High-brightness light-emitting diodes (LED) based on AlGaInP combines the possibility to achieve high efficiency with the flexibility of tuning the emission wavelength over a large range of the visible spectrum. For optimizing the device characteristics an accurate determination of the electronic properties, like e. g. the voltage drop across the semiconductor layer sequence, is desirable. We demonstrate the potential of Kelvin Force Microscopy for quantitative investigations of the voltage drop across the heterostructure layers of an operating AlGaInP LED. The surface potential was measured for external biases between -2.0 V and +1.86 V. By subtracting the zero bias result the voltage drop could be extracted quantitatively. In the low voltage regime, most of the voltage drops in the active layer. Above +1.5 V an additional voltage drop occurs on the p-side of the device, i. e. outside the active layer sequence, which reduces the efficiency of the LED. By comparing experimental data with simulations we will discuss possible mechanisms of these findings.
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.
High brightness AlGaInP thin-film resonant cavity LEDs with an emission wavelength around 650 nm are presented. The combination of a thin-film waveguide structure and a resonant cavity with an omnidirectional reflector (ODR) leads to significantly higher efficiencies compared to standard resonant cavity LED (RCLED) structures. Preliminary devices based on this configuration show external quantum efficiencies of 23% and 18% with and without encapsulation, respectively, despite a non-ideal detuning. These devices exhibit a narrow far-field pattern and are therefore adapted for applications requiring high brightness emitters such as for example plastic optical fiber communications. By opting for a negative detuning, i.e. a cavity resonance that is red-shifted compared to the intrinsic emission spectrum, even higher efficiencies should be achievable.
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.