Stuart Brinkley

Efficient and Color-Tunable Oxyfluoride Solid Solution Phosphors for Solid-State White Lighting

A solid solution strategy helps increase the efficiency of Ce{sup 3+} oxyfluoride phosphors for solid-state white lighting. The use of a phosphor-capping architecture provides additional light extraction. The accompanying image displays electroluminescence spectra from a 434-nm InGaN LED phosphor that has been capped with the oxyfluoride phosphor.

Robust thermal performance of Sr2Si5N8:Eu2+: An efficient red emitting phosphor for light emitting diode based white lighting

An important component to the advent of solid state lighting technology is the development of inorganic crystalline phosphors for efficient conversion of photons from blue light emitting diodes (LEDs) to other visible wavelengths for greater color rendering and “warmer” white lighting. We present the results of a recently developed rare earth doped nitride-based red emitting phosphor, Sr2Si5N8:Eu2+, combined with GaN-based blue emitting LEDs and YAG:Ce phosphor for improved white lighting applications. A unique remote phosphor packaging approach was used in all testing to isolate LED performance from phosphor performance. Luminous efficacies were achieved at 94 lm/W with an improved color rendering index (CRI) of 72, mixing red phosphor with YAG:Ce. The Sr2Si5N8:Eu2+ red emitting phosphor was found to have a low temperature sensitivity (only 28% power reduction at 150 °C) and greater luminous performance at low concentrations in the encapsulant by weight relative to other typical red emitting phosphors.

Characterization of blue-green m-plane InGaN light emitting diodes

High indium content blue-green (460–520 nm) m-plane InGaN light emitting diodes (LEDs) were grown on low defect-density m-plane GaN substrates. Systematic studies were performed on packaged blue-green LED lamps by using a range of well and barrier thicknesses. Photoluminance and electroluminance peak wavelengths increased while the well width was increased from 2 to 4 nm. The highest output power was achieved for well width of 2.5 nm. The output power improved significantly with the increase in barrier thickness. Nearly blueshift-free emission was observed in all LEDs from 1–400 A/cm2 current density under pulsed operation.

A green-yellow emitting oxyfluoride solid solution phosphor Sr2Ba(AlO4F)1−x(SiO5)x:Ce3+ for thermally stable, high color rendition solid state white lighting

A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1−x(SiO5)x has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce3+ activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green (λem = 523 nm) to yellow (λem = 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 ± 5% for some of the examples in the series. Excellent thermal properties were found for the x = 0.5 sample, with the photoluminescence intensity at 160 °C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED (λmax = 400 nm) exhibit high color rendering white light with Ra = 70 and a correlated color temperature near 7000 K. The value of Ra could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.

30-mW-Class High-Power and High-Efficiency Blue Semipolar (1011) InGaN/GaN Light-Emitting Diodes Obtained by Backside Roughening Technique

The first 30-mW-class semipolar blue light-emitting diode (LED) on a free-standing (1011) GaN substrate has been demonstrated by using microscale periodic backside structures. The light extraction efficiency and corresponding output power were greatly enhanced, by up to 2.8-fold (bare chip) compare with conventional devices. At a driving current of 20 mA, the LED showed an output power of 31.1 mW and an external quantum efficiency of 54.7%. Semipolar GaN LED technology is now comparable to commercial c-plane blue LED technology, not only in terms of internal material properties but also in terms of chip processing techniques.

Polarized spontaneous emission from blue-green m-plane GaN-based light emitting diodes

The polarization of spontaneous emission was investigated for various indium compositions and quantum wells on m-plane oriented gallium nitride (GaN) light emitting diodes (LEDs) grown on bulk-GaN substrates. Internal light scattering and depolarization was mitigated with application of absorber materials to the LED die. The polarization ratio (ρ) was measured under electrical injection for devices with InGaN active regions emitting up to 520 nm and observed as high as 96%. Values of ρ were independent of drive current. The valence band energy separation (ΔE) was characterized using spectral measurement and temperature dependent optical analysis of valence band hole distributions.

Substitution of oxygen by fluorine in the GdSr 2 AlO 5 :Ce 3+ phosphors: Gd 1-x Sr 2+x AlO 5-x Fx solid solutions for solid state white lighting

Solid solutions between two isotypic host compounds: GdSr(2)AlO(5) and Sr(3)AlO(4)F; Gd(1-x)Sr(2+x)AlO(5-x)F(x):Ce(3+) (GSAF:Ce(3+)), have been prepared across the complete solid solution range x. Depending on x, the series display considerable optical tunability of emission wavelengths in the range 574 nm to 474 nm, which is attributed to the decreased crystal field splitting arising from increased host ionicity with fluorine addition. Applying the GSAF:Ce(3+) phosphors on InGaN LEDs (lambda (max) = 405 nm and 450 nm) permits white lighting sources to be prepared. The characteristics of these are reported.

Polarized light extraction in m-plane GaN light-emitting diodes by embedded photonic-crystals

The polarization-preserving property of photonic crystals (PhCs) is combined to the superior extraction efficiency of embedded PhCs to enhance polarized light emission in m-plane GaN light-emitting diodes. As guided modes in m-plane GaN are mostly polarized along the a-direction (E∥a), their efficient extraction is achieved by one-dimensional embedded PhCs also aligned to the a-direction. A better diffraction efficiency is obtained by air-gap PhCs which required the growth of high quality, defect-free, m-plane GaN coalesced over the embedded air-grooves.

Chip Shaping for Light Extraction Enhancement of Bulk c-Plane Light-Emitting Diodes

The enhancement of light extraction from light-emitting diodes (LEDs) grown on bulk c-plane gallium nitride (GaN) through chip shaping was investigated. Photolithography and diamond scribing determined the individual LED chip geometries (triangles, parallelograms, and rectangles). A light-absorbing material was applied to the sidewalls of individual LED dies systematically and measured in an integrating sphere to determine the amount of photon extraction from the sidewalls. The systematic use of the absorber was repeated in commercial ray tracing software yielding almost identical results and allowed the determination of the extraction efficiency for each chip geometry. The total extraction efficiencies were 47.7, 52.6, and 53.1% for the rectangle, parallelogram, and triangle, respectively.

Improving color rendition in solid state white lighting through the use of quantum dots

Quantum dots have the potential to be used in solid state white lighting applications as an additional down-converting component to better represent wavelengths in the red spectral region, leading to higher quality white light with improved color rendering. In this contribution, we report on color characteristics of phosphor-converted white light-emitting diodes that utilize inorganic garnet-based phosphors, with quantum dots incorporated appropriately. Devices were fabricated using red-emitting CdSe/ZnS core/shell quantum dots (λem = 560 nm or λem = 590 nm) in conjunction with yellow-emitting Y3Al5O12:Ce3+ or green-emitting Lu3Al5O12:Ce3+ and a blue-emitting InGaN LED (λem = 450 nm). Several phosphor/quantum dot geometries were examined, including encapsulating the phosphor powder and quantum dots together in silicone resin, or in two separate layers of silicone resin, with either the quantum dots closest to the LED or the phosphor particles closest to the LED. The quantum dots were most efficient when encapsulated with the phosphor particles in the resin. Devices with Y3Al5O12:Ce3+ and quantum dots (λem = 590 nm) achieved a correlated color temperature of 4000 K, color rendering index of 81, and luminous efficacy of 57 lm W−1, while devices with Lu3Al5O12:Ce3+ and quantum dots (λem = 590 nm) achieved a correlated color temperature of 5700 K, a color rendering index of 90, and luminous efficacy of 22 lm W−1. The results obtained suggest that the use of quantum dots may allow for solid state white lighting devices with high color rendition.