Osram Opto Semiconductors (distributed by Braemac and ) says it has developed an indium gallium nitride-based substrate technology that can produce high luminosity, blue thin-film LEDs. These could eventually be used in non-conventional LED applications such as vehicle headlighting that have traditional been ruled out due to the device’s insufficient brightness.
The substrate technology-called ‘ThinGaN’-is said to make it possible to extract up to 75 percent of the internally generated light from a chip while also offering an efficient method for extracting the actual chip from a semiconductor crystal.
The standard substrate material for the indium gallium nitride (InGaN) blue LEDs is based on silicon carbide (SiC). Typical substrate thickness is 250 µm and it is said to be both chemically and mechanically stable.
However, SiC cannot be completely removed-with either wet chemical or dry chemical plasma processes-without also destroying a critical 5-µm thin InGaN epitaxial layer.
The company claims to have solved the problem by tailoring the epitaxial procedure specifically to the SiC material and developing a pulsed UV laser method for gentle substrate removal. In what is said to be 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 is then cleanly separated from the substrate.
Key factors behind the high efficiency of ThinGaN LEDs are an optimised surface roughness, low optical losses in the LED itself and mirror metallisation in the form of a reflector tailored to this material combination. Prototypes of 5-mm radial blue LEDs are quoted to have achieved brightness values of up to 16 mW (at 460 nm) at an operating current of 20 mA. Full scale production is expected to start in 2004.