Semiconductor lighting enters the boardroom

28 Aug 2002

Osram Opto Semiconductors has just settled a long-running litigation battle with Nichia of Japan over patents for blue and white light-emitting diodes. Johan van der Linden profiles the German company that is one of the pioneers of gallium nitride technology.

From Opto & Laser Europe September 2002

The news that Nichia and Osram Opto Semiconductors (OOS) have finally agreed to settle their differences over gallium nitride (GaN) based opto-semiconductors must have been welcomed by Osram's executives. The technology in question is the basis for the industrial manufacture of blue-emitting LEDs and lasers - devices that are likely to play an important role in optical data storage and display applications. The GaN/InGaN-based semiconductors can also be combined with phosphor convertors to generate white light. A patent cross-licence agreement now opens the way for the devices to see more widespread adoption.

In March 2001 OOS became the first company in Europe to manufacture a GaN blue laser-diode operating in continuous-wave mode at room temperature. Until then, the market had been cornered by Nichia.

Rüdiger Müller, CEO of OOS, commented on the end of the dispute: "We are sure that both partners will profit from this agreement as the patent dispute had brought an air of uncertainty to the market."

Joint venture

Based in Regensburg, Germany, OOS is a fully owned subsidiary of Osram, one of the world's largest lamp manufacturers. The firm was founded in January 1999 as a joint venture between Osram and Infineon Technologies (both spin-offs from Siemens). The venture brought Osram's optoelectronics activities together with similar operations at Infineon.

Less than two years later, in August 2001, Osram purchased the 49% share of OOS that belonged to Infineon. Today, it continues to use its former partner's sales infrastructure. OOS currently has a staff of more than 4000, of whom about 1100 are employed at the Regensburg headquarters (300 people have been hired over the last 12 months). Most of the firm's remaining employees work at its processing facility in Penang, Malaysia. The latter location also serves as a front-end facility for manufacturing organic LEDs, based on a licensing agreement with UK-based polymer LED pioneer Cambridge Display Technology.

Revenue from OOS accounted for about 7% of Osram's overall group sales of € 4.5 bn in 2001. Wolf Dieter Bopst, president of the Osram group, has no doubts about the potential for growing this figure through the use of LEDs in general lighting applications: "Optical semiconductors will revolutionize the world of lighting," he said. "LEDs last longer and are much smaller, more rugged and more economical than incandescent lamps."

Already a world leader in interior lighting for the automotive industry - LEDs are now thought to hold a market share of around 75% in this sector - OOS is aiming even higher: "Our vision is to be number one in opto-semiconductor illumination, sensing and visualization," said Stephan Kreusel, distribution manager at OOS.

LED brightness is crucial for existing markets such as traffic lights, full-colour displays, cellphone backlights, and vehicle rear lights. OOS has devoted a considerable chunk of its research and development resources to improving brightness. In particular, light-extraction efficiency has been greatly enhanced. An LED's external quantum efficiency is defined as the product of its internal quantum efficiency (number of created photons) and the extraction efficiency.

Improving brightness - recognized as a challenge ever since the first LEDs were conceived in the late 1950s - meant reducing the refractive index difference and/or restructuring the light-emitting surface to increase the spatial cone in which no total internal reflection occurs. OOS has come up with a cluster of innovative designs to achieve this, including implementing back-surface or epitaxial reflectors, as well as reducing absorption inside the semiconductor substrate material.

Record luminous efficiency In October 2001 it was discovered that a combination of surface-structuring and thin-carrier bonding, followed by the removal of the GaAs growth substrate, produced a record luminous efficiency of more than 50 lm/W at 615 nm from an AlGaInP-based LED. The design of the LED uses a metal coating as a bonding film that is partially alloyed to act both as an electrical contact and a highly reflective mirror.

Josef Attenhauser, process integration manager at the firm, says that because OOS is the only company that is using 4 inch GaAs epitaxial wafers, this bonding process is particularly advantageous. However, OOS is still using 2 inch silicon carbide substrates for its InGaN-based product line, through a purchasing agreement with the US company Cree.

The Regensburg production facility currently covers an area of about 2000 m², but just over a year ago OOS began construction work on a 40,000 m² state-of-the-art semiconductor component front-end manufacturing plant. With € 120 m invested in the first phase of the project, the plant will be operational by the end of this year.

The ultra-bright LED series is already used as display lighting for mobile communications and in intelligent traffic-control modules. One of the company's latest products for the automotive market is a central brake-light that is just 3.2 mm thick - generating substantial cost savings in the assembly plant, because rear-window mounting becomes much easier. LED lifetime of much more than 10,000 h additionally cut the vehicle's maintenance costs.

General lighting White LEDs for general lighting purposes still face tough competition from the classical sources (temperature radiation, low-pressure and high-pressure discharge sources), but they also clearly complement these technologies. With its mother company being a major player in the classical lamp field, OOS head of engineering Günter Waitl believes the LED market is fundamentally driven by demand for colour sources, rather than white light.

Low cost and high wall-plug efficiency are the most important issues in general lighting. Epitaxy remains an expensive technology in terms of both initial investment and operation costs, and the use of phosphors for light conversion can reduce efficiency and reliability. However, for some applications the output power is the only factor that really counts.

With a standard incandescent bulb producing slightly more than 100 lm (with an efficiency of less than 10 lm/W), it is clear why some believe that LEDs have a bright future in general lighting applications. Energy savings are often quoted as the major driving force for the replacement of conventional white light sources by LEDs. However, fluorescent lamps with high conversion efficiencies (more than 50 lm/W) in a wide variety of application-specific emission spectra also exist.

An important future role for semiconductor line-type emitters in general lighting could be the "colour-tuning" of base sources like halogen lamps (brightness level in the order of 25 lm/W). "Don't forget that what we call natural light is a spectrum, which changes constantly over the day as well as the seasons," emphasized Waitl. Specific-colour LEDs could be added to white-light lamps to simulate this change in natural light.

Another emerging market for high-brightness LEDs is in illuminated corporate signs, such as those used in shops and petrol stations. Many of these signs are registered trademarks that contain a particular colour specification, and a major problem for such LED modules is creating uniformity in power and colour. In the yellow wavelength range, in particular, the human eye's sensitivity limits the purity margin to less than 1 nm for assembled sources, which makes extensive testing and labelling procedures necessary.

A significant amount of research activity at OOS has been committed to the development of wavelength converter materials. One of the purposes of this is to manufacture coloured LEDs for fashionable limited-series mobile phones or car dashboards.

For more information
Osram Opto Semiconductors www.osram-os.com