Manufacturing LEDs: Developing and exploiting an edge

LEDs are ideal for horticultural lighting, combining high efficiency with the opportunity to target a spectral range. Sanan has a 30 percent share of the Chinese market for horticultural lighting. Chipmakers are chasing better margins via greater economies of scale, carefully targeted product portfolios and superior device performance

The LED industry is not in the greatest of health. Despite a slight rise in global revenue, market conditions are tough. Recent, severe price erosion is making it challenging to operating with good margins, and any LED chipmaker striving to do so will have to develop and exploiting an edge over its peers.

At the seventh CS International Conference, held in Brussels on 7-8 March, three of the world's biggest LED chipmakers offered an insight into how they are trying to succeed in today's market. The Chinese colossus Sanan Optoelectronics revealed its advantages when it comes to economies of scale, resulting from tremendous manufacturing capacity, while Philips Lumileds and Osram Opto Semiconductors, two industry stalwarts, explained how they are setting themselves apart from the chasing pack by producing devices with superior performance.

Colossal capacity

Delivering the keynote talk in the session Optimising Light Emitters, Sanan's Chief Technology Officer, Troy Hsu, began his presentation with a positive take on the LED lighting market, claiming: "There is lots of space for lighting growth in the next few years".

To back up this view, Hsu shared data from two market researchers. He said that Digitimes had forecast that the global LED lighting market would be worth $24.6 billion in 2016, with this form of lighting having a penetration rate of 31 percent; and quoted reports from LEDinside, suggesting that the LED lighting market will grow from $29.6 billion in 2016 to $33.1 billion in 2017, when it will have a penetration rate of 52 percent.

This growth is good news, but note that increases in LED sales will be modest, and declining. LEDinside suggests that the LED market will grow by 4.2 percent from 2016 to 2017, before slowing to year-on-year growth of initially 2.6 percent, followed by 2.7 percent, 1.2 percent, and then a 0.1 percent decline.

Hsu discussed the expansion of LED manufacturing in China, and the dominance of Sanan. China's share of the global LED industry has rocketed from 27 percent in 2013 to 46 percent last year, with Sanan's share of this accounting for more than 55 percent.

At CS International 2017, Chief Technology Officer of Sanan, Troy Hsu (left), delivered the keynote talk in the session Optimising Light Emitters. He spoke about the company's expansion plans, which are claimed to transform the chipmaker into the biggest in the world. Speaking in the same session, Martin Behringer (centre), leader of chip research at Osram Opto Semiconductors, showed how improvements in crystal quality can combat droop, and Oleg Shchekin (right), Senior Director of Device Architecture, Technology Research and Development at Lumileds, discussed the role of the entendue in LED performance.

This strong position requires substantial manufacturing capacity. Sanan certainly has that, with more than 6000 employees shared over four production sites that are equipped with the equivalent of 259 Veeco K465i MOCVD reactors.

Sanan's target is to have the greatest epi-capacity in the world, and it is certainly well on the way to that goal. In 2017, it plans to increase the number of MOCVD tools to more than 376, and it "maybe up to 400".

The capabilities of Sanan extend beyond visible LEDs to include the production of various substrates, infra-red, UV and microwave devices, and chips for optical communication. Backing up this activity are 640 global patents, plus another 571 that are pending.

To illustrate the great range of expertise at Sanan, Hsu described the company's core technology, in terms of epitaxy, substrates and devices for different material systems. The company has strength in GaN epitaxy, the production of sapphire and SiC substrates and the production of LEDs based on this; it has capabilities in GaAs epitaxy, the growth of GaAs and germanium substrates, and the fabrication of microwave devices and solar cells; and it has expertise in InP epitaxy, InP substrates and the production of devices for optical communications.

The LEDs produced by China's leading chipmaker span the deep UV to the infra-red: AlGaN-based devices emit at 260 nm to 280 nm, products based on AlInGaN cover 365 nm to 535 nm, AlGaInP chips extend from 564 nm to 618 nm, and AlInGaAs LEDs emit from 610 nm to 940 nm.

Sanan's portfolio is vast, due to the range of devices it produces at different wavelengths, and it is enjoying significant success in many sectors. It has 70 percent of the domestic market for lateral chips used for backlighting, along with 40 percent of the market for general lighting, which it serves with a variety of devices, including high-voltage variants. Hsu explained that these products, operating at 9 V, could be used to replace two or three die.

When it comes to displays, Sanan has a 70 percent share of the domestic market for indoor displays, and accounts for 50 percent of the outdoor display sector. According to Hsu, strengths of LEDs that serve this application include a high brightness in the green, a long reverse-bias operating lifetime and good immunity to electrostatic damage.

To cater for those that miss the warm glow of the incandescent bulb, Sanan has released a form of lateral device that it describes as its filament series. Chips have efficacies of 160 lm/W to 230 lm/W, and offer uniform dimming.

Sanan has a very strong position in the domestic market for flip-chip LEDs. The company's high-brightness products, which can be used for automobile headlights and camera flash, have a lower forward voltage and higher light output than those of their rivals, claimed Hsu. The demand for this class of device is so high that a three-fold expansion in capacity is taking place during the start of this year.

The company also produces a range of vertical LEDs with powers of 1 W to 30 W. They are targeting projection, industrial, automotive and entertainment markets. These devices can handle current densities of up to 4 A mm-², and are available as white emitters with colour temperatures of 2700 K to 7000K, and single-colour emitters in the UV, blue, green, red, deep red and far red.

