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Ultrathin (5–50 nm) epitaxial superconducting niobium nitride (NbN) films were grown on AlN-buffered c-plane Al2O3 by an industrial scale physical vapor deposition technique at 400°C. Both X-ray diffraction and scanning electron microscopy analysis show high crystallinity of the (111)-oriented NbN films, with a narrow full-width-at-half-maximum of the rocking curve down to 0.030°. The lattice constant decreases with decreasing NbN layer thickness, suggesting lattice strain for films with thicknesses below 20 nm. The superconducting transition temperature, the transition width, the upper critical field, the irreversibility line, and the coherence length are closely correlated to the film thickness. This work realized high quality ultrathin epitaxial NbN films by an industry-scale PVD technology at low substrate temperature, which opens up new opportunities for quantum devices.

Plasmonic nanostructures are successfully demonstrated in solar cells due to their broad spectra-selective resonance in the range of ultraviolet to near-infrared, and thus light absorption can be mostly improved and power conversion efficiency (PCE) further. Here, we demonstrate plasmonic dye-sensitized solar cells (DSSCs) using collapsible Au nanofingers to build photoanode to enhance light absorption. In this plasmonic DSSCs, by balancing local field enhancement due to gap-plasmon resonance and dye fluorescence quenching, the optimal gap size in collapsed Au/Al2O3/Au nanofingers is designed by twice the Al2O3thickness and then deposited a TiO2layer as photoanode. The results show that the PCE of DSSCs is mostly improved as compared to DSSCs with photoanode of Au/Al2O3/TiO2films, which can be ascribed to the coupled local field enhancement within the sub-nanometer gaps. In addition, fluorescence of dyes on plasmonic nanofingers is nearly 10 times higher than plain Au/Al2O3/TiO2films, which further proves the dye absorption enhancement. These plasmonic nanofingers enable the precise engineering of gap-plasmon modes and can be scaled up to wafer scale with low cost by the nanoimprint lithography technique, which suggests the feasibility of applying our result in constructing the photoanode for other types of solar cells.

Transparent paper is a kind of promising and environmentally friendly material. In this study, we show that transparent paper can be fabricated in an ultra-fast and low-cost way. This low-cost top-down method only takes three steps of cell separation, lignin removal, and cold pressing to obtain a high-quality transparent paper. The fabrication time is further reduced, and the resulted transparent paper shows high transparency up to 90.3%. The application as a substrate material for transparent and flexible electronic devices is demonstrated by emulating the printed circuit on the prepared transparent paper. This top-down method will greatly promote the market-oriented applications of transparent paper as an environment friendly material.

Highly efficient broadband light management to enhance the light trapping inside active layer is critical for many energy conversion devices such as thin film solar cells and photoelectrochemical cells. In this work, we demonstrate highly transparent, mesoporous wood composite via fast extraction of lignin along naturally formed low tortuosity channels followed by fast filling of polymers. The transparent wood displays a high optical transmittance and at the same time a high haze in a broad wavelength range between 400 nm and 1100 nm. With such unique optical properties, the transparent wood composite with cellulose nanofibers can be utilized for a range of optoelectronics, especially for solar cells and wide-angle lighting where light management is crucial to enhance device operation efficiency. We demonstrate that the newly developed transparent wood composite can function as a broad range light management layer and substantially improve the overall energy conversion efficiency by as much as 18% when simply coated with a GaAs thin film solar cell. Our research on wood based light management material provides an attractive platform for future development embracing green, disposable optoelectronic devices with efficient light management.

Optoelectronic devices are ubiquitously built on substrates. To increase the efficiencies of light coupling into and out of optoelectronic devices, such as thin film solar cells and flexible lighting, a substrate with high transmittance and high haze is desired. Unfortunately, optical transmittance and optical haze are usually contrasting to each other in common substrates: plastic is highly transparent but with a low optical haze, whereas paper has a high optical haze but a low total transmittance. Herein, we combine these two materials through a simple templated infiltration approach to achieve a new type of substrate, plastic–paper, which has a high optical transmittance (>85%) and high transmittance haze (>90%) in a broadband wavelength. The plastic–paper has an ultra-flat surface, is mechanically flexible, durable in different solvents and compatible with standard processing in semiconductors, which are shown by organic light emitting diodes (OLED) fabricated directly onto the plastic–paper substrate. Plastic–paper leads to improved light coupling into and out of optoelectronic devices and demonstrates an improvement in efficiency for both OLED and typical GaAs solar cells. The fabrication method is also fully scalable with roll-to-roll production. The newly developed low-cost, high-performance transparent and hazy substrate is attractive for a range of optoelectronic devices.

