The concept of an AlGaInP thin-film light emitting diode includes a structure of semiconductor layers with low optical absorption on which a highly reflective mirror is applied. After bonding this wafer to a suitable carrier, the absorbing GaAs substrate is removed. Subsequently, electrical contacts and an efficient light scattering mechanism for rays propagating within the chip is provided. To achieve high efficiency operation it is crucial to optimize all functional parts of the device, such as the mirror, contacts, and active layer. Different mirrors consisting of combinations of dielectrics and metals have been tested. New chip designs have been evaluated to reduce the absorption at the ohmic contacts of the device. For efficient light scattering, the surface roughness of the at the emission window has to be optimized.
Using these structures, and a thin active layer consisting of five compressively strained quantum wells, an external quantum efficiency of 40% is demonstrated at 650 nm. Further improvement is expected.
Since the AlGaInP material system can provide only poor carrier confinement for active layers emitting in the yellow wavelength regime, the internal efficiency of these LEDs is comparably low. In order to reduce the problem of carrier leakage, a yellow active region usually consists of some hundred nanometers of active material. To circumvent the problem of this highly absorbing active layer, a separation of the light generation and the area of light extraction is suggested for yellow thin-film LEDs. First results are presented in this paper.