Tag Archives: letter-leds-tech

High-power white LEDs face the same problem that Michigan Stadium faces on game day — too many people in too small of a space. Of course, there are no people inside of an LED. But there are many electrons that need to avoid each other and minimize their collisions to keep the LED efficiency high. Using predictive atomistic calculations and high-performance supercomputers at the NERSC computing facility, researchers Logan Williams and Emmanouil Kioupakis at the University of Michigan found that incorporating the element boron into the widely used InGaN (indium-gallium nitride) material can keep electrons from becoming too crowded in LEDs, making the material more efficient at producing light.

This is the crystal structure of a BInGaN alloy. Using atomistic calculations and high-performance supercomputers at the NERSC facility, Logan Williams and Emmanouil Kioupakis at the University of Michigan predicted that incorporating boron into the InGaN active region of nitride LEDs reduces or even eliminates the lattice mismatch with the underlying GaN layers while keeping the emission wavelength approximately the same. The lattice matching enables the growth of thicker active regions and increases the efficiency of LEDs at high power. Credit: Michael Waters and Logan Williams

This is the crystal structure of a BInGaN alloy. Using atomistic calculations and high-performance supercomputers at the NERSC facility, Logan Williams and Emmanouil Kioupakis at the University of Michigan predicted that incorporating boron into the InGaN active region of nitride LEDs reduces or even eliminates the lattice mismatch with the underlying GaN layers while keeping the emission wavelength approximately the same. The lattice matching enables the growth of thicker active regions and increases the efficiency of LEDs at high power. Credit: Michael Waters and Logan Williams

Modern LEDs are made of layers of different semiconductor materials grown on top of one another. The simplest LED has three such layers. One layer is made with extra electrons put into the material. Another layer is made with too few electrons, the empty spaces where electrons would be are called holes. Then there is a thin middle layer sandwiched between the other two that determines what wavelength of light is emitted by the LED. When an electrical current is applied, the electrons and holes move into the middle layer where they can combine together to produce light. But if we squeeze too many electrons in the middle layer to increase the amount of light coming out of the LED, then the electrons may collide with each other rather than combine with holes to produce light. These collisions convert the electron energy to heat in a process called Auger recombination and lower the efficiency of the LED.

A way around this problem is to make more room in the middle layer for electrons (and holes) to move around. A thicker layer spreads out the electrons over a wider space, making it easier for them to avoid each other and reduce the energy lost to their collisions. But making this middle LED layer thicker isn’t as simple as it sounds.

Because LED semiconductor materials are crystals, the atoms that make them up must be arranged in specific regular distances apart from each other. That regular spacing of atoms in crystals is called the lattice parameter. When crystalline materials are grown in layers on top of one another, their lattice parameters must be similar so that the regular arrangements of atoms match where the materials are joined. Otherwise the material gets deformed to match the layer underneath it. Small deformations aren’t a problem, but if the top material is grown too thick and the deformation becomes too strong then atoms become misaligned so much that they reduce the LED efficiency. The most popular materials for blue and white LEDs today are InGaN surrounded by layers of GaN. Unfortunately, the lattice parameter of InGaN does not match GaN. This makes growing thicker InGaN layers to reduce electron collisions challenging.

Williams and Kioupakis found that by including boron in this middle InGaN layer, its lattice parameter becomes much more similar to GaN, even becoming exactly the same for some concentrations of boron. In addition, even though an entirely new element is included in the material, the wavelength of light emitted by the BInGaN material is very close to that of InGaN and can be tuned to different colors throughout the visible spectrum. This makes BInGaN suitable to be grown in thicker layers, reducing electron collisions and increasing the efficiency of the visible LEDs.

Although this material is promising to produce more efficient LEDs, it is important that it can be realized in the laboratory. Williams and Kioupakis have also shown that BInGaN could be grown on GaN using the existing growth techniques for InGaN, allowing quick testing and use of this material for LEDs. Still, the primary challenge of applying this work will be to fine tune how best to get boron incorporated into InGaN at sufficiently high amounts. But this research provides an exciting avenue for experimentalists to explore making new LEDs that are powerful, efficient, and affordable at the same time.