Efficient solar harvesting for ultrathin amorphous silicon (α-Si) films with a thickness of less than 100 nm is critical to the performance of solar cells, since the very short carrier-diffusion length of α-Si and the Staebler–Wronski effect restrict their thickness. In this work, we numerically investigate energy harvesting in metamaterial-based solar cells, in which an ultrathin α-Si film is sandwiched between a silver (Ag) substrate and a square array of Ag nanodisks, and combined with an indium tin oxide (ITO) anti-reflection layer. It is found that only a 20 nm-thick α-Si film is able to absorb over 50% solar energy in the spectral range from 300 to 800 nm at normal incidence, and the amount of absorbed light is equivalent to a photocurrent of about 13.4 mA cm−2. This broadband absorption is achieved by the spectral design on the overlapped absorption peaks which are caused by the excitations of two lowest-order Fabry–Perot (FP) resonances in the α-Si and ITO layers and a magnetic resonance arising from the plasmon hybridization between Ag disks and the substrate. The absorption performance of our structure is less dependent on the incident angle θ and polarization of light when θ 70° (20°) for P (S) polarization.

New-type silica Λ-shape wedge arrays (SWAs) were fabricated by a combined process of template fabrication, hot embossing and sol–gel replication. Using silica needle arrays (SNAs) as template, the configuration of Λ-shape was transferred from SNAs to PMMA/PC molds by hot-pressing process. Then they were casted to form silica Λ-shape wedge arrays by sol–gel process. Using this procedure, high quality silica Λ-shape wedge arrays can be obtained with designed sizes and shapes over a large area. The taper height and interspace of SWAs can be tuned by the imprint depth during the hot-embossing process. This technology provides a cost-effective method to fabricate Λ-shape silica wedges. By changing the primal template, other patterned silica substrate can also be fabricated, such as grating, pillar and spherical structures. These silica based microstructure can find many applications in semiconductor devices, solar cells and optics etc.

The light output of 530 nm green GalnN/GaN light-emitting diodes on sapphire has been nearly doubled by the insertion of a 130-nm GalnN underlayer (UL) between the n-GaN electron injection layer and the quantum-well (QW) active region. Under variation of the alloy composition, best results were obtained for an x = 6.3% Ga1-xInxN UL. By low-temperature depth-resolved cathodoluminescence spectroscopy, an interplay of the impurity-related donor-acceptor pair recombination, the UL, and the QW emission has been observed. We propose that the resonance and level alignments between the defect and UL levels reroute excitation toward radiative recombination in the QWs.

We report the growth and structural characteristics of green and yellow (529-576 nm) GaInN/GaN light emitting diodes (LEDs) grown on two types of c-plane substrates - bulk GaN and sapphire. In this longer wavelength range, depending on the substrate, we find different strain relaxation mechanisms within the GaInN/GaN quantum well (QW) region. In optimized epitaxy, structures on sapphire that contain a low density of threading dislocations (TDs) within the n-GaN show virtually no generation of additional misfit dislocations (MDs) (

The junction temperature of homoepitaxial green and blue GaInN/GaN quantum well light emitting diode (LED) dies is analyzed by micro-Raman, photoluminescence, cathodoluminescence mapping, and forward-voltage methods and compared to finite element simulations. Dies on GaN substrate and sapphire were analyzed under variable drive current up to 200 mA (246 A/cm2). At 100 mA, dies on bulk GaN remain as cool as 355 K (83 °C) while dies on sapphire heat up to 477 K (204 °C). The efficiency droop and spectral line shift in green LEDs with increasing current density can now be separated into electrical and thermal contributions.

An enhancement of radiative recombination in GaInN/GaN heterostructures is being pursued by a reduction of defects associated with threading dislocations and a structural control of piezoelectric polarization in the active light-emitting regions. First, in conventional heteroepitaxy on sapphire substrate along the polar c-axis of GaN, green and deep green emitting light-emitting diode (LED) wafers are being developed. By means of photoluminescence at variable low temperature and excitation density, internal quantum efficiencies of 0.18 for LEDs emitting at 530 nm and 0.08 for those emitting at 555 nm are determined. Those values hold for the high current density of 50 A/cm 2 of high-power LED lamps. In bare epi dies, we obtain efficacies of 16 lm/W. At 780 A/cm 2 we obtain 22 lm when measured through the substrate only. The 555 nm LED epi material under pulsed photoexcitation shows stimulated emission up to a wavelength of 485 nm. This strong blue shift of the emission wavelength can be avoided in homoepitaxial multiple quantum well (MQW) and LED structures grown along the non-polar a- and m-axes of low-dislocation-density bulk GaN. Here, wavelength-stable emission is obtained at 500 and 488 nm, respectively, independent on excitation power density opening perspectives for visible laser diodes.

