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EuroLED Focuses On The Biggest Issues In Solid-state Lighting

Vast swings in MOCVD reactor shipments, stagnation in LED sales in the build up to the solid-state lighting revolution and technologies to slash the cost of chips for LED light bulbs featured at euroLED 2012. Richard Stevenson reports.

Bridgelux aims to slash the GaN chip costs by producing devices on 200 mm silicon and processing wafers through under-utilised silicon fabs

The LED industry is changing. Expenditure on capital equipment is rebounding after falling off a cliff; chip prices are in decline, with shipments soon to surge as penetration into the general illumination market starts to ramp; and some manufacturers are preparing to launch GaN-on-silicon devices that should drive down the prices of solid-state lighting products.

All of these trends were discussed at euroLED 2012, a two-day conference held on 14-15 June that combined a series of invited talks with an exhibition featuring 95 companies. This gathering, which was held at the spacious National Exhibition Centre on the outskirts of Birmingham, UK, kicked off with an overview of the latest LED market trends. According to analyst Jamie Fox from IMS Research (which has recently been acquired by IHS), the LED industry is between cycles, with sales of devices for TV backlighting nearing their peak and solid-state lighting still in its infancy.

“We are waiting for the lighting market to take off," explained Fox. “Prices are currently a little too high for mass adoption, but 2013 and 2014 could be really big years."

Back in 2010, GaN-based LED sales hit an all-time high of $8 billion, a hike of almost $3 billion over the previous year, thanks to massive growth in the number of devices deployed in TV backlighting. Last year, falling prices and weakening demand for LEDs for TV backlighting led to a slight fall in worldwide LED revenue compared to 2010. However, this year the uptake of products in the lighting sector should spur a rise in revenues that should continue for the next two years to more than $10 billion per annum. “Even being conservative, we expect the market to increase by the end of this year," remarked Fox.

Sales of LEDs to makers of TV backlights should net about $2 billion in 2012, rise slightly in 2013, but fall back a little by 2016. This rise and fall in sales will occur because more and more TVs are being built with LED backlighting, but the number of chips for each unit is in decline. The number of LEDs in each unit is being reduced due to improvements in the performance of the LED-based system, in terms of the internal and external quantum efficiencies of the chip, the efficiency of light guides, panel transmissivity and optical film efficiency. Average LED revenue per TV is also declining, due to a move away from just using light guides for backlighting. Low-cost TVs are now featuring a more direct lighting technology, which can halve the number of die required. Another trend is the adoption of larger, less efficient die that can trim the number of lightbars.

Data from IMS shows that the number of LEDs continues to fall in various sizes of TV. In the latter half of 2009, the average number of LEDs in 32-inch, 40-inch and 46-inch sets was 216, 288 and 350, respectively; by the start of the year all these numbers had fallen by roughly 50 percent. According to Fox, the proportion of TVs backlit with LEDs is rising rapidly. Last year penetration stood at 38 percent, this year it should hit 63 percent, and next year it will account for 93 percent of the market. Fast forward another year to 2014, and all TVs will feature LED backlighting.

Lighting takes centre stage

Another significant milestone is also tipped to occur in 2014 – sales of LEDs to the general illumination market will overtake those to TV backlighting manufacturers. When this happens, the number of LEDs shipped to the lighting sector will still be far less than that dispatched to backlighting manufacturers, but each product for the lighting sector will command a far higher price tag. Fox and his colleagues predict that just 2.4 percent of lamps and luminaires sold this year will be based on LEDs. This figure will rise throughout the decade, with massive year-over year increases to 2014, and penetration exceeding 40 percent in 2020.

This revolution in lighting will drive a massive increase in the production of packaged LEDs with dimensions of 1 mm by 1 mm. This year 3.5 billion will made, and this will climb by roughly an order of magnitude by 2020. Economies of scale will play a minor part in reducing the cost of making these high-power LEDs. However, the bigger factors behind the fall in average selling price will be overcapacity within the industry and growth in the number of competitors, particularly from China. According to IMS, these factors will contribute to a rapid decline in the average selling price of an LED suitable for deployment in a lamp from $0.48 in 2012 to just $0.10 in 2020.

Despite these price falls, total sales of LEDs for general lighting will rise, thanks to a massive growth in shipments. Packaged LED lighting sales will net $1.8 billion this year, and are expected to peak at $4.5 billion in 2016. The long lifetime of LEDs will then cause this market to steadily decline, but it will still be worth more than $3 billion by the end of this decade.

Data from market research firm IMS highlights the growth in LED lighting that will take place during this decade

While the chip making industry waits for the LED lighting market to take off, shipments of MOCVD tools for GaN epi-growth are fluctuating wildly. In the third quarter of 2010 they reached an all-time high of more than 200 reactors, before nose-diving to just 41 tools in the first quarter of this year. According to Fox, that quarter should be the nadir for MOCVD suppliers, and 281 tools are likely to be shipped this year.

Assuming a five-year life for MOCVD tools, it is possible that another 2300 tools could be sold between 2013 and 2016 to meet the growing demand for LED chips. However, the average lifetime of these tools will only be that short if manufacturers can convince chipmakers that their latest tools combine a substantial hike in yield and LED performance with a lower cost-of-ownership and a wider process window.

