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This paper analyses the influence of the sapphire substrate on stress in GaN epilayers in the temperature range between 4K and 600K. Removal of the substrate by a laser assisted liftoff technique allows, for the first time, to distinguish between stress and other material specific temperature dependencies. In contrast to the prevailing assumption in the literature, that the difference in the thermal expansion coefficients is the main cause for stress it is found that the substrate has a rather small influence in the examined temperature range. The measured temperature dependence of stress is in contradiction to the published values for the thermal expansion coefficients for sapphire and GaN.

We have studied the pressure dependence of photoluminescence for various InGaN quantum wells of different Indium compositions. In contrast to classical quantum well systems such as GaAs/AlGaAs, the pressure coefficients of the photoluminescence peak position of wells of similar parameters can vary greatly, depending on conditions of growth. Generally, the pressure coefficient of the emission peak is much lower than the one predicted for a GaN-InN system. We show that this behavior can be related in most cases to the influence of the thick GaN layer compressibility on the strain in the InGaN quantum wells. However, in some cases (Nichia devices), the pressure coefficient of the emission peak seems to be unrelated to the shift of the energy gap of the material, suggesting the involvement of strongly localized electronic states

A photoluminescence from a multiquantum GaN/InxGa1−xN/GaN well structure (x varies between 0.1 to 0.4) was investigated at various temperatures, pumping powers. While the temperature dependence of the peak position indicates normal band to band character of radiative recombination, the large pumping power induced “blue”shift of the peak position (up to 200 meV) can be observed. This kind of shift cannot be easily explained by the band tailing effect but is most likely the result of the screening of the strain-induced piezoelectric field. By evaluating the theoretical values of the piezoelectric filelds in the quantum well, we can show that in order to account for the experimental results we have to assume the partial relaxation of the strain.

We propose a new approach to explain the unusually low pressure coefficients of the luminescence peaks observed in single-quantum-well InGaN-based light emitting diodes manufactured by Nichia Chemical Industries. In view of the most recent first principles band structure calculations for InN under hydrostatic pressure, we find that it is possible to reproduce the measured low (12–16meV/GPa) pressure coefficients of the luminescence by assuming the formation of zincblende InN inclusions in the InGaN quantum well layers. These cubic inclusions, surrounded by the usual wurtzite material, should act like quantum dots giving rise to enhanced electron localization. Obtained this way, the pressure shift of the luminescence peaks in blue and green InGaN-based emitters should be close to 14meV/GPa, in good agreement with our experimental results. This explanation of the observed low pressure coefficients in these devices is consistent with recent independent evidence for InN inclusions in InGaN epilayers.

We have studied the electroluminescence and photoluminescence of Nichia single-quantum-well Al0.2Ga0.8N/In0.45Ga0.55N/GaN green light-emitting diodes over a broad range of temperatures (15-300 K) and currents (0.2 μA - 2 A). The most striking behavior is an anomalous temperature shift of both photo- and electroluminescence, with the emission peak moving towards higher energies with increasing temperature. This blue shift is opposite to that of the energy gap of the active layer, which practically excludes interband transitions as responsible for the observed optical transitions. We suggest that population effects within the band tails can account for the observed anomaly. We also determined that the current flowing through the p-n junction is dominated by carrier tunneling, the omnipresent effect in the GaN-based optoelectronic devices.

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.

We performed a degradation study of high-brightness Nichia single-quantum well AlGaN-InGaN-GaN green light-emitting diodes (LEDs). The devices were subjected to high current electrical stress with current pulse amplitudes between 1 A and 7 A and voltages between 10 V and 70 V with a pulse length of 100 ns and a 1 kHz repetition rate. The study showed that when the current amplitude was increased above 6 A, a fast (about 1 s) degradation occurred, with a visible discharge between the p- and n-type electrodes. Subsequent failure analysis revealed severe damage to the metal contacts which lead to the formation of shorts in the surface plane of the diode. For currents smaller than 6 A, a slow degradation was observed as a decrease in optical power and an increase in the reverse current leakage. After between 24 and 100 hours however, a rapid degradation occurred which was similar to the rapid degradation observed at higher currents. The failure analysis results indicate that the degradation process begins with carbonization of the plastic encapsulation material on the diode surface, which leads to the formation of a conductive path across the LED and subsequently to the destruction of the diode itself.

Studies of electrical characteristics of nitride-based light emitting diodes (LEDs) are of interest as they can shed light on carrier transport across the p-n heterojunction. In addition, they provide a convenient way of investigating degradation processes associated with high electrical stress. We present electrical characteristics of Nichia NLPB500 blue LEDs based on AlGaN/InGaN/GaN material system in the temperature range of 9-340 K. Two components of current are identified. High-density current stress leads to diode degradation by shunt formation caused by metal electromigration. At 8 K, blue emission was observed at currents as small as 20 nA.