We demonstrate homoepitaxial growth of GaInN/GaN-based green (500–560 nm) light emitting diodes (LEDs) on a-plane and m-plane quasi-bulk GaN prepared by hydride vapor phase epitaxy (HVPE). We find that in order to achieve an emission peak wavelength beyond 500 nm, a minimum InN-fraction of ∼14% is needed for both, a- and m-plane quantum wells (QWs), while ∼8% are enough for c-plane-oriented QWs. Besides increasing the InN-fraction in these non-polar QWs, widening the QW also proves to effectively shift the emission to longer wavelengths without loosing efficiency with the benefit of maintaining a low InN-fraction.

In this paper, we rely on the formation of novel macroporous polystyrene (PS)/silica masks to prepare two-dimensional (2D) arrays of metallic split ring (SR) and full ring (FR) microstructures on silicon substrates. In these PS/silica porous templates, 2D silica particle arrays were made to anchor on silicon wafers on which PS membranes with 2D pore arrays are molded and made movable via a fluid flow perturbation after partial etching of the silica particles. By using the porous masks in combination with physical deposition, both metallic FRs and SRs on silicon substrates can be prepared. The obtained SRs have a nearly U-shaped cavity and a circular outer profile. The present method could open an alternative but low-cost and parallel approach for obtaining artificial magnetic metamaterials operating in near infrared.

GaInN/GaN multiple quantum well light-emitting diode structures in polar c -axis and non-polar m -axis growth have been compared in terms of luminescence properties. Grown under identical conditions, under low excitation density the c -axis structure has a luminescence maximum at 558 nm while the m -axis structure shows a maximum at 488 nm and shows superluminescence at 485 nm under high photoexcitation density. Under the same conditions, on increasing the excitation power, the peak intensity increases 40 fold in the m -axis structure without any variation of the emission wavelength. In similar but separately grown c -axis structures without a p-side, luminescence shifts from 555 nm at low excitation density to superluminescence at 485 nm under high excitation. The coincidence, of the superluminescence wavelength in the polar structure with the stable peak wavelength in the non-polar one, suggests that the wavelength shift in the polar structure is due to its piezoelectric polarization. The absence of such effects in the m -axis-grown structure therefore suggests a stronger dipole matrix element, potentially enabling higher quantum efficiencies and suitability for high efficiency light-emitting diode and laser diode designs in the green spectral region.

The efficiency of green GaInN/GaN light emitting diodes (LEDs) is known to drop under high current densities. To explore the reason, 535 nm emitting GaInN/GaN multiple quantum well (MQW) and GaN epilayers were characterized using Z-scan techniques under comparable excitation conditions with a continuous wave laser. Two wavelengths, 514 nm and 488 nm, were selected right below and above the apparent optical absorption edge. At 514 nm, a very large nonlinear absorption coefficient β = 2.6 cm/W was obtained. This leads to a 20% absorption enhancement at a photon flux of 21 kW/cm2. We attribute this to nonlinear free-carrier absorption. On the other hand, at 488 nm, a nonlinear saturable absorption with β = –1.7 cm/W was observed. This induced transparency indicates photon bleaching in the MQW. Apparently, free carrier dynamics strongly affects optical nonlinearity, while nonlinear absorption provides a small contribution to the limitations of current green LEDs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We perform a transmission electron microscopy (TEM) characterization of blue light-emitting diode epiwafers grown homoepitaxially on a c-plane GaN substrate and compare with such grown on sapphire. We find a threading dislocation (TD) density as low as 2 × 10 8 cm -2 in homoepitaxial growth, which is 1/30 of that in sapphire-based material. A unique type of inverted pyramid defect with diameter ∼650 nm was observed. It originates from the homo-interface in edge-type TDs. TDs were also found to be generated within the quantum wells without a precursor TD defect. Otherwise, high-resolution TEM suggests extremely homogeneous quantum wells and barriers in terms of composition and well width. Besides plan-view TEM, wet chemical etching on n-type GaN was carried out in hot phosphoric acid to evaluate the TD density more quickly. The two methods prove to be equivalent with respect of determined TD densities.

