 thin films by means of a simple sputter-coater operated at a base pressure of 1 x 10(-3) mbar. During sputtering air is introduced through a fine valve so that the sputtering gas is a mixture of air/Ar. High-resolution electron microscopy shows the formation of amorphous SiO(x) thin films for the as-deposited samples. The index x approaches 1 when the ratio of the partial pressure of air/Ar tends to 0.1. On the other hand, pure silica is formed when the ratio of the partial pressure of air/Ar approaches 0.5. The films in the as-deposited state show intense green-yellow photoluminescence. This fades away with short annealing under air at 950 degrees C. If on the other hand, prolonged annealing is performed under Argon atmosphere at 1000 degrees C, red-infrared photoluminescence is recorded due to the formation of Si nanocrystals embedded in SiO2. This simple method could be suitable for the production of thin SiO(x) films with embedded nanocrystals for optoelectronic or photovoltaic applications.     

Study on modulating the indium composition in InGaN quantum wells to improve the luminous efficiency of GaN LED

This study primarily used metal-organic chemical vapor deposition to grow gallium nitride (GaN) light-emitting diode (LED) structures with InGaN quantum wells (QWs). During the InGaN QW growing process, an identical concentration of trimethylindium gas was prepared and introduced at different times (Before(B), Middle(M), and After(A)) into the QW structures for an investigation of the variation in GaN LED luminous efficacy. Because of segregation resulting from the different concentrations of In content of the InGaN QWs during the process and because of the stress resulting from lattice mismatch between atoms, the interaction between segregation and stress forms quantum dots (QDs). Under processes with the appropriate parameters, the QDs can improve the luminous efficacy of GaN LEDs. Postprocess LEDs were measured for their electroluminescence, photoluminescence, cathodoluminescence, thermal stability, light output power, and external quantum efficiency. The QW structures were analyzed and observed using high-resolution transmission electron microscopy. The results revealed that the Before (B) LED had the greatest light output power at 46.6 mW, an increase of approximately 15.6%. Thermal annealing was then used to treat the LED at 850 °C, after which the photoluminescence intensity increased by 1.7 times.

     

Electron Holographic Study of Semiconductor Light‐Emitting Diodes

Semiconductor light‐emitting diodes (LEDs), especially GaN‐based heterostructures, are widely used in light illumination. The lack of inversion symmetry of wurtzite crystal structures and the lattice mismatch at heterointerfaces cause large polarization fields with contributions from both spontaneous polarization and piezoelectric polarization, which in turn results in obvious quantum confined stark effect. It is possible to alleviate this effect if the local electrostatic fields and band alignment induced charge redistribution can be quantitatively determined across the heterostructures. In this Concept, the applications of electron holography to investigate semiconductor LEDs are summarized. Following the off‐axis electron holography scheme, the GaN‐based LED heterostructures including InGaN/GaN‐based quantum wells, other GaN‐based quantum wells, and other forms of GaN‐based LED materials are discussed, focusing on the local potential drops, polarization fields, and charge distributions. Moreover, GaAs‐based LED heterostructures are briefly discussed. The in‐line electron holography scheme emphasizes the capability of large area strain mapping across LED heterostructures with high spatial resolution and accuracy, which is combined with quantitative electrostatic measurements and other advanced transmission electron microscopy characterizations to provide an overall nanometer scale perspective of LED devices for further improvement in their electric and optical properties.     

InGaN-based nano-pillar light emitting diodes fabricated by self-assembled ITO nano-dots

InGaN/GaN based nano-pillar light emitting diodes (LEDs) with a diameter of 200-300 nm and a height of 500 nm are fabricated by inductively coupled plasma etching using self-assembled ITO nano-dots as etching mask, which were produced by wet etching of the as-deposited ITO films. The peak PL intensity of the nano-pillar LEDs was significantly higher than that of the as-grown planar LEDs, which can be attributed to the improvement of external quantum efficiency of the nano-pillar LEDs due to the large sidewall of the nano-pillars. We have also demonstrated electrical pumping of the InGaN/GaN based nano-pillar LEDs with a self-aligned TiO2 layer as a passivation of sidewall of the nano-pillars.     

