The III-V semiconductor system AlGaInP is today the only material suitable for the fabrication of high-brightness LEDs in the yellow to red color range. The material can be grown lattice matched on GaAs-wafers and allows to tune the bandgap wavelength from 560 nm to 650 nm. The performance of conventional AlGaInP LEDs is limited by two fundamental problems, the high reflective index of the semiconductor and the absorption of visible light of the GaAs-substrate. Due to the large difference in refractive index between the LED chip and the surrounding medium more than 90% of the generated light is totally reflected inside the device, where it is eventually converted to heat. In addition, all light emitted or reflected towards the substrate is lost by absorption. Despite these problems, the quantum efficiency of AlGaInP-LEDs has been increased to several tens of percent using thick GaP-window layers on top of the LED structure and by replacing the original GaAs substrate by a transparent GaP-wafer using direct wafer-bonding. The best performance in terms of efficiency is achieved, when sawing the transparent wafer into dies with the shape of inverted pyramids.
© 2003 Optical Society of AmericaPDF Article