Bulbs based on filament LEDs have the look of an incandescent, but the efficiency of a solid-state source. Manufacturers of these tiny LEDs included Sanan.

Within the UV range, Sanan produces devices that span the UVA (360 nm to 405 nm) and the UVC (265 nm to 300 nm). The former, which are claimed to deliver world-class performance and can be used for curing, zapping bugs and horticulture, include 280 nm devices that produce more than 30 mW at 350 mA. In the lab, devices are emitting 70 mW. Products with a 50 mW output are planned for release in the third quarter of this year.

Sanan's red, orange and yellow LEDs, which are based on the AlInGaP material system, are being use for outdoor displays, automotive lighting, and horticulture. It also produces AlGaAs-based, infra-red LEDs that can handle drive currents up to 1.5 A mm-², and are targeting surveillance systems, touch panels, vehicle night vision, medical equipment, iris recognition and data sensing.

Summarising the efforts of the company, Hsu explained that Sanan will soon have the greatest capacity of any LED chipmaker in the world, thanks to a 70 percent expansion this year. The current focus is not on standard, visible LEDs, but those emitting in the UV and infra-red, as well as micro-devices and emitters for the automotive market.

Combatting droop

An insight into improving the performance of the LED was provided by Martin Behringer, leader of chip research at Osram Opto Semiconductors.

Behringer began his talk by pointing out that today's LEDs can have an internal quantum efficiency in excess of 85 percent at room-temperature. However, the efficiency peaks at a low current density "“ typically just a few A cm-². "We want to operate at a very high current density, and that's far away from the peak, so we lose ten to fifteen percent."

According to Behringer, this loss "“ known as droop "“ is well understood, being caused by Auger recombination. He argued that loss can be reduced by either: lowering the current density, via the introduction of new architectures; or increasing the crystal quality of the device. Using the latter approach, engineers at Osram have improved the internal quantum efficiency over the entire operating range, compared to devices from 2012 and 2015.

Another option for overcoming droop is to switch to the third dimension "“ that is, to produce LEDs from nanowires, which operate at lower current densities. Merits of this approach are not limited to increased efficiency, but extend to lower costs, optical and thermal benefits, an increased active area per wafer area, and the opportunity to use a close-coupled, micro-grain phosphor. What's more, as the emission size is equal to the chip size and the packages can be very small, the technology is scalable, as devices can be placed right up to one another in arrays.

Blue nanowire devices developed by Osram produce homogeneous emission, exhibit minor current crowding close to the current-spreading chips, and have an external quantum efficiency, estimated from wafer-level measurements, of 10 percent at 70 mA.

Like Sanan, Osram is keen to compete in the ultra-violet market. UVA LEDs are in development, producing wall plug efficiencies in excess of 50 percent at up to 1.5 A. These devices are capable of emitting 4.5 W mm-² when driven at 3 A. In the deep UV, high-performance is far harder to realise, with Behringer listing challenges that include doping aluminium-rich AlGaN, realising a high quantum efficiency, extracting most of the light out of the device, and ensuring its reliability.

One option for increasing the external quantum efficiency in all forms of LED is to stack quantum wells on top of one another, and insert tunnel junctions between them. Behringer explained that this promises to produce infra-red LEDs that combine an operating voltage of 1.7 V "“ far less than that of the battery "“ with an external quantum efficiency in excess of 100 percent, thanks to using each electron twice.

Profiling emission

Speaking within the same session, Oleg Shchekin, Senior Director of Device Architecture, Technology Research and Development at Lumileds, outlined approaches to deliver high efficiency at high power densities.

Shchekin began by highlighting the tremendous progress in LED efficiency, which has led to cool-white LED packages with efficacies of over 150 lm/W, and warm-white variants not that far behind.

According to data presented by Shchekin, retaining the conventional design, the phosphor-converted LED, would allow further gains in efficacy to around 240 lm/W. However, switching to a combination of a blue LED, a green phosphor, and a red LED would lift the ceiling to about 300 lm/W, while 350 lm/W might be possible by combining red, green, blue and amber LEDs.

Two classes of LED were discussed in detail: conventional mid-power LEDs, which have high extraction efficiency die, low operating power densities for a high efficiency, and a large reflective cup that reduces optical losses; and conventional high-power LEDs, which feature high-extraction thin-film flip-chip die, a highly reflective and thermally conductive sub-mount, and a die footprint of 2 mm² or more to maintain a high internal quantum efficiency.

"Unfortunately, not all light is useful," warned Shchekin, before going on to explain the concept of the entendue, which is the product of the emitting area of the source, the emission solid angle and the square of the refractive index.

The lower the entedue, the greater the freedom of the designer to optimise light utilisation and system size. Both conventional mid-power and high-power LEDs are poor is this regard, having a high entendue that stems from a large source area "“ and in the case of the latter device, also the silicone dome.

Superior structures for combining high-luminance with a low-etendue are chip-scale packages, and isolated thin-film flip chips and vertical thin-film chips. Challenges with these architectures include droop, due to the higher current densities.

Shchekin discussed approaches to combat droop associated with both the chip and the phosphor, and also technologies for increasing extraction efficiency (both are detailed in the feature "Strategies for better, brighter LEDs" on p.36").

It is clear from this talk, and those of Hsu and Behringer, that the leading LED makers are striving hard to improve the capability of their products, and widen their portfolio. Profits are harder to come by, but that's not stopping performance from increasing.