Michelle Bourke, production business group director at Oxford Instruments commented, “The PlasmaPro100 Sapphire is designed specifically to address the harsh chemistries required forHBLED materials, delivering fast etch rates uniformly on wafers up to 200mm in diameter. At Oxford Instruments we strive to provide the most innovative, cost effective and reliable process solutions for our customers. This latest system is designed to encompass all these requirements.”

According to Oxford Instruments, the PlasmaPro100 Sapphire’s technology promises manufacturers the tools to deliver more efficient, lower cost lighting that is needed worldwide to assist this lighting revolution.

In theory, using silicon as a substrate would lower material cost and enable economies of scale in mass production from the larger wafer diameters. However, the quality of nitride semiconductors on silicon suffer from the larger lattice mismatch compared with conventional, more expensive substrates of free-standing GaN, sapphire or silicon carbide (SiC).

The researchers note that producing longer-wavelength nitride semiconductor LEDs is challenging due to the difficulty of producing good-quality indium gallium nitride (InGaN) with higher indium concentration. Although growth on silicon is well established in nitride semiconductor-based transistor development, the researchers point out that it is only fairly recently that similar growth methods have been applied to LED device material.

The researchers used metal-organic chemical vapor deposition (MOCVD) to grow the initial template, grown on 2-inch silicon . The template consisted of aluminium nitride (AlN) nucleation, 8 pairs of aluminium nitride/gallium nitride (AlN/GaN) layers to create a superlattice (SL) as stress-balancing interlayer, and a 2μm GaN buffer.

The researchers deposited layers of SiO2 and indium tin oxide (ITO) and etching the ITO with hydrochloric (HCL) acid solution to form a mask, and finally used plasma etching to form Silicon dioxide (SiO2) nanorods. The density of nanorods was 2x109/cm2, giving a surface coverage of 35%. The nanorods acted as a mask in GaN re-growth with reduced dislocation density and improved crystalline quality.

Then the researchers grew the LED structure through MOCVD with re-growth of 800nm of GaN around the nanorods, an AlN/GaN SL interlayer, 2μm of n-type GaN, a 5-period multiple quantum well (MQW), and 200nm of p-GaN. The re-grown GaN had a dislocation density of 8x108/cm2, which was described by the researchers as “one of the lowest values reported for GaN-on-Si substrates, as determined by TEM”.

Materials suitable for emitting yellow (565nm) and green (505 and 530nm) light were prepared and formed into 300μm x 300μm LED chips.

As was to be expected, the light output power (LOP) decreased as the wavelength increased (Figure 2). At 20mA, the output at 505nm was 1.18mW. The respective values for 530nm and 565nm were 0.30mW and 74μW, respectively. Saturation of the light output power was achieved at 7.60mW (200mA), 2.72mW (180mA) and 0.52mW (160mA), for the 505nm, 530nm and 565nm devices, respectively.

The researchers stated, “This is the first report of fabricated 565nm yellow InGaN/GaN MQW LEDs on a silicon substrate, and the LOP of the 505nm LEDs was much higher than that for the LEDs on Si reported in the past.”

Apart from the improved material quality, the researchers believe that the nanorods also provide a scattering enhancement of light extraction from the devices.

The researchers note that developing an LED that emits a broad spectrum of warm white light on par with sunlight has proven difficult. Conventional White LEDs produce light by passing electrons through a semiconductor material, coupled with phosphors that glow when excited by radiation from the LED.

"It's hard to get one phosphor that makes the broad range of colors needed to replicate the sun," commented John Budai, a scientist in ORNL's Materials Science and Technology division. "One approach to generating warm-white light is to hit a mixture of phosphors with ultraviolet radiation from an LED to stimulate many colors needed for white light."

Budai is working with a team of scientists from University of Georgia and Oak Ridge and Argonne national laboratories to understand a new group of crystals that might produce the right blend of colors for white LEDs as well as other uses. Zhengwei Pan's group at UGA grew the nanocrystals using europium oxide and aluminum oxide powders as the source materials because the rare-earth element europium is known to be a dopant, or additive, with good phosphorescent properties.

"What's amazing about these compounds is that they glow in lots of different colors—some are orange, purple, green or yellow," Budai said. "The next question became: why are they different colors? It turns out that the atomic structures are very different."

Budai and the other scientists have used x-rays from Argonne's Advanced Photon Source to studying the atomic structure of the materials. Budai says that two of the three types of crystal structures in the group of phosphors had never been seen before, which can probably be attributed to the crystals' small size.

"Only the green ones were a known crystal structure," Budai said. "The other two, the yellow and blue, don't grow in big crystals; they only grow with these atomic arrangements in these tiny nanocrystals. That's why they have different photoluminescent properties."

X-ray diffraction analysis is helping the scientists figure out the arrangement of the atoms in each of the different crystal types. The different-colored phosphors exhibit distinct diffraction patterns when they are hit with x-rays, depending on the crystal structure. So the diffraction patterns can be used to analyze the crystal structures.

"What that means in terms of how the electrons around the atoms interact to make light is much harder," Budai said. "We haven't completely solved that yet. That's the continuing research. We have a lot of clues, but we don't know everything."

