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A comprehensive study of surface passivation effect on porous fluorescent silicon carbide (SiC) was carried out to elucidate the luminescence properties by temperature dependent photoluminescence (PL) measurement. The porous structures were prepared using an anodic oxidation etching method and passivated by atomic layer deposited (ALD) Al2O3 films. An impressive enhancement of PL intensity was observed in porous SiC with ALD Al2O3, especially at low temperatures. At temperatures below 150 K, two prominent PL emission peaks located at 517 nm and 650 nm were observed. The broad emission peak at 517 nm was attributed to originate from the surface states in the porous structures, which was supported by X-ray photoelectron spectra characterization. The emission peak at 650 nm is due to donor-acceptor-pairs (DAP) recombination via nitrogen donors and boron-related double D-centers in fluorescent SiC substrates. The results of the present work suggest that the ALD Al2O3 films can effectively suppress the non-radiative recombination for the porous structures on fluorescent SiC. In addition, we provide the evidence based on the low-temperature time-resolved PL that the mechanism behind the PL emission in porous structures is mainly related to the transitions via surface states.

We report an adhesive bonding approach using hydrogen silsesquioxane (HSQ) for silicon carbide (SiC) samples. A hybrid light-emitting diode (LED) was successfully fabricated through bonding a near-ultraviolet (NUV) LED grown on a commercial 4H-SiC substrate to a free-standing boron-nitrogen co-doped fluorescent-SiC epi-layer. The bonding quality and the electrical performance of the hybrid LED device were characterized. Neither voids nor defects were observed which indicates a good bonding quality of the proposed HSQ approach. A strong warm white emission was successfully obtained from the hybrid LED through an electric current injection of 30 mA. Funding Agencies|Innovation Fund Denmark [4106-00018B]

Development of antibiotic resistance is a major challenge for antibiotics as an effective treatment approach of infectious diseases and pathogenic microbes with resistance to antibiotics will become difficult to be treated. Therefore, a new therapy method, which is safe and can inactivate pathogenic microbes effectively without developing a resistance, is highly needed. Ultraviolet irradiation is well known for its ability of effective microbial inactivation and it is widely used in sterilization of inanimate objects based on conventional ultraviolet light sources. Meanwhile, applying ultraviolet irradiation in human disinfection application is an emerging and rapidly progressing field. This review focuses on recent studies in ultraviolet based disinfection methods including both animal and human studies. We will introduce different microbial inactivation mechanisms, which are associated with the ultraviolet irradiation wavelength. Relevant research work will be summarized with a focus on their microbial inactivation effect and safety issues.

AbstarctWe report for the first time a NUV light to white light conversion in a N-B co-doped 6H-SiC (fluorescent SiC) layer containing a hybrid structure. The surface of fluorescent SiC sample contains porous structures fabricated by anodic oxidation method. After passivation by 20 nm thick Al2O3, the photoluminescence intensity from the porous layer was significant enhanced by a factor of more than 12. Using a porous layer of moderate thickness (~10 µm), high-quality white light emission was realized by combining the independent emissions of blue-green emission from the porous layer and yellow emission from the bulk fluorescent SiC layer. A high color rendering index of 81.1 has been achieved. Photoluminescence spectra in porous layers fabricated in both commercial n-type and lab grown N-B co-doped 6H-SiC show two emission peaks centered approximately at 460 nm and 530 nm. Such blue-green emission phenomenon can be attributed to neutral oxygen vacancies and interface C-related surface defects generated dring anodic oxidation process. Porous fluorescent SiC can offer a great flexibility in color rendering by changing the thickness of porous layer and bulk fluorescent layer. Such a novel approach opens a new perspective for the development of high performance and rare-earth element free white light emitting materials.

The influence of thickness of atomic layer deposited Al2O3 films on nanotextured fluorescent 6H-SiC passivation is investigated. The passivation effect on the light emission has been characterized by photoluminescence and time-resolved photoluminescence at room temperature. The results show that 20nm thickness of Al2O3 layer is favorable to observe a large photoluminescence enhancement (25.9%) and long carrier lifetime (0.86ms). This is a strong indication for an interface hydrogenation that takes place during post-thermal annealing. These result show that an Al2O3 layer could serve as passivation in fluorescent SiC based white LEDs applications.

We have investigated the use of nanopillar structures on high indium content InGaN amber multiple quantum well (MQW) samples to enhance the emission efficiency. A significant emission enhancement was observed which can be attributed to the enhancement of internal quantum efficiency and light extraction efficiency. The size-dependent strain relaxation effect was characterized by photoluminescence, Raman spectroscopy and time-resolved photoluminescence measurements. In addition, the light extraction efficiency of different MQW samples was studied by finite-different time-domain simulations. Compared to the as-grown sample, the nanopillar amber MQW sample with a diameter of 300 nm has demonstrated an emission enhancement by a factor of 23.8.

