Electrical and optical properties of Nichia double-heterostructure blue light-emitting diodes, with In 0.06 Ga 0.94 N:Zn, Si active layer, are investigated over a wide temperature range from 10 to 300 K. Current-voltage characteristics have complex character and suggest the involvement of various tunneling mechanisms. At small voltages ͑and currents͒, the peak wavelength of the optical emission shifts with the applied bias across a large spectral range from 539 nm ͑2.3 eV͒ up to 443 nm ͑2.8 eV͒. Light emission takes place even at the lowest temperatures, indicating that a complete carrier freeze-out does not occur.

We investigate electrical and optical characteristics ofNichia NLPB-500 doubleheterostructure blue light-emitting diodes (LEDs), measured over a wide temperature range from 10 to 300K. Current-voltage characteristics have complex character and suggest involvement of at least two different tunneling mechanisms. The peak energy of the optical emission follows the applied bias for voltages between 2.3-2.6 V and can be tuned in large spectral range from 2.3 up to 2.8 eV (yellow to blue). This behavior can be understood invoking the photonassisted tunneling model which was previously successfully applied to highly doped GaAs LEDs. Even at the lowest temperatures, light emission still continues while the increase in the series resistance does not exceed a few tens of k~, which indicates absence of complete carrier freeze-out.

GaNIIn0.05Ga5N/A10.15Ga.85N double heterostructures doped with Zn and Si, used in Nichia LEDs, are investigated. Electrical, electroluminescent and photoluminescent properties are presented and discussed. Blue photoluminescence (PL) is analyzed to obtain optical transition parameters (phonon coupling strength and zero-phonon line position) involved in formation of the impurityrelated emission band. With a minor modification ofparameters for Zn centers in GaN, a satisfactory fit is achieved for PL spectra.

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The reliability of devices fabricated in GaN and related alloys, especially under high current densities as would be found in lasers, has yet to be fully characterized. Our previous work [1] investigated the degradation of GaN-based blue light emitting diodes (LEDs) under high pulsed current stress. This work indicated a possible correlation between the high crystal defect density and failures caused by metal migration along these defect tubes. To assess the impact of this data on devices under more normal conditions, several LEDs from both older and more recent production lots were placed in a controlled temperature and current environment for several thousand hours. The test started with a constant 20 mA current for the first 1000 hours and continued for another 1650 hours at various currents up to 70 mA, all at a temperature of 23 °C. During this test, one of the older generation LED's output degraded by more than 50%. Subsequent failure analysis showed that this was caused by a crack which isolated part of the active region from the p-contact. The remaining LEDs were returned to life testing where the temperature was subsequently increased by 5 °C after each 500 hours of testing. The output from one of the newer LEDs dreiven at 70 mA degraded to 55% of its original value after 3600 hours and a second newer LED degraded by a similar amount after 4400 hours. The first failure, LED #16, did not exhibit a significant change in its I-V characteristics indicating that a change in the package transparency was a likely cause for the observed degradation. The second failure, LED #17, did show a noticeable change in its I-V characteristics. This device was subsequently returned to life testing where the degradation process will be monitored for further changes.