We present a junction temperature analysis of GaInN/GaN quantum well (QW) light-emitting diodes (LEDs) grown on sapphire and bulk GaN substrate by micro-Raman spectroscopy. The temperature was measured up to a drive current of 250 mA (357 A/cm2). We find better cooling efficiency in dies grown on GaN substrates with a thermal resistance of 75 K/W. For dies on sapphire substrates we find values as high as 425 K/W. Poor thermal performance in the latter is attributed to the low thermal conductivity of the sapphire. Three-dimensional finite-element simulations show good agreement with the experimental results, validating our thermal model for the design of better cooled structures.

Department of Electrical, Computer, and Systems Engineering, Rensselaer, Troy, NY 12180, USA Received 19 September 2006, accepted 29 November 2006 Published online 31 May 2007 PACS 73.40.Kp, 73.61.Ey, 78.60.Hk, 78.67.De, 85.60.Jb The electroluminescence of GaInN/GaN multiple quantum well light-emitting diode dies is analyzed at variable low temperatures. We compare the external quantum efficiency of three dies of nominally identi-cal structure but strongly different RT performance at 520 nm. For all dies, the external quantum efficien-cy increases as the temperature is lowered. A maximum is reached for all near 158 K while for lower temperatures as low as 7.7 K, the efficiency continuously drops. The low-temperature efficiency is the lowest for the highest performing die at RT. The peak energy exhibits a blue shift from RT to 158 K fol-lowed by a red shift for lower temperatures. In the same low-temperature range, a secondary emission peak appears near 390 nm that resembles a donor-acceptor pair transition. The pronounced efficiency ma-ximum is tentatively assigned to competition of carrier transport properties and the non-radiative lifetime in the active region.

We analyze GaInN based light emitting diodes emitting in the range of 400-550nm using a new intensity-quantitative spectroscopic cathodoluminescence mapping method. Spectroscopic information of arbitrary sample locations is generated from sequences of quantitative image scans. From the temperature dependence of the intensity, we derive thermal activation energies of the dominant loss processes. Those compare well with the hole binding energies in the piezoelectric and quantized quantum well structures.

We demonstrate homoepitaxial growth of GaInN/GaN-based light emitting diodes (LED) on quasi-bulk GaN with an atomically flat polished surface. The threading dislocation densities of the epitaxial layers were 2-5 x 10 8 cm -2 which was one order of magnitude less than those grown on c-plane sapphire substrate. The growth defects introduced during the epitaxial process were also one order of magnitude smaller than those grown on the sapphire substrate. The crystalline quality and the optical properties of the epitaxial layer and device performance were much improved. The optical output power of the light emitting diode increased by more than one order of magnitude compared to those on sapphire substrate.

A charge profiling of GaInN/GaN hetero-polarization structures typical for high power 525 nm green light emitting diodes is presented. We identify three individual charge maxima of some 1.17 x 10 12 cm -2 electrons with a line width as narrow as 17 nm corresponding to three individual but widely separated quantum wells. The sheet charge density indicates negligible screening of the polarization charges. The voltage required to deplete each well is found to be ΔU = -0.4 ± 0.1 V and approximates the voltage drop across the polarization dipole of the quantum well. The modulation frequency dependence reveals a characteristic time constant of 1 μs for the dominant trapping and ionization process. Such analysis of high performance green light emitters proves a suitable tool for further device optimization.

Plasmonic metal nanoparticles are a category of plasmonic materials that can efficiently convert light into heat under illumination, which can be applied in the field of solar steam generation. Here, this study designs a novel type of plasmonic material, which is made by uniformly decorating fine metal nanoparticles into the 3D mesoporous matrix of natural wood (plasmonic wood). The plasmonic wood exhibits high light absorption ability (≈99%) over a broad wavelength range from 200 to 2500 nm due to the plasmonic effect of metal nanoparticles and the waveguide effect of microchannels in the wood matrix. The 3D mesoporous wood with numerous low-tortuosity microchannels and nanochannels can transport water up from the bottom of the device effectively due to the capillary effect. As a result, the 3D aligned porous architecture can achieve a high solar conversion efficiency of 85% under ten-sun illumination (10 kW m−2). The plasmonic wood also exhibits superior stability for solar steam generation, without any degradation after being evaluated for 144 h. Its high conversion efficiency and excellent cycling stability demonstrate the potential of newly developed plasmonic wood to solar energy-based water desalination.