LEDs have a large potential in many dental and oral applications. Areas such as photo polymerization, fluorescence imaging, photodynamic therapy, and photoactivated disinfection are important future candidates for LED based diagnostics and treatment in dentistry.

Porous silicon carbide (B–N co-doped SiC) produced by anodic oxidation showed strong photoluminescence (PL) at around 520 nm excited by a 375 nm laser. The porous SiC samples were passivated by atomic layer deposited (ALD) aluminum oxide (Al2O3) films, resulting in a significant enhancement of the PL intensity (up to 689%). The effect of thickness, annealing temperature, annealing duration and precursor purge time on the PL intensity of ALD Al2O3 films was investigated. In order to investigate the penetration depth and passivation effect in porous SiC, the samples were characterized by X-ray photoelectron spectroscopy (XPS) and time-resolved PL. The optimized passivation conditions (20 nm Al2O3 deposited at 160 °C with purge time of 20 s, followed by an annealing for 5 min at 350 °C) for porous SiC were achieved and the results indicate that surface passivation by ALD Al2O3 thin films is a very effective method to enhance the luminescence efficiency of porous SiC.

Homoepitaxial layers of fluorescent 4H-SiC were grown on 4 degree off-axis substrates by sublimation epitaxy. Luminescence in the green spectral range was obtained by co-doping with nitrogen and boron utilizing donor-acceptor pair luminescence. This concept opens possibilities to explore green light emitting diodes using a new materials platform.

Polycrystalline doped SiC act as source for fluorescent SiC. We have studied the growth of individual grains with different polytypes in the source material. We show an evolution and orientation of grains of different polytypes in polycrystalline SiC ingots grown by the Physical Vapor Transport method. The grain influence on the growth rate of fluorescent SiC layers grown by a sublimation epitaxial process is discussed in respect of surface kinetics.

Surface nanocones on 6H-SiC have been developed and demonstrated as an effective method of enhancing the light extraction efficiency from fluorescent SiC layers. The surface reflectance, measured from the opposite direction of light emission, over a broad bandwidth range is significantly suppressed from 20.5% to 1.0 % after introducing the sub-wavelength structures. An omnidirectional light harvesting enhancement (>91%), is also achieved which promotes fluorescent SiC as a good candidate of wavelength converter for white light-emitting diodes.

We report an approach of using an interlayer of single layer graphene (SLG) for electroluminescence (EL) enhancement of an InGaN/GaN-based near-ultraviolet (NUV) light-emitting diode (LED) with an aluminum-doped zinc oxide (AZO)-based current spreading layer (CSL). AZO-based CSLs with and without a SLG interlayer were fabricated on the NUV LED epi-wafers. The current-voltage (I-V) characteristic and the EL intensity were measured and compared. We find that the LED without the SLG interlayer can possess a 40% larger series resistance. Furthermore, a 95% EL enhancement was achieved by the employment of the SLG interlayer.

Enhanced transmittance of 6H-SiC with nanocone structures were achieved by using self-assembled Au nanoparticles as etching mask. HF passivation process of nanocone structures was investigated to further improve the transmittance. The max transmittance of structured SiC is significantly improved by 10%.

Surface plasmonics from metal nanoparticles have been demonstrated as an effective way of improving the performance of low-efficiency light emitters. However, reducing the inherent losses of the metal nanoparticles remains a challenge. Here we study the enhancement properties by Ag nanoparticles for InGaN/GaN quantum-well structures. By using a thin SiN dielectric layer between Ag and GaN we manage to modify and improve surface plasmon coupling effects, and we attribute this to the improved scattering of the nanoparticles at the quantum-well emission wavelength. The results are interpreted using numerical simulations, where absorption and scattering cross-sections are studied for different sized particles on GaN and GaN/SiN substrates.

We demonstrate a time-efficient and low-cost approach to fabricate Si3N4 coated nanodome structures in fluorescent SiC. Nanosphere lithography is used as the nanopatterning method and SiC nanodome structures with Si3N4 coating are formed via dry etching and thin film deposition process. By using this method, a significant broadband surface antireflection and a considerable omnidirectional luminescence enhancement are obtained. The experimental observations are then supported by numerical simulations. It is believed that our fabrication method will be well suitable for large-scale production in the future.