The stacked color sensor


November 16, 2017

The human eye has three different types of sensory cells for the perception of colour: cells that are respectively sensitive to red, green and blue alternate in the eye and combine their information to create an overall colored image. Image sensors, for example in mobile phone cameras, work in a similar way: blue, green and red sensors alternate in a mosaic-like pattern. Intelligent software algorithms calculate a high-resolution colour image from the individual colour pixels.

However, the principle also has some inherent limitations: as each individual pixel can only absorb a small part of the light spectrum that hits it, a large part of the light is lost. In addition, the sensors have basically reached the limits of miniaturization, and unwanted image disturbances can occur; these are known as color moiré effects and have to be laboriously removed from the finished image.

Transparent only for certain colors

Researchers have therefore been working for a number of years on the idea of stacking the three sensors instead of placing them next to each other. Of course, this requires that the sensors on top let through the light frequencies that they do not absorb to the sensors underneath. At the end of the 1990s, this type of sensor was successfully produced for the first time. It consisted of three stacked silicon layers, each of which absorbed only one colour.

This actually resulted in a commercially available image sensor. However, this was not successful on the market because the absorption spectra of the different layers were not distinct enough, so part of the green and red light was absorbed by the blue-sensitive layer. The colors therefore blurred and the light sensitivity was thus lower than for ordinary light sensors. In addition, the production of the absorbing silicon layers required a complex and expensive manufacturing process.

Empa researchers have now succeeded in developing a sensor prototype that circumvents these problems. It consists of three different types of perovskites – a semiconducting material that has become increasingly important during the last few years, for example in the development of new solar cells, due to its outstanding electrical properties and good optical absorption capacity. Depending on the composition of these perovskites, they can, for example, absorb part of the light spectrum, but remain transparent for the rest of the spectrum. The researchers in Maksym Kovalenko’s group at Empa and ETH Zurich used this principle to create a color sensor with a size of just one pixel. The researchers were able to reproduce both simple one-dimensional and more realistic two-dimensional images with an extremely high color fidelity.

Accurate recognition of colors

The advantages of this new approach are clear: the absorption spectra are clearly differentiated and the colour recognition is thus much more precise than with silicon. In addition, the absorption coefficients, especially for the light components with higher wavelengths (green and red), are considerably higher in the perovskites than in silicon. As a result, the layers can be made significantly smaller, which in turn allows smaller pixel sizes. This is not crucial in the case of ordinary camera sensors; however, for other analysis technologies, such as spectroscopy, this could permit significantly higher spatial resolution. The perovskites can also be produced using a comparatively cheap process.

However, more work is still needed in order to further develop this prototype into a commercially usable image sensor. Key areas include the miniaturisation of pixels and the development of methods for producing an entire matrix of such pixels in one step. According to Kovalenko, this should be possible with existing technologies.

Perovskites are such a promising material in research that the prestigious journal Science has published a special edition about them. It includes a review article by the Empa/ETH research group led by Maksym Kovalenko about the current state of research and potential uses of lead halide perovskites nanocrystals.

These have properties that make them a promising candidate for the development of semiconductor nanocrystals for various optoelectronic applications such as television screens, LEDs and solar cells: they are inexpensive to manufacture, have a high tolerance to defects and can be tuned precisely to emit light in a specific colour spectrum.

Seoul Semiconductor has developed an ultra-compact LED driver series with a power density 5X higher than conventional LED drivers. Based on Seoul Semiconductor’s patented Acrich technology, the MicroDriver Series delivers more than 24W of output power with a power density of 20W/cubic inch cubic inch, compared to existing drivers at 3-5W/cubic inch. Measuring just 1.5″ x 1.1″ x 0.8″ (38mm x 28mm x 20.5mm), the MicroDriver is 80% smaller than conventional LED drivers, giving lighting designers the ability to develop ultra-thin and novel luminaires with flicker-free operation.