We report the development of non-polar light emitting diodes (LEDs) covering the emission spectra from 480 to 520 nm (dominant wavelength), i.e. from cyan to the green region. The devices are obtained via GaInN-based homoepitaxy on non-polar m -plane GaN bulk substrate. Owing to the absence of piezoelectric polarization, these LEDs exhibit stable emission color with a wavelength shift less than 3 nm when the drive current density is changed from 0.1–30 A/cm2. However, we observe a decreasing electroluminescence efficiency as the emission wavelengths increases from cyan to green. We tentatively attribute this to a higher density of defects in the longer wavelength structures. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In 535-565 nm green and deep green LED dies on c-plane sapphire, we correlate the structural morphology with photoluminescence performance. Two classes of material with active region roughness of 4 nm and 0.4 nm (RMS) are analyzed in transmission electron microscopy. In the rough material, a high density of edge-type dislocations is identified that originates within the quantum wells (QWs). They initiate V-defects that exhibit {101} growth facets with a second set of narrow QWs and barriers. As V-defects widen with progressive growth, the width of QW and barriers on the c-plane increases. In the smooth material, however, no V-defects can be found. QWs and barriers are highly uniform and homogenous throughout the structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Under external laser excitation, we studied the electroluminescence (EL) of GaInN/GaN green light emitting diodes (LEDs) grown on sapphire by metal organic vapor phase epitaxy. The EL intensity was found to be greatly enhanced under the photon bias. This enhancement strongly supersedes the photoluminescence (PL) signal. The EL enhancement varied with injection carrier density. The relative EL enhancement under 325-nm laser bias starts with a high ratio and decreases monotonically. This increase of the EL intensity is tentatively attributed to a balance between photocarrier screening and carrier drift within the active region. These findings elucidate the transition from highly efficient recombination at low current density to the region of efficiency droop at high current densities. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate pn-junction heating in 525 nm green GaInN/GaN quantum well light emitting diode dies by micro-Raman and photoluminescence spectroscopy. We compare dies grown on GaN and sapphire substrates of different sizes at current densities up to 267 A/cm2. Analysis of the experiments was supported by realistic three-dimensional finite-element simulations of the thermal properties. Sapphire based LED dies reach a junction temperature as high as 240 °C at a current density of 133 A/cm2, while those on bulk GaN only heat to 83 °C. This corresponds to a thermal resistance of 428 K/W and 63 K/W, respectively. By help of this calibration, the role of junction temperature in the spectral light output performance is identified. From the numerical simulation, dimensions and thermal conductivity of the substrate are identified as the dominant control parameters. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The strong correlation between V-defect density in the active region and light output power of GaInN based green and deep green light emitting diodes (LEDs) has been evaluated. The formation of such defects is confirmed as the major cause for rough growth morphology, drift of well and barrier widths, emission linewidth broadening, large color shift with current density, and thus poor light emission performance. We have optimized the epitaxial processes for the active region and successfully eliminated such defects from the entire device structure. The electroluminescence intensities from these V-defect-free devices with dominant wavelengths ranging from 520 to 570 nm, improve by a factor of two under high injection current density up to 100 A/cm2. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficiency droop and green gap are terms that summarize performance limitations in GaInN / GaN high brightness light emitting diodes (LEDs). Here we summarize progress in the development of green LEDs and report on time resolved luminescence data of polar c -plane and non-polar m -plane material. We find that by rigorous reduction of structural defects in homoepitaxy on bulk GaN and V -defect suppression, higher efficiency at longer wavelengths becomes possible. We observe that the presence of donor acceptor pair recombination within the active region correlates with lower device performance. To evaluate the aspects of piezoelectric polarization we compare LED structures grown along polar and non-polar crystallographic axes. In contrast to the polar material we find single exponential luminescence decay and emission wavelengths that remain stable irrespective of the excitation density. Those findings render high prospects for overcoming green gap and droop in non-polar homoepitaxial growth.

Among many other requirements, energy efficient building materials require effective daylight harvesting and thermal insulation to reduce electricity usage and weatherization cost. The most commonly used daylight harvesting material, glass, has limited light management capability and poor thermal insulation. For the first time, transparent wood is introduced as a building material with the following advantages compared with glass: (1) high optical transparency over the visible wavelength range (>85%); (2) broadband optical haze (>95%), which can create a uniform and consistent daylight distribution over the day without glare effect; (3) unique light guiding effect with a large forward to back scattering ratio of 9 for a 0.5 cm thick transparent wood; (4) excellent thermal insulation with a thermal conductivity around 0.32 W m−1 K−1 along the wood growth direction and 0.15 W m−1 K−1 in the cross plane, much lower than that of glass (≈1 W m−1 K−1); (5) high impact energy absorption that eliminates the safety issues often presented by glass; and (6) simple, scalable fabrication with reliable performance. The demonstrated transparent wood composite exhibits great promise as a future building material, especially as a replacement of glass toward energy efficient building with sustainable materials.