Donor-acceptor co-doped silicon carbide layers are promising light converters for novel monolithic all-semiconductor LEDs due to their broad-band donor-acceptor pair luminescence and potentially high internal quantum efficiency. Besides appropriate doping concentrations yielding low radiative lifetimes, high nonradiative lifetimes are crucial for efficient light conversion.Despite the excellent crystalline quality that can generally be obtained by sublimation epitaxy according to XRD measurements, the role of defects in f-SiC is not yet well understood. Recent results from room temperature photoluminescence, charge carrier lifetime measurements by microwave detected photoconductivity and internal quantum efficiency measurements suggest that the internal quantum efficiency of f-SiC layers is significantly affected by the incorporation of defects during epitaxy. Defect formation seems to be related to nitrogen incorporation from the growth ambient while nitrogen doping from the source yielded better results regarding quantum efficiency. To investigate the presence of different types of defects in f-SiC layers and their impact on the fluorescent properties of f-SiC, this study will focus on defect characterization of f-SiC layers grown under different process conditions, especially different growth ambient and using differently doped source material. The results may help to identify critical process parameters and reduce the concentration of relevant defects.

Ag nanodisks are fabricated on GaN-based LED to enhance emission efficiency. Nanosphere lithography is used to obtain a periodic nano-structure, and a photoluminescence enhancement of 2.7 is reported with Ag nanodisk diameter of 330 nm.

An approach of fabricating pseudoperiodic antireflective subwavelength structures on silicon carbide by using self-assembled Au nanopatterns as etching mask is demonstrated. The nanopatterning process is more time-efficiency than the e-beam lithography or nanoimprint lithography process. The influences of the reactive-ion etching conditions and deposited Au film thickness to the subwavelength structure profile and its corresponding surface reflectance have been systematically investigated. Under the optimal experimental conditions, the average reflectance of the silicon carbide in the range of 390x02013;784 nm is dramatically suppressed from 21.0x00025; to 1.9x00025; after introducing the pseudoperiodic nanostructures. A luminescence enhancement of 226x00025; was achieved at an emission angle of 20x000B0; on the fluorescent silicon carbide. Meanwhile, the angle-resolved photoluminescence study presents a considerable omnidirectional luminescence enhancement.

Porous 6H-SiC samples with different thicknesses were fabricated through anodic etching in diluted hydrofluoric acid. Scanning electron microscope images show that the dendritic pore formation in 6HSiC is anisotropic, which has different lateral and vertical formation rates. Strong photoluminescence was observed and the etching process was optimized in terms of etching time and thickness. Enormous enhancement as well as redshift and broadening of photoluminescence spectra were observed after the passivation by atomic layer deposited Al2O3 and TiO2 films. No obvious luminescence was observed above the 6H-SiC crystal band gap, which suggests that the strong photoluminescence is ascribed tosurface state produced during the anodic etching.

In the present work, antireflective sub-wavelength structures have been fabricated on fluorescent 6H-SiC to enhance the white light extraction efficiency by using the reactive-ion etching method. Broadband and omnidirectional antireflection characteristics show that 6H-SiC with antireflective sub-wavelength structures suppress the average surface reflection significantly from 20.5 % to 1.01 % over a wide spectral range of 390-784 nm. The luminescence intensity of the fluorescent 6H-SiC could be enhanced in the whole emission angle range. It maintains an enhancement larger than 91 % up to the incident angle of 70 degrees, while the largest enhancement of 115.4 % could be obtained at 16 degrees. The antireflective sub-wavelength structures on fluorescent 6H-SiC could also preserve the luminescence spectral profile at a large emission angle by eliminating the Fabry-Perot microcavity interference effect.

We demonstrate a light-extraction approach using a whispering gallery resonators array. The wavelength-scale resonant dielectric nanospheres support whispering gallery modes, which can be coupled with the confined waveguide modes inside the bulk material, thus dramatically improving light extraction. Broadband light-extraction enhancement across the entire visible spectral range is achieved by exciting three low-order and low-quality-factor resonances. As an example, the broadband extraction enhancement of about 50% is obtained for the emission of fluorescent SiC at all the tested angles. The experimental results are supported by numerical simulations. Our light-extraction strategy could enable the manufacturing of high-throughput, nondestructive, and affordable optical coating in a variety of optical devices.

The strong photoluminescence from f-SiC was achieved after the optimization of the B and N concentrations. Surface nanostructures were successfully applied to enhance the extraction efficiency. f-SiC is a promising wavelength convertor for white LEDs.