“The new MicroDriver Series LED drivers will have a significant impact on external converters, enabling lighting design engineers to dramatically reduce the size, weight and volume of their luminaires,” explained Keith Hopwood, executive vice-president at Seoul Semiconductor. “This breakthrough in size reduction for the MicroDriver Series is the result of the company’s continuing investment in Acrich high voltage LED technology, delivering benefits for customers in smaller size, increased efficiency and lower costs.”

The MicroDriver Series LED drivers are ideal for lighting designs such as wall sconces, vanity lights, downlights, and flush-mounted lighting fixture applications. The MicroDriver Series’ smaller size facilitates the conversion of these applications to LED light sources, which was not previously possible due to bulky conventional LED drivers, making halogen lamp replacement possible without the need for a large volume recess for the driver, or a reduction in light output.

The MicroDriver Series LED drivers are ideal for luminaire designs up to 2,400 lumens, and their compact size enables integration of the lighting control circuitry with the external converter. This gives lighting designers the capability to mount more light sources on the board or reduce the total size of the fixture and mounting plate.

The resulting decrease in the LED drivers’ physical size has significant business implications for the lighting industry, giving lighting designers the ability to shrink the size of light fixtures by as much as 20%, which reduces shipping and storage costs. Because conventional LED drivers are both heavy and bulky, they are typically shipped via sea freight from manufacturers in Asia to European and North American fixture companies, with transit times up to six weeks. The MicroDriver Series LED drivers are small and lightweight enough to make airfreight practical and economical, reducing lead time and streamlining the overall supply chain.

The MicroDriver Series is rated to IP66, and is available in 10 models, rated for 8 – 24W in 120V or 230V versions, for LED assemblies from 900-2400 lumens. The drivers are CE recognized, provide flicker-free operation for phase-cut dimmers, and are compliant to California Title 24, enabling lighting designers to meet the most challenging design requirements, including low flicker, high power factor, Class B EMI and 2.5kV surge.

Seoul Semiconductor exhibited its new SunLike Series LEDs, the world’s first LED to produce light that closely matches the spectrum of natural sunlight, at the recent Professional Lighting Design Conference (PLDC), held in Paris, France from Nov. 1 – 4. The new LED technology, first unveiled in Frankfurt, Germany in June of this year, is generating interest from many global lighting companies, who are developing new lighting products using SunLike Series LEDs.

New products from leading lighting designers powered by Seoul Semiconductor’s SunLike LED technology were on display at PLDC 2017, which attracted more than 2000 attendees. A number of these companies signaled their intention to launch these new SunLike-powered lighting products in the market.

The director of Seoul Semiconductor’s Lighting Divison, Mr. Yo Cho, was invited as a keynote speaker at the PLDC’s opening event, where he presented SunLike Series LED technology. “Because the SunLike Series LEDs are designed to deliver light that closely matches sunlight’s natural spectrum, they provide an optimized light source that maximizes the benefits of natural light,” said Mr. Cho. “Thus, the colors and texture of objects can be viewed more accurately, as they would be seen under natural sunlight.”

According to Dr. Kibum Nam, head of Seoul Semiconductor R&D Center and Chief Technology Officer, “SunLike Series LEDs have the potential to drive a revolution in lighting – overcoming the limits of artificial light sources by implementing light closer to the natural spectrum of sunlight. Seoul will open a new era of natural spectrum lighting with the launch of more SunLike LED technology.”

SunLike Series natural spectrum LEDs may also play a key role in minimizing the negative effects of artificial lighting. While conventional LED technology produces light with a pronounced blue “spike” in its spectral output, SunLike LEDs implement a more uniform spectrum that more closely matches natural sunlight, lowering this blue light spike. Some recent research indicates that this blue light spike may produce negative effects when viewed for prolonged periods of time during night-time hours, potentially interfering with natural human biorhythms. By employing new light sources powered by SunLike Series LEDs, lighting designers will be able to deliver a healthier light experience.