In absence of piezoelectric polarization along the growth axis, a- and m-plane green GaInN light emitting diodes manifest stable emission wavelength -- independent of the injection current density. The shift of the dominant wavelength is less than 8 nm when varying the forward current density from 0.1 to 38 A/cm2. Furthermore, the light emitted from the growth surface of such non-polar structures shows a very degree of linear polarization. This is attributed to a strong valance band splitting in such anisotropically strained wurtzite GaInN quantum wells . Such light emitting diodes show a high potential for energy efficient display applications.

Received 2 September 2009; revised manuscript received 29 January 2010; published 22 March 2010 The creation of symmetrical pairs of inclined dislocations was observed in the GaInN/GaN quantum wells QWs of c-axis grown green light-emitting diodes LEDs on low-defect density bulk GaN substrate, but not in green LEDs on sapphire substrate with high threading dislocation TD density. Pairs of dislocations start within 20 nm of the same QW and incline 18° – 23° toward two opposite 11 ¯ 00 directions or in a 120° pattern. We propose that in the absence of TDs, partial strain relaxation of the QWs drives the defect formation by removal of lattice points between the two dislocation cores. In spite of those inclined dislocation pairs, the light output power of such green LEDs on GaN is about 25% higher than in LEDs of similar wavelength on sapphire.

The green spectral region provides a formidable challenge for energy efficient light emitting diodes. In metal organic vapor phase epitaxy we developed GaInN/GaN quantum well material suitable for 500 - 580 nm LEDs by rigorous defect reduction and thrive for alloy uniformity. We achieve best results in homoepitaxy on polar c-plane, and non-polar a -plane and m -plane bulk GaN. By the choice of crystal orientation, the dipole of piezoelectric polarization in the quantum wells can be optimized for highest diode efficiency. We report progress towards the goal of reduced efficiency droop at longer wavelengths.

We have performed the epitaxial growth of GaInN based deep green (540–570 nm) light emitting diodes (LEDs) by introducing processes to avoid the formation of V-defects in the active region of the GaInN/GaN multiple quantum wells. For 541 and 559 nm LED wafers, the light output power (LOP) at 90 A/cm2 reaches as high as 14 and 8 mW, respectively. These values are 4 times larger than those of typical deep green LED wafers with average V-defect density of 2–5 × 108cm–2. Under pulsed operation (pulse width of 50 μs, duty cycle 1%), the external quantum efficiency exhibits a maximum of 39% at 16 A/cm2 in 558 nm LED die of size (350 μm)2. This type of V-defect-free LED die also shows excellent lifetime behaviour. In 168 hr stress test of 30 mA at 80 °C, LOP declines less than 5%. Apparently, eliminating V-defects from the active region significantly enhances the reliability of deep green LEDs. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of green emission Ga0.80In0.20N/GaN multi-quantum well and light emitting diode have been investigated by using photoluminescence, cathodoluminescence, electroluminescence, and photoconductivity. The temperature dependent photoluminescence and cathodoluminescence studies show three emission bands including GaInN/GaN quantum well emission centered at 2.38 eV (~ 520 nm). The activation energy of the non-radiative recombination centers was found to be ~ 60 meV. The comparison of photoconductivity with luminescence spectroscopy revealed that optical properties of quantum well layers are strongly affected by the quantum-confined Stark effect.

Electroluminescence of GaInN/GaN multiple-quantum-well (QW) light-emitting diodes emitting in the green spectral region is analyzed at variable low temperature. Spectra are dominated by QW emission at RT throughout 7.7 K. Below 150 K, donor-acceptor pair recombination appears that must be assigned to residual impurities in either the barriers or the p-layers. The current-voltage behavior reveals shunt paths that carry up to several mA at low bias voltages. Below 20 K those paths are frozen out, but the device still emits predominantly from the QW. The peak energy exhibits a blue shift from RT to 158 K followed by a red shift from 158 K to 7.7 K. The deeper binding energy at low temperatures can hardly be affect by the injection current indicating that saturation of low-density states cannot be responsible for the transition between red and blue shifts.