The preparation and application of co-doped polycrystalline SiC as source in sublimation growth of fluorescent layers is a complex topic. Photoluminescence topographies of luminescent 6H-SiC layers and their corresponding source crystals have been studied in order to investigate the dependence of the epitaxial growth on the source material. It is shown that the homogeneity concerning the dopant incorporation and the layer luminescence intensity does not depend on the characteristics of the PVT grown source material. Therefore co-doped polycrystalline SiC is a promising source material in fast sublimation growth of luminescent 6H-SiC.

Recently it was shown a new approach based on all-semiconductor material technology which is composed with a near ultra-violet GaN LED excitation source and fluorescent silicon carbide (f-6H-SiC) substrate which generates a visible broad spectral light by N and B dopants and an efficient donor to acceptor pair recombination [1,2]. This combination can achieve higher electric-light conversion efficiency and high color rendering in comparison with today’s used blue GaN LED based and phosphors. The devices are promising candidates for general lightning applications and may obtain stability/reproducibility, and potentially low cost in high performance LEDs. However, there are still many problems to obtain best optimization for f-6H-SiC material since neither carrier transport, nor the carrier recombination is known in such co-doped carbides. From the existing data of carrier lifetimes in the SiC materials it is impossible to calculate requirements for epilayer thicknesses, for surfaces and interfaces that can provide sink for non-intentional losses of emission probability. In this work we report on carrier lifetime studies in f-6H-SiC epitaxial growth layers that are co-doped by N and B impurities. Epitaxial samples were grown by a sublimation growth process using a control of source materials. Variable concentration of B and N dopants was uniform over epitaxial thicknesses 45-60 m as was obtained by SIMS measurements (Table 1). Samples had different PL intensity at 300 K. Free-carrier-absorption technique under co-linear and orthogonal probe geometry was used to measure carrier lifetimes in the layers under variable injection conditions. Same results are shown in Fig. 1 exaggerating the fact that longer electron lifetime responsible for higher emission and n-type doping should prevail the p-type doping in active layer of 6H-SiC. An appropriate model for explaining experimental findings will be presented together with an appropriate model for the LED device.

Different sizes of Ge nanocrystals embedded in a SiO2 matrix were formed by PECVD, and analyzed by TEM. Size effect of Ge nanocystals was demonstrated by Raman spectroscopy after excluding the thermal effect.

We have theoretically investigated the influence of antireflective sub-wavelength structures on a monolithic white light-emitting diode (LED). The simulation is based on the rigorous coupled wave analysis (RCWA) algorithm, and both cylinder and moth-eye structures have been studied in the work. Our simulation results show that a moth-eye structure enhances the light extraction efficiency over the entire visible light range with an extraction efficiency enhancement of up to 26 %. Also for the first time to our best knowledge, the influence of sub-wavelength structures on both the color rendering index (CRI) and the correlated color temperature (CCT) of the monolithic white LED have been demonstrated. The CRI of the monolithic white LED could be improved from 92.68 to around 94 by applying a cylinder structure, and the CCT could be modified in a very large range with appropriate design of the cylinder structure.

Growth of 3C or 6H-SiC epilayers on low off-axis 6H-SiC substrates can be mastered by changing the size of the on axis plane formed by long terraces in the epilayer using geometrical control. The desired polytype can be selected in thick (~200 µm) layers of both 6H-SiC and 3C-SiC polytypes on substrates with off-orientation as low as 1.4 and 2 degrees. The resultant crystal quality of the 3C and the 6H-SiC epilayers, grown under the same process parameters, deteriorates when lowering the off-orientation of the substrate.

Surface-patterning technologies have enabled the improvement of currently existing light-emitting diodes (LEDs) and can be used to overcome the issue of low quantum efficiency of green GaN-based LEDs. We have applied nanosphere lithography to fabricate nanopillars on InGaN∕GaN quantum-well LEDs. By etching through the active region, it is possible to improve both the light extraction efficiency and, in addition, the internal quantum efficiency through the effects of lattice strain relaxation. Nanopillars of different sizes are fabricated and analyzed using Raman spectroscopy. We have shown that nanopillar LEDs can be significantly improved by applying a combination of ion-damage curing techniques, including thermal and acidic treat- ment, and have analyzed their effects using x-ray photoelectron spectroscopy. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.JNP.9.093062)

A cost-effective method for fabricating antireflective subwavelength structures on silicon carbide is demonstrated. The nanopatterning is performed in a 2-step process: aluminum deposition and reactive ion etching. The effect, of the deposited aluminum film thickness and the reactive ion etching conditions, on the average surface reflectance and nanostructure landscape have been investigated systematically. The average reflectance of silicon carbide surface is significantly suppressed from 25.4% to 0.05%, under the optimal experimental conditions, in the wavelength range of 390-784 nm. The presence of stochastic nanostructures also changes the wetting properties of silicon carbide surface from hydrophilic (47°) to hydrophobic (108°).