MBE grown InGaN quantum dots and quantum wells: effects of in-plane localization

InGaN/GaN heterostructure samples were grown by molecular beam epitaxy using ammonia as a nitrogen precursor. The growth of InGaN/GaN self-assembled quantum dots was monitored in situ by reflection high energy electron diffraction intensity oscillations. Atomic force microscopy scans showed a very high density of InGaN islands, 1×10¹¹ cm⁻², well above the dislocation density. This could explain the increased radiative efficiency of these samples compared to homogeneous quantum wells. Light emitting diodes (LEDs) with InGaN active layers buried in GaN were realized. Electroluminescence and photocurrent spectra of these LEDs evidence a strong Stokes shift that can be attributed to high localization of carriers in InGaN layers.     

Fabrication of three-dimensional autocloned photonic crystal on sapphire substrate

We applied the laser interference lithography method to form a patterned sapphire substrate (PSS). A three-dimensional photonic crystal was formed by autocloning the PSS with alternate Ta2O5/SiO2 coatings. A high total integrated reflectance (TIR) band was obtained around the 410 to 470?nm wavelength range that matched the emission spectrum of the gallium nitride (GaN) light-emitting diode (LED) for application in manipulating the light extraction of the sapphire-based GaN LED.     

Enhancement of Light Output Power from LEDs Based on Monolayer Colloidal Crystal

One of the major challenges for the application of GaN‐based light emitting diodes (LEDs) in solid‐state lighting lies in the low light output power (LOP). Embedding nanostructures in LEDs has attracted considerable interest because they may improve the LOP of GaN‐based LEDs efficiently. Recent advances in nanostructures derived from monolayer colloidal crystal (MCC) have made remarkable progress in enhancing the performance of GaN‐based LEDs. In this review, the current state of the art in this field is highlighted with an emphasis on the fabrication of ordered nanostructures using large‐area, high‐quality MCCs and their demonstrated applications in enhancement of LOP from GaN‐based LEDs. We describe the remarkable achievements that have improved the internal quantum efficiency, the light extraction efficiency, or both from LEDs by taking advantages of diverse functions that the nanostructures provided. Finally, a perspective on the future development of enhancement of LOP by using the nanostructures derived from MCC is presented.     

Synthesis and characteristics of tb-doped Y2SiO5 nanophosphors and luminescent layer for enhanced photovoltaic cell performance

Nanophosphors based on green emitting terbium doped yttrium silicates with the general formula Y2SiO5:Tb3+ with a size of 30-60 nm were synthesized by the hydrothermal method. The prepared nanophosphors were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy and fluorescence spectroscopy. It was found that the nanophosphors crystallize in an X1-type monoclinic structure (P2(1)/c) and absorb UV light from 220 to 300 nm which they then down-convert into visible-light (strong green emission at around 545 nm (5D4-->7F(J. As TiO2-based dye-sensitized solar cells exhibit their maximum incident photon to current efficiency at around 500-600 nm, the wavelength-modulation characteristics of the nanophosphors can be efficient for dye-sensitized solar cell systems. Therefore, the Y2SiO5:Tb3+ nanophosphors were introduced into a TiO2-based dye-sensitized solar cell and their effects on the performance of the solar cell were investigated.     

Investigating entrained air voids and Portland cement hydration with low-temperature scanning electron microscopy

This paper describes the application of low temperature scanning electron microscopy to the materials science of Portland cement. The details of low-temperature scanning electron microscopy are described, along with a number of specimen preparation techniques. There are three main research topics presented in this paper: (1) ice morphology in entrained air voids, (2) development of air voids during early hydration and (3) progression of hydration in Portland cement. The first research focus examines ice in air voids at freezing temperatures, and various cement paste ages. The second research focus tracks the development of the air voids during the first hour of hydration. In the third research focus, the progression of hydration with and without accelerating and retarding admixtures is described. Each of these research programs demonstrates how low-temperature scanning electron microscopy can be an effective tool in Portland cement research.     