Interest in the link between light sources and human health is higher than ever before, as evidenced by the winners of this year’s Nobel Prize in Physiology, Professor Jeffrey C. Hall, University of Maine; Professor Michael Morris Rosbach, Brandeis University; and Professor Michael Young, Rockefeller University. These researchers are credited with seminal discoveries about the cellular mechanisms for circadian biology.

The trick is to be able to use beryllium atoms in gallium nitride. Gallium nitride is a compound widely used in semiconductors in consumer electronics from LED lights to game consoles. To be useful in devices that need to process considerably more energy than in your everyday home entertainment, though, gallium nitride needs to be manipulated in new ways on the atomic level.

“There is growing demand for semiconducting gallium nitride in the power electronics industry. To make electronic devices that can process the amounts of power required in, say, electric cars, we need structures based on large-area semi-insulating semiconductors with properties that allow minimising power loss and can dissipate heat efficiently. To achieve this, adding beryllium into gallium nitride – or ‘doping’ it – shows great promise,” explains Professor Filip Tuomisto from Aalto University.

Sample chamber of the positron accelerator. Credit: Hanna Koikkalainen

Sample chamber of the positron accelerator. Credit: Hanna Koikkalainen

Experiments with beryllium doping were conducted in the late 1990s in the hope that beryllium would prove more efficient as a doping agent than the prevailing magnesium used in LED lights. The work proved unsuccessful, however, and research on beryllium was largely discarded.

Working with scientists in Texas and Warsaw, researchers at Aalto University have now managed to show – thanks to advances in computer modelling and experimental techniques – that beryllium can actually perform useful functions in gallium nitride. The article published in Physical Review Letters shows that depending on whether the material is heated or cooled, beryllium atoms will switch positions, changing their nature of either donating or accepting electrons. “Our results provide valuable knowledge for experimental scientists about the fundamentals of how beryllium changes its behaviour during the manufacturing process. During it – while being subjected to high temperatures – the doped compound functions very differently than the end result,” describes Tuomisto.

If the beryllium-doped gallium nitride structures and their electronic properties can be fully controlled, power electronics could move to a whole new realm of energy efficiency.

“The magnitude of the change in energy efficiency could as be similar as when we moved to LED lights from traditional incandescent light bulbs. It could be possible to cut down the global power consumption by up to ten per cent by cutting the energy losses in power distribution systems,” says Tuomisto.

Researchers have developed a technique that allows users to collect 100 times more spectrographic information per day from microfluidic devices, as compared to the previous industry standard. The novel technology has already led to a new discovery: the speed of mixing ingredients for quantum dots used in LEDs changes the color of light they emit – even when all other variables are identical.

Researchers have discovered that the speed of mixing ingredients for quantum dots used in LEDs changes the color of light they emit -- even when all other variables are identical. Credit: Milad Abolhasani

Researchers have discovered that the speed of mixing ingredients for quantum dots used in LEDs changes the color of light they emit — even when all other variables are identical. Credit: Milad Abolhasani

“Semiconductor nanocrystals are important structures used in a variety of applications, ranging from LED displays to solar cells. But producing nanocrystalline structures using chemical synthesis is tricky, because what works well on a small scale can’t be directly scaled up – the physics don’t work,” says Milad Abolhasani, an assistant professor of chemical and biomolecular engineering at North Carolina State University and corresponding author of a paper on the work.

“This challenge has led to an interest in continuous nanomanufacturing approaches that rely on precisely controlled microfluidic-based synthesis,” Abolhasani says. “But testing all of the relevant variables to find the best combination for manufacturing a given structure takes an extremely long time due to the limitations of the existing monitoring technologies – so we decided to build a completely new platform.”

Currently, microfluidic monitoring technologies are fixed in place, and monitor either absorption or fluorescence. Fluorescence data tells you what the crystal’s emission bandgap is – or what color of light it emits – which is important for LED applications. Absorption data tells you the crystal’s size and concentration, which is relevant for all applications, as well as its absorption bandgap – which is important for solar cell applications.