We report nonlinear optical investigation of green emission GaInN/GaN multi-quantum structures grown along c- and m-axes on sapphire and bulk GaN substrates, respectively. Under intense pulsed photo excitation, we observed strong superluminescence near the lasing condition in c-plane grown quantum well structures with full width at half maximum of 6 nm. The superluminescence couples out of the edge of the sample in a mode pattern consistent with gain in a high mode of the waveguide. The wavelength of the superluminescence is 474 nm. The threshold intensity of superluminescence was found to be 156 kW/cm2. Increasing pump intensity leads to a strong photoluminescence blueshift as large as 380 meV in samples grown along the c-axis on sapphire substrate, while under the same excitation conditions, the blue shift for the m-axis grown structure on bulk GaN substrate is less than 10 meV. The large emission blueshift is hereby attributed to the internal piezoelectric field in the c-axis grown structure. We observe a gain value of 20 cm-1 together with internal absorption losses of 2.3 – 6.0 cm-1 for the superluminescent samples.

In an attempt to identify the performance limiting factors of green GaInN/GaN light emitting diodes (LEDs) we analyze a large number of LED dies fabricated from over 160 epitaxy runs covering the wavelength range from 390 – 580 nm on a quantitative scale of the emission power. As a function of drive current, we analyze the external quantum efficiency (EQE) with particular emphasis on the low current range. We observe three very distinct populations of EQE behaviors. We find that the EQE maximum value has a strong correlation with the forward current in this point, while it has weak correspondences with both, the forward voltage in this point, and the dominant wavelength at 20 mA. The very good linear relationship between the forward voltage in green LED dies at 1 mA and at 20 mA is also observed. All those characteristics are meaningful traits to ultimately converge to a physical electronic transport and bandstructure model for the next step enhancement of green and deep green GaInN/GaN LED performance.

Antireflective surfaces are often realized by minimizing the refractive index contrast between the air and substrate using subwavelength microstructures. In this paper, we introduce another kind of geometry-induced antireflective surface using multireflection to suppress reflection to a very low extent. This surface is composed of micro-∧-shape array with the wedge size much larger than the wavelength of the incident light. Simulation and experimental results show that the micro-∧-shape array can effectively suppress the reflection within wide incident angles and large wavelength ranges. The enhanced light adsorption caused by the greatly increased light path length within the micro-∧-shape array is responsible for the antireflection phenomena. Such antireflective surface may find various applications, in solar cells, for example.

Green GaInN/GaN quantum well light-emitting diode (LED) wafers were grown on nanopatterned c-plane sapphire substrate by metal-organic vapor phase epitaxy. Without roughening the chip surface, such LEDs show triple the light output over structures on planar sapphire. By quantitative analysis the enhancement was attributed to both, enhanced generation efficiency and extraction. The spectral interference and emission patterns reveal a 58% enhanced light extraction while photoluminescence reveals a doubling of the internal quantum efficiency. The latter was attributed to a 44% lower threading dislocation density as observed in transmission electron microscopy. The partial light output power measured from the sapphire side of the unencapsulated nanopatterned substrate LED die reaches 5.2 mW at 525 nm at 100 mA compared to 1.8 mW in the reference LED.

Linearly polarized light emission is analyzed in nonpolar light-emitting diodes (LEDs) covering the blue to green spectral range. In photoluminescence, m-plane GaInN/GaN structures reach a polarization ratio from 0.70 at 460 nm to 0.89 at 515 nm peak wavelength. For a-plane structures, the polarization ratio is 0.53 at 400 nm and 0.60 at 480–510 nm. In electroluminescence the polarization ratio is 0.77 at 505 nm in 350×350 µm2m-plane devices at 20 mA. Such a device should allow 44% power saving compared with nonpolarized c-plane LEDs combined with a polarizing filter, as commonly used in LED-backlit liquid crystal displays.

We report the development of 480 nm cyan and 520 nm green light emitting diodes (LEDs) with a highly stable emission wavelength. The shift is less than 3 nm when the drive current density is changed from 0.1 to 38 A/cm2. LEDs have been obtained in GaInN-based homoepitaxy on nonpolar m-plane GaN bulk substrates. For increasing emission wavelength we find a large number of additional dislocations generated within the quantum wells (2×108 to ∼1010 cm2) and a decrease in the electroluminescence efficiency. This suggests that the strain induced generation of defects plays a significant role in the performance limitations.

