Analysis of internal quantum efficiency of high-brightness AlGaInP LEDs

The internal quantum efficiency of light emitting diodes (LEDs) based on the AlGaInP material system was analysed. This study formulated a comparatively simple rate equation model to gain a quantitative understanding of the contributions of the different loss mechanisms. On the experimental side a series of LED structures was grown with multiple (Al_xGa_1-x)_0.5In_0.5P quantum wells embedded in In_0.5Al_0.5P confining layers. The emission wavelength was adjusted varying the aluminum content, from 650 nm down to 560 nm in the green spectral range. Simple test structures were processed and the optical output power measured over a certain range of operating currents and temperatures. On the other hand, in the computation of the external quantum efficiency it is important to note that due to re-absorption and re-emission of light in the active layer (´photon recycling´) the extraction efficiency becomes dependent on the internal quantum efficiency. This effect was accounted for by performing ray-tracing simulations for these specific LED structures. For small internal efficiencies the extraction efficiency decreases by a factor of 3. Including this effect in the model a good fit to the experimental data over a wide temperature and current range could be achieved.