To monitor both fluorescence and absorption you’d need two separate monitoring points. And, being fixed in place, people would speed up or slow down the flow rate in the microfluidic channel to control the reaction time of the chemical synthesis: the faster the flow rate, the less reaction time a sample has before it hits the monitoring point. Working around the clock, this approach would allow a lab to collect about 300 data samples in 24 hours.

Abolhasani and his team developed an automated microfluidic technology called NanoRobo, in which a spectrographic monitoring module that collects both fluorescent and absorption data can move along the microfluidic channel, collecting data along the way. The system is capable of collecting 30,000 data samples in 24 hours – expediting the discovery, screening, and optimization of colloidal semiconductor nanocrystals, such as perovskite quantum dots, by two orders of magnitude. Video of the automated system can be seen at https://www.youtube.com/watch?v=FBQoSDdn_Uk.

And, because of the translational capability of the novel monitoring module, the system can study reaction time by moving along the microfluidic channel, rather than changing the flow rate – which, the researchers discovered, makes a big difference.

Because NanoRobo allowed researchers to monitor reaction time and flow rate as separate variables for the first time, Abolhasani was the first to note that the velocity of the samples in the microfluidic channel affected the size and emission color of the resulting nanocrystals. Even if all the ingredients were the same, and all of the other conditions were identical, samples that moved – and mixed – at a faster rate produced smaller nanocrystals. And that affects the color of light those crystals emit.

“This is just one more way to tune the emission wavelength of perovskite nanocrystals for use in LED devices,” Abolhasani says.

NC State has filed a provisional patent covering NanoRobo and is open to exploring potential market applications for the technology.

MagnaChip Semiconductor Corporation(NYSE: MX), a designer and manufacturer of analog and mixed-signal semiconductor products, announced today it now offers a 0.35 micron 700V Ultra-High Voltage process technology (UHV) that reduces mask counts, manufacturing time and cost for power-related AC-DC products. This UHV process technology offers 700V nLDMOS, 700V JFET, and 5.5V CMOS devices that are suitable for manufacturing AC-DC converter ICs and LED driver ICs.

The demand for AC-powered products in home appliances continues to increase, creating the need for highly efficient and cost-competitive AC-DC converter ICs, AC-DC chargers and LED driver ICs.  MagnaChip’s 0.35 micron 700V UHV process technology is a suitable match to manufacture these types of power-related products.

MagnaChip provides various types of UHV technology to meet the diverse demands of the customers. HP35ULB700, the newly developed UHV process, eliminates five photolithography steps through process simplification compared with MagnaChip’s previous generation of UHV technology, making it possible to reduce manufacturing cost and to accelerate the time to market. Among the devices offered in HP35ULB700 are 700V low Ron nLDMOS, 500V nLDMOS, 700V JFET, 5.5V CMOS, BJT, 700V resistor, BP cap, and MIM and fuse. All these devices enable the integrated solution of AC-DC converter ICs and LED driver ICs. The 700V low Ron nLDMOS devices offer improved specific-on-resistance of 150 mohm·cm2. In addition, the devices enable various design schemes, including the possibility to separate or connect the source and the bulk in nLDMOS.

YJ Kim, MagnaChip’s Chief Executive Officer, commented, “Our  0.35 micron 700V UHV technology  provides our foundry customers with a high-performance, highly efficient manufacturing process for AC-DC converter ICs and LED driver ICs for various LED lighting applications.” Mr. Kim added, “To meet the diverse customer requirements, MagnaChip will continue to develop new UHV technologies such as customer-specific UHV processes with additional option devices.”

The huge increase in computing performance in recent decades has been achieved by squeezing ever more transistors into a tighter space on microchips.

However, this downsizing has also meant packing the wiring within microprocessors ever more tightly together, leading to effects such as signal leakage between components, which can slow down communication between different parts of the chip. This delay, known as the “interconnect bottleneck,” is becoming an increasing problem in high-speed computing systems.

One way to tackle the interconnect bottleneck is to use light rather than wires to communicate between different parts of a microchip. This is no easy task, however, as silicon, the material used to build chips, does not emit light easily, according to Pablo Jarillo-Herrero, an associate professor of physics at MIT.