We report the development of 520–540nm green light emitting diodes (LEDs) grown along the nonpolar a axis of GaN. GaInN∕GaN-based quantum well structures were grown in homoepitaxy on both, a-plane bulk GaN and a-plane GaN on r-plane sapphire. LEDs on GaN show higher, virtually dislocation-free crystalline quality and three times higher light output power when compared to those on r-plane sapphire. Both structures show a much smaller wavelength blue shift for increasing current density (

A 392 nm GaInN-based UV light emitter has been demonstrated by homoepitaxial growth technique aiming to improve quantum efficiency by crystalline perfection of the heterostructures on naturally stable semipolar (11–22) bulk GaN.

In this study, we characterized the structural defects in blue and green GaInN/GaN LEDs grown on c-plane bulk GaN and sapphire substrates. Low density large V-defects with diameters around 600 nm were found in the blue LEDs on bulk GaN. They were initiated by edge-type threading dislocations (TDs) around the homoepitaxial growth interface. On the other hand, a high density 7×109 cm−2 of smaller V-defects with sidewalls on {1101} facets was observed in the active region of green LEDs on sapphire. Their diameter ranges from 150 to 200 nm. Misfit dislocations (MDs) generated in the quantum wells are found to initiate these V-defects. With optimizing the epitaxial growth conditions, the generation of MDs and their smaller V-defects was largely suppressed. As a result, the light output power improved by one order of magnitude. For green LEDs on bulk GaN, another unique type of defect was found in the active region: an inclined dislocation pair (IDP). In it a pair of dislocations propagate at a tilt angle of 18 to 23° from the [0001] growth direction towards 100>. This defect seems to be a path of strain relief in the high indium composition quantum wells.

Characterization of operational AlGaInN heterostructure light emitting diodes (LEDs) is critical to their performance optimization and time-to-failure analysis. Typically, device performance data needs to be corroborated with structural information such as layer thicknesses, charge profiles, and the absolute location of the pn-junction. Here, non-destructive testing by capacitance-voltage profiling is being applied to AlGaInN LED structures. Within a large set of samples with different active layer geometry, we observe distinct layers of high mobile charge accumulation. We correlate those with layer thicknesses derived from an x-ray diffraction analysis of the corresponding epiwafers. In this way, we identify the charge maxima as the upper and lower interfaces of the p-type AlGaN electron blocking layer to the neighboring GaN layers. By means of this successful analysis, we now have the opportunity to monitor epi process performance and stability as well as device degradation progress quasi-continuously over the device lifetime in a non-destructive mode.

With an external laser excitation, the electroluminescence (EL) of GaInN/GaN green light emitting diodes (LEDs) grown on sapphire by metal organic vapor phase epitaxy has been investigated. The EL was found significantly enhanced under this bias and the difference can not merely be attributed to additional photoluminescence (PL). Under 325 nm photon bias, the EL enhancement starts at its highest value and decreases along with an increase of the LED current. Under 408 nm and 488 nm bias, it increases first to a value smaller than that of the 325 nm bias and then decreases with a much lower rate along the current increase. The EL enhancement is attributed to the more efficient carrier injection into quantum wells (QWs) resulting from the screening of the QW polarization by photon bias. Therefore, an enhanced balance of majority and minority carriers was obtained resulting in a better radiative recombination rate. Meanwhile, the current-voltage characteristics show a negative current first and then a voltage reduction as forward voltage increases. The reverse photocurrent indicates carrier loss due to the solar cell effect in our LED device while at high current region the carrier loss is attributed to another effect controlled by the external electrical field. At the balance point of those two effects, the EL enhancement is the highest. These findings clarify the transition from highly efficient radiative recombination at low current density to the region of efficiency droop at high current densities.

GaInN based light emitting diodes (LEDs) play an important role as energy efficient light sources in solid state lighting. A controversial discussion addresses the origin of lateral light emission variations and their correlation with either of the identified defects, e.g., threading dislocations and V-defects. In order to establish any possible correlation of defects and luminescence centers, we analyze three UV, blue and green LED dies by microscopic mapping of spectroscopic cathodoluminescence and secondary electrons at variable low temperature from 7 K to room temperature. Particular effort is being placed on a quantitative analysis of the luminescence signal. Image intensities are not being scaled and offset for highest contrast as otherwise typical for imaging mode. In standard configuration, we analyze image areas of (0.037 mm)2 with pixel resolution of 72 nm. Following regions of strong and weak emission we find that remain bright and dark respectively even at low temperature. Those variations increase with the mean emission wavelength of the LEDs and with temperature. The largest peak wavelength variation associated with the intensity contrast was observed in the green LEDs and amounts to 5 nm. Here the peak wavelength is higher in the dark spots than in the bright ones. This finding corresponds to the general trend when comparing the lower efficiency in longer wavelength green emitters to the blue ones.