The market leader of MOCVD tools is changing hands. “Aixtron was ahead, but Veeco has caught up and we predict it will lead in 2012," revealed Fox. Veeco’s success stems from the high uptime of its popular K465i reactor, and its high level of penetration in the Chinese market, which has recently accounted for more than half of all shipments. Some of the biggest customers in recent times are Epistar affiliates based in China, such as ULED, Epicrystal (Epistar Changzhou) and Kaifa Jung Lighting (Epistar Xiamen).

Improving LED performance

One company that has always been at the forefront of GaN LED development is Japanese firm Nichia. At euroLED 2012, Deputy Managing Director for Nichia Europe, Hideki Kaneguchi, updated delegates on the company’s recent progress, both in the lab and in production.

The company is still to better its record efficacy of 249 lm/W at 20 mA, a figure first reported at Photonics West in 2009. The best devices in the fab fall well short of this, delivering up to 170 lm/W. The internal quantum efficiency of these LEDs already exceeds 80 percent, and Kaneguchi argued that increases in efficacy must now come from better light extraction – from both the chip and the package.

He also compared the performance of the company’s record-breaking ‘small chip’ LED with a 1 mm2device from the lab producing 180 lm/W. The larger variant has a higher forward voltage – 3.21 V compared to 2.89 V – and its external quantum efficiency is 73.6 percent, down from 84.3 percent. Peak emission for both LEDs is similar – the small chip emits at 444 nm, 3 nm longer than its bigger cousin.

According to Kaneguchi, Nichia will soon release new versions of four LED products from its portfolio. These successors will be: Low-watt LEDs that have an efficacy of 140 lm/W at 65 mA, and produce a peak output of about 50 lumens; multi-chip devices operating at an efficacy of 125 lm/W at 350 mA, and delivering an output of up to 240 lumens; powerful, single-chip LEDs with a peak output of 380 lumens, and an efficacy of 115 lm/W at 350 mA; and chip-on-board packages with a ceramic base that can deliver up to 2200 lumens and emit 110 lm/W at 32 mA per chip.

This wide range of products gives flexibility to the manufacturers of LED light bulbs. For example, they can select a single chip-on-board package for their bulb, or turn to ten single die.

Bulbs that result can combine high efficacy with a colour quality exceeding that of a tri-phosphor fluorescent lamp. This produces Raand R9values of 85 and 8; in comparison, conventional and ultra-high-CRI LEDs from Nichia have Raand R9 values of 74 and 18, and 97 and 96, respectively. Values for Raof over 90 allow bulbs to be used for speciality and professional lighting.

Nichia’s spokesman finished his talk by outlining key challenges for white LEDs and their ultraviolet siblings. Targets for white emission include a higher CRI (Ragreater than 95), tighter colour bins, a forward voltage of just 2.8 V and an efficacy of initially 180 lm/W and subsequently 200 lm/W. In addition, the company is aiming for a thermal droop of no less than 95 percent at a junction temperature of 85 °C. “Minimizing this gap is a big challenge for us," admitted Kaneguchi.

In the ultraviolet regime, increasing power is the primary goal. Devices emitting tens of milliwatts in the 255-300 nm range are planned, as well as those producing several watts at longer wavelengths.

From sapphire to silicon

To generate a healthy profit as LED prices fall, device manufacturers will have to cut chip production costs. Switching the substrate from sapphire to silicon is one way to do this, and this approach brings additional benefits, according to Tom Van den bussche, European Marketing Director of Bridgelux. Silicon substrates enable wafer processing on 200 mm silicon lines; they open the door to the manufacture of larger die, which could lead to increased integration; and they could enable new designs of LED.

According to Van den bussche, fabrication of GaN-on-silicon LEDs typically begins with the growth of strain management layers on the substrate, followed by deposition of the light-emitting structure. A mirror is deposited on top of this epitaxial stack, before a second silicon substrate is bonded to the top of the structure. The resulting composite is then flipped over, and the substrate used for epi-growth removed to expose a GaN surface, which is etched to increase light extraction. Adding contacts completes the LED production process. Switching from sapphire to silicon tends to lead to a hike in defect density, due to a greater lattice mismatch between the substrate and epilayers. However, the insertion of a stack of layers that manage strain and reduce defects enables the threading dislocation density in Bridgelux’s LED epistructures to be as low as just 3-6 x 108 cm-2.

The dislocation densities in Bridgelux’s GaN-on-silicon LEDs are very low, enabling
the devices to deliver high efficacies

High material quality enables the fabrication of LEDs with impressive characteristics, including reduced droop, a very low operating voltage and low thermal resistance. Driven at 350 mA, blue-emitting chips operate at 2.82 V and deliver 0.59 W at a wall plug efficiency (WPE) of 59 percent. Crank up the current to 1 A by increasing the bias to 3.21 V, and the chip emits 1.52 W at a WPE of 47 percent.

Van den bussche presented data showing that Bridgelux LEDs significantly outperform GaN-on-silicon rivals developed by Osram Opto Semiconductors, Samsung and Lattice Power. The performance of Osram’s LEDs is closest to that of those made by Bridgelux – the output power of Osram’s blue chip is slightly higher, but its emission wavelength of 438 nm is too short for optimal pumping of a phosphor. Bridgelux is the only company to report the efficacy of a warm-white GaN-on-silicon LED, which produces 127 lm/W.

Prototype production of LEDs on silicon is underway at Bridgelux, and commercial launch is slated for 2013. When this class of LED hits the market, it will usher in a new era for the LED industry, and hope to spur the production of LED bulbs with a price tag that is attractive to most consumers. Exactly what that price will be is debatable – maybe it will be a major talking point at the next euroLED meeting, scheduled for June 2013.

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