Now, in a paper published today in the journal Nature Nanotechnology, researchers describe a light emitter and detector that can be integrated into silicon CMOS chips. The paper’s first author is MIT postdoc Ya-Qing Bie, who is joined by Jarillo-Herrero and an interdisciplinary team including Dirk Englund, an associate professor of electrical engineering and computer science at MIT.

The device is built from a semiconductor material called molybdenum ditelluride. This ultrathin semiconductor belongs to an emerging group of materials known as two-dimensional transition-metal dichalcogenides.

Unlike conventional semiconductors, the material can be stacked on top of silicon wafers, Jarillo-Herrero says.

“Researchers have been trying to find materials that are compatible with silicon, in order to bring optoelectronics and optical communication on-chip, but so far this has proven very difficult,” Jarillo-Herrero says. “For example, gallium arsenide is very good for optics, but it cannot be grown on silicon very easily because the two semiconductors are incompatible.”

In contrast, the 2-D molybdenum ditelluride can be mechanically attached to any material, Jarillo-Herrero says.

Another difficulty with integrating other semiconductors with silicon is that the materials typically emit light in the visible range, but light at these wavelengths is simply absorbed by silicon.

Molybdenum ditelluride emits light in the infrared range, which is not absorbed by silicon, meaning it can be used for on-chip communication.

To use the material as a light emitter, the researchers first had to convert it into a P-N junction diode, a device in which one side, the P side, is positively charged, while the other, N side, is negatively charged.

In conventional semiconductors, this is typically done by introducing chemical impurities into the material. With the new class of 2-D materials, however, it can be done by simply applying a voltage across metallic gate electrodes placed side-by-side on top of the material.

“That is a significant breakthrough, because it means we do not need to introduce chemical impurities into the material [to create the diode]. We can do it electrically,” Jarillo-Herrero says.

Once the diode is produced, the researchers run a current through the device, causing it to emit light.

“So by using diodes made of molybdenum ditelluride, we are able to fabricate light-emitting diodes (LEDs) compatible with silicon chips,” Jarillo-Herrero says.

The device can also be switched to operate as a photodetector, by reversing the polarity of the voltage applied to the device. This causes it to stop conducting electricity until a light shines on it, when the current restarts.

In this way, the devices are able to both transmit and receive optical signals.

The device is a proof of concept, and a great deal of work still needs to be done before the technology can be developed into a commercial product, Jarillo-Herrero says.

The researchers are now investigating other materials that could be used for on-chip optical communication.

Most telecommunication systems, for example, operate using light with a wavelength of 1.3 or 1.5 micrometers, Jarillo-Herrero says.

However, molybdenum ditelluride emits light at 1.1 micrometers. This makes it suitable for use in the silicon chips found in computers, but unsuitable for telecommunications systems.

“It would be highly desirable if we could develop a similar material, which could emit and detect light at 1.3 or 1.5 micrometers in wavelength, where telecommunication through optical fiber operates,” he says.

To this end, the researchers are exploring another ultrathin material called black phosphorus, which can be tuned to emit light at different wavelengths by altering the number of layers used. They hope to develop devices with the necessary number of layers to allow them to emit light at the two wavelengths while remaining compatible with silicon.

“The hope is that if we are able to communicate on-chip via optical signals instead of electronic signals, we will be able to do so more quickly, and while consuming less power,” Jarillo-Herrero says.

An interdisciplinary team of scientists at the U.S. Naval Research Laboratory (NRL) has uncovered a direct link between sample quality and the degree of valley polarization in monolayer transition metal dichalcogenides (TMDs). In contrast with graphene, many monolayer TMDs are semiconductors and show promise for future applications in electronic and optoelectronic technologies.

In this sense, a ‘valley’ refers to the region in an electronic band structure where both electrons and holes are localized, and ‘valley polarization’ refers to the ratio of valley populations — an important metric applied in valleytronics research.

