Influence of thermal stress on I-V characteristics and low-frequency noise of AlGaInP UHB-LEDs

Light emitting AlGaInP double heterostructures were exposed to thermal stress at 670 K in order to investigate the influence of aging processes on the I-V characteristics and the low frequency noise. The results reveal a strong dependence of the degradation mechanisms on the operating current. UHB-LEDs, which were only thermally stressed for 9 h, showed nearly no degeneration of the device performance. These thermally stressed diodes can be described by a simple series combination of an ideal light emitting diode and a series resistance during the whole stress duration. In contrast to this, the samples thermally stressed during operation at 20 mA degenerated abruptly after more than 4 h. For the explanation of the I-V dependence on the stress during operation a model was developed. Due to the applied electric field in combination with heat, dark-regions were generated. The development of these non radiating zones is associated with dislocations because of the directional diffusion of dopants into lattice interstitials and strain at the hetero-interfaces. After 9 h, the samples stressed during operation did not emit light up to currents of 100 mA. Furthermore, the low frequency noise spectroscopy proofed itself to be a very sensitive diagnostic tool for detection of aging processes