World record in luminous efficacy:

Thin-film technology now also for blue LEDs

With blue thin-film LEDs it is now possible to extract up to 75% of the internally generated light from the chip. Osram Opto Semiconductors is the first company in the world to solve the physical problems of implementing the thin-film concept for blue LEDs. The new ThinGaN technology, which is based on indium gallium nitride, will promote applications of far-reaching significance such as the use of LEDs in vehicle headlights. Series production is due to start in 2004.

Light is generated inside LED chips with a high degree of efficiency. The greatest challenge with LEDs is finding an efficient way of extracting the light from the semiconductor crystal. The substrate, which is used as the germ for crystal growth, absorbs a large proportion of the generated light. In the case of thin-film technology, the substrate is still used for growing the crystal but once this process has finished the substrate is removed. What’s left behind is only a thin light-generating layer. More light can escape than is the case with conventional technologies.

The standard substrate material for blue LEDs based on InGaN is SiC (silicon carbide) with a typical substrate thickness of 250 µm. It is chemically and mechanically very stable and cannot be completely removed with either a wet chemical process or dry chemical plasma processes without also destroying the 5 µm thin InGaN epitaxial layer. Alternatively, sapphire (Al2O3) is suitable as a standard substrate for InGaN epitaxy. However, the material properties of sapphire cause major distortions and defects during InGaN growth, which reduce the internal luminous efficacy. Osram Opto Semiconductors has tailored the epitaxial procedure specifically to this material and at the same time developed a method for careful removal of the substrate material. In the world’s first production-scale laser lift-off plant, the light from a pulsed UV laser splits up the semiconductor material into its components. It can then be cleanly separated from the sapphire substrate.

Other key factors for the exceptionally high efficiency of the thinGaN LEDs are the optimised surface roughness, low optical losses in the LED itself and mirror metallisation as a reflector tailored to this material combination. Prototypes of 5 mm radial LEDs achieve brightness values of up to 16 mW for the blue products (460 nm) at an operating current of 20 mA.

About Osram Opto Semiconductors
Osram Opto Semiconductors GmbH, Regensburg, is a wholly owned subsidiary of Osram, one of the world’s three largest lamp manufacturers, and offers its customers a range of solutions based on semiconductor technology for lighting, sensor and visualisation applications. The company operates facilities in Regensburg (Germany), San José (USA) and Penang (Malaysia). In fiscal year 2002 (ending September) total sales of Euro 309 million were generated by some 3370 employees.

Further information is available at www.osram-os.com.

Press contact:

OSRAM

Marion Reichl
Tel: +49-941-850-1693
Fax: +49-941-850-4441693
Email: marion.reichl@osram-os.com