Upper Panel: schematic of optical excitation in the K valley of WS2 monolayers. Lower Panel: Photoluminescence (PL) intensity map of a triangular monolayer island of WS2 and the associated valley polarization map demonstrate the clear inverse relationship. Each map covers a 46 x 43 micron area. The regions exhibiting smallest PL intensity and lowest quality are found at the center of the flake and radiate outward toward the three corners. These regions correspond to the highest valley polarization. Credit: US Naval Research Laboratory

Upper Panel: schematic of optical excitation in the K valley of WS2 monolayers. Lower Panel: Photoluminescence (PL) intensity map of a triangular monolayer island of WS2 and the associated valley polarization map demonstrate the clear inverse relationship. Each map covers a 46 x 43 micron area. The regions exhibiting smallest PL intensity and lowest quality are found at the center of the flake and radiate outward toward the three corners. These regions correspond to the highest valley polarization. Credit: US Naval Research Laboratory

“A high degree of valley polarization has been theoretically predicted in TMDs yet experimental values are often low and vary widely,” said Kathleen McCreary, Ph.D., lead author of the study. “It is extremely important to determine the origin of these variations in order to further our basic understanding of TMDs as well as advance the field of valleytronics.”

Many of today’s technologies (i.e. solid state lighting, transistors in computer chips, and batteries in cell phones) rely simply on the charge of the electron and how it moves through the material. However, in certain materials such as the monolayer TMDs, electrons can be selectively placed into a chosen electronic valley using optical excitation.

“The development of TMD materials and hybrid 2D/3D heterostructures promises enhanced functionality relevant to future Department of Defense missions,” said Berend Jonker, Ph.D., principal investigator of the program. “These include ultra-low power electronics, non-volatile optical memory, and quantum computation applications in information processing and sensing.”

The growing fields of spintronics and valleytronics aim to use the spin or valley population, rather than only charge, to store information and perform logic operations. Progress in these developing fields has attracted the attention of industry leaders, and has already resulted in products such as magnetic random access memory that improve upon the existing charge-based technologies.

The team focused on TMD monolayers such as WS2 and WSe2, which have high optical responsivity, and found that samples exhibiting low photoluminescence (PL) intensity exhibited a high degree of valley polarization. These findings suggest a means to engineer valley polarization via controlled introduction of defects and nonradiative recombination sites

“Truly understanding the reason for sample-to-sample variation is the first step towards valleytronic control,” McCreary said. “In the near future, we may be able to accurately increase polarization by adding defect sites or reduce polarization by passivation of defects.”

Results of this research are reported in the August 2017 edition of the American Chemical Society’s Nano, The research team is comprised of Dr. Kathleen McCreary, Dr. Aubrey Hanbicki, and Dr. Berend Jonker from the NRL Materials Science and Technology Division; Dr. Marc Currie from the NRL Optical Sciences Division; and Dr. Hsun-Jen Chuang who holds an American Society for Engineering Education (ASEE) fellowship at NRL.

WIN Semiconductors Corp (TPEx:3105), the world’s largest pure-play compound semiconductor foundry, has released an optimized version of its 0.25µm gallium nitride technology, NP25, that provides superior DC and RF transistor performance. NP25 is a 0.25µm-gate GaN-on-SiC process, and offers users the flexibility to produce both fully integrated amplifier products as well as custom discrete transistors. In production since 2014, the optimized 0.25µm process offers enhanced RF performance with fast switching time, higher gain and increased power added efficiency for demanding power applications through Ku-band

Optimized NP25 transistors exhibit more ideal DC and RF IV characteristics and provide 2 dB higher maximum stable gain. Increased gain leads directly to higher power density and PAE under a range of tuning and bias conditions. This performance-optimized process is fully qualified and supported with a comprehensive design kit and transistor models.

The WIN NP25 technology is fabricated on 4-inch silicon carbide substrates and operates at a drain bias of 28 volts. At 10GHz, NP25 provides saturated output power of 5 watts/mm with 19 dB linear gain and over 65% power added efficiency. These performance metrics make the NP25 process well suited for a variety of high power, broad bandwidth and linear transmit functions in the radar, satellite communications, and wireless infrastructure markets.