The use of Germanium as an alternative substrate for the growth of
AlInGaP LEDs provides several technical advantages such as lower
substrate costs and the possibility of fabricating As-free AlInGaP
devices. The LED layer structures are grown in a multiwafer MOVPE
reactor on 4 inch Ge substrates. The growth conditions, such as
temperature and substrate orientation, influence the LED external
efficiency and its degradation behavior. In particular, it is
found that during growth Ge is incorporated into the layers, which
strongly affects the LED efficiency. Moreover a defect annealing
occurs during regular operation resulting in an increased
efficiency. Electrical characterization as well as deep level
transient spectroscopy are performed in order to characterize the
nonradiative recombination centers. In addition a quantitative
analysis of the external quantum efficiency, before and after
degradation, is carried out and the relative change in the
nonradiative recombination rate is evaluated.
In Thinfilm LEDs, the substrate absorption of the generated light is avoided by a metal reflector between the light emitting layer and the substrate. The light extraction can be further enhanced by buried microreflectors or surface texturing. We demonstrate that the combination of these technologies gives prospects equal or superior to all other known approaches in terms of luminous efficiency and luminance. At a peak wavelength of 617 nm, we have obtained a luminous efficiency of 95.7 lm/W at 20 mA. We further analyze the internal and light extration efficiencies of our LEDs using raytracing simulations as well as a theoretical model for the internal efficiency. This analysis shows quantitatively that the efficient light extraction from InGaAlP thinfilm LEDs becomes more and more difficult when approaching shorter wavelengths.
Operation-induced degradation of internal quantum efficiency of high-brightness (AlxGa1-x)0.5In0.5P light-emitting devices (LEDs) is analysed experimentally and theoretically. A test series of LEDs was grown by MOCVD with identical layer sequence but different Aluminum content x in the active AlGaInP layer resulting in devices emitting light between 644 nm and 560 nm. The analysis yields the wavelength dependence of both the nonradiative recombination constant A as well as the carrier leakage parameter C of devices before and after aging. While test devices with λ>615 nm are very stable, LEDs with shorter emission wavelengths exhibit both an increase of A and a slight decrease of C upon aging. Possible degradation mechanisms are discussed.