Enhanced performance of GaN-based LEDs via electroplating of a patterned copper layer on the backside

InGaN/GaN multi-quantum well light-emitting diodes (LEDs) are conventionally grown on a sapphire substrate due to a lack of compatible substrates with a high compressive strain. This is a result of the relatively large lattice, and thermal expansion coefficient mismatches between GaN and sapphire. The compressive strain is considered to be a major obstacle to further improve next-generation high-performance GaN-based LEDs. In this paper, we have designed, electroplated, and tested an efficient substrate using a patterned copper (Cu) layer on the backside of sapphire to relax the compressive strain in a GaN epilayer. The patterned Cu layer has a significant function in that it supports the GaN/sapphire LEDs with an external tensile stress. The external tensile stress is capable of compensating for the compressive strain in the GaN/sapphire LEDs by controlling the curvature of the wafer bowing. This patterned Cu layer, when applied to the GaN/sapphire LEDs, suppresses the compressive strain by up to 0.28 GPa. The GaN-based LEDs on this innovative and effective sapphire/Cu substrate offer improved optical and electrical performance.     

Controlling electron overflow in phosphor-free InGaN/GaN nanowire white light-emitting diodes

We have investigated for the first time the impact of electron overflow on the performance of nanowire light-emitting diodes (LEDs) operating in the entire visible spectral range, wherein intrinsic white light emission is achieved from self-organized InGaN quantum dots embedded in defect-free GaN nanowires on a single chip. Through detailed temperature-dependent electroluminescence and simulation studies, it is revealed that electron leakage out of the device active region is primarily responsible for efficiency degradation in such nanowire devices, which in conjunction with the presence of nonradiative surface recombination largely determines the unique emission characteristics of nanowire light-emitting diodes. We have further demonstrated that electron overflow in nanowire LEDs can be effectively prevented with the incorporation of a p-doped AlGaN electron blocking layer, leading to the achievement of phosphor-free white light-emitting diodes that can exhibit for the first time virtually zero efficiency droop for injection currents up to ~2200 A/cm(2). This study also provides unambiguous evidence that Auger recombination is not the primary mechanism responsible for efficiency droop in GaN-based nanowire light-emitting diodes.     

Various nanostructures on macroscopically large areas prepared by tunable ion-swelling

Various nanostructures were fabricated by ion irradiation on large area (100) Si surfaces covered by colloidal Langmuir-Blodgett films as nanolithographic masks. The ordered structure of the Langmuir-Blodgett monolayer composed from spherical St?ber silica particles of 200 nm and 450 nm diameter offer the possibility to form local surface swelling patterns during the ion bombardment step. Utilizing the dependence of the surface morphology on the irradiation parameters the tunability of nanostructuring was studied for 40 keV Ar+ and 500 keV Xe2+ ions. We show that the periodicity of the resulted surface pattern is determined by the size of the masking particles, while the height of nanostructures can be tuned by the ion fluence. The quality of projection of the nanomask contours to the substrate-the contrast of masking-can be set by choosing appropriate ion energy, thereby determining the curvature of the surface pattern. Moreover, deformation of the nanomask due to ion-nanoparticle interactions should be taken into account since these effects can be also utilized for tailoring various structures. The silica masking layers before and after ion irradiation and the resulting Si surface patterns were investigated by field emission scanning electron microscopy and atomic force microscopy analysis.     

Enhanced performance of GaN-based light-emitting diodes by using a p-InAlGaN/GaN superlattice as electron blocking layer

The electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) with various kinds of electron blocking layers (EBLs) are analyzed numerically. The results indicate that an enhanced hole injection efficiency and a reduced electron leakage could be achieved with the GaN-based LED where a p-InAlGaN/GaN superlattice (SL) was employed as EBL as compared with the conventional GaN-based LEDs using rectangular p-AlGaN EBL or p-AlGaN/GaN SL EBL. Moreover, it was found that the efficiency droop could be significantly improved at high injection current density for GaN-based LEDs with p-InAlGaN/GaN SL EBL.     

Effect of ammoniating temperature on microstructure of one-dimensional GaN nanorods with Tb intermediate layer

GaN nanorods have been successfully synthesized on Si (111) substrates by magnetron sputtering through ammoniating Ga₂O₃/Tb thin films. The influence of ammonating temperatures on microstructure, morphology and light emitting properties of GaN nanorods was ananlyzed in detail using X-ray diffraction, X-ray photoelectron spectroscopy, FT-IR spectrophotometer, scanning electron microscopy, high- resolution transmission electron microscopy, and photoluminescence spectroscopy. The results demonstrate that the GaN nanorods are single crystalline and exhibit hexagonal wurtzite symmetry. The highest crystalline quality was achieved at 950 °C for 15 min with the size of 100–150 nm in diameter, which have an excellent light emitting properties. A small red-shift occurs due to band-gap change caused by the tensile stress.     

Structural and optical characteristics of InN/GaN multiple quantum wells grown by metalorganic chemical vapor deposition

The structural and electrical properties of InN/GaN multiple quantum wells, which were grown by metalorganic chemical vapor deposition, were characterized by transmission electron microscopy (TEM) and electroluminescence measurements. From the TEM micrographs, it was shown that the well layer was grown like a quantum dot. The well layer is expected to be the nano-size structures in the InN multiple quantum well layers. The multi-photon confocal laser scanning microscopy was used to investigate the optical properties of the light emitting diode (LED) structures with InN active layers. It was found that the two-photon excitation was possible in InN system. The pit density was measured by using the far-field optical technique. In the varied current conditions, the blue LED with the InN multiple quantum well structures did not have the wavelength shift. With this result, we can expect that the white LEDs with the InN multiple quantum well structures do not show the color temperature changes with the variations of applied currents.     

Structural and optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition

Silicon nanocrystals (Si-nc) embedded in SiO2 matrix have been prepared by high temperature thermal annealing (1000-1250 degrees C) of substoichiometric SiOx films deposited by plasma-enhanced chemical vapor deposition (PECVD). Different techniques have been used to examine the optical and structural properties of Si-nc. Transmission electron microscopy analysis shows the formation of nanocrystals whose sizes are dependent on annealing conditions and deposition parameters. The spectral positions of room temperature photoluminescence are systematically blue shifted with reduction in the size of Si-nc obtained by decreasing the annealing temperature or the Si content during the PECVD deposition. A similar trend has been found in optical absorption measurements. X-ray absorption fine structure measurements indicate the presence of an intermediate region between the Si-nc and the SiO2 matrix that participates in the light emission process. Theoretical observations reported here support these findings. All these efforts allow us to study the link between dimensionality, optical properties, and the local environment of Si-nc and the surrounding SiO2 matrix.     

Submicrometre resolved optical characterization of green nanowire-based light emitting diodes

The electroluminescent properties of InGaN/GaN nanowire-based light emitting diodes (LEDs) are studied at different resolution scales. Axial one-dimensional heterostructures were grown by plasma-assisted molecular beam epitaxy (PAMBE) directly on a silicon (111) substrate and consist of the following sequentially deposited layers: n-type GaN, three undoped InGaN/GaN quantum wells, p-type AlGaN electron blocking layer and p-type GaN. From the macroscopic point of view, the devices emit light in the green spectral range (around 550 nm) under electrical injection. At 100 mA DC current, a 1 mm2 chip that integrates around 10(7) nanowires emits an output power on the order of 10 μW. However, the emission of the nanowire-based LED shows a spotty and polychromatic emission. By using a confocal microscope, we have been able to improve the spatial resolution of the optical characterizations down to the submicrometre scale that can be assessed to a single nanowire. Detailed μ-electroluminescent characterization (emission wavelength and output power) over a representative number of single nanowires provides new insights into the vertically integrated nanowire-based LED operation. By combining both μ-electroluminescent and μ-photoluminescent excitation, we have experimentally shown that electrical injection failure is the major source of losses in these nanowire-based LEDs.