The atomic-scale analysis is helping the research team improve the phosphorescent crystals. Different factors in the growth process such as temperature, powder composition, and types of gas used can change the final product. A fundamental understanding of all the parameters could help the team to perfect the recipe and improve the crystals' ability to convert energy into light. The scientists note that improving the material's luminescence efficiency is key to making it useful for commercial LED products and other applications.

Budai concluded, "You can keep growing the crystals and measuring them, or you can understand why it's doing what it's doing, and figure out how to make it better. That's what we're doing—basic research. We have to figure out nature first."

The agreement includes an expanded licensing and manufacturing supply relationship. Bridgelux says it will continue to develop and market its GaN-on-Sapphire LED products as a fabless solid state lighting company. The companies began their collaboration in early 2012, and later in 2012 Toshiba became an investor in Bridgelux. As part of the previously announced agreement, Toshiba hired Bridgelux’s GaN-on-Silicon development team. In turn, Bridgelux reportedly retains a majority of its revenue generating operations as a fabless LED company.

“We are thrilled to be moving into the next stage of our joint work with Toshiba to advance GaN-on-Silicon-based solid state lighting technologies,” said Brad Bullington, CEO of Bridgelux. “As we outlined last month, Bridgelux will focus on commercializing, productizing and bringing to market GaN-on-Silicon technologies alongside a proven global scale semiconductor manufacturer. At the same time, we remain committed to our GaN-on-Sapphire business and look forward to continuing to provide world-class innovation and service to our customers.”

The company claims that this puts QLEDs near the fundamental efficiency limit of the technology which the company says is 20 percent for quantum dots.These results are two times higher efficiency than previously reported state-of-the-art efficiency of a QLED device. QD Vision says its current and luminous power efficiency are better than the best evaporated OLED result of the same color coordinate, and significantly better than what solution-processed OLEDs have thus far achieved.

In comparison, Pacific Northwest National Laboratory (PNNL) recently reported 11 percent external quantum efficiency for a blue organic light emitting diode (OLED) at 800 cd/m2. However Phosphor-based OLEDs are apparently not included in the company's comparison statement.

“This paper clearly demonstrates the fundamental efficiency advantage that QLEDs have over any other emissive display technology. Achieving this milestone is a great breakthrough and the result of years of hard work and dedication to achieving what others may have thought impossible,” said QD Vision co-founder Seth Coe-Sullivan.

While at an earlier stage of development and commercialization than QD Vision’s Color IQTM products, QD Vision says that its QLED performance is already suitable for use in certain products that require precision color solutions in an ultra-slim form factor, including monochrome visible and infrared displays, and lighting devices for machine and night vision applications.

The researchers note that green-emitting nitride semiconductor LED structures tend to suffer from low light output due to the difficulty in producing the high-indium-content indium gallium nitride (InGaN) needed for longer-wavelength light emission. In addition to the material quality challenge, strain induced by the lattice mismatch with pure GaN leads to large piezoelectric effects, giving electric fields that tend to pull electrons and holes apart, reducing rates of recombination into photons (i.e. the quantum-confined Stark effect, or QCSE), thus reducing quantum efficiency.

The Chinese team inserted a layer of lower-indium-content InGaN before the high-In-content light-emitting layer. Simulations suggested that such a layer could reduce the strain-dependent electric fields in the active light-emitting multiple quantum well (MQW) structure.

MOCVD on C-plane sapphire was used to produce epitaxial material with a low-In-content InGaN shallow quantum well (SQW) step. A 325nm helium-cadmium laser was used to excite the photoluminescence spectra of the materials at low temperature (85K) and room temperature (298K). One effect of the SQW was to reduce the width of the spectral peak full-width at half maximum (FWHM) at 85K from 16.7nm for the conventional LED material to 13.1nm for the SQW material. The 298K measurement reduced the conventional FWHM of 20.1nm to 15.7nm. The peak intensity was also higher with the SQW structure, therefore the SQW material had improved crystal quality.

The peak height for the SQW material at 298K was 55.1% that at 85K. The corresponding ratio for the conventional structure was 24.1%. The higher ratio for the SQW material indicates a higher rate of radiative recombination and higher internal quantum efficiency (IQE).

The electroluminescence was measured in an integrating sphere, giving light output power–current–voltage (L–I–V) results. The voltage performance is similar in the SQW and conventional devices. However, the light output at 150mA is 28.9% greater in the SQW LED (49.3mW) over the conventional device (38.4mW).

The researchers point out that improved overlap of the electron and hole wavefunctions in the device leads to improved recombination into photons. The external quantum efficiency (EQE) increased from 10.2–13.3% over the conventional LED performance.

Company CTO Boris Epelbaum commented, "Further diameter increase in our patented tungsten based furnaces is not limited as we are using SiC as initial seed."

Wafer polishing drastically improved as well for the AlN substrates. "The corresponding wafers feature surface roughness of less than 0.3 nm and are highly UV transparent," said Octavian Filip, director of wafering.

Hitachi Cable GaN-template reportedly can solve this problem because the n-type GaN layer is already grown on the template. Hitachi Cable says that this can reduce the time required for growth by about half compared with conventional methods. The GaN-templates are also said to be suitable for high-output LEDs which require large currents because they allow both low resistance and high crystal formation,

The firm has developed single-crystal free-standing GaN substrates used for blue-violet lasers and developed HVPE-growth technology and machines for mass-production of GaN substrates. Template sized 2”, 4” and 6” are available with 8” templates in development.