We investigated donor–acceptor-pair emission in N–B-doped 6H-SiC samples by using photoluminescence (PL) and angle-resolved PL. It is shown that n-type doping with concentrations larger than 1018 cm−3 is favorable for observing luminescence, and increasing nitrogen results in stronger luminescence. A dopant concentration difference greater than 4×1018 cm−3 is proposed to help achieve intense PL. Angular-dependent PL was observed that was attributed to the Fabry–Perot microcavity interference effect, and a strong luminescence intensity in a large emission angle range was also achieved. The results indicate that N–B-doped fluorescent SiC is a good wavelength converter in white LED applications.

In the present work, we investigated donor-acceptor-pair emission in N-B doped fluorescent 6H-SiC, by means of photoluminescence, Raman spectroscopy, and angle-resolved photoluminescence. The photoluminescence results were interpreted by using a band diagram with Fermi-Dirac statistics. It is shown that with N and B concentrations in a range of 1018 cm−3 the samples exhibit the most intense luminescence when the concentration difference (n-type) is about 4.6x1018 cm−3. Raman spectroscopy studies further verified the doping type and concentrations for the samples. Furthermore, strong luminescence intensity in a large emission angle range was achieved from angle-resolved photoluminescence. The results indicate N-B doped fluorescent SiC as a good wavelength converter in white LEDs applications.

We fabricate a multi-colored quantum-well structure as a prototype towards monolithic white light-emitting diodes, and modify the emission intensities of different colors by introducing dielectric and Ag nanoparticle coating.

Most of the commercial white light-emitting diode (LED) light sources are made from phosphor coated blue-emitting gallium nitride (GaN) chips. This type white LED light source always has tradeoff between luminous efficacy and color rendering index (CRI). Furthermore, yellow-emitting phosphor decays much faster than the semiconductor chip, so the white color will turn into bluish over the time. This paper will propose a new type white LED light source: using fluorescent silicon carbide (SiC) to take the place of phosphor. This new type LED has the following advantages: a) SiC is a wide bandgap semiconductor material , so it is stable; b) Fluorescent SiC has very wide emission spectrum, and it could generate white light with very high CRI; c) It is a better substrate than sapphire for the GaN growth in terms of lattice match and thermal conductivity. This paper will cover: the growth of fluorescent SiC, its optical characterization, nanostructuring of the SiC surface for extraction efficiency enhancement, and surface passivation for further efficiency enhancement.

Nanopillar InGaN/GaN green light-emitting diode arrays were fabricated by using self-assembled nanopatterning and dry etching process. Both internal and external quantum efficiency were increased due to strain relaxation and enhanced light extraction.

The technology breakthrough in high-efficiency blue nitride based LEDs has enabled the rapid market penetration of energy-saving LED white light sources. In this presentation, silicon carbide (SiC) has been explored to emit yellow light and blue light as a new wavelength conversion material for a new type of white LED light source taking advantages of abundancy and good thermal conductivity of SiC. Strong yellow emission from boron and nitrogen co-doped SiC has been demonstrated at room temperature. Blue emission has been investigated from both aluminum and nitrogen co-doping of SiC and porous SiC. Strong blue emission from Al-N co-doped SiC has only been observed at low temperature, while strong blue emission from porous SiC has been achieved at room temperature after optimization of pores formation conditions and surface passivation. White light with color rendering index as high as 81 has been achieved from B-N co-doped SiC with a porous surface under optical pump. A prototype of hybridly integrated warm white is achieved by adhesive bonding of a blue LED on SiC substrate with B-N co-doped fluorescent SiC. At last, a monolithically integrated warm white light source through directly growing near ultraviolet LED on fluorescent SiC by metalorganic vapor chemical vapor deposition is demonstrated. Our original results cover growth of fluorescent SiC, formation of porous SiC, optical characterization of these materials with regard to photoluminescence, low temperature photoluminescence, time resolved photoluminescence and thermally stimulated luminescence, material emission mechanism analysis, carrier dynamics analysis, warm white light source fabrication and characterization. Basing on these results, perspectives are shared.

Antireflective structures were fabricated on fluorescent 6H-SiC for white LEDs to enhance the extraction efficiency. Average surface reflectance decreased from 22.1 % to 5.1 % over a broad range, and luminescence intensity was